Organ-specific and hormone-dependent expression of genes for serine carboxypeptidases during development and following germination of rice grains.

Several cDNA clones encoding either serine carboxypeptidases or related proteins of Oryza sativa L. were identified, and the abundance of the corresponding mRNA in immature and germinated grains was examined. The deduced amino acid sequence of each cDNA included key sequences, such as a pentapeptide (G-X-S-X-G/A) that is conserved among many serine carboxypeptidases, and the putative protein products were classified as two general and one novel type of cereal serine carboxypeptidases. Two general types exhibited considerable homology to type I and type III carboxypeptidases of cereal plants. The novel type encoded a serine carboxypeptidase-like protein that was very similar to type III carboxypeptidases of barley and wheat but had slight differences in both the N- and the C-terminal sequences. The mRNAs of each of these carboxypeptidases were observed in immature grains, and they decreased during maturation. The abundance of mRNA for each class of carboxypeptidase increased again following germination with the same time course and in a tissue-specific manner. The mRNAs for type I and type III-like carboxypeptidases were abundant in germinated embryos composed of leaf, root, and scutellum, whereas the mRNA for type III carboxypeptidase was conspicuous in endosperm that contained the aleurone layer. Altered amounts of mRNA in deembryonated half-grains in response to phytohormones, such as gibberellic acid and abscisic acid, were only detectable in the case of type III carboxypeptidase. Southern blot analysis using rice genomic DNA revealed the simple organization of each gene for these three classes of carboxypeptidases.