Abstract
The apple tree canker is caused by Valsa mali, which produces major pathogenic factors involving multiple cell wall-degrading enzymes (CWDEs) and toxins. The β-glucosidases are among the main CWDEs, and thus, they play important roles in the virulence of necrotrophic pathogens. However, the specific roles of β-glucosidases in the virulence of V. mlai remain largely unknown. In this study, we identified a β-glucosidase gene, VmGlu2, which was upregulated during the V. mali infection. We found that VmGlu2 protein had high enzyme activity of β-glucosidase using p-nitrophenyl-β-D-glucopyranoside (pNPG) as a substrate, while the VmGlu2 could convert phloridzin to phloretin with the release of glucose. The deletion and overexpression of VmGlu2 showed no effect on vegetative growth, but gene deletion mutants of V. mlai showed significantly reduced pycnidia formation. The gene deletion mutants had lower β-glucosidase activities and toxin levels as compared to the wild-type strain. Therefore, these mutants showed a reduced virulence. Moreover, the overexpression of VmGlu2 did not affect toxin levels, but it significantly enhanced β-glucosidase activities, which resulted in an increased pathogenicity. Thus, we conclude that VmGlu2 is required for the full virulence of V. mali. These results provide valuable evidence to the complex role of CWDEs in the fungal pathogenicity.
Highlights
The hydrolytic enzymes of pathogens are important factors in pathogenesis because they determine the accumulation of secondary metabolites and degradation of host plant tissues (Morrissey and Osbourn, 1999; Schirawski et al, 2006; Kubicek et al, 2014)
Our results indicate that VmGlu2 has a major role in the virulence of V. mali and provides important information for us to understand the pathogenicity of necrotrophic fungus
National Center for Biotechnology Information (NCBI)’s conserved domain database showed that VmGlu2 had a βglucosidase (Bg1B) conserved domain, which belonged to a glycoside hydrolases 1 (GH1) family (Supplementary Figure 1B)
Summary
The hydrolytic enzymes of pathogens are important factors in pathogenesis because they determine the accumulation of secondary metabolites and degradation of host plant tissues (Morrissey and Osbourn, 1999; Schirawski et al, 2006; Kubicek et al, 2014). The carbohydrate active enzymes degrade cell wall polymers cell wall-degrading enzymes (CWDEs), and facilitate pathogens to invade plant tissues for nutrient acquisition (Hématy et al, 2009; Zhao et al, 2013). These enzymes that regulate the breakdown of cellulose, xylan, and pectin, are important for phytopathogenic fungi, because they lack specialized penetration structures (Kikot et al, 2009; Gibson et al, 2011). There are evidences that the disruption or modification of genes encoding CWDEs may reflect their direct involvement in the infection and disease (Zhang et al, 2009; Yu et al, 2018). The specific roles of the majority of CWDEs in the virulence remain largely unknown
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
