Mechanisms of phloem loading. (June 2018) - Zhang, Cankui
Highlights: SWEET transporters were shown to facilitate diffusion of sugars into the apoplast. Gene expression analysis illuminated stepwise development of transfer cells. The first transporters for phloem loading of amino acids were discovered. Modeling provided insight into phloem loading through plasmodesmata. Abstract : The complex form of higher plants requires continuous, balanced transport of nutrients in the phloem. The initial step of transferring sugars, amino acids, and other materials from photosynthetic cells to the conducting sieve tubes is known as phloem loading. Three phloem loading mechanisms have been described. The first involves release of sucrose into the apoplast and subsequent retrieval by the phloem. The initial release step in this process is now known to be mediated by a new class of transporters, the SWEET proteins. In the other two loading mechanisms, polymer trapping and diffusion, sucrose passes into the phloem through cytoplasmic channels, the plasmodesmata. Recent models have shed additional light on these mechanisms and their ability to sustain the growth of even the tallest trees.