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Special Paper |
School of Life Science, Xiamen University, Xiamen 361005, P. R. of China; Department of Botany and Microbiology, University of Oklahoma, Norman, Oklahoma 73019 USA
Received for publication October 11, 2006. Accepted for publication May 8, 2007.
ABSTRACT
Calcium has an essential signaling, physiological, and regulatory role during sexual reproduction in flowering plants; elevation of calcium amounts is an accurate predictor of plant fertility. Calcium is present in three forms: (1) covalently bound calcium, (2) loosely bound calcium typically associated with fixed and mobile anions (ionic bonding); and (3) cytosolic free calcium–an important secondary messenger in cell signaling. Pollen often requires calcium for germination. Pollen tube elongation typically relies on external calcium stores in the pistil. Calcium establishes polarity of the pollen tube and forms a basis for pulsatory growth. Applying calcium on the tip may alter the axis; thus calcium may have a role in determining the directionality of tube elongation. In the ovary and ovule, an abundance of calcium signals receptivity, provides essential mineral nutrition, and guides the pollen tube in some plants. Calcium patterns in the embryo sac also correspond to synergid receptivity, reflecting programmed cell death in one synergid cell that triggers degeneration and prepares this cell to receive the pollen tube. Male gametes are released in the synergid, and fusion of the gametes requires calcium, according to in vitro fertilization studies. Fusion of plant gametes in vitro triggers calcium oscillations evident in both the zygote and primary endosperm during double fertilization that are similar to those in animals.
Key Words: angiosperm sexual reproduction • calcium • cell signaling • double fertilization • gamete interactions • programmed cell death
When compatible pollen lands on the stigma, pollen grains germinate, forming tubes, which typically elongate through a pollen aperture. The commonly haploid pollen tube continues to elongate within the sporophytic (commonly diploid) tissue of the pistil, proceeding through the style and, once within the ovary, on to the placenta. The tube ultimately reaches an ovule and enters the haploid female gametophyte (commonly known as the embryo sac). The male gametes are released into the embryo sac, are transported to an appropriate location, and then fuse with their respective female target cell where nuclear fusion and fertilization occurs. In this process, complex interactions occur between the male gametic cells, female gametic cells, and the pistil, permitting attraction and precise positioning of the pollen tube in the style, appropriate ovule targeting, and release of male gametes (commonly called sperm cells) inside the embryo sac. Release of the male gametes occurs at a precise location within the embryo sac, facilitating gamete translocation and placing sperm cells in a unique position for gamete recognition. Two male gametes are present within each pollen tube, and each male gamete fuses with a single female target cell. One male gamete fuses with the egg cell, forming the zygote and subsequent embryo lineage, whereas the other male gamete fuses with the female polar nuclei within the central cell, forming the nutritive endosperm.
In the past 30 yr, numerous studies on pollen tube elongation and pollen-pistil interactions have been published. Recent research into sexual reproduction of flowering plants has increasingly begun to investigate fertilization mechanisms on a biochemical, cell biological, and molecular basis. The role of calcium in this process has received particular attention, as calcium appears to provide a universal signal with pleiotropic effects involving attraction, long and short-distance communication, cellular fusion, and cell signaling. Techniques allowing the isolation and maintenance of pollen and other reproductive cells have been slow to develop because angiosperm gametes are inaccessible and difficult to manipulate. Studies on calcium metabolism in these cells may help us to answer questions about plant fertility and fertilization. This review is among the first to examine the role of calcium in sexual reproduction in flowering plants—a topic that has not been separately reviewed in prior reviews on plant calcium biology.
Calcium is a necessary ion in plant development, with myriad physiological functions. Calcium is tightly compartmentalized in specific states of availability within plants and is required for normal growth and development. In its ionic form, it is known to interact directly and indirectly with calmodulin to regulate other proteins through signal transduction pathways as a second messenger and to serve as a metabolic factor or cofactor in various cellular activities. Calcium mediates cell physiological processes through multiple independent pathways. For many years, calcium regulation in plant physiological processes has been an active topic of research, and results have often been reviewed; some general reviews include Bush (1993
, 1995
), Gilroy et al. (1993), and Sander et al. (1999). Literature reviews summarizing calcium signaling during pollen tube elongation reflect the particularly active research currently proceeding in this area (Hepler, 1997
; Taylor and Hepler, 1997
; Zhang and Cass, 1997
; Malhó, 1998a
; Trewavas and Malhó, 1998
; Yang, 1998
; Franklin-Tong, 1999a
, b
; Robinson and Messerli, 2002
; McClure and Franklin-Tong, 2006
). Among examinations of calcium involvement in sexual plant reproduction, however, the study of calcium interactions in the pistil has been relatively neglected. This current review reports on the role of calcium during sexual plant reproduction, including: (1) recent research regarding the function of calcium in pollen tubes and pollen tube guidance; (2) spatial and temporal characteristics of calcium localization in pistils; and (3) general aspects of calcium function during gametic interactions and fertilization in angiosperms. The role of calcium in early plant development is discussed elsewhere (Anil and Rao, 2001
).
TYPES OF CALCIUM IN CELLS
Calcium in cells exists in several states. The total calcium of a cell is generally purported to range between 0.1 and 10 mM (Nagai et al., 1975
; Spanswick and Williams, 1965
) and can be divided into free calcium, bound calcium, and stored (loosely-bound) calcium. Free calcium is located in the cytosol, is freely soluble, and is present at concentrations of about 200 nM (Brinley et al., 1977
; Bush, 1995
; Sanders et al., 1999
). Bound calcium has a stronger affinity with macromolecules, which often means that it forms covalent chemical bonds requiring substantial energy to break. Such tightly bound calcium cannot be precipitated by competition with low affinity anions, such as antimonate. This tightly bound calcium can be detected by a wide range of non-specific methods, including X-ray analysis (Chandler and Battersby, 1976
; Van Iren et al., 1979
) and proton-induced X-ray emission (PIXE, Reiss et al., 1983
) (Fig. 1). The highest concentration of bound calcium is found in common crystalline forms, such as calcium oxalate. Loosely-bound calcium is the prevalent form of calcium stored in most cells. This form of calcium is typically present at concentrations above 10–6 M and is in dynamic equilibrium with the free ionized form, Ca2+. Stored calcium is often sequestered in the cell wall, localized in specific cellular organelles, or is associated with proteins that use calcium as a coenzyme or that control calcium concentration. When physiological conditions in the cell alter the concentration of Ca2+, the solubility coefficient (Ksp) predicts the highest concentration of ionic free calcium that can accumulate before it spontaneously begins forming ionic bonds. If Ca2+ exceeds the Ksp for a given calcium-binding molecule, then excess calcium is bound; however, if Ca2+ falls below the Ksp, Ca2+ is dynamically released from its ionically bound state. Thus, high concentrations of free Ca2+ can be dynamically depleted above the Ksp by reversible ionic binding, effectively controlling the free calcium concentration. The effective Ksp of the binding molecule, which is often a protein, may be dramatically altered in binding and functionality by changes in molecular or ionization state, including those induced by pH and other environmental factors. Protein alterations through enzymatic modification, or linkage of control molecules, including phosphorylation, also control calcium binding. Any such changes in ionic state of the molecule may alter its ability to bind calcium. Because they occur in an unbound soluble form, some of these calcium binding proteins can serve as transport molecules, acquiring calcium from storage and releasing it where calcium is required (Tandler et al., 1970
).
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). In the tobacco ring cells, programmed cell death is preceded by the accumulation of abundant fine druses in vacuoles, which may contribute to triggering cell death.
Dynamically sequestered calcium, as mentioned before, is typically ionically rather than covalently bound. This calcium store is dynamic and may therefore be readily converted to (and from) free calcium through interaction with a wide variety of control proteins—in this sense such proteins have calmodulin-like properties. Presumably, a wider examination of expressed transcripts and proteomic data in pollen (Hruba et al., 2005
; Sheoran et al., 2006
) and other tissues (Swanson et al., 2005
) may aid in identifying specific protein candidates for such calcium binding; information about such products is likely to be readily available for bioinformatic data mining in international genome and proteome related databases. To avoid the potentially toxic effects of free Ca2+, the concentration of Ca2+ in the cytoplasm and nucleus in resting cells is three to four lower than in other cellular compartments (Bush, 1993
; Gilroy et al., 1993
). However, when the cell is stimulated by external stimuli, free calcium is released, calcium channels open, and free Ca2+ enters through the plasma membrane. Sequestered calcium is also released from the ER, vacuole, and mitochondria (Marmé, 1989
; Trewavas and Malhó, 1998
). Wick and Hepler (1982)
characterized this non-covalently sequestered calcium as loosely bound calcium because of its exchangeable property.
CALCIUM EFFECTS ON POLLEN GERMINATION
When pollen grains are germinated in small numbers in vitro, the rates of germination and elongation are often less than in larger scale pollen culture. This population effect can be overcome by adding more pollen, but it may also be overcome by adding more calcium to the medium (Kwack and Brewbaker, 1961
). Confirmation of this phenomenon has been obtained in 86 species representing 39 plant families (Brewbaker and Kwack, 1963
), so this represents a widespread calcium dependency among flowering plants.
Uptake of calcium by germinating pollen grains has been directly demonstrated using the radioisotope 45Ca2+ (Bednarska, 1989b
), which is rapidly incorporated into pollen. Accumulation of calcium in the membranes of germinating pollen is readily illustrated using the fluorescence microscopic probe, chlorotetracycline (CTC) (Polito, 1983
). At the onset of pear-pollen hydration, the first faint CTC fluorescence was located at the periphery of the pollen grain, associated with germinal apertures. Following hydration, fluorescence continued to intensify near the pore. Calcium accumulation was also demonstrated in the apertures of lily pollen using similar techniques, reflecting a distinct polarization of calcium (Reiss and Herth, 1978
). One hypothesis for calcium uptake is that pollen hydration may trigger increased turgor that allows uptake through stretch-activated calcium channels (Feijó et al., 1995
). This could, in turn, maintain calcium polarization in the pollen grain, amplify polarization of calcium distribution in the pollen tube, and create a sustainable gradient.
During pollen germination, a calcium effect has also been confirmed using calcium antagonists to inhibit germination (Bednarska, 1989b
), although pollen grains in some plants can germinate in media without calcium. Presumably, the latter plants have some calcium stores in the pollen wall, which may be released into the medium during pollen hydration (Shivanna and Johri, 1985
; Heslop-Harrison, 1987
). In rice pollen grains, abundant calcium exists within the pollen grain before germination (Tian et al., 1998
).
CALCIUM EFFECTS ON POLLEN TUBE ELONGATION
Exogenous calcium in the immediate environment affects both pollen germination and tube growth. A famous assay done by Mascarenhas and Machlis (1962)
revealed a chemotropic response of pollen tubes of Antirrhinum majus to external calcium gradients in vitro. In cultured pollen tubes of Tradescantia virginana, the optimum concentration for elongation was from 10–4 to 10–3 mol/L calcium in the medium (Picton and Steer, 1983
). In other plants, pollen tubes grew poorly if calcium concentrations in the medium contained higher or lower concentrations (Morré and Vanderwoude, 1974
; Reiss and Herth, 1980
; Steer and Steer, 1989
; Obermeyer and Weisenseel, 1991
). In culturing pollinated styles of tobacco, pollen tube growth also required supplemental calcium in the medium (Tian and Russell, 1997b
). When the external calcium concentration exceeds 5 x 10–6 M, actin filaments in the pollen tube of Lilium longiflorum become fragmented and cytoplasmic streaming stops (Kohno and Shimmen, 1987
, 1988
). In media containing low concentrations of calcium, generally the pollen tube apex swells, and the cell wall at the tip becomes very thin and discontinuous. In media containing high concentrations of calcium, however, the wall at the tip of the tube becomes overly thickened. Lowering calcium concentrations appears to diminish the elongation of the tube by causing an excessive quantity of vesicles to accumulate at the tip, inducing apical swelling (Morré and Vanderwoude, 1974
; Picton and Steer, 1983
; Steer and Steer, 1989
). High calcium concentrations appear to accelerate vesicle fusion at the pollen tube tip, but cytoskeletal elements may be altered, contributing to a thickened wall at the tube tip.
External effects of calcium on pollen tube growth have been confirmed by adding calcium channel blockers or antagonists into the medium (Bednarska, 1989b
; Heslop-Harrison and Heslop-Harrison, 1990
). Pollen tube growth is inhibited by experimentally opening calcium channels using the calcium ionophore A23187 in the medium. Ionophore A23187 disturbs the calcium gradients inside and outside the pollen tube, permitting Ca2+ to pass freely through the plasma membrane (Reiss and Herth, 1978
, 1979b
; Picton and Steer, 1983
; Bednarska, 1989a
).
Calcium distribution in pollen tubes
Intracellular calcium has been measured using a variety of localization techniques. Each method displays unique sensitivities and weaknesses, and the method by which the probes are applied vary in their invasiveness and reliability. As different techniques have emerged, they have been applied to accumulate complementary data on the different forms of calcium. Using the radioisotope 45Ca2+ and radioautography, Jaffe et al. (1975)
demonstrated that externally absorbed calcium first accumulated at the pollen tube apex. Proton-induced X-ray emission (PIXE), which measures total calcium, indicated that concentrations in pollen tubes of Lilium longiflorum range from 1300 ppm calcium at the tip to 800 ppm at the base (Reiss et al., 1983
). Chlorotetracycline (CTC) labeling reveals primarily membrane-bound calcium and indicates that membrane-sequestered calcium displays a similar calcium gradient (Reiss and Herth, 1978
, 1979a
, 1985
; Picton and Steer, 1983
; Polito, 1983
; Bednarska, 1989a
). According to ultrastructural examination, this membrane-bound calcium in the pollen tube is mainly localized in the endoplasmic reticulum (Reiss and McConchie, 1988
; Cresti and Tiezzi, 1990
).
The detection of free calcium is the most difficult, given the mobility of unbound calcium and its ability to dramatically change the cytoplasm at exceedingly low functional concentrations. In animals, transient spatial and temporal changes in the quantity of free calcium ions (Ca2+) are still considered to be among the first detectable events in a signal transduction cascade arising from stimulation of the outside of the cell (Tang et al., 2000
; Sanders et al., 2002
). In that case, the binding of ligand and receptor evokes an internal release of free calcium that serves as a potent second messenger triggering numerous downstream effects. In pollen tubes of Lilium longiflorum, concentrations of free calcium estimated using quin-2 fluorescence are reported as 10–7 mol/L near the tip and 2 x 10–8 mol/L at the base of the tube (Nobiling and Reiss, 1987
). Free Ca2+ concentrations measured using the fura-2 fluorescence indicator were about 10 times lower, with calcium concentrations of 1.7 to 2.6 µmol/L at the tip and 60 to 100 nmol/L at 100 µm behind the tip (Obermeyer and Weisenseel, 1991
). Fura-2 dextran, which is a membrane- impermeable injected label, provided Ca2+ measurements that were nearly one-third lower at the tube tip (490 nmol/L at the extreme tip) and 170 nmol/L at 10–20 µm behind the tip. The free-calcium gradient reportedly drops sharply within 10–20 µm of the tube tip (Miller et al., 1992
). Although values of free calcium in these reports differ markedly, a free-calcium gradient appears consistently regardless of technique. Several more recent reports have indicated that the Ca2+ gradient is restricted to just the apical 20 µm of the pollen tube tip. Changes in free calcium beyond the first 20 µm of the pollen tube were significantly lower (Pierson et al., 1994
, 1996
; Malhó and Trewavas, 1996
; Franklin-Tong et al., 1997
; Hepler, 1997
; Holdaway-Clarke et al., 1997
; Messerli and Robinson, 1997
). The most modern approach uses the recombinant yellow cameleon calcium reporter. This technique avoids the loading problems of the former free calcium probes and is temporally sensitive enough to detect fluctuations in the calcium gradient of the pollen tube tip. Also, its protein reporter can be targeted using specific peptide additions to selected subcellular compartments (Watahiki et al., 2004
).
Interestingly, the Ca2+ gradient exists only in growing pollen tubes. Upon death or the addition of calcium channel blockers or antagonists into the medium, the calcium gradient disappears from the tip of the pollen tube, and the tube stops elongating. When such arrested tubes are transferred to a medium without calcium blockers or antagonists, they rapidly reestablish a Ca2+ gradient and reinitiate elongation. Strongly concentrated Ca2+ gradients in the pollen tube tip closely correspond with vigorous tip elongation (Rathore et al., 1991
; Obermeyer and Weisenseel, 1991
; Miller et al., 1992
; Li et al., 1994
; Malhó et al., 1994
, 1995
; Pierson et al., 1994
; Feijó et al., 1995
).
Oscillation of the calcium gradient and pulsatory growth of the pollen tube
Pierson et al. (1994)
used more rapid, ratiometric calculations with a confocal microscope and confirmed the steepness and depth of the tip-focused intracellular Ca2+ gradient of lily pollen tubes, which varied from >3.0 µM at the apex to 
0.2 µM within 20 µm from the tip. Although levels of intracellular Ca2+ were assumed to remain essentially stable over time, further examination revealed that, based on measurements using a Ca2+-specific vibrating electrode, extracellular calcium entered the pollen tube at influx rates between 1.4 and 14 pmol·cm–2 s–1. These rates of extracellular calcium influx into the pollen tube were uneven but reasonably periodic. The tubes elongated in pulses that corresponded with periodic deposition of cell wall components (Pierson et al., 1995
). Ratiometric ion imaging of the intracellular Ca2+ gradient indicated that the high point of the gradient fluctuated in magnitude from 0.75 to more than 3 µM during 60-s measuring intervals. Elevation of the calcium gradient appeared to be correlated with an increased rate of tube growth (Pierson et al., 1996
). Holdaway-Clarke et al. (1997)
confirmed that the tip-focused Ca2+ gradient oscillates with the same periodicity as pollen tube growth, but learned that the pulses are slightly out of phase. The extracellular calcium influx is delayed by about 11 s compared with the oscillation of the Ca2+ gradient (Fig. 2). They therefore inferred that the delay in the extracellular inward current may be related to the cyclic depletion of Ca2+ stores in the pollen tube immediately before a similarly out-of-phase cycle of replenishment.
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; Trewavas, 1999
). Pulsatory tube elongation, however, is not required, even in plants that characteristically display pulsatory pollen tube behavior. Geitmann and Cresti (1998)
found that inorganic Ca2+ channel inhibitors La3+ and Gd3+ could trigger the pulsating pollen tubes to abandon their rhythm and continue to elongate with steady rates. Organic inhibitors of Ca2+ channels, nifedipine and verapamil, slowed pulse frequency but did not block pulses. These results strongly suggest that at least two types of Ca2+ channels occur in the plasma membrane of the tube tip. If apical elongation of the pollen tube is inhibited, periodic membrane traffic may still occur with nearly the same periodicity as was exhibited during normal tube elongation (Parton et al., 2003
).
Pulsatory exocytosis and cyclic Ca2+ correspond well with pulsatory patterns of pollen tube elongation, but it is becoming increasingly evident that models of oscillatory pollen tube elongation will not be explained only by cyclic cell wall loosening under turgor (Messerli and Robinson, 2003
). Cellular oscillations apparently involve an integration of plasma membrane ion fluxes that constitutes a "cytosolic choreography" of proton and calcium fluxes that operates in concert with intake fluxes of anions, including potassium and chloride (Feijó et al., 2001
; Zonia et al., 2001
). Chloride efflux at the pollen tube apex is in turn affected by fluxes in inositol 3,4,5,6-tetrakisphosphate with momentary interruptions of chloride efflux associated with elongation maxima (Zonia et al., 2002
). Pollen tube behavior is also influenced by other ions, notably K+, which appears to be controlled by gated channels (Fan et al., 2001
; Mouline et al., 2002
). Increasing information is available concerning the involvement of Rop GTPase-dependent regulatory proteins (Fu et al., 2001
; Baxter-Burrell et al., 2002
; Gu et al., 2003
), Rop1ps (Zhao and Ren, 2006
), Rop2 GTPases (Jones et al., 2002
), and Rac-like GTPases (Cheung et al., 2003
) in the control of endo- and exocytosis in the pollen tube apex (Camacho and Malhó, 2003
).
Dynamic calcium concentrations play a central role in increasingly complicated models of growth oscillations in pollen tubes, both in a direct physiological role and more importantly in signaling (Holdaway-Clarke et al., 2003
). Fluctuations in the pollen tube tip-focused calcium gradient are not reflected in nuclear calcium levels according to analysis using the recombinant yellow cameleon calcium reporter (Watahiki et al., 2004
). Clearly, the occurrence of calcium oscillations in pollen tubes has become an important part of our current model for understanding calcium oscillation in higher plant systems (see reviews by Evans et al., 2001
; Gradmann, 2001
).
ROLE OF THE FUNCTIONAL CLASSES OF CALCIUM IN POLLEN TUBE ELONGATION
The three types of calcium in plant cells—bound, loosely bound, and free calcium—although interchangeable, clearly have different functions and use different controlling molecules during pollen tube elongation. These multiple forms of calcium, their spatial localization, and temporal characteristics compound the complexity of calcium mediation during pollen tube elongation (Hepler, 1997
; Malhó, 1998a
, b
; Yang, 1998
; Franklin-Tong, 1999a
). The following sections summarize some significant advances in our understanding of calcium regulation in the context of pollen tube elongation.
Cytosolic free calcium function during pollen tube elongation
The ability of the pollen tube to reorient tip growth is at the heart of understanding how tubes can so precisely target a single cell within the female embryo sac. Hypotheses that pollen tubes grow toward the ovule by such tropistic mechanisms as chemo-, thigmo- or electrotropism have met with at best equivocal success. What molecular changes would precede and trigger pollen tube curvature? Calcium gradients have long been implicated in regulating pollen tube tip elongation, but their role in determining the orientation of tube growth was not well supported by experimental evidence until the 1990s.
Rui Malhó's group provided evidence that the directionality of Agapanthus umbellatus pollen tubes could be modified by iontophoretic introduction of calcium and by weak electrical fields, in which the pollen tubes elongate toward the cathode. Introducing a localized gradient of ionophore A23187, which is believed to open calcium ion channels, causes the pollen tube tip to reorient toward the A23187. When the Ca2+ channel blocker GdCl3 was added to the growth medium, tip elongation became oriented away from GdCl3. The region of highest concentration of cytosolic free calcium was closely aligned with tip reorientation and the direction of future elongation.
A further demonstration of the effect of free calcium was obtained by microinjecting caged Ca2+ into living pollen tubes. These injected pollen tubes were then eccentrically irradiated with UV light near their tips, causing photolysis of the cages and release of free Ca2+ within the pollen tube at that location. A resulting transient rise in free Ca2+ induced a reorientation of tip elongation toward the site of irradiation. Tip growth resumed near the irradiated region and caused a sustained reorientation of tip elongation. The site of tip reorientation corresponded closely with the local release of Ca2+. This pattern was reinforced by a decline in Ca2+ levels on the opposite side of the tube, completing the reorientation (Malhó et al., 1994
, 1995
; Malhó and Trewavas, 1996
). Thus, areas of rich Ca2+ within a gradient can reorient tip growth and thereby establish the directionality of future pollen tube elongation. A related study demonstrated that a kinase in the pollen tube apex may also be involved in regulating localized Ca2+ channel activity (Moutinho et al., 1998
).
The signal-transduction intermediate inositol 1,4,5-trisphosphate (InsP3) can reportedly trigger calcium release from stores in the cell, as well as serve as a second messenger controlling numerous cellular processes (Berridge, 1993
). To examine the effect of locally introduced InsP3 on pollen tube elongation, Malhó (1998c)
microinjected caged InsP3 into pollen tubes of Agapanthus umbellatus, then photolysed the cages with spot irradiation. Releasing caged InsP3 in the apex, subapical, and nuclear regions of pollen tubes resulted in a transient increase of Ca2+ in these regions. When caged InsP3 was released in the nuclear or subapical region, the growth axis of pollen tube was realigned. An undesirable consequence was that photolysis elicited transitory increases in tube elongation, causing tips to burst. These results clearly indicate that calcium is a unique and separate part of the signal transduction pathway controlling tube guidance. The addition of cAMP, which affects the localized release of Ca2+, may reorient tip elongation as well (Moutinho et al., 2001
).
Calcium in pollen tube wall
Among the primary targets of exogenously provided 45Ca2+ is the pollen tube wall (Brewbaker and Kwack, 1963
), where the radioisotope incorporates into carboxyl groups in the pectic components of the cell wall (Kwack, 1967
). Calcium associated with pollen tube walls is frequently abundant but rarely is incorporated directly at the tip, because calcium-crosslinked pectin may behave as a rigid and insoluble gel (McNeil et al., 1984
). The bipartite pollen tube wall consists of an inner sheath of callose and an outer region of fibrillar layers containing abundant pectin. The tip of the pollen tube is composed of a single pectin layer (Heslop-Harrison, 1987
). In immunocytochemical studies, at least two types of pectin were labelled in the pollen tube wall: a polygalacturonic acid form of pectin and methyl-esterified pectin. Acid pectins capable of crosslinking Ca2+ are localized in the subapical region of the pollen tube. In contrast, richly methyl-esterified pectins are particularly abundant at the pollen tube tip, where they may inhibit Ca2+ crosslinking. Secretory vesicles in the pollen cytoplasm also appear to contain abundant methyl-esterified pectin (Li et al., 1994
; Geitmann et al., 1995
; Jauh and Lord, 1996
; Geitmann and Parre, 2004
). Tube elongation represents a delicate balance between the ability of the tube wall to withstand bursting and the high native osmolality of the pollen cytoplasm (Cresti and Tiezzi, 1992
; Feijó et al., 1995
). When secretory vesicles release their methyl-esterified pectin at the tube tip, their contents merge to become incorporated into the tube wall. Upon formation, this tip region is the most elastic in the tube and is also likely to be the expansion point under internal turgor pressure. Following tube tip expansion, methyl-esterified pectin migrates basipetally from the tip region. Pectin is deesterified and becomes crosslinked with Ca2+, which contributes much to the rigidity and mechanical strength of the pollen wall as well as providing turgor pressure in the other regions of the tube (Li et al., 1994
). In vivo, the region of the pollen tube wall adjacent to the micropyle in ovules of tobacco and Plumbago contains abundant calcium-induced precipitates, indicating high concentrations of loosely bound calcium (Tian and Russell, 1997a
; Tian et al., 2000
).
Other calcium functions during pollen tube growth
The tip gradient of calcium in the living pollen tube is essential to vesicle secretion at the tube tip. As discussed before, all attempts to alter the gradient also alter the pattern of exocytosis. The elevated concentration of calcium near the secretory membrane appears to be essential for vesicle fusion, as indicated by its involvement in animal as well as plant systems (Almers, 1990
; Zorec and Tester, 1992
; Battey and Blackbourn, 1993
). During membrane fusion, the role of Ca2+ appears to be to draw the docked vesicle membrane into apposition with the plasma membrane by counterbalancing the native negative charge of their constituent phospholipids (Battey et al., 1999
). Vesicle secretion at the tip of the pollen tube is an intense, localized form of exocytosis that, perhaps not surprisingly, involves an especially steep gradient of elevated calcium. In pollen tubes, highly active Golgi bodies produce abundant vesicles that migrate to their fusion site via actomyosin associations. The vesicles fuse with the plasma membrane at the tip of the pollen tube, secreting both wall and plasma membrane material for expansion (Hepler et al., 1994
). These areas of elevated Ca2+ in lily pollen tubes closely resemble conditions in other sites of intense exocytosis (Roy et al., 1999
).
Maintenance of the calcium gradient in the tip is characteristic of vigorous pollen tube elongation, but mechanistic details are still not clear. Calcium concentrations outside the pollen need to exceed those at the tip, suggesting that tightly localized calcium channel activity may be sufficient to sustain an exceedingly tight gradient. Even a cursory examination of elongating pollen tubes reveals intense cytoplasmic streaming in the tube, but there is a transition in this pattern of movement from directional to saltatory movement toward the tip (Heslop-Harrison et al., 1991
). High concentrations of calcium at the tip induce fragmentation of actin filaments and inhibit cytoplasmic streaming at the extreme tip because of F-actin instability (Kohno and Shimmen, 1987
, 1988
). The high concentration of calcium within the final 20 µm of cytoplasm at the tube tip appears to create a relatively calm environment for vesicle transport and fusion (Steer and Steer, 1989
). The negatively charged vesicles (Heslop-Harrison et al., 1991
) may then continue their forward migration through attraction to the positively charged Ca2+-rich tip. Although the high calcium concentration inhibits cytoskeleton formation in this region, it promotes the directed transportation of secretory vesicles and their adherence and fusion with the plasma membrane of the tube tip.
Sperm cells in growing pollen tubes are typically located somewhat distant from the tip, for instance 90 µm from the tip in maize, a location where calcium concentrations are reasonably low (G. Zhang et al., 1997
). This condition seems to improve the long-term viability of sperm cells. Maintaining isolated sperm cells requires a suitable concentration of exogenous calcium for membrane integrity (perhaps <1 mM), but the concentration is critical or the useful lifespan of viable sperm cells is very brief (G. Zhang et al., 1992
, 1995
).
CALCIUM DISTRIBUTION IN STIGMATIC TISSUE
When a compatible pollen grain lands on the stigma, it germinates and forms a pollen tube with an elongating tip. Using chlorotetracycline (CTC), Tirlapur and Shiggaon (1988)
found abundant membrane-bound calcium in papillae of Ipomoea batatas. Bednarska (1989c)
, who also used CTC, found a similar result in Ruscus aculeatus L. and confirmed this with X-ray microanalysis, further observing that germinating pollen of Primula officinalis and R. aculeatus absorb calcium from the stigma (Bednarska, 1991
). Studies using potassium antimonate precipitation indicate that loosely bound calcium in sunflower is more abundant on the receptive surface of the stigma, especially outside and inside the papillae, than on nonreceptive surfaces (J. Zhang et al., 1995
). Abundant calcium antimonate precipitates are also reported in the intercellular matrix of the stigmatic tissue of cotton (J. Zhang et al., 1997
) and on the stigmatic surface of Brassica oleracea following pollination, particularly when the pollen grain lands and tube germinates on the stigmatic papillae (Elleman and Dickinson, 1999
). The occurrence of abundant calcium in and on stigmatic surfaces is in accord with the need for extracellular calcium for pollen germination and tube elongation in vitro (Brewbaker and Kwack, 1963
).
CALCIUM DISTRIBUTION IN STYLAR TISSUE
After pollen germination, the tubes penetrate the stigmatic surface and grow into the transmitting tissue of the style. Mascarenhas and Machlis (1962)
found a chemotropic response of pollen tubes to calcium and observed a gradient of calcium concentrations in pistil tissues of Antirrhinum majus proceeding from the stigma to the ovule. Based on in vitro assays, they concluded that the tubes responded to a calcium gradient in the pistil, which causes tubes to elongate toward the embryo sac. Calcium gradients have been confirmed in the pistils of a number of flowering plants, including Gladiolus gandavensis (Day et al., 1971
), but calcium gradients were absent in several other flowering plants (Glenk et al., 1971
; Mascarenhas, 1975
). More recent assays using antimonate have confirmed that loosely bound calcium stores occur in the transmitting tissue of a number of plants, including Brassica napus (Mao et al., 1992
), Helianthus annuus L. (J. Zhang et al., 1995
), Gossypium hirsutum L. (J. Zhang et al., 1997
), and Lilium longiflorum (Zhao et al., 2004
). Calcium-induced antimonate precipitates were mainly localized in the apoplastic system of the transmitting tissue, i.e., in the intercellular matrix and cell wall, where pollen tubes elongate. In contrast, fewer calcium precipitates were localized within the cells of the parenchymatous tissue. The occurrence of abundant calcium in the transmitting tissue suggests that the calcium required for tube elongation in vitro is obtained from the rich calcium environment of the style during in vivo growth. Calcium as a directional cue, however, appears to be effective only in the pollen tubes of a restricted number of plants because numerous flowering plants are insensitive to such a calcium signal (Johnson and Preuss, 2002
; Higashiyama et al., 2003
).
Calcium is also implicated in the self-incompatibility response of self incompatible (SI) plants. When S-glycoprotein is extracted from self-incompatible stigmas of Papaver rhoeas and is introduced into the culture medium, the concentration of calcium increases in tubes, and this increase appears to inhibit tube elongation (Franklin-Tong et al., 1993
). However, when either compatible or heat-denatured incompatible stigmatic S-glycoproteins were introduced into the culture medium, no rise in Ca2+ levels of pollen tubes was detected, and elongation continued normally (Franklin-Tong et al., 1993
). Caged Ca2+ introduced into pollen tubes did not alter tube elongation, but upon cage photolysis, a similar inhibition of tube elongation was observed accompanying artificially elevated internal levels of Ca2+. Photoactivation of caged InsP3 elicits a similar inhibitory response (Franklin-Tong et al., 1996
). The potential effect of contamination by other stylar components was eliminated by cloning the SI-gene and expressing it in E. coli; the purified gene product also elicited the same result. Thus, the S-protein alone appears to be sufficient for triggering the Ca2+ signal during the pollen SI response (Franklin-Tong et al., 1995
).
Imaging the calcium directly established that S-protein added to the medium resulted in an influx of extracellular Ca2+ at the "shank" of the pollen tube. The influx of extracellular Ca2+ appears to play a role in the Papaver SI response (Straatman et al., 2001
). The addition of the Ca2+-antagonist Verapamil or the calcium channel blocker La3+ allowed pollen of Secale to overcome the self-incompatibility response and permitted elongation into the style (Wehling et al., 1994
). Calcium is believed to regulate special transduction reactions during the self-incompatibility response, likely by controlling the activity of protein kinases. Ca2+ influx differs between compatible and self-incompatible plants, suggesting an active role for the influx of extracellular Ca2+ in the SI response (Franklin-Tong et al., 2002
). In Nicotiana alata, a plant with gametophytic self-incompatibility, self-pollen tube elongation was inhibited by an S-related ribonuclease in the stylar transmitting tissue (McClure et al., 1990
). Presumably, calcium-dependent protein kinases from the pollen tube activated the S-ribonuclease of the transmitting tissue, resulting in the digestion of RNA in the pollen tube and the inhibition of tube growth (Kunz et al., 1996
). Immunological studies have indicated the occurrence of calmodulin and calmodulin-like and calreticulin-like proteins that could be involved in Ca2+-related cell signaling during pollen–pistil interactions (Lenartowska et al., 2001
, 2002
; Rudd and Franklin-Tong, 2001
, 2003
; Snedden and Fromm, 2001
).
CALCIUM DISTRIBUTION IN OVULAR SOMATIC TISSUE
After pollen tubes emerge from the transmitting tissue and enter the ovary, they elongate onto the placental surface within the ovarian locule. As pollen tubes travel along the placental surface toward the proximal end of the ovary, individual pollen tubes diverge from the masses by 90° before elongating through the last several micrometers to enter the ovular micropyle. In all plants investigated to date (pearl millet, Chaubal and Reger, 1992a
; sunflower, J. Zhang et al., 1995
; tobacco, Tian and Russell, 1997a
; cotton, J. Zhang et al., 1997
; Brassica napus, Yu et al., 1998
; Plumbago zeylanica, Tian et al., 2000
; Crocus,Chichiriccó et al., 2002
), the micropyle appears to contain abundant levels of calcium, which correlate closely with fertility and may serve as an attractant in some plants. Accumulations of calcium-induced precipitates of antimonate at the entrance of the micropyle exceeded calcium concentrations in the integument and nucellus in ovules of tobacco and Plumbago zeylanica. Calcium distribution appeared to be polarized within the micropylar nucellus, which contained more calcium-induced precipitates than did the chalazal nucellus and thus contribute to an apparent calcium distribution gradient (Tian and Russell, 1997a
; Tian et al., 2000
).
Some spatial and temporal characteristics are evident in the distribution of loosely bound calcium in the ovule. Calcium concentration in the micropylar regions of each ovule are highest as the pollen tube approaches the ovules in tobacco and Plumbago zeylanica. After the tip of the pollen tube passes, calcium levels quickly decline. Flowers in which pollination is withheld, however, retain higher concentrations of micropylar calcium, with calcium levels decreasing slowly over time. That the gradient is retained suggests that micropylar calcium gradients remain. Delayed pollination tests also demonstrate that the ovule continues to attract pollen tubes (Tian and Russell, 1997a
; Tian et al., 2000
). Accumulating calcium concentrations may be a valuable indicator of fertility (Chudzik and Sniezko, 2003
).
Generally, only one pollen tube enters each ovule in angiosperms. In ovaries with only a single ovule, subsequent pollen tubes stop growing once one pollen tube grows into the ovule. However, once multiovulated ovaries are penetrated by pollen tubes, arriving tubes simply bypass the penetrated ovules to grow toward receptive ovules. After the pollen tube penetrates the ovule, rapid decreases in micropylar calcium correlate closely with loss of receptivity and may prevent supernumerary pollen tubes from entering the ovule. Presumably, the disappearance of micropylar calcium renders penetrated ovules unattractive to the remaining pollen tubes. This latent calcium signal, controlled by penetration of the ovule, can be rapidly dissipated.
CALCIUM DISTRIBUTION IN THE EMBRYO SAC
The embryo sac represents the end of the journey for the pollen tube. Because in most angiosperms the pollen tube enters the receptive synergid, calcium distribution in the synergids has been the subject of multiple studies conducted using a variety of calcium detection techniques. The earliest examination of the chemical content of synergids was a study by Jensen (1965)
in which sections of the embryo sac of cotton were microincinerated and then examined using dark field microscopy. Abundant ash, presumed to be essentially elemental calcium based on the temperature used, was found localized within the synergids. Since this early work, other techniques have been used to document the abundance of calcium in the synergid of other species. When total calcium in the embryo sacs of wheat and pearl millet before and after pollination was assayed using energy-dispersive X-ray analysis, concentrations greatly exceeded typical physiological measurements; critical-point dried synergid vacuoles contained over 20% calcium by mass (Chaubal and Reger, 1990
, 1992a
). CTC labeling of membrane-bound calcium in embryo sac cells of tobacco and Petunia hybrida indicated intense fluorescent signal in the receptive synergid (Huang and Russell, 1992
; Tirlapur et al., 1993
). Loosely bound calcium is similarly abundant, according to localizations using potassium antimonate (Chaubal and Reger, 1992a
, 1993
, 1994
; He and Yang, 1992
; Tian and Russell, 1997a
; Yu et al., 1998
; Kristóf et al., 1999
). With potassium antimonate, changes in calcium labeling in the receptive synergid can be detected before degeneration and before other ultrastructural cues (Tian and Russell, 1997a
). Calcium-induced antimonate precipitates also provide an opportunity to map loosely-bound calcium based on precipitate distribution, size, and quantity (Tian et al., 2000
; Yu et al., 1998
).
In all flowering plants examined in recent years, except Plumbago zeylanica (which lacks synergids), the highest calcium levels within the embryo sac occur in synergids—and typically, the most abundant label is localized in the receptive synergid. Calcium in the synergid appears to be distributed in a micropylar–chalazal gradient. The highest concentration of calcium appears to occur in the filiform apparatus in wheat, pearl millet, tobacco, and Brassica napus (Chaubal and Reger, 1992a
, b, 1994; Tian and Russell, 1997a
; Yu et al., 1998
). In sunflower, however, higher concentrations of calcium appear to occur at the chalazal rather than the micropylar end of the synergid (He and Yang, 1992
). In all of these plants, the embryo sac contains two synergids, only one of which receives the pollen tube and sperm cells. In mature embryo sacs of wheat, pearl millet, sunflower, and B. napus, both sister synergids contain nearly the same calcium content (Chaubal and Reger, 1990
, 1992a
, b; He and Yang, 1992
; Yu et al., 1998
). In tobacco and Petunia hybrida, however, the two synergids contain different levels of membrane-bound calcium (Huang and Russell, 1992
; Tirlapur et al., 1993
) as well as loosely bound calcium (Tian and Russell, 1997a
). The degenerated synergid of tobacco contains higher levels of calcium than the persistent synergid (Huang and Russell, 1992
). According to Mogensen and Suthar (1978)
, synergid differences originate during the first day after pollination. Localization of calcium using potassium pyroantimonate confirms a correlation between calcium differences between the two synergids before and during pollen tube penetration (Tian and Russell, 1997a
). The pollen tube penetrates the synergid that has the most label, and the pattern of calcium labeling characteristic of the persistent synergid is retained in the unpenetrated synergid after fertilization (Tian and Russell, 1997a
).
Abundance of calcium in the synergid is closely related with pollen tube entrance. Although no differences in calcium content are evident in the two synergids in unpollinated flowers of sunflower, calcium content after pollination increases in the synergid that degenerates and receives the pollen tube (He and Yang, 1992
). Similarly, calcium content and distribution are similar in the sister synergids of B. napus before pollination, but calcium precipitates in both synergids increase by 2.4 times after pollination, and a difference in size of precipitate granules appears to differentiate the two synergids. The synergid that will accept the pollen tube contains smaller granules than the persistent synergid (Yu et al., 1998
). In tobacco, calcium levels in the synergid appear to remain elevated until after a pollen tube enters the receptive synergid, when the high level of calcium in the filiform apparatus appears to decrease (Tian and Russell, 1997a
). Following pollination in Torenia fournieri, the egg apparatus produces a labyrinthine structure on the micropylar surface that contains high levels of calcium. Once the synergid is penetrated by the pollen tube, however, calcium content decreases in the penetrated synergid (Kristóf et al., 1999
), and the ovule no longer appears to attract pollen tubes (Higashiyama et al., 1998
). In this context, calcium may be only a small part of a larger process of programmed cell death that occurs in the synergid as part of pollen tube arrival and discharge (Christensen et al., 2002
).
A continuing interaction between elongating pollen tubes and receptive synergids is evident in terms of calcium signaling. Calcium in the synergid, especially in the filiform apparatus, directly attracts elongating pollen tubes, whereas elongating pollen tubes may stimulate calcium accumulation in the embryo sac (Tian and Russell, 1997a
). Similar to calcium dynamics in the micropyle, penetration by a pollen tube induces rapid calcium depletion in the synergids, which may prevent extra pollen tubes from entering the embryo sac (Chaubal and Reger, 1990
, 1992b
).
Once the pollen tube penetrates the embryo sac, calcium in the degenerated synergid may play a role in sperm cell transport and fusion. In flowering plants, the two sperm cells and vegetative nucleus form a functional association known as the "male germ unit" (Dumas et al., 1984
). This assemblage favors the transportation of the male gametes within the tube and assures their simultaneous delivery, but must be severed upon arrival in the embryo sac. Because the assemblage is presumably maintained by cytoskeletal elements (Palevitz and Tiezzi, 1992
), calcium in the synergid may be involved in the breakdown of cytoskeletal elements from the pollen tube and the preparation of the sperm cell surface for fusion.
Among the cells of the embryo sac, the egg cell appears to retain consistently low levels of calcium (Chaubal and Reger, 1992a
, b; Tian and Russell, 1997a
; Yu et al., 1998
; Tian et al., 2000
). However, in Plumbago zeylanica, which lacks synergids, the egg cell in the mature embryo sac contains abundant calcium, especially in its filiform apparatus (Tian et al., 2000
). This result supports the idea that calcium attracts pollen tubes to the ovule and into the embryo sac in vivo and confirms that the egg cell of P. zeylanica has indeed acquired some synergid-like functions (Cass and Karas, 1974
; Russell, 1982
). After fertilization in rice, calcium-induced antimonate precipitates increase by 4.4-fold in the cytoplasm of the zygote and by 10.5-fold in the nucleoplasm. Calcium levels in two-celled proembryos, however, quickly decline in the cytoplasm, but remain abundant in nucleoli and cell walls. An abundance of calcium was reported in the apoplast of the embryo sac after fertilization and during early embryogenesis (Zhao et al., 2002
).
The central cell also contains significant accumulations of calcium, particularly at the micropylar end of the cell (Tian and Russell, 1997a
; Tian et al., 2000
; Yang et al., 2002
). After the second sperm cell fuses with the central cell, labeled calcium also quickly decreases in the newly formed endosperm. There seems to be no direct relationship between calcium in the central cell and chemotropic activity of pollen tubes in vivo. Calcium gradients near the synergids presumably control the attraction of pollen tubes, whereas calcium in the central cell may relate more specifically to the stimulation of gamete fusion and the early development of the endosperm, which precedes that of the embryo. Because the central cell is a major nutritional store for the embryo sac, high levels of calcium may also facilitate the absorption of nutrients into the embryo sac. Abundant calcium has been demonstrated to be necessary for accumulation and transport of amino acids (Rickauer and Tauner, 1986
) and for carbohydrate metabolism (Brauer et al., 1990
). In late stages of microspore development in rice, the anther locule also contains abundant calcium, which appears to be related to rapid starch accumulation (Tian et al., 1998
). Additional experimental evidence will be needed to further our understanding of the role of calcium in the central cell.
Knowledge on the function of the antipodal cells is very limited in most plants. In many, the antipodal cells degenerate prior to embryo sac maturity and have no obvious function in the embryo sac. In these plants, antipodal cells presumably contain low levels of calcium, which distinguishes them from the other cells of the embryo sac (Tian and Russell, 1997a
). Although antipodal cells in Plumbago zeylanica may contain some calcium granules, most are localized in the single large vacuole, with no apparent gradients formed. After fertilization of the central cell, antipodal cells degenerate as the endosperm begins to develop (Tian et al., 2000
).
CALCIUM AND FUSION OF GAMETES
Calcium is known to induce somatic fusion and appears to contribute to gametic fusion as well. In an in vitro fertilization study in maize, the highest fusion success was obtained using 5 mM Ca2+ in the fusion medium (Faure et al., 1994
). Fusion was highly successful in sperm–egg cell pairs (79.8%), whereas only 1.8% of sperm–mesophyll protoplast combinations fused. Under these circumstances, gametes adhered and fused within 1 min. This fusion specificity has been interpreted as indicative of gametic cell recognition (Faure et al., 1994
). Because lowering or eliminating calcium from the medium inhibited fusion and raising the calcium concentration did not improve fusion success, Faure et al. (1994)
concluded that 5 mM calcium stimulated but did not coerce fusion. In contrast, fusion may be coerced at high calcium concentrations (50 mM), particularly when accompanied by high pH (Kranz and Lörz, 1994
). Under these conditions, fusions among a variety of cell types are possible, apparently overriding any existing cell recognition systems. Although fusions of isolated tobacco sperm and egg cells could be accomplished using polyethylene glycol without calcium, the addition of 5 mM CaCl2 improved success in preliminary in vitro fertilization experiments (Tian and Russell, 1997c
). Lower levels of calcium are typically used accompanying electrofusion (e.g., 2 mM CaCl2 in rice; Uchiumi et al., 2006
).
Calcium alone, however, cannot replace the need to have gametes at the appropriate position in the cell cycle to initiate successful embryogenesis; this cell phase at which fusion normally occurs differs among angiosperms (Friedman, 1999
). Although tobacco cells readily form callus and plants are easily regenerated from callus, tobacco gametes isolated at G1 phase did not successfully form embryos (Tian and Russell, 1997c
). A detailed examination of the role of the cell cycle in tobacco fertilization showed that sperm cells underwent S phase upon arrival at the ovule and completed S phase within the synergid. Gametic fusion then occurred at the beginning of the G2 phase, and completion of S phase in the female gametes was synchronized with the male gametes (Tian et al., 2005
). In tobacco, only gametes in which the sperm and egg cells fused at G2 phase formed successful embryos, whereas male gametes of tobacco isolated from the pollen tube at G1 phase formed fusion products that failed to divide. Cell cycle congruity is thus a prerequisite for successful in vitro fertilization, even if calcium requirements are met (Wang et al., 2006
).
Ultrastructural studies of embryo sacs have frequently revealed highly electron dense materials between the egg and central cells near the site of future gamete fusion. Although it is unclear whether these materials are directly involved in fusion in vivo (Russell, 1992
), with the antimonite technique, abundant calcium precipitated at the fusion site in tobacco (Tian and Russell, 1997a
), Brassica napus (Yu et al., 1998
), and Plumbago zeylanica (Tian et al., 2000
). Presumably, an environment containing sufficient calcium favors the fusion of male and female gametes.
Spontaneous fusion of sperm cell pairs—a condition thought to be rare in nature—may be stimulated in sperm cells in vitro. Using solutions containing 4.27 mM calcium, Roeckel and Dumas (1993)
found spontaneous sperm cell fusions that formed large, initially multinucleated cells in isolated maize gametes. Under certain culture conditions, media containing as low as 1 mM calcium were able to induce sperm cell fusions consisting of as many as 20 nuclei (G. Zhang et al., 1997
). Without calcium in the medium, however, spontaneous fusion of tobacco sperm cells infrequently occurred and took >30 min to complete. In fusion media containing calcium, the frequency was markedly increased, with cells fusing in as little as 10 s, suggesting a direct stimulation of sperm cell fusion (Tian and Russell, 1998
). The spontaneous ability of sperm cells to fuse among themselves decreases as the cells mature, suggesting maturational changes in the sperm cell surface during development. Within 20 h of sperm cell formation, spontaneous sperm–sperm fusion is no more frequent than at the time of gamete fusion and requires enzymatic treatment (Tian and Russell, 1998
).
Because calcium-mediated prefertilization events are possible in angiosperm gametes, Williams et al. (1997a
, b
) examined the uptake kinetics of Ca2+ and the effect of exogenous calcium on protein phosphorylation and the Ca2+-binding proteins, calmodulin and calreticulin, in sperm cells of maize (Zea mays) in the presence and absence of Ca2+. Calcium uptake kinetics indicated that Ca2+ influx in maize sperms is mediated by a combination of active (oxidative phosphorylation-dependent) Ca2+ transporters and Ca2+ channels (Williams et al., 1997a
). With the addition of 1 mM Ca2+, the phosphorylation of a 18-kDa protein increased 13-fold after 12 h incubation, calmodulin content increased by 136% in as little as 1 h, and calreticulin increased by 34% after 3 h, as compared to the controls (Williams et al., 1997b
).
Calcium conditions during in vivo fertilization have not been directly examined in the intact egg cell because of its relative inaccessibility. A calcium requirement, however, seems likely. Because sperm cells usually enter the embryo by means of the synergid, the calcium conditions of this region may represent the normal environment for gamete fusion. If this is the case, the amount of calcium in the immediate environment of the synergid may be remarkably high (Higashiyama, 2002
), but the concentration of calcium actually participating in fusion is unknown.
ACTIVATION OF THE EGG CELL AND CALCIUM SIGNALING
In animals, the entry of the sperm into the egg cell triggers embryogenesis, but eggs can also be activated by other disturbances in the surface of the egg. The most conspicuous initial step in egg activation is a significant increase in the level of cytosolic free calcium and dramatic oscillations in calcium influx within the fertilized egg cell (Ridgway et al., 1977
; Swann and Ozil, 1994
; Tang et al., 2000
; Sanders et al., 2002
). In the brown alga Fucus serratus, fertilization of the egg cell is accompanied by uptake of calcium from the external medium, which appears to be necessary for egg cell activation (Roberts and Brownlee, 1995
). Calcium uptake by itself can also trigger egg activation, because the addition of the calcium ionophore A23187 to the culture medium will cause nearly half of treated mouse oocytes to undergo parthenogenesis (Nakasaka et al., 2000
).
In flowering plants, changes in membrane-bound Ca2+ in maize egg cells are detectable using chlorotetracycline. Enhanced labeling occurs at the egg plasma membrane and around the egg nucleus prior to gamete fusion (Tirlapur et al., 1995
). Following in vitro fertilization of a single egg, membrane-bound Ca2+ labeling with chlorotetracycline increases in the cytoplasm and around the nucleus, but the Ca2+ signal at the plasma membrane decreases until it is not detectable (Digonnet et al., 1997
).
Dramatic changes in free calcium also occur during in vitro fertilization of flowering plant gametes. In vitro fertilized egg cells of maize displayed significant changes in free Ca2+ (Digonnet et al., 1997
), which are apparently triggered by fusion (Fig. 3). Cytosolic free Ca2+ was unchanged during gamete positioning and adhesion. Once fusion occurred, however, free Ca2+ increased for several minutes and then decreased. This transient calcium elevation apparently triggers an influx in extracellular free Ca2+, which is non-invasively detectable using an extracellular Ca2+-selective vibrating probe (Antoine et al., 2000
). On the egg cell surface prior to fusion, the influx of Ca2+ was close to zero. Following gamete fusion, however, a Ca2+ influx was evident near the site of sperm cell entry. The onset of Ca2+ influx appears to precede the elevation of cytoplasmic Ca2+ by approximately 40–120 s. The addition of 10 µM Gd+3 prior to fusion is sufficient to block sperm incorporation, suggesting that the Ca2+ influx is required for incorporation. The fusion point, however, displays a highly localized sperm-dependent and Gd+3-insensitive Ca2+ influx (Antoine et al., 2001
).
|
were able to trigger calcium oscillations in mouse eggs and therefore suggested that the activating protein may be found in both plants and animals. Injection of a sperm extract in Torenia central cells triggers a calcium oscillation and an accumulation of calcium that mimics endosperm activation. Interestingly, this effect can also be produced by the addition of caged inositol 1,4,5-trisphosphate. Once the cages are photolysed and the chemical is released, inositol 1,4,5-trisphosphate appears to mimic the effects of free calcium oscillation (Han et al., 2002
). Stimulation of the female target cells by sperm extract suggests that sperm cells contain a trigger for releasing egg stores of Ca2+ that may activate a myriad of Ca2+ channels in both plants and animals. Inositol 1,4,5-trisphosphate is known in other systems to stimulate the release of Ca2+ from cellular stores (Depass et al., 2001
), and the observation that inositol 1,4,5-trisphosphate can substitute for sperm extract suggests that Ca2+ accumulation and oscillation may have its origin in signal-transduction signals transmitted by the male during gamete fusion.
In plants as well as animals, there is evidence that fertilization induces the first embryonic events by opening calcium channels and triggering Ca2+ influx and then calcium oscillation and accumulation, which in turn may trigger further aspects of egg activation (Antoine et al., 2001
). These results indicate that spatial and temporal changes in free calcium concentration in fertilized egg cells may be the initial steps in egg cell activation during fertilization in higher plants (Lord and Russell, 2002
; Weterings and Russell, 2004
).
CONCLUSION AND FUTURE PROSPECTS
Calcium fulfills many essential roles in sexual reproduction of flowering plants. Interactions between male and female gametes and gametophytes involve different calcium forms, occur at different scales of organization, and require different forms of calcium regulation at each phase. Changing spatial and temporal characteristics of calcium distribution in reproductive tissue and cells reflect the critical role that calcium plays during flowering plant reproduction (Digonnet et al., 1997
).
Calcium distributions in pollen tubes closely relate to pollen tube function, polarity, and tip elongation. External stimulation that is locally applied appears to induce asymmetric Ca2+ distribution in the tube tip and triggers reorientation of the axis of elongation. Subcellular distribution of calcium in the pollen tube occurs along a steep dynamic gradient that is related to tube elongation and cell wall synthesis. When pollen tubes are grown in media containing unsuitable concentrations of calcium or when chemicals are added that block or open calcium channels, the gradient disappears and tube elongation immediately ceases. Although pollen contains some stores of calcium, the much larger pistil of flowering plants contains more abundant calcium stores that support tube elongation requirements in vivo and attract pollen tubes to the ovules and embryo sacs. The spatial and temporal characteristics of calcium distribution and of post-fertilization calcium depletion suggest that calcium accumulation in the ovule provides directional cues that may be impaired if pollination does not occur. This specific form of calcium depletion appears to effectively inhibit fertilization by multiple pollen tubes and may place supernumerary pollen tubes into developmental stasis when all ovules are fertilized. High levels of calcium in degenerated synergids may also function to attract pollen tubes, but further direct experimental evidence is needed. Taken together, these results indicate that the calcium ion plays a critical role during plant reproduction.
Significant progress has been made in understanding the role of calcium in sexual plant reproduction, but there are numerous remaining problems. First, many of the results reported were obtained in vitro. In vitro observations are presumed to reflect the natural state of pollen tubes and to represent gamete behavior, but significant and perhaps critical functional differences may be overlooked. Pollen tubes elongate faster in vivo, tubes live longer, and sperm cells (in bicellular species) form sooner and more reliably than they do in vitro. Confirmatory evidence would either reassure or extend our current knowledge. Confirmatory results using techniques other than antimonate precipitation should be used to localize loosely bound calcium in pistillate tissues. Even though calcium signaling may be involved in significant short- or long-distance communication, the role of free calcium in pistils is extremely difficult to study directly.
The role of free calcium during embryo sac function is essentially unknown, despite its potential significance during in vivo calcium signaling that may occur between male and female cells during pollen tube growth. In vivo studies of free calcium in ovulo should be particularly useful for expanding our understanding of the role of calcium during fertilization. Particularly attractive as study subjects are the handful of flowering plants that have naturally emergent embryo sacs in which the synergids and egg extend beyond the micropyle (viz., Torenia fournieri, T. hirsuta, Philadelphus coronarius, Galium lucidum). These plants provide ideal and intact living materials for examining calcium signals in male and female cells in vivo. The use of in vivo male and female cells to study the behavior and effects of calcium should contribute significantly to our understanding of fertilization mechanisms in angiosperms.
FOOTNOTES
1 The authors thank the U.S. Department of Agriculture (NRICGP grant no. 99-37304-8097) and the National Natural Science Foundation of China (grant no. 30670126) for partial support of the research reported in this review. 
4 Author for correspondence (e-mail: hqtian{at}xmu.edu.cn
; phone: +86-592-2186486; fax: +86-592-2181015
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- and Lß-peaks of antimony (Sb) overlapping with the K
