Genomics analysis of genes expressed in maize endosperm identifies novel seed proteins and clarifies patterns of zein gene expression

We analyzed cDNA libraries from developing endosperm of the B73 maize inbre d line to evaluate the expression of storage protein genes, This study show ed that zeins are by far the most highly expressed genes in the endosperm, but we found an inverse relationship between the number of zein genes and t he relative amount of specific mRNAs. Although a-zeins are encoded by large multigene families, only a few of these genes are transcribed at high or d etectable levels. In contrast, relatively small gene families encode the ga mma- and beta -zeins, and members of these gene families, especially the ga mma -zeins, are highly expressed. Knowledge of expressed storage protein ge nes allowed the development of DNA and antibody probes that distinguish bet ween closely related gene family members. Using in situ hybridization, we f ound differences in the temporal and spatial expression of the alpha-, gamm a-, and delta -zein gene families, which provides evidence that gamma -zein s are synthesized throughout the endosperm; before alpha- and delta -zeins. This observation is consistent with earlier studies that suggested that ga mma -zeins play an important role in prolamin protein body assembly. Analys is of endosperm cDNAs also revealed several previously unidentified protein s, including a 50-kD gamma -zein, an 18-kD alpha -globulin, and a legumin-r elated protein. Immunolocalization of the 50-kD gamma -zein showed this,pro tein to be located at the surface of prolamin-containing protein bodies, si milar to other gamma -zeins. The 18-kD alpha -globulin, however, is deposit ed in novel, vacuole-like organelles that were not described previously in maize endosperm.