Targeted promoter editing for rice resistance to Xanthomonas oryzae pv. oryzae reveals differential activities for SWEET14-inducing TAL effectors.

As a key virulence strategy to cause bacterial leaf blight, Xanthomonas oryzae pv. oryzae (Xoo) injects into the plant cell DNA-binding proteins called transcription activator-like effectors (TALEs) that bind to effector-binding elements (EBEs) in a sequence-specific manner, resulting in host gene induction. TALEs AvrXa7, PthXo3, TalC and Tal5, found in geographically distant Xoo strains, all target OsSWEET14, thus considered as a pivotal TALE target acting as major susceptibility factor during rice-Xoo interactions. Here, we report the generation of an allele library of the OsSWEET14 promoter through stable expression of TALE-nuclease (TALEN) constructs in rice. The susceptibility level of lines carrying mutations in AvrXa7, Tal5 or TalC EBEs was assessed. Plants edited in AvrXa7 or Tal5 EBEs were resistant to bacterial strains relying on the corresponding TALE. Surprisingly, although indels within TalC EBE prevented OsSWEET14 induction in response to BAI3 wild-type bacteria relying on TalC, loss of TalC responsiveness failed to confer resistance to this strain. The TalC EBE mutant line was, however, resistant to a strain expressing an artificial SWEET14-inducing TALE whose EBE was also edited in this line. This work offers the first set of alleles edited in TalC EBE and uncovers a distinct, broader range of activities for TalC compared to AvrXa7 or Tal5. We propose the existence of additional targets for TalC beyond SWEET14, suggesting that TALE-mediated plant susceptibility may result from induction of several, genetically redundant, host susceptibility genes by a single effector.