Hormonal Regulation of Expression of Two Cysteine Endopeptidase Genes in Rice Seedlings

Two cDNA probes (pRP60 and pRP80) were used to examine the hormonalregulation of expression of two cysteine endopeptidase genesin germinated whole seeds and de-embryonated seeds of rice.The pRP60 protein is a major rice cysteine endopeptidase namedREP-1 that digests rice glutelin, the main seed storage protein.The pRP80 protein has features of a cysteine endopeptidase buthas not yet been confirmed to be one. Neither mRNA was detectablein dry seeds, and the levels per seed increased sharply uponimbibition, reached peaks at d 6 to d 9, and then decreased.Both mRNAs were expressed in a seed-specific manner, but theaccumulation of pRP60 mRNA was about ten times greater thanthe other. When seeds were de-embryonated and incubated, theamounts of both mRNAs were reduced to very low levels, but inthe presence of 0.01 to 1 µM. gibberellic acid (GA₃) bothagain reached high levels. Addition of abscisic acid (ABA) oruniconazole, an inhibitor of gibberellin biosynthesis, partlyeliminated the effect of GA₃. Protein immunoblot analysis showedthat the accumulation of the REP-1 protein in seeds was regulatedby GA₃ and ABA similarly to that of pRP60 mRNA, but the changein pRP60 mRNA with time after the onset of imbibition precededthat of the REP-1 protein. (Received May 28, 1997; Accepted September 8, 1997)