Synthesis of 3-deoxyanthocyanidin phytoalexins in sorghum occurs independent of light

Abstract

Aetiolated mesocotyls of sorghum were inoculated with the anthracnose pathogen,Colletotrichum sublineolum,and placed in the light or in the dark for 120 h. A time course study showed that each of the four known 3-deoxyanthocyanidin phytoalexins accumulated regardless of whether seedlings had been incubated in the light or the dark. Analyses of extracts by high performance liquid chromatography and plasma desorption mass spectrometry confirmed the presence of each of the phytoalexins. The phytoalexins were never detected in the control, uninoculated plants, regardless of the light treatment. Total RNA was extracted from the mesocotyls, and a time course of RNA hybridization revealed accumulation of transcripts for phenylalanine ammonia-lyase (PAL) and chalcone synthase at times consistent with those expected to be required for phytoalexin synthesis. PAL transcripts accumulated to higher levels in light-grown than in dark-grown seedlings. Importantly, chalcone synthase transcripts accumulated in response to inoculation and accumulation was independent of light. The accumulation of phytoalexins in the dark and the increase of RNA transcripts question the assumption that light is always required to initiate phenolic compound synthesis at the level of PAL. The results suggest that in sorghum the induction of the synthesis of PAL transcripts and the resultant synthesis of the 3-deoxyanthocyanidin phytoalexins occurs as a response to fungal infection and probably is separate from the induction of PAL and phenolic compound synthesis which occurs as a response to light.