Effect of RNAi suppression of the gossypol pathway on resistance to necrotrophic fungal seedling disease pathogens in cotton

Abstract

We previously demonstrated that blocking the gossypol pathway by RNAi suppression of the early pathway biosynthetic enzyme CYP82D hydroxylase resulted in enhanced resistance to the hemibiotrophic Fusarium wilt pathogens. The enhanced resistance was attributed to the induction of a linked lacinilene defense pathway. Others reported that RNAi suppression of the same gene, which was named as SILENCE-INDUCED STEM NECROSIS (SSN), caused spontaneous hypersensitive response-like cell death resulting in a lesion mimic phenotype and increased resistance to hemibiotrophic Verticillium wilt pathogens. We observed no lesion formation on hypocotyls when the CYP82D RNAi plants were grown in pasteurized soil at warm temperatures that are unfavorable to seedling disease development. Therefore, RNAi suppression of CYP82D is not associated with spontaneous hypersensitive-like cell death. When grown in unpasteurized soil at cool temperatures that are favorable for seedling disease development, more than 87% of the CYP82D RNAi plants developed necrotic hypocotyl lesions, compared to 3% of the wild type plants that developed lesions. Furthermore, at least 83% of the necrotic CYP82D RNAi plants harbored at least one of the isolates of Fusarium spp, Rhizopus spp., or Aspergillus spp. that are often implicated in the seedling diseases of cotton. Pathogenicity assays with the seedling disease pathogen Rhizoctonia solani AG4 confirmed that blocking the gossypol defense pathway by CYP82D RNAi increased host susceptibility to the necrotrophic pathogen. The concentrations of induced lacinilene products detected in the roots and hypocotyls of RNAi plants when treated with Rhizoctonia were far lower than the concentrations seen previously in these tissues induced by Fusarium wilt pathogens. Thus, the increased susceptibility of the RNAi plants to Rhizoctonia was probably due to the combined effect of low concentrations of lacinilenes and the blockage of the gossypol pathway by the RNAi. Developing cotton host resistance to wilt diseases without compromising resistance to seedling disease pathogens requires further understanding of the interaction between signaling pathways involved in responses to hemibiotrophs and necrotrophs.