POST-SIEVE ELEMENT TRANSPORT OF PHOTOASSIMILATES IN SINK REGIONS

Photoassimilate transport from the sieve elements to the recipient sin k cells, principally in the form of sucrose, provides a link between s ink metabolism and compartmentation with phloem import. Phloem unloadi ng has focused attention on photoassimilate transport across the sieve element boundary. However, post-sieve element transport can be of equ al or greater significance. Three cellular pathways of sieve element u nloading and post-sieve element transport are identified. These are ap oplastic, symplastic and symplastic interrupted by an apoplastic step. The symplastic path is considered to be the common path, while the re maining pathways serve specialized functions. In particular, the apopl astic step isolates the sieve element transport function from the effe cts of solute concentration or osmotic changes in the sink cells. Swit ching between apo- and symplastic routes within a given sink has been found to be linked with such changes. Plasmodesmatal transport undoubt edly 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. Efflu x across the plasma membranes of the sieve element-companion cell (se- cc) complexes and other vascular cells occurs by passive diffusion. Al ong 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 se ed is passive in cereals and energy-coupled in certain grain legumes. Accumulation of sugars from the apoplast of all sinks with an apoplast ic step universally occurs by a plasma membrane-bound sugar/proton sym port mechanism. Regulation of symplastic transport could be mediated b y a combination of sink metabolism and compartmentation coupled with c hanges in the transport properties of the interconnecting plasmodesmat a.