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(American Journal of Botany. 2007;94:1041-1045.)
© 2007 Botanical Society of America, Inc.

Brief Communication

Mannitol-enhanced, fluid-phase endocytosis in storage parenchyma cells of celery (Apium graveolens; Apiaceae) petioles1

Ed Etxeberria, Pedro Gonzalez and Javier Pozueta-Romero

University of Florida, IFAS, Horticulture Department, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, Florida 33850 USA; Agrobioteknologia Instituta, Consejo Superior de Investigaciones Científicas, Nafarroako Gobernua and Nafarroako Unibertsitate Publikoa, Mutiloako etorbidea zembaki gabe, 31192 Mutiloabeti, Nafarroa, Spain

ABSTRACT

We recently demonstrated the occurrence of a sucrose-enhanced, fluid-phase endocytic (FPE) mechanism of nutrient uptake in heterotrophic cells. In the present work, the possible enhancement/induction of FPE by photoassimilates other than sucrose was investigated by measuring the incorporation of the fluorescent endocytosis marker d-TR (dextran-Texas red, 3000 mw) into celery (Apium graveolens) petiole storage parenchyma (CSP), a tissue that transports and accumulates mannitol. Mannitol uptake in these cells is biphasic, with a hyperbolic phase at concentrations below 20 mM and a linear phase above 20 mM external solute concentration. In the absence of mannitol, or in its presence at concentrations within the hyperbolic phase, CSP cells accumulated low levels of d-TR. Conversely, d-TR accumulation by CSP cells was greatly enhanced in the presence of mannitol at concentrations within the linear phase. At high external mannitol concentration, d-TR accumulation was prevented by the endocytic inhibitors LY294002 and latrunculin B. In addition, d-TR uptake was temperature dependent under high mannitol concentration. Microscopic observations revealed that d-TR accumulated in the vacuole. These data support the occurrence of an FPE mechanism in CSP cells that participates in trapping and transport of photoassimilates to the vacuole. The FPE mechanism is enhanced by high mannitol concentrations.

Key Words: apoplast • biphasic uptake curve • sugar carriers • sugar uptake • vacuole storage






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