PthA4^AT , a 7.5-repeats transcription activator-like (TAL) effector from Xanthomonas citri ssp. citri, triggers...

Transcription activator-like effectors (TALEs) are important effectors of Xanthomonas spp. that manipulate the transcriptome of the host plant, conferring susceptibility or resistance to bacterial infection. Xanthomonas citri ssp. citri variant A^T (X. citri A^T ) triggers a host-specific hypersensitive response (HR) that suppresses citrus canker development. However, the bacterial effector that elicits this process is unknown. In this study, we show that a 7.5-repeat TALE is responsible for triggering the HR. PthA4^AT was identified within the pthA repertoire of X. citri A^T followed by assay of the effects on different hosts. The mode of action of PthA4^AT was characterized using protein-binding microarrays and testing the effects of deletion of the nuclear localization signals and activation domain on plant responses. PthA4^AT is able to bind DNA and activate transcription in an effector binding element-dependent manner. Moreover, HR requires PthA4^AT nuclear localization, suggesting the activation of executor resistance (R) genes in host and non-host plants. This is the first case where a TALE of unusually short length performs a biological function by means of its repeat domain, indicating that the action of these effectors to reprogramme the host transcriptome following nuclear localization is not limited to 'classical' TALEs.