SPATIAL AND TEMPORAL EXPRESSION OF SUCROSE TRANSPORT-RELATED GENES INDEVELOPING COTYLEDONS OF VICIA-FABA L

Dry-matter accumulation by developing, cotyledons of grain legumes inc ludes a mandatory influx of photoassimilates, largely in the form of s ucrose, from the seed apoplasm across the plasma membranes of the coty ledon cells. This study examined the temporal and spatial expression o f an H+/sucrose symporter, a P-type H+-ATPase, and a sucrose-binding p rotein (SEP) in cotyledons of Vicia faba L. throughout their developme nt. The flux of city matter and sucrose symporter activity exhibited i dentical temporal trends. These were a marked increase during cotyledo n expansion to a plateau maintained until cotyledon maturity. Thereaft er both parameters declined precipitously. The temporal changes in suc rose symporter activity were accounted for by shifts in its V-max. Tra nscript levels of the H+/sucrose symporter followed a similar temporal pattern to the sucrose symporter activity suggesting regulation by ge ne expression. Equivalent conclusions were drawn for SEP and the H+-AT Pase expression during cotyledon expansion. Thereafter, during seed fi lling, the transcript levels of SEP and H+-ATPase did not closely foll ow that found for the sucrose symporter. A progressive wave of gene ex pression in the abaxial epidermal cells spread from the cotyledon regi on juxtaposed to the non-vascular region of the seed coat at the pole distal from the funicle. The pattern of expression progressed most rap idly along the median longitudinal plane of the cotyledons and more sl owly outward to their margins. The densities of SEP and H+-ATPase, ins erted into the plasma membranes of the abaxial epidermal cells. increa sed throughout cotyledon expansion. Gene expression (sucrose symporter ) and membrane insertion of the gene products (SEP. H+-ATPase) were cl osely associated with the initiation and development of wall ingrowths in the abaxial epidermal cells.