Bacterial plant-pathogenic Xanthomonas strains translocate TAL effectors into plant cells to function as specific transcription factors via a novel DNA-binding domain. Several TAL effectors are important virulence factors, but only little is known about their plant target genes. Members of the SWEET/nodulin-3 gene family are candidate targets of TAL effectors from rice-pathogenic Xanthomonas oryzae pv. oryzae (Xoo) strains because induction of two SWEET genes has been shown to be essential for disease development. Here, we identified Tal5 as a novel TAL effector from an African Xoo strain that induces expression of OsSWEET14. Interestingly, the Tal5 target box differs from those of the already known TAL effectors TalC, AvrXa7 and PthXo3 which also induce expression of OsSWEET14. To test if OsSWEET14 is a bona fide plant virulence target, we constructed artificial TAL effectors (ArtTALs) that recognize DNA sequences in the OsSWEET14 promoter region that differ from those of the four natural TAL effectors. These ArtTALs efficiently complemented an Xoo talC mutant demonstrating that induction of this particular plant gene is crucial to support Xoo virulence. This proof of concept demonstrates the feasibility to use designer virulence factors as a tool to identify potentially relevant plant targets. To decipher if other members of the rice SWEET family can function as virulence targets, we constructed a series of ArtTALs that specifically target individual members of the rice SWEET family. Our results demonstrate that only five phylogenetically close SWEET proteins which presumably act as sucrose transporters can support for Xanthomonas virulence.
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