LaeA Controls Virulence and Secondary Metabolism in Apple Canker Pathogen Valsa mali.

Yaqiong Feng,Nana Wang,Yuxing Wu,Lili Huang,Hongxia Du,Zhiyuan Yin,Liangsheng Xu

doi:10.3389/fmicb.2020.581203

Abstract

Apple Valsa canker is a destructive disease caused by the ascomycete Valsa mali and poses a serious threat to apple production. Toxins synthesized by secondary metabolite biosynthetic gene clusters (SMBGCs) have been proven to be crucial for pathogen virulence. A previous study showed that V. mali genome contains remarkably expanded SMBGCs and some of their genes were significantly upregulated during infection. In this study, we focus on LaeA, a known regulator of secondary metabolism, for its role in SMBGC regulation, toxin production, and virulence of V. mali. Deletion of VmLaeA led to greatly reduced virulence with lesion length reduced by 48% on apple twigs. Toxicity tests proved that toxicity of secondary metabolites (SMs) produced by VmLaeA deletion mutant (ΔVmlaeA) was markedly decreased in comparison with wild-type (WT). Transcriptomic and proteomic analyses of WT and ΔVmlaeA indicated that a portion of transporters and about half (31/60) SMBGCs are regulated by VmLaeA. Function analysis of eight gene clusters including PKS7, PKS11, NRPS14, PKS16, PKS23, PKS31, NRPS/PKS33, and PKS39 that were differentially expressed at both transcriptional and translational levels showed that four of them (i.e., PKS11, PKS16, PKS23, and PKS31) were involved in pigment production and NRPS14 contributed to virulence. Our findings will provide new insights and gene resources for understanding the role of pathogenicity-related toxins in V. mali.

Highlights

Apple Valsa canker, a destructive disease in apple trees which leads to severe yield losses, is caused by the ascomycete Valsa mali (Lee et al, 2006; Li et al, 2013; Wang X. et al, 2014)
A. nidulans LaeA homologous was identified in V. mali by BLAST and termed VmLaeA
Phenotypic analysis showed that VmlaeA exhibited no apparent difference on vegetative growth after cultivation on potato dextrose agar (PDA) medium for 48 h (Figure 1A)

Summary

Introduction

Apple Valsa canker, a destructive disease in apple trees which leads to severe yield losses, is caused by the ascomycete Valsa mali (Lee et al, 2006; Li et al, 2013; Wang X. et al, 2014). The non-host-specific toxin botrydial, produced by Botrytis cinerea, could induce chlorosis and collapse of French bean tissues, and deletion of the botrydial biosynthetic genes would lead to reduced virulence (Deighton et al, 2001; Siewers et al, 2005) Another well-known toxin fumonisin B1 produced by several Fusarium spp. can induce host cell death and is necessary for the development of disease symptoms on maize seedlings (Myung et al, 2012). These toxins are all crucial weapons for phytopathogenic fungi and are generally synthesized by secondary metabolite biosynthetic gene clusters (SMBGCs)

Methods

Results

Conclusion