Plant-pathogenic Xanthomonas infects a wide variety of host plants and causes many devastating diseases on crops. Transcription activator-like effectors（TALEs） are delivered by a type III secretion system（T3 SS） of Xanthomonas into plant nuclei to directly bind specific DNA sequences（TAL effector-binding elements, EBEs） on either strand of host target genes with an unique modular DNA-binding domain and to bidirectionally drive host gene transcription. The target genes in plants consist of host susceptibility（S） genes promoting disease（ETS） and resistance（R） genes triggering defense（ETI）. Here we generally summarized the discovery of TALEs in Xanthomonas species, their functions in bacterial pathogenicity in plants and their target genes in different host plants, and then focused on the newly revealed modes of protein action in triggering or suppressing plant defense.
