Alterations in gene expression in sorghum induced by the host-specific toxin from Periconia circinata

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Abstract
Susceptibility of sorghum to the fungal pathogen P. circinata and sensitivity to its host-specfic toxin are determined by the semidominant allele at the pc locus. Pretreatment of susceptible seedlings with cycloheximide or cordycepin for 4 h before treatment with the toxin protected the seedlings against toxin-induced loss of electrolytes and prevented development of disease symptoms. In vivo incorporation of [3H]leucine into protein was inhibited 91% and 47% by cycloheximide and cordycepin, respectively, but was not affected by the toxin. Gel electrophoresis and fluorography of in vitro-labeled proteins extracted from non-treated and toxin-treated root tips of near-isogenic susceptible and resistant lines revealed a selective increase in radioactivity of a protein band at MW 16,000 only in preparations from toxin-treated susceptible root tips. Two-dimensional gel electrophoresis separated the MW 16,000 band into 4 proteins and confirmed the increased rate of synthesis. Products of in vitro translation were substantially enriched with the 4 MW 16,000 proteins when total RNA from toxin-treated susceptible root tips was used in a cell free protein-synthesizing system. Because the proteins that increase are common to both susceptible and resistant genotypes, the toxin apparently interferes with a regulatory function, perhaps a function of the pc locus, and thereby alters gene expression in the susceptible genotypes. The data suggest but do not establish that phytotoxicity results from the increased rate of synthesis of the specific proteins.