Whitefly-induced tomato volatiles enhance the virulence of Lecanicillium lecanii

Abstract

Entomopathogenic fungi (EPF) are a group of microorganisms that have potential for replacing synthetic chemical pesticides. However, EPF virulence is often insufficient, and therefore adoption of EPF biopesticides has been relatively limited. Previous studies have shown that herbivore-induced plant volatiles (HIPVs) promoted the virulence of Lecanicillium lecanii, and that extracellular proteases and chitinases are important virulence factors of entomopathogenic fungi. We therefore put forward the hypothesis that HIPVs promote L. lecanii virulence by inducing extracellular protease or/and chitinase activity. Lecanicillium lecanii was treated with tomato HIPVs induced by various numbers/densities of Bemisia tabaci, the effect of HIPVs on L. lecanii virulence and activities of virulence-related enzymes (extracellular proteases and chitinases) was analyzed. Changes in enzyme activities were confirmed by measuring the variations in Pr1 and Chit II genes expression of virulence-related enzymes under the influence of HIPVs. Finally, the correlation between virulence and virulence-related enzymes or genes was analyzed to explore the mechanism of promoting HIPVs in L. lecanii virulence enhancement. The results showed that volatiles produced by the tomato plants induced by 50 B. tabaci adults (50-HIPVs) significantly enhanced the virulence of L. lecanii. When exposed to both 50-HIPVs and L. lecanii, the mortality rate of B. tabaci reached 79.0% within 7 days compared to 28.87% in the control group. Additionally, 50-HIPVs significantly promoted the activity of chitinase and the relative expression of Chit II genes, which consequently increased by 62.74% and 48.55%, respectively. However, each group of HIPVs had no promoting effect on extracellular protease and Pr1 gene expression. Correlation analysis results showed that the virulence of L. lecanii is significantly correlated with the activity of chitinase and relative expression of Chit II genes. Thus, HIPVs induced virulence of L. lecanii by increasing chitinase activity. This study demonstrates important methods to enhance the virulence of L. lecanii. Moreover, from the perspective of chemical ecology, the findings provide theoretical support for field application of EPF, and also reveal the impact of HIPVs on EPF from a biochemical perspective.