Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 24, 1999
Publication Date: N/A
Interpretive Summary: This paper presents research results that indicate that the biocontrol fungus Trichoderma virens controls cotton seedling disease caused by the soilborne pathogen Rhizoctonia solani by inducing resistance in the cotton host. The biocontrol agent induces resistance by producing a compound that stimulates the cotton plant to produce terpenoid compounds that are toxic to the disease causing agent. The paper demonstrates that the terpenoid compounds produced by cotton are poisonous to the fungus, and it shows that strains of the biocontrol agent that do not stimulate terpenoid production in the plant are not effective as biocontrol agents.
Technical Abstract: Research on the mechanisms employed by the biocontrol agent Trichoderma virens to suppress cotton seedling disease incited by Rhizoctonia solani has shown that mycoparasitism and antibiotic production are not major contributors to successful biocontrol. In this study we examined the possibility that seed treatment with T. virens stimulates defense responses, including the synthesis of terpenoids in cotton roots, and what the role of these compounds might be in disease control. Extract and analysis of cotton roots and shoots from T. virens treated seed showed that terpenoid synthesis and peroxidase activity was increased in cotton roots, but not in the hypocotyls of treated plants nor the untreated controls. Bioassay of the terpenoids for toxicity to R. solani showed that pathway intermediates desoxyhemigossypol (d-HG) and hemigossypol (HG) were strongly inhibitory to the pathogen, while the final product gossypol (G) was toxic only at much higher concentration. Strains of T. virens and T. koningii were much more resistant to HG and G, and they thoroughly colonized the cotton roots. A comparison of biocontrol efficacy and induction of terpenoid synthesis in cotton roots by strains of T. virens, T. koningii, T. harzianum and protoplast fusants indicated that there was a strong correlation (+0.89) between these two phenomena. It therefore appears that induction of terpenoid synthesis in cotton roots by T. virens may be an important mechanism in the biocontrol by this fungus of R. solani-incited cotton seedling disease.