Physiological and molecular basis of phytoalexin biosynthesis in sorghum

3-Deoxyanthocyanidins are the principal phytoalexins that accumulate in sorghum. The phytoalexin response was induced in response to attempted infection by Cochliobolus heterostrophus, a non-pathogen of sorghum or Colletotrichum sublineolum, the causal agent of the sorghum anthracnose. In the resistant responses (non-host or host), fungal development in the hosts was contained during early stages of pathogenesis. Accumulation of phytoalexins was localized in the area of infection and was associated with the restriction of fungal development in the host. When compared to the compatible interaction, defense responses in the incompatible interaction with C. sublineolum could be characterized by a greater and faster accumulation of phytoalexins and an earlier induction of defense-related genes. The pathogen was able to colonize the susceptible host with the proliferation of primary and secondary hyphae. In addition to the quantitative and timing differences, there were qualitative differences between the incompatible and compatible interactions in their phytoalexin response. Thus, the resistant cultivar accumulated a complex phytoalexin mixture including luteolinidin and 5-methoxyluteolinidin, neither of which was produced by the susceptible cultivar. These compounds were also shown to exhibit higher fungitoxicity than other sorghum phytoalexin components. Activation of the biosynthesis of 3-deoxyanthocyanidin phytoalexins was found to be under precise metabolic control. Light-induced anthocyanin accumulation was repressed in plants which were exposed to stimuli of light and pathogen induction simultaneously. Anthocyanidin structural genes encoding flavanone 3-hydroxylase, dihydroflavone 4-reductase, and anthocyanidin synthase were all down-regulated following inoculation. This would increase the availability of flavonoid substrates for the synthesis of the structurally related 3-deoxyanthocyanidin phytoalexins. Chalcone synthase (CHS) is a key branchpoint enzyme in the phenylpropanoid pathway from which flavonoid compounds, including 3-deoxy-anthocyanidins and anthocyanidins, originate. Accumulation of CHS mRNA was stimulated by light and pathogen induction. A family of seven chalcone synthase genes was isolated from a sorghum genomic bacterial artificial chromosome library. Expression analysis by reverse transcription-polymerase chain reactions indicated that there was a subset of CHS genes responsive to pathogen induction only