Expression of two sucrose synthetase genes in endosperm and seedling cells of maize: evidence of tissue specific polymerization of protomers

Abstract

The shrunken (sh) locus on chromosome nine in maize has been considered to be endosperm specific at the biochemical and the phenotypic levels of expression. The recent evidence concerning the presence of Sh encoded transcripts in the seedling (Springer et al. 1985) led us to examine this aspect at the protein level. The results from immuno-blot analyses show that the Sh encoded sucrose synthetase-1 protein (SS-1) is indeed present in seedlings. The expression of the gene SS-1 in seedling roots is verifiable by two criteria: 1) Various sh-null mutants that lack the SS-1 protein in the developing endosperm are also devoid of this protein in seedling root extracts. Only a single band, presumably encoded by the SS-2 locus, is seen in these mutants. 2) The electrophoretic variant of the sh mutant encoding a slow migrating SS-1 protein in the endosperm exhibits a concomitantly altered pattern of SS isozymes in seedling roots. A notable feature of SS-1 and SS-2 expression is that a unique tissue specific difference between endosperm and seedling roots is retained. The Sh root extracts show a total of five SS isozymes due to protomeric interaction of the SS1-SS2 tetramers present in the same cell. Endosperm extracts of the same genotype show only SS-1 and SS-2 homomers. The lack of heteropolymers is indicative of spatial and/or temporal separation of SS-1 and SS-2 expression in the endosperm cells. We suggest that these observations provide a new dimension and a useful tool for the analysis of many sh mutants and their revertants due to various transposable elements.