One plant's killer is another's simple nuisance; this difference can be encoded by a single gene. For instance, Xanthomonas strains that inject a protein called AvrBs3 into plant cells cause severe disease in several crop plants. But plants that carry the Bs3 gene resist those particular bugs by actively killing off infected cells before the pathogen spreads too much. The mechanistic basis of this resistance was not known.
Kay et al. now show that the AvrBs3 protein helps bacteria reprogram the plant cell's metabolism by directly activating plant genes as would a plant transcription factor. AvrBs3 bound directly to the promoter of a master regulator of plant cell size known as upa20. Its encoded protein—also a transcription factor—caused cells to swell, which probably helps squeeze the bacteria out of infected tissue and into new terrain. The group also identified the promoter sequence in the plant gene bound by the bacterial activator.
Römer et al. then showed that some plants take advantage of AvrBs3's transcriptional abilities. They found that the plant resistance gene, Bs3, contained the promotor sequence identified by Kay et al. This trick made bacterial AvrBs3 activate transcription of the bug's killer: accumulation of Bs3—an unusual flavin monooxygenase—triggered cell death.
“As far as I know,” says Bonas, “this is the first example of a resistance factor that is a promoter element rather than a protein.”