Activation of three targets by a TAL effector confers susceptibility to bacterial blight of cotton
Xanthomonas spp. employ transcription activator-like effectors (TALEs) to promote pathogenicity by activating host susceptibility (S) genes. Cotton GhSWEET10 is an S gene targeted by a TALE in an early isolate of Xanthomonas citri pv. malvacearum (Xcm), but not by recent field Xcm isolates. To understand the pathogenicity shift in Xcm and its adaptation to cotton, we assembled the whole genome and the TALE repertoire of three recent Xcm Texas field isolates. A newly evolved TALE, Tal7b, activated different GhSWEET genes, GhSWEET14a and GhSWEET14b. Simultaneous activation of GhSWEET14a and GhSWEET14b resulted in pronounced water-soaked lesions. Transcriptome profiling coupled with TALE-binding element prediction identified a pectin lyase as an additional Tal7b target, quantitatively contributing to Xcm virulence alongside GhSWEET14a/b. CRISPR-Cas9-based gene editing supported the function of GhSWEETs as S genes in cotton bacterial blight and the promise of disrupting the TALE-binding site in these genes to control the disease. Collectively, our findings elucidate the rapid evolution of TALEs in Xanthomonas field isolates and highlight the virulence mechanism wherein TALEs induce multiple S genes simultaneously to promote pathogenicity.