A FLAVONOID 7-O-METHYLTRANSFERASE IS EXPRESSED IN BARLEY LEAVES IN RESPONSE TO PATHOGEN ATTACK

We have shown previously that transcripts corresponding to the cDNA cl one pBH72-F1, with similarities to O-methyltransferases (OMT), accumul ated in barley leaves in response to attack by the pathogenic fungus B lumeria graminis (Plant Mol Biol 26 (1994) 1797). To investigate the a ccumulation pattern in the defence response and the organ localization of the pBH72-F1-encoded polypeptide (F1-OMT), an antiserum was raised against Escherichia coli expressed F1-OMT. The 43 kDa protein was abs ent in normal leaves but accumulated strongly in response to pathogen attack. The F1-OMT protein accumulated faster in barley lines inoculat ed with an avirulent B. graminis isolates compared to a virulent isola te. Additionally, F1-OMT related proteins were detected in developing kernels. F1-OMT was expressed as a functional enzyme in E. coli and th e substrate specificity was investigated. The enzyme exhibited OMT act ivity towards flavonoid aglycones with the highest activity against ap igenin (4',5,7-trihydroxyflavone). In contrast, caffeic acid did not s erve as substrate for F1-OMT. The product of F1-OMT was analyzed by HP LC and GC-MS and found to be genkwanin (4',5-dihydroxy-7-methoxyflavon e). Initial velocity data were best represented by a sequential bi-bi mechanism, and kinetic parameters of K-SAM = 10.9 mu M, K-apigenin = 4 .6 mu M and a specific activity of 0.45 mu kat/g were obtained. Barley F1-OMT, apigenin 7-O-methyltransferase, is suggested to be involved i n the production of a methylated flavonoid phytoalexin.