Molecular Dissection of the Pathogen-Inducible 3-Deoxyanthocyanidin Biosynthesis Pathway in Sorghum

3-Deoxyanthocyanidins are the unique phytoalexins synthesized by sorghum in response to fungal inoculation. They are structurally related to anthocyanins but the final steps of their pathogen-inducible biosynthesis are not fully understood. We have identified new flavonoid structural genes from the recently completed sorghum BTx623 genome sequence. The biochemical functions of the different expressed sorghum genes were established in planta by complementation in the appropriate Arabidopsis transparent testa mutants. There is a family of nine chalcone synthase genes which are all inducible by fungal inoculation in sorghum seedlings. Specific dihydroflavonol 4-reductase (DFR) genes responsive to conditions which stimulated anthocyanin accumulation (SbDFR1) or 3-deoxyanthocyanidin production (SbDFR3) were identified. Recombinant SbDFR1 and SbDFR3 were found to function as typical DFRs by accepting dihydroflavonol substrates. On the other hand, both DFRs showed substantially lower but detectable NADPH-dependent activities toward flavanones. Reduction of flavanones to flavan-4-ols is a reaction step required for 3-deoxyanthocyanidin production. Flavanone 3-hydroxylase (F3H) converts flavanones to dihydroflavonols for anthocyanin biosynthesis. In sorghum seedlings, expression of two F3H genes was either absent or strongly suppressed during the accumulation of 3-deoxyanthocyanidins. Under such conditions, most flavanones are expected to be reduced by the pathogen-induced SbDFR3 for the formation of flavan-4-ols. Our work also revealed that 3-deoxyanthocyanidin accumulation and SbDFR3 expression were induced by methyl jasmonate treatment in sorghum roots but the stimulation effects were antagonized by salicylic acid.