Molecular evidence for biochemical diversification of phenolamide biosynthesis in rice plants

Two phenolamides (PAs), p-coumaroylputrescine and feruloylputrescine strongly accumulate in rice (Oryza sativa cv. Nipponbare) leaves subjected to attack of chewing and sucking herbivores. Here we identified and characterized in vitro three novel rice genes that mediated coumaroyl-CoA/feruloyl-CoA conjugation to polyamines, putrescine and agmatine. Interestingly, two genes were highly specific for their polyamine substrates, encoding putrescine N-hydroxycinnamoyltransferase and agmatine N-hydroxycinnamoyltransferase, while the third enzyme could use both polyamines and it was therefore annotated as putrescine/agmatine N-hydroxycinnamoyltransferase. All genes were preferentially expressed in rice roots and developing flowers, and in addition, the putrescine/agmatine N-hydroxycinnamoyltransferase transcripts were strongly induced by wounding in the young rice leaves. Because the wound response of this gene was only partially suppressed in the jasmonoyl-L-isoleucine deficient plants (Osjar1), it suggests that its upregulation (as well as inducible PAs in rice) may be largely independent of jasmonoyl-L-isoleucine signaling pathway. The finding of three closely related genes with a similar and/or overlapping activity in PA biosynthesis provides another striking example of rapid diversification of plant metabolism in response to environmental stresses in nature.