POST-SIEVE ELEMENT TRANSPORT OF SUCROSE IN DEVELOPING SEEDS

Developing seeds of cereals and grain legumes have proven to be useful experimental models to examine post-sieve element assimilate transpor t in sink tissues. Morphologically, these seeds offer well-defined sin ks in which the processes of sucrose import plus efflux and influx plu s metabolism may be examined independently. In all cases, sucrose is d elivered through the phloem to the maternal seed tissues. Unloading fr om the sieve element-companion cell complexes is symplastic. Subsequen tly, sucrose moves through a symplastic route to cells responsible for sucrose efflux to the seed apoplast. The efflux cells are located at, or near, the maternal/filial interface. Sucrose is retrieved from the seed apoplast by the outermost cell layers of the filial tissues. Sub sequent transfer of sucrose to the sites of storage in the filial tiss ues is confined principally to a symplastic route. Sucrose efflux from the maternal tissues appears to be passive in cereals and energy depe ndent in grain legumes, possibly through a sucrose/proton antiport sys tem. Sucrose influx across the plasma membranes of the filial cells is energy dependent and, for grain legumes, is energy coupled through a sucrose/proton symporter. Studies on the control of post-sieve element transport of sucrose have focused largely on the membrane transport s teps. The role of phytohormones as modulators of sucrose transport is uncertain. In grain legumes, efflux from the maternal cells could be r egulated by rates of sucrose utilisation in the filial tissues through a turgor homeostat mechanism located in the efflux cells.