AUTHOR=Huang Yan , Yu Chunlei , Sun Cuicui , Saleem Muhammad , Li Pingliang , Li Baohua , Wang Caixia 

TITLE=β-Glucosidase VmGlu2 Contributes to the Virulence of Valsa mali in Apple Tree

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.695112

DOI=10.3389/fmicb.2021.695112

ISSN=1664-302X

ABSTRACT=<p>The apple tree canker is caused by <italic>Valsa mali</italic>, 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 <italic>V. mlai</italic> remain largely unknown. In this study, we identified a β-glucosidase gene, V<italic>mGlu2</italic>, which was upregulated during the <italic>V. mali</italic> 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 <italic>VmGlu2</italic> showed no effect on vegetative growth, but gene deletion mutants of <italic>V. mlai</italic> 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 <italic>VmGlu2</italic> did not affect toxin levels, but it significantly enhanced β-glucosidase activities, which resulted in an increased pathogenicity. Thus, we conclude that <italic>VmGlu2</italic> is required for the full virulence of <italic>V. mali</italic>. These results provide valuable evidence to the complex role of CWDEs in the fungal pathogenicity.</p>