Post-sieve element transport of photoassimilates in sink regions

Photoassimilate transport from the sieve elements to the recipient sink cells, principally in the form of sucrose, provides a link between sink metabolism and compartmentation with phloem import. Phloem unloading has focused attention on photoassimilate transport across the sieve element boundary. However, post-sieve element transport can be of equal or greater significance. Three cellular pathways of sieve element unloading and post-sieve element transport are identified. These are apoplastic, symplastic and symplastic interrupted by an apoplastic step. The symplastic path is considered to be the common path, while the remaining pathways serve specialized functions. In particular, the apoplastic step isolates the sieve element transport function from the effects of solute concentration or osmotic changes in the sink cells. Switching between apo- and symplastic routes within a given sink has been found to be linked with such changes. Plasmodesmatal transport undoubtedly involves a diffusive component, but whether bulk flow contributes to the symplastic flux of photoassimilate from the sieve elements to the recipient sink cells is yet to be established unequivocally. Efflux across the plasma membranes of the sieve element-companion cell (se-cc) complexes and other vascular cells occurs by passive diffusion. Along the axial route, retrieval from the phloem apoplast is mediated by sucrose/proton symport. However, this mechanism is absent in terminal sinks. Non-vascular efflux from the maternal tissues of developing seed is passive in cereals and energy-coupled in certain grain legumes. Accumulation of sugars from the apoplast of all sinks with an apoplastic step universally occurs by a plasma membrane-bound sugar/proton symport mechanism. Regulation of symplastic transport could be mediated by a combination of sink metabolism and compartmentation coupled with changes in the transport properties of the interconnecting plasmodesmata.