| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
What's this? |
Brief Communication |
2Department of Biology and Intercollege Program in Plant Physiology, 208 Mueller Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 USA; and 3Department of Biology, 304 Life Sciences Center, University of Missouri–Columbia, Columbia, Missouri 65211 USA
Received for publication April 7, 2006. Accepted for publication August 24, 2006.
ABSTRACT
Plants, like animals, suffer from a variety of diseases that are transmitted via their sexual organs. In many species, the flowers senesce rapidly after pollination or fertilization. In ongoing studies of the impacts of a transposon insertional mutation in the gene that encodes the most abundant isoform of a major group-1 pollen allergen of maize, we found that pollen tubes with the mutant allele grow significantly slower in vivo than pollen with the wild-type allele. Here, we report that under field conditions, maize silks (styles) pollinated with pollen bearing the slower-growing mutant allele take significantly longer to senesce, and the resulting ears (infructescences) have dramatically higher incidence of "fungal ear rot" disease than silks pollinated with pollen bearing the wild-type allele. Because ear rot fungi gain access to the developing ear by growing on and through the silks, we propose that accelerated senescence of silks after fertilization is a defense against pathogens such as those causing ear rot. In addition, we divided the silks on each ear into two halves and experimentally varied the type of pollen (wild type, mutant, unpollinated) that was placed onto each half of the silks. Senescence of unpollinated silks was accelerated when ovaries on the other half of the ear were fertilized.
Key Words: beta-expansin • ear rot fungi • flower longevity • pathogen defense • silk senescence • style senescence • Zea mays
The lifespan of unpollinated flowers varies considerably among species, ranging from a few hours in Cucurbita species to several weeks in some orchid species (Primack, 1985
; Ashman and Schoen, 1994
; Winsor et al., 2000
). In many species, pollen deposition, fertilization, and/or pollen removal hasten floral senescence (Devlin and Stephenson, 1984
, 1985
; Richardson and Stephenson, 1989
; Stead, 1992
; O'Neill, 1997
; Bell and Cresswell, 1998
). Ashman and Schoen (1994)
and Schoen and Ashman (1995)
have proposed that interspecific differences in the lifespans of unpollinated flowers are determined by the daily costs of maintaining an open flower relative to the costs of producing a new flower and the expected fitness payoff in terms of pollen dissemination and pollen accumulation on the stigma. In this context, the accelerated senescence of flowers associated with pollen deposition and pollen removal can be viewed as conditional intraspecific adjustments in floral lifespan that save resources (Evanhoe and Galloway, 2002
).
Shykoff et al. (1996)
noted that in natural populations of two species of dioecious Silene, the male plants had both longer floral lifetimes and a higher incidence of the pollinator-vectored fungus Microbotryum violaceum (Pers.) Deml. & Oberw. (= Ustilago violacea Pers.) than the female plants. They proposed that differences in the infection rate between the two sexes could be due to differences in floral longevity and later demonstrated that male and female plants differ in risk of infection per contact (Kaltz and Shykoff, 2001
). Ashman and Schoen (1996)
agreed that plant–pathogen interactions could also influence the evolution of floral longevity and suggested that future theoretical efforts should incorporate the rate at which infectious spores arrive at flowers and the extent to which the resulting infections diminish future reproductive success. If differences in floral longevity among species and between genders in dioecious species can be influenced by plant pathogens, then it is reasonable to speculate that rapid floral senescence following pollination/fertilization may function to prevent infection by pathogens via the sexual organs.
In the ears of maize (Zea mays L.) plants, each silk (style) is attached to a different ovary (kernel) on the ear. Unpollinated silks remain receptive and continue to grow (elongate) under field conditions for approximately 8 days before senescence occurs (i.e., the silks turn brown and curl). However, within 56 h after pollination, the silks stop growing and begin to senesce (Bassetti and Westgate, 1993
). In maize, silk senescence is a response to fertilization (rather than pollination per se), and senescence is often used by plant breeders as an indicator of fertilization. Maize silks are also one of the preferred foods of corn rootworm beetles (Diabrotica spp.) but, after senescence, the brown silks are no longer appealing to the beetles (Peters et al., 1996
). These beetles can directly depress seed production by damaging the silks, and they are excellent vectors for a variety of fungi that cause smut and ear rot (Ingold, 1953
; Dowd, 1995
). Worldwide, the most common ear rots are caused by Fusarium graminearum and F. moniliforme and their sexual stages (Gibberella zeae and G. fujikuroi) (deLeon and Jeffers, 2004
). Virtually all maize fields of the midwestern and eastern USA have some level of infection by these pathogens, but infection levels are greatest during cool and moist years (deLeon and Jeffers, 2004
). These filamentous fungi (and other ear rots) enter the ear by growing through and around the fresh silks (Dowd, 1998
), and most ear rots can be transmitted to the next generation through the seeds (McGee, 1988
).
Over the past 6 years, we have been investigating the roles of a major group-1 pollen allergen of maize in pollen development, pollen germination, and pollen tube growth. The group-1 pollen allergens are the main causative agents of hay fever and seasonal asthma induced by grass pollen (Malley et al., 1962
; Johnson and Marsh, 1965a
, b
) and they constitute a clade within the ß-expansin or EXPB family (Cosgrove et al., 1997
; Sampedro and Cosgrove, 2005
). Group-1 pollen allergens in maize are secreted proteins with cell-wall loosening activity that is specific for grass cell walls, and they are expressed specifically in pollen, where they represent 
4% of the total extracted protein (Cosgrove et al., 1997
; Wu et al., 2001
; Li et al., 2003
). Four maize group-1 allergen isoforms have been characterized, the most abundant is called Zea m1d (Li et al., 2003
). Several highly similar genes code for this isoform, and we refer to them collectively as subclass EXPB1 (Valdivia, 2005
). We have characterized a maize line with a Mutator (Mu) transposon insertional mutation in the most abundant isoform of EXPB1 (see Valdivia, 2005
for details). For the sake of clarity, in this work we will refer to this gene as EXPB1.
Pollen with the expb1::mu allele did not differ significantly in viability or growth rate in vitro from pollen with the EXPB1 allele. However, pollen tubes with the expb1::mu allele grew significantly slower in vivo and sired fewer seeds under competitive conditions than pollen with the EXPB1 allele. Here, we report the results of a serendipitous field study and two experimental studies indicating that the time between pollen deposition and senescence in maize silks can have dramatic effects on the incidence of fungal ear rot and that the senescence of unpollinated silks is accelerated when other ovaries on the cob are fertilized.
MATERIALS AND METHODS
Study plants
We obtained transposon insertional lines from Pioneer Hi-Bred International (Meeley and Briggs, 1995
; Benson et al., 1995
; Brutnell, 2002
) and identified a mu insertion in the gene that encodes for EXPB1 (GenBank accession AY197353). These plants were backcrossed to the non-Mutator parental line (FR696 inbred line) for three generations, and plants heterozygous for the mu insertion were self-pollinated. The resulting seeds were planted and screened for the presence of the transposon insertion using PCR with EXPB1-specific primers that span the transposon insertion (Valdivia, 2005
). From these plants via self pollinations, we created true breeding mutant (expb1/expb1) and wild-type (EXPB1/EXPB1) lines as well as heterozygous plants by crossing the true breeding lines.
Serendipitous field experiment
In May of 2003, we planted four adjacent 20 x 60 m plots at the Pennsylvania State University Agricultural Experiment Station at Rock Springs, PA. The plots were separated by 5 m. In the first and third plots, we sowed maize that was true breeding for the EXPB1 allele, and in the remaining plots we sowed both heterozygous plants and plants that were true breeding for the expb1 allele. Plants were sprayed with insecticide on a biweekly schedule. However, when the plants were producing tassels and silks, the combination of wet conditions and a desire to not expose the pollen to insecticide resulted in a 5-week hiatus in insecticide application. To obtain seeds for ongoing and future studies of the function of the EXPB1 gene, we crossed all three types of plants to the wild-type plants using standard maize breeding techniques (see website http://www.maizedb.org/IMP/WEB/pollen.htm, compiled by L. Vincent, Maize Mapping Project, University of Missouri–Columbia [accessed April 2003]). We performed 103 total pollinations—several of each type of pollination on each of the two plots with EXPB1/EXPB1 plants. No pollinations were made on the outer three rows in each plot and only one ear per plant was pollinated. In short, ears were bagged before silk emergence. The tip of the husks and the silks were cut off when the silks were first visible (i.e., all silks were trimmed to the same length) and rebagged. Fresh pollen was collected at anthesis from several individuals of each genotype by shaking the tassels. Anthers and debris were removed by filtering pollen through a sieve. The silks were pollinated 1–2 d after trimming. By this time, the silks had grown an additional 1.5–2.0 cm. The bags did not prevent corn rootworm beetles and other insects from gaining access to the silks. In the autumn the mature ears were harvested by hand, and the seeds were removed and counted. At the time of seed removal, we scored each ear for the incidence of ear rot disease (which was common in the wet summer of 2003) so that we could avoid using these seeds in future studies.
Silk collapse experiment
To investigate the onset of silk senescence following pollination, in 2004 we collected and cleaned the pollen from each of the three types of plants (as described earlier) and then filled microcentrifuge tubes to 100 µL volumes with the pollen. Twelve ears on the true-breeding plants for the wild-type allele were then pollinated with 100 µL of pollen from each of the three types of plants using the same pollination technique as described (36 total pollinations). A 100-µL volume of pollen was used because we showed previously (see Valdivia, 2005
) that this volume is sufficiently large to fertilize all or most of the ovules, but sufficiently small to ensure that pollen with the wild-type allele (that produces faster growing tubes) and pollen with the mutant allele (that produces slower-growing tubes) both fertilize ovules. Because we found previously that fertilization of the ovules in the central region of the cob occurs between 22–24 h after deposition of pollen with the wild-type allele, we harvested four ears that were pollinated by each of the three types of plants at 24, 36, and 48 h after pollination. The husks were carefully removed from these ears, and the base of 30 silks (just above the ovary) from the central region of the cob was examined with a hand lens. The base of each silk was scored as either collapsed or not collapsed (see Fig. 1).
|
|
Of the 103 mature ears that were produced in the 2003 field study, we found that 72% (N = 25) of the ears pollinated with pollen from expb1/expb1 plants, 70% (N = 30) of the ears pollinated with pollen from heterozygous plants, and 25% (N = 48) of the ears pollinated with pollen from EXPB1/EXPB1 plants had symptoms of fungal ear rot. A 
2 test revealed that the probability that an ear will develop rot is not independent of the type of pollen deposited onto the silks (2 = 21.6; df = 2; P < 0.0001).
The silk collapse experiment revealed that less than 5% of the silks had collapsed by 24 h after pollination and >85% of all silks had collapsed by 48 h after all three types of pollinations. However, a one-way analysis of variance revealed that a significant effect of pollen donor on the proportion of silks that had collapsed at 36 h after pollination (F2,9 = 57.8; P < 0.0001). (The data were arc-sine square root transformed prior to analysis to meet the assumption of normality [SAS, 2002
]). We found collapsed silks near the juncture of the silk with the ovary in 49% of the silks pollinated with pollen from expb1/expb1 plants, 60% of the silks pollinated with pollen from heterozygous plants, and 85% of the silks on ears pollinated with pollen from EXPB1/EXPB1 plants (Figs. 1 and 3). These findings show that the onset of senescence is delayed when the pollen parent is either heterozygous or homozygous for the expb1 allele.
|
|
Plant evolutionary biologists have argued that interspecific differences in the longevity of flowers and floral organs have been influenced by a variety of factors including their postpollination role(s) in pollinator attraction (e.g., Jones and Cruzan, 1999
), trade-offs among floral construction costs, maintenance costs, and expected fitness payoffs (pollen deposition and donation) (Ashman and Schoen, 1994
; Schoen and Ashman, 1995
), and trade-offs between expected fitness payoffs and the risk of pathogen infection (Shykoff et al., 1996
; Kaltz and Shykoff, 2001
). Similarly, variation in floral longevity associated with pollen deposition, pollen removal, and/or fertilization has been viewed as a means of conserving resources when the residual fitness payoffs of a flower drop below a minimum threshold (e.g., Evanhoe and Galloway, 2002
).
The experiments reported here show that the onset of maize silk senescence is delayed approximately 12 h when the silks are pollinated by the slower-growing pollen expressing the expb1 allele (Fig. 3). Under field conditions in 2003, ears pollinated by expb1/expb1 and EXPB1/expb1 plants had significantly higher levels of fungal ear rot than ears pollinated by the EXPB1/EXPB1 plants. Under field conditions, the spores of the fungi causing ear rot can arrive at silks via air currents and/or they are transmitted by vectors such as Diabroticite beetles and other insects that feed on the silks (Ingold, 1953
; Dowd, 1995
). After arrival, these fungi gain access to the ear by growing in and around the silks (Dowd, 1998
). We suggest that the lower rate of infection following pollination by the faster-growing pollen tubes of EXPB1/EXPB1 plants is due to the accelerated rate of silk senescence on these ears. Furthermore, we propose that accelerated senescence of silks following fertilization is a defense against pathogens such as those causing ear rot.
Although the higher levels of ear rot infection following pollination by true-breeding expb1 plants and heterozygous plants may have alternative explanations (see Valdivia, 2005
), the data from the half-ear experiments are also consistent with the notion that accelerated silk senescence functions in disease protection. In both half-ear experiments, the onset of silk senescence (the cessation of silk growth) following the deposition of expb1 is similar to that of unpollinated silks, and silk senescence is delayed compared to those silks pollinated with EXPB1 pollen (Fig. 4). However, when half of the ovaries have been fertilized by EXPB1 pollen in the EXPB1/EXPB1 treatment of the half-ear experiment, both the unpollinated silks and silks pollinated with expb1 pollen senesce sooner than when the other half of the silks are unpollinated. That is, the silks from unfertilized ovaries senesce faster when they pose a risk to developing seeds.
The collapse of the base of the silk following fertilization is most likely due to a physiological signal(s) originating in the ovary (Stead, 1992
). Our data from the half-ear experiment indicate that there is also some physiological cross-talk (i.e., transmission of signals necessary to initiate senescence) among the ovaries on an ear that accelerates the senescence of silks on unfertilized ovaries when half of the silks have been fertilized by EXPB1 pollen. In many species, an ethylene burst originating from the ovary is associated with the senescence of floral organs (see reviews by Stead, 1992
; O'Neill, 1997
). A gaseous hormone, such as ethylene, would be especially effective as a signal for interovary cross-talk because the ears of maize are tightly packaged within the husks.
Although ours is a small, 1-year study using a cultivated species, accelerated senescence following pollination, pollen removal, and fertilization has been documented in many species (both cultivated and wild), and many plant pathogens are known to be transmitted via floral organs (e.g., Antonovics, 2005
). If, as our data suggest, rapid floral senescence following pollination/fertilization decreases the probability of pathogen infection, then it is reasonable to predict that species with short floral durations and/or rapid postpollination senescence are species that regularly face challenges from pathogens that infect their hosts via the sexual organs. In this regard, experiments that manipulate the timing of senescence (perhaps by manipulating the ethylene burst via ethylene inhibitors and promotors) would be particularly insightful.
FOOTNOTES
1 The authors thank T. Omeis for use of the Buckhout Greenhouse; R. Oberheim and the Department of Horticulture for use of the Agricultural Experiment Station at Rock Springs, PA; S. Chopra, P. McSteen, and D. Braun for field space, helpful discussion, and guidance; D.M. Durachko, E. Wagner, M. Perich, J. Sampedro, R. Carey, T. Kinney, N. Sella Kapu, and J. Mena-Ali for assistance in the field, greenhouse, and lab. This project was made possible in part through use of technology developed by Pioneer Hi-Bred International, Inc. and was supported by NSF grant DEB 02-35217 and USDA grant 2005-35320-15251 to A.G.S. and by NIH grant GM060397 and DOE grant DE-FG02-84ER13179 to D.J.C.; E.R.V. was supported in part by an NSF Graduate Research Training grant to D.J.C. 
4 Author for correspondence (as4{at}psu.edu
)
LITERATURE CITED
Antonovics J.. 2005. Plant venereal diseases: insights from a messy metaphor. New Phytologist 165: 71-80.[CrossRef][Web of Science][Medline]
Ashman T.-L. Schoen D. J.. 1994. How long should flowers live?. Nature 371: 788-791.[CrossRef]
Ashman T.-L. Schoen D. J.. 1996. Flower lifespan and disease risk. Nature 379: 780.[CrossRef]
Bassetti P. Westgate M. E.. 1993. Emergence, elongation, and senescence of maize silks. Crop Science 33: 271-275.
Bell S. A. Cresswell J. E.. 1998. The phenology of gender in homogamous flowers: temporal change in residual sex function of flowers of oil-seed rape (Brassica napus). Functional Ecology 12: 298-306.[CrossRef][Web of Science]
Benson R. J. Johal G. S. Crane V. C. Tossberg J. T. Schnable P. S. Meeley R. B. Briggs S.. 1995. Cloning and characterization of the maize An1 gene. Plant Cell 7: 75-84.[Abstract]
Brutnell T. P.. 2002. Transposon tagging in maize. Functional & Integrative Genomics 2: 4-12.
Cosgrove D. J. Bedinger P. Durachko D. M.. 1997. Group I allergens of grass pollen as cell wall-loosening agents. Proceedings of the National Academy of Sciences, USA 94: 6559-6564.
deLeon C. Jeffers D.. 2004. Maize diseases: a guide for field identification, 4th ed CIMMYT Publications, Mexico City, Mexico.
Devlin B. Stephenson A. G.. 1984. Factors that influence the duration of the staminate and pistillate phases of Lobelia cardinalis flowers. Botanical Gazette 145: 323-328.[CrossRef]
Devlin B. Stephenson A. G.. 1985. Sex differential floral longevity, nectar secretion, and pollinator foraging in a protandrous species. American Journal of Botany 72: 303-310.[CrossRef][Web of Science]
Dowd P. F.. 1995. Sap beetles and mycotoxins in maize. Food Additives and Contaminates 12: 497-508.
Dowd P. F.. 1998. Involvement of arthropods in the establishment of mycotoxigenic fungi under field conditions. In K. K. Sinhaand and D. Bhatagnar [eds.] Mycotoxins in agriculture and food safety 307-350 Marcel Dekker, New York, New York, USA.
Evanhoe L. Galloway L. F.. 2002. Floral longevity in Campanula americana (Campanulaceae): a comparison of morphological and functional gender phases. American Journal of Botany 89: 587-591.
Ingold C. T.. 1953. Dispersal in fungi Oxford University Press, London, UK.
Johnson P. Marsh D. G.. 1965a. "Isoallergens" from rye grass pollen. Nature 206: 935-937.[Medline]
Johnson P. Marsh D. G.. 1965b. The isolation and characterization of allergens from the pollen of rye grass (Lolium perenne). European Polymer Journal 1: 63-77.[CrossRef]
Jones C. E. Cruzan M. B.. 1999. Floral morphological changes and reproductive success in deer weed (Lotus scoparius, Fabaceae). American Journal of Botany 86: 273-277.
Kaltz O. Shykoff J. A.. 2001. Male and female Silene latifolia plants differ in per contact risk of infection by sexually transmitted disease. Journal of Ecology 89: 99-109.[CrossRef]
Li L. C. Bedinger P. A. Volk C. Jones A. D. Cosgrove D. J.. 2003. Purification and characterization of four beta-expansins (Zea m 1 isoforms) from maize pollen. Plant Physiology 132: 2073-2085.
Malley A. Reed C. E. Lietze A.. 1962. Isolation of the allergen from timothy pollen. Journal of Allergy and Clinical Immunology 33: 84-93.
McGee D. C.. 1988. Maize diseases: a reference source for seed technologists APS Press, St. Paul, Minnesota, USA.
Meeley R. B. Briggs S.. 1995. Reverse genetics for maize. Maize Genetics Cooperation Newsletter 69: 67-82.
O'Neill S. D.. 1997. Post-pollination regulation of flower development. Annual Review of Plant Physiology and Plant Molecular Biology 48: 547-574.[CrossRef][Web of Science][Medline]
Peters L. L. Meinke L. J. Witkowski J. F.. 1996. Insects that feed on corn ears Available at website http://ianrpubs.unl.edu/insects/g839.htm#DAMBYAD [accessed June 2005].
Primack R. B.. 1985. Longevity of individual flowers. Annual Review of Ecology and Systematics 16: 15-37.[CrossRef][Web of Science]
Richardson T. R. Stephenson A. G.. 1989. Pollen removal and pollen deposition affect the duration of the staminate and pistillate phases in Campanula rapunculoides. American Journal of Botany 76: 532-538.[CrossRef][Web of Science]
Sampedro J. Cosgrove D. J.. 2005. The expansin superfamily. Genome Biology 6: 242.1-242.11.
SAS Institute.. 2002. SAS user's guide SAS Institute, Cary, North Carolina, USA.
Schoen D. J. Ashman T.-L.. 1995. The evolution of floral longevity: resource allocation to maintenance versus construction of repeated parts in modular organisms. Evolution 49: 131-139.
Shykoff J. A. Bucheli E. Kaltz O.. 1996. Flower lifespan and disease risk. Nature 379: 779.
Stead A. D.. 1992. Pollination induced flower senescence: a review. Plant Growth Regulation 11: 13-20.
Valdivia E. R.. 2005. Role of maize expansins in reproductive development Ph.D. dissertation, Pennsylvania State University, University Park, Pennsylvania, USA.
Winsor J. A. Peretz S. Stephenson A. G.. 2000. Pollen competition in a natural population of Cucurbita foetidissima (Cucurbitaceae). American Journal of Botany 87: 527-532.
Wu Y. Meeley R. B. Cosgrove D. J.. 2001. Analysis and expression of the alpha-expansin and beta-expansin gene families in maize. Plant Physiology 126: 222-232.
CiteULike Complore Connotea Del.icio.us Digg Facebook Reddit Technorati Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  J. H. Williams Amborella trichopoda (Amborellaceae) and the evolutionary developmental origins of the angiosperm progamic phase Am. J. Botany, January 1, 2009; 96(1): 144 - 165. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
