Plasmodesmata and Phloem Loading

Cornell University, Ithaca NY

Investigators

Abstract

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.