AN ANALYSIS OF THE MODE OF ACTION OF THE HOST-SPECIFIC TOXINS PRODUCED BY PERICONIA CIRCINATA

THOMAS JOSEPH WOLPERT, Purdue University

Abstract

Pathogenicity of the fungus Periconia circinata, causal agent of milo disease, is strictly correlated with the ability of isolates to produce a low molecular weight phytotoxin. The toxin induced an efflux of electrolytes and caused milo disease symptoms in the susceptible genotype but not in the near-isogenic resistant genotype. However, pretreatment of susceptible seedlings with low concentrations of toxin for 10 hr followed by treatment with high concentrations virtually prevented electrolyte leakage, yet seedlings developed symptoms at a normal rate, suggesting that electrolyte leakage is not essential for disease development. Further, treatment of seedlings with citrinin or fusaric acid resulted in losses of electrolytes two- to three-fold greater than treatment with toxin, but seedlings developed symptoms 2 to 3 days later than those treated with toxin. Thus, symptom development was not correlated with the rate or amount of electrolyte leakage, suggesting that physiological response other than electrolyte leakage are more significant to the development of visible disease symptoms. Pretreatment of susceptible seedlings with cycloheximide or cordycepin for 4 hr before treatment with toxin protected seedlings against toxin-induced loss of electrolytes and prevented development of milo disease symptoms. In vitro incorporation of ('3)H-leucine was inhibited 91% and 47% by cycloheximide and cordycepin, respectively, but was not affected by toxin. Gel electrophoresis and fluorography of in vivo labelled proteins extracted from nontreated and toxin-treated root tips revealed a selective increase in radioactivity of a protein band at 16,000 daltons only in preparations from susceptible, toxin-treated root tips. Two-dimensional gel electrophoresis separated the 16,000 dalton band into four proteins. Products of in vitro translation were substantially enriched with the four 16,000 dalton proteins when total RNA from toxin-treated root tips was used in a wheat germ or rabbit reticulocyte lysate cell-free protein synthesizing system. The results indicate that P. circinata toxin directly or indirectly alters gene expression in susceptible genotypes of sorghum by increasing the rate of transcription of selected RNAs. The data suggest, but do not establish, that phytotoxicity results from the increased rate of synthesis of specific proteins.

Degree

Ph.D.

Subject Area

Plant pathology

COinS