Sink-Source Transition in Tobacco Leaves

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Leaves of higher plants are converted from sinks to sources of photoassimilate as they mature. This lab has shown that termination of photoassimilate import (end of the sink phase) in tobacco is coincident with, but not due to, achievement of positive carbon balance. This suggests that phloem unloading is symplastic (through plasmodesmata) and ceases when plasmodesmatal connections to surrounding cells are either lost or blocked. Interruption of symplastic flow would also allow the cells of the phloem to achieve the high solute content needed for export by preventing leakage. Prior results support the hypothesis that unloading is passive and symplastic. Additional experiments will be conducted to further substantiate the symplastic phloem unloading route in tobacco sink leaves by testing the effects of breaking plasmodesmatal connections, preventing symplastic transport with high pH and inhibitor treatments, and refining the present morphometric analysis of the unloading pathway to allow stimulation of transport using data on solute flux. High resolution EM studies will be conducted to determine whether plasmodesmata along the unloading pathway become physically obstructed. Phloem loading will also be studied to determine whether an apoplastic step is involved.%%% These studies on the physiology and structure of a developing leaf are providing information on how sugars made during photosynthesis are first stored in leaves and then mobilized to form grains and fruits. This has obvious implications for a better understanding of crop productivity. It is surprising how much plant structure remains to be elucidated. This project is filling an important information gap in plant biology.***//