Biological activities of an abscisic acid analog in barley, cress, and rice

Biological activities of an abscisic acid (ABA) analog, RCA-7a [1-(3-carboxyl-5-methylphenyl)-1-hydroxy-2,6,6- trimethyl-4-oxo-2-cyclohexene], carrying a phenyl group in the side chain are reported. (+)-RCA-7a was approximately 3-fold less active than (+)-ABA in the inhibition of alpha-amylase induction by gibberellic acid (GA(3)) in embryo-less barley grains. In aleurone layers, RCA-7a inhibited GA(3)-induced alpha-amylase mRNA accumulation and induced the accumulation of dehydrin mRNA, but less effectively than ABA. It also activated a dehydrin gene promoter in barley aleurone protoplast transient assays. A higher concentration of RCA-7a than ABA was required for similar levels of inhibition of the growth of cress plants at 1 week after sowing; however, RCA-7a remained effective for at least 3 weeks. In contrast, ABA rapidly lost its effectiveness and became less effective than RCA-7a by 3 weeks. RCA-7a also significantly retarded the growth of rice plants. These greenhouse pot experiments showed that RCA-7a had more useful characteristics than ABA for practical applications. These and our previous results indicate that RCA-7a has an ABA-like activity in various biological processes in plants and has potential as a novel plant growth regulator.