TAL Effector Adaptation and Virulence Evolution in Bacterial Blight of Cotton

Abstract

Bacterial blight of cotton (BBC) is a destructive foliar disease caused by the pathogen Xanthomonas citri pv. malvacearum (Xcm). The primary virulence strategy of Xanthomonas spp. involves the use of transcription activator-like (TAL) effectors. BBC has been largely controlled with the use of resistant cotton varieties. However, a recent resurgence of BBC cases suggests Xcm has evolved to overcome resistance. A previous study has revealed that Xcm targets clade III GhSWEET genes encoding bidirectional sugar transports. Specifically, the historic US Xcm isolate H1005 induces GhSWEET10, however recent Xcm isolates collected from BBC-infected fields appear to primarily upregulate GhSWEET14a and GhSWEET14b. It is likely resurging Xcm isolates have evolved novel TAL effectors to target alternative susceptibility (S) genes. However, what remains unknown is the TAL effector repertoire of new Xcm field isolates, how these TAL effectors cause disease, and how these TAL effectors evolved. The primary goal of this study is to understand the molecular and genetic basis of TAL effector-driven pathogen virulence and deploy this knowledge to improve cotton disease resistance. The central repeat regions (CRR) influence DNA-binding specificity and are largely similar between reemerging virulent and nonvirulent isolates. However, TAL7b in virulent isolates exhibits a CRR dissimilar from nonvirulent isolates and has predicted binding sites within the promoter regions of GhSWEET14a and GhSWEET14b. Consistently, whole-genome transcriptomic analysis of TAL7b-regulated genes indicated that GhSWEET14a and GhSWEET14b are markedly induced by TAL7b. In addition, the pathogenicity analysis of the Xcm mutant variant with the TAL7b deletion showed reduced virulence, particularly with the water-soaking lesion development compared to the wild-type Xcm strain. Transformation of TAL7b into the XcmΔTAL7b mutant restored virulence. The data suggests that TAL7b, a newly evolved TAL effector in Xcm field isolates, is the primary virulence factor responsible for inducing disease symptoms in recent BBC cases. Additionally, we have also generated transgenic BBC resistant cotton using CRISPR/Cas9-mediated genome editing of multiple S genes within the GhSWEET gene family. We anticipate this information will help further explain how Xcm virulence evolves in the field to overcome resistance and ultimately provide foundations for disease monitoring, control, and prevention.