Plant Cell Physiology

Few processes are as vital to plant productivity as the uptake of photoassimilates into heterotrophic plant cells. Whether intended for short or long term storage, uptake of photoassimilates not only supply energy and building blocks for growth and development, but often excess carbon is stored in diverse cellular structures amassing massive reserves. Furthermore, these same reserves, contained in the vacuole(s), amyloplasts or elaioplasts, become sources of energy to support metabolism during postharvest shelf life and to support germination and sprouting of seeds, tubers, hypocotyls and other reproductive organs. In this laboratory, we concentrate our efforts to the study of the mechanisms of sucrose uptake into the vacuole of herterotrophic plant cells and its mobilization at times of high energy demands.

We have recently described an endocytic mechanism for sucrose uptake into heterotrophic plant cells. Although just now we are beginning to understand some of the complexities of this transport system, we are certain that endocytsis as a means of nutrient uptake is induced by sucrose (or other sugars) which is (are) eventually deposited in the vacuole. Our discoveries are based on physiological studies and confocal laser fluorescent microscopy of Citrus juice cells (Fig. 1) and sycamore cultured cells (Fig. 2). With the use of fluorescent endocytic markers, endocytic inhibitors, artificial nano-particles (Fig. 3) and quantum nano-spheres (Fig. 4) we have shown that uptake of a large portion of photoassimilates occurs by means of fluid-phase endocytosis.

Separately, we are investigating the coordinated action between fluid phase endocytosis and membrane-bound sugar carriers. Using the fluorescent glucose derivative NBDG in combination with fluorescent dextran (mw 3,000) we were able to demonstrate the parallel uptake of extracellular metabolites into heretotrophic cells. The use of cytoplasts (Fig. 5) was essential in the study that showed uptake through plasmalemma-bound carriers and vesicle mediated endocytosis can occur simultaneously to transport metabolites to different destinations. Details of the coordinated action between these two systems are currently being investigated.

Other studies presently being carried out focus on the mechanisms of sugar accumulation in Citrus fruits as consequence of imposing a mild water stress. In field studies, we have demonstrated that mild water stress enhances the accumulation of soluble solids in citrus fruits, increase acid content and delay flowering. Postharvest shelflife of fruit from mildly-water stressed tees in currently being investigated.

Funded by a 3-year USDA-TSTAR grant, we are also analyzing various metabolic parameters believed to have direct influence in citric acid accumulation in Citrus fruits. Low fruit acidity is a recurring problem in Florida Citrus and these studies are essential for the future design of research to find a long term solution.


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