Spatial and temporal expression of sucrose transport-related genes in developing cotyledons ofVicia faba L.

Abstract

Dry-matter accumulation by developing cotyledons of grain legumes includes a mandatory influx of photoassimilates, largely in the form of sucrose, from the seed apoplasm across the plasma membranes of the cotyledon cells. This study examined the temporal and spatial expression of an H+/sucrose symporter, a P-type H+-ATPase, and a sucrose-binding protein (SBP) in cotyledons ofVicia faba L. throughout their development. The flux of dry matter and sucrose symporter activity exhibited identical temporal trends. These were a marked increase during cotyledon expansion to a plateau maintained until cotyledon maturity. Thereafter both parameters declined precipitously. The temporal changes in sucrose symporter activity were accounted for by shifts in its Vmax. Transcript levels of the H+/sucrose symporter followed a similar temporal pattern to the sucrose symporter activity suggesting regulation by gene expression. Equivalent conclusions were drawn for SBP and the H+-ATPase expression during cotyledon expansion. Thereafter, during seed filling, the transcript levels of SBP and H+-ATPase did not closely follow that found for the sucrose symporter. A progressive wave of gene expression in the abaxial epidermal cells spread from the cotyledon region juxtaposed to the non-vascular region of the seed coat at the pole distal from the funicle. The pattern of expression progressed most rapidly along the median longitudinal plane of the cotyledons and more slowly outward to their margins. The densities of SBP and H+-ATPase, inserted into the plasma membranes of the abaxial epidermal cells, increased throughout cotyledon expansion. Gene expression (sucrose symporter) and membrane insertion of the gene products (SBP, H+-ATPase) were closely associated with the initiation and development of wall ingrowths in the abaxial epidermal cells.