A Xanthomonas effector protein contributes quantitatively to virulence by inducing at least two minor Susceptibility genes

Transcription Activator-Like Effector (TALE) Tal12a is widespread in strains of Xanthomonas campestris pv. campestris ( Xcc ) causing black rot on Brassica crops. We sought to determine whether and how Tal12a contributes to disease.

Transcriptomic analysis of cauliflower leaves infected with Xcc strains expressing Tal12a was combined with TALE-binding element prediction and heterologous expression assays in Nicotiana benthamiana in order to identify candidate Susceptibility ( S ) genes. Artificial TALEs (arTALEs) were used to validate the contribution of candidate target genes to susceptibility.

Tal12a enhances bacterial virulence and growth in cauliflower. Transcriptomic analysis revealed 380 upregulated cauliflower genes, from which nine were selected as candidate targets. Expression of sugar transporter genes BoSWEET13 and BoSWEET14c was induced though likely indirectly. Five genes were confirmed as direct targets of Tal12a. Functional assays showed that BoIAA7c auxin-dependent transcriptional regulators and BoSWEET14c each independently contribute to disease, but not to bacterial proliferation.

This work identifies the first susceptibility genes in cauliflower. Tal12a enhances susceptibility in cauliflower thanks to a complex transcriptional reprogramming comprising both direct and indirect tal12a targets, some of which act as minor S genes.