Possible involvement of phosphoinositide-Ca2+ signaling in the regulation of alpha-amylase expression and germination of rice seed (Oryza sativa L.)

We have studied the effects of neomycin, a potent inhibitor of inositol pho spholipid-specific phospholipase C (PLC), on the germination of rice seed a nd the gibberellin-induced expression of alpha-amylase in the aleurone laye r and the scutellar tissues. It was shown that, in the absence of exogenous Ca2+, neomycin markedly reduced the germination speed and seedling growth of rice seeds and inhibited the gibberellin-induced expression of alpha-amy lase in both secretory tissues. In addition, neomycin decreased the formati on of inositol 1,4,5-trisphosphate (IP3) in the gibberellin-treated aleuron e layer and the scutellar tissues. However, the inhibitory effects on the g ermination speed and the expression of alpha-amylase activity were overcome by supplementation of Ca2+. In addition, gibberellin elevated the level of IP3, and ABA prevented the gibberellin-induced formation of IP3, although ABA alone did not alter the IP3 level. The expression of a membrane-bound P LC molecule in rice aleurone layer was shown to be induced by gibberellin, and the gibberellin-induced expression of PLC was markedly delayed by treat ment with ABA, These results strongly suggest that the phosphoinositide-Ca2 + signal transduction pathway may play an important role in the gibberellin -induced expression of alpha-amylase molecules closely related to the germi nation processes of rice seed.