Carbon partitioning: more and more players acting on phloem loading
Résumé
Phloem transport between source and sink organs is coupled to the translocation of carbohydrates and other metabolites over long distance. Sucrose is the main photoassimilate used for carbon distribution. In apoplasmic phloem loader species such as Arabidopsis, energized-sucrose transporters mainly orchestrate sucrose loading in the phloem. In Arabidopsis, one of them, SUC2, has been shown to be central. It acts both on sugar uptake in minor veins and sucrose retrieval along the transport phloem pathway. Other actors, such as SWEET proteins, were recently shown to participate to the loading process, potentially acting on efflux from mesophyll cells into phloem cells. Another potential pathway for sugar loading is the cell-to-cell transfer via plasmodesmata, a mechanism driven by hydrostatic pressure gradients initiated in mesophyll cells. Depending on plant species, symplasmic and/or apoplasmic mechanisms can operate, although there is probably an intimate interplay between both pathways. Our main goal is the identification in Arabidopsis of novel actors acting on phloem loading. Our approach has been based on the analysis of candidate genes identified on transcriptome profiling of the phloem tissues, which allowed the identification of several new factors. We also recently developed approaches to analyze the composition of the phloem sap exudate in order to propose of an integrated view of carbon partitioning.