The citrus industry is an important agribusiness sector in Brazil; however, despite its economic strength, the sector is facing several challenges due mostly to the need of controlling numerous citrus diseases including citrus canker, caused by the bacterial pathogens Xanthomonas citri and Xanthomonas aurantifolii. We have been studying the interaction of citrus plants with these pathogens and have found that while X. citri causes canker in all commercial sweet orange varieties, X. aurantifolii pathotype C is a pathogen of the Mexican lime only, and in sweet oranges it triggers a hypersensitive reaction (HR). The molecular basis for this differential pathogenicity is not yet known, however, we have noticed that X. citri and X. aurantifolii C have each a distinct set of the so-called "TAL" (transcription activator-like) effectors. For instance, we have evidence indicating that the TAL effectors from X. aurantifolii (PthCs 1 and 2) act as avirulence (Avr) factors in sweet orange whereas the effector proteins PthA2 and 4 from X. citri are associated with canker development. To elucidating the function of the X. citri and X. aurantifolii TAL effectors as pathogenicity and/or avirulence factors in different hosts, we plan to obtain mutants of X. citri with deletions in each of the four pthA genes. These mutants will be complemented with the corresponding genes; nevertheless, a mutant with the four pthA genes deleted will also be created and used to evaluate the contribution of each TAL effector from X. citri and X. aurantiifolii in pathogenicity/avirulence. In addition, we expect to complement another X. citri mutant, which lacks any functional TAL effector, with the pthA and pthC genes we have.
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