Citrus bacterial canker (CBC), caused by Xanthomonas citri subsp. citri (
Xcc), causes dramatic losses to the citrus industry worldwide.
Transcription activator-like effectors (TALEs), which bind to effector binding elements (EBEs) in host promoters and activate transcription of downstream host genes, contribute significantly to
Xcc virulence. The discovery of the biochemical context for the binding of TALEs to matching EBE motifs, an interaction commonly referred to as the TALE code, enabled the in silico prediction of EBEs for each TALE
protein. Using the TALE code, we engineered a synthetic resistance (R) gene, called the
Xcc-TALE-trap, in which 14 tandemly arranged EBEs, each capable of autonomously recognizing a particular
Xcc TALE, drive the expression of Xanthomonas avrGf2, which encodes a bacterial effector that induces plant cell death. Analysis of a corresponding transgenic Duncan grapefruit showed that transcription of the cell death-inducing executor gene, avrGf2, was strictly TALE-dependent and could be activated by several different
Xcc TALE
proteins. Evaluation of
Xcc strains from different continents showed that the
Xcc-TALE-trap mediates resistance to this global panel of
Xcc isolates. We also studied in planta-evolved TALEs (eTALEs) with novel
DNA-binding domains and found that these eTALEs also activate the
Xcc-TALE-trap, suggesting that the
Xcc-TALE-trap is likely to confer durable resistance to
Xcc. Finally, we show that the
Xcc-TALE-trap confers resistance not only in
laboratory infection assays but also in more agriculturally relevant field studies. In conclusion, transgenic plants containing the
Xcc-TALE-trap offer a promising sustainable approach to control CBC.