Enhancement of antifungal metabolite production from gamma-ray induced mutants of some Trichoderma species for control onion white rot disease

Abstract

The influence of gamma-irradiation on improving production of anti-fungal metabolites and biological proficiency of the biocontrol agent Trichoderma species against Sclerotium cepivontm, the causal organism of onion white rot disease, was studied. Mutants of Trichoderma harzianllm, T. viride and T. koningii were generated by exposing their sporulated cultures to 20 and 50 kilo-rad doses of gamma irradiation. The isolated mutants dispJayed variation in growth and antagonistic activity. Two mutants with better growth rate, sporulation and antagonistic effect against the pathogen were selected from each species. Antifungal metabolites were extracted from the culture filtrates of the Trichoderma isolates, fractionated by PAGE or HPLC and assayed. Results showed that all selected mutant isolates produced highly active exo-enzymes compared to the wild type. Mutants derived from T. harziallum and T. koningii had the highest isozyme band umber and quantity of chitinase and β-1,3 glucanase. Mutants of T. viride also showed higher levels of cellulase and chitinase isozymes. Fractionation of the culture filtrates via HPLC revealed that production of the metabolites included phenolic compounds and antibiotics, including gliotoxin, trichodennine and viridin, were greatly increased over the wild-type in all selected mutants. In soil naturally infested with S. cepivorum, Trichoderma mutants significantly reduced the onion white rot disease incidence and improved the plant yield. Trichoderma mutants were effective in reducing the pathogen growth in rhizosphere soil Ornpared to the wild type strains. At the same time, population counts of mutants were dramatically increased in rhizosphere soil for 18 weeks compared to 7 weeks in wild types.