Spatial and temporal expression of the two sucrose synthase genes in maize: immunohistological evidence

We describe the spatial and temporal immunohistological distributions of the two sucrose synthases, SS1 and SS2, encoded by the Sh and Sus genes, respectively, in different parts of the maize plant. The two similar isozymes were differentially localized in developing endosperm cells through the combined uses of a shrunken (sh) mutant lacking the SS1 protein and the SS1 and SS2 antisera. The accumulation of SS1 protein always coincided with starch deposition in the Sh endosperm cells, whereas in the sh endosperm, the centrally located cells were lost at or during the most critical phase of starch biosynthesis. The SS2 specific cells, including aleurone layer and the basal endosperm transfer cells in both genotypes, were not associated with detectable starch deposition. Such heterogeneity was indicative of two cell types separable by gene expression, and of differential in vivo roles of the two isozymes in the endosperm. In young roots, the expression of both SS encoding genes was predominantly in the vascular cylinder region. These data fulfill a previous prediction, based on the genetic analyses, that the expression of the SS genes is spatially and/or temporally separated in endosperm cells but not in root cells.