SPATIAL REGULATION IN THE EXPRESSION OF STRUCTURAL AND REGULATORY STORAGE-PROTEIN GENES IN ZEA-MAYS ENDOSPERM

Endosperm development in maize seed involves the multiplication, enlargement, and differentiation of cells with consequent accumulation of storage products. The storage protein genes, encoding zeins, and glutelins (multigene families) are expressed and developmentally regulated by different loci. Wild-type lines and genotypes carrying mutations at loci affecting zein synthesis (o2, o7, fl2, and pro1) were characterized at the molecular level and investigated by Northern analysis in order to define the expression of structural and regulatory genes. In situ hybridization in both wild-type and mutant lines was performed to visualize the spatial distribution of transcripts representing each gene family, during endosperm development. The zein and glutelin mRNAs are expressed in all endosperm cells, except for the aleurone layer. However, each mRNA type accumulates at a different level in the various endosperm regions, thus allowing to recognize specific territories of expression for each storage protein mRNA within the tissue. The spatial expression patterns appear early for each gene type and are maintained during the course of endosperm development. Also, the quantitative distribution of the same transcripts in endosperm of mutant lines is specific for each mutant and different from that of the wild-type. Furthermore, the amount of the O2 transcript, present in the nucleus and cytoplasm of wild-type cells, varies substantially in the different o2 mutations considered, in one mutant almost exclusively confined within the nucleus. These data suggest a specific control of the spatial expression of storage protein genes and a heterogeneous molecular composition of protein bodies throughout the endosperm tissue.