A CHALCONE SYNTHASE WITH AN UNUSUAL SUBSTRATE PREFERENCE IS EXPRESSEDIN BARLEY LEAVES IN RESPONSE TO UV-LIGHT AND PATHOGEN ATTACK

A cDNA clone was isolated by differential hybridization from a library prepared from barley leaves inoculated with the fungus Blumeria grami nis f.sp. hordei (Bgh). The open reading frame of the insert (designat ed HvCHS2) encoded a polypeptide with 72-79% identity to chalcone synt hases (CHS) and 65-68% identity to stilbene synthases. Alignments of t he amino acid sequence of HvCHS2 with the consensus sequence of naring enin-CHS (EC 2.3.1.74) reveals significant differences between HvCHS2 and naringenin-CHS. HvCHS2 transcripts accumulate strongly in barley l eaves in response to inoculation with Bgh, whereas only insignificant accumulation of barley naringenin-CHS (CHS1) transcripts is seen upon the inoculation. The accumulation of HvCHS2 transcripts is also elicit ed by UV light. To compare the activity of HvCHS2 with the activity of CHS1, the two enzymes were expressed in Escherichia coli. Both HvCHS2 and CHS1 catalyse the formation of chalcones. However, HvCHS2 and CHS 1 differ in their substrate requirements. CHS1 uses cinnamoyl-CoA and 4-coumaroyl-CoA at comparable rates whereas feruloyl-CoA is a poor sub strate for this enzyme. In contrast, HvCHS2 converts feruloyl-CoA and caffeoyl-CoA at the highest rate whereas cinnamoyl-CoA is a poor subst rate. Thus, HvCHS2 is a novel pathogen and UV light induces homoeriodi ctyol/eriodictyol CHS involved in the direct production of flavonoids possessing multi-substituted B-rings.