Plasmodesmata and Phloem Loading

该项目支持对叶片韧皮部加载机制的鉴定研究。通过光合作用产生的糖被装载到细脉的韧皮部中，然后出口到生长器官。有两种已知的韧皮部加载机制。质外体加载涉及跨膜运输，而共体加载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，大概是以不同的方式。根据目前的模式，胞间连丝计数可以可靠地识别胞质外体和共体负载物种。另一种假说是，质外体负载是普遍存在的，除了那些运输水苏糖或其他类似大小的碳水化合物的物种，以及通过聚合物捕获来负载的物种。韧皮部负载研究主要基于对氯汞苯磺酸对跨膜运输的特定抑制，将对5种转运蔗糖但根据胞间连丝计数被推测为共生体负载的物种进行研究。据预测，这些植物是通过质外体加载的。根据植物的生长习性，小脉韧皮部的胞间连丝频率反映了叶片的胞间连丝数目，而与韧皮部生理没有必然的关系，这一假说将被进一步研究。

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.