Plasmodesmata and Phloem Loading

本项目支持叶片韧皮部加载机制鉴定的研究。光合作用产生的糖被装入小静脉的韧皮部，出口到生长器官。有两种已知的韧皮部加载机制。外胞质装载涉及跨膜运输，而共塑装载则完全通过胞间连丝连接的细胞质进行。能够区分这些机制是很重要的，因为它们受到环境压力的抑制，可能以不同的方式。根据目前的模式，胞浆和共塑负荷物种可以通过胞浆胞丝计数可靠地识别。在本研究中需要验证的另一种假设是，除了那些通过聚合物捕获转运水苏糖或其他类似大小的碳水化合物的物种外，外体负荷是普遍存在的。韧皮部负荷研究，主要基于对氯脲苯磺酸对跨膜运输的特异性抑制，将在五种转运蔗糖的物种上进行，但根据胞间连丝计数推定它们是共塑负荷者。预测是这些植物通过外质体装载。进一步的研究将验证小叶脉韧皮部的胞间连丝频率反映叶片中整体胞间连丝数量的假设，这取决于植物的生长习惯，而不一定与韧皮部生理有关。

This project supports studies on the identification of phloem loading mechanisms in leaves. Sugars manufactured by photosynthesis are loaded into the phloem of minor veins for export to growing organs. There are two known phloem loading mechanisms. Apoplastic loading involves trans-membrane transport while symplastic loading takes place entirely through the plasmodesmata-connected cytoplasm. It is important to be able to distinguish between these mechanisms because they are inhibited by environmental stress, presumably in different ways. According to the current paradigm, apoplastic- and symplastic-loading species can be identified reliably by plasmodesmatal counts. An alternative hypothesis to be tested in this investigation is that apoplastic loading is universal, except for those species that translocate stachyose, or other carbohydrates of similar size, and load by polymer trapping. Phloem loading studies, based primarily on the specific inhibition of trans-membrane transport by p-chloromercuribenzenesulfonic acid, will be conducted on five species that translocate sucrose but have been putatively identified as symplastic loaders on the basis of plasmodesmatal counts. The prediction is that these plants load via the apoplast. Further studies will be conducted to test the hypothesis that plasmodesmatal frequencies in minor vein phloem reflect overall plasmodesmatal numbers in leaves, depending on plant growth habit, and are not necessarily related to phloem physiology.