Sink-Source Transition in Tobacco Leaves

高等植物的叶片在成熟时从吸收光合物质的汇转化为吸收光合物质的源。本实验表明，烟草中光同化物进口的终止（汇期结束）与正碳平衡的实现是一致的，但不是由于正碳平衡的实现。这表明韧皮部的卸荷是共塑的（通过胞间连丝），当胞间连丝与周围细胞的连接丢失或受阻时卸荷停止。阻断共塑性流动还可以使韧皮部的细胞通过防止泄漏而获得出口所需的高溶质含量。先前的结果支持卸载是被动和共塑性的假设。进一步的实验将通过测试高pH和抑制剂处理对破坏胞间连丝连接、阻止共塑运输的影响，进一步证实烟草汇叶中共塑韧皮部卸载途径。并改进目前卸载途径的形态计量学分析，以允许使用溶质通量数据刺激运输。将进行高分辨率的EM研究，以确定沿卸载路径的胞间连丝是否被物理阻塞。还将研究韧皮部负荷，以确定是否涉及外体步骤。这些对发育中的叶片的生理和结构的研究提供了关于光合作用过程中产生的糖如何首先储存在叶片中，然后被动员形成籽粒和果实的信息。这对于更好地了解作物生产力具有明显的意义。令人惊讶的是，还有多少植物结构有待阐明。该项目填补了植物生物学领域的一个重要信息空白

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.***//