Citrus canker, caused by the bacterium Xanthomonas axonopodis pv. citri (Xac), is a disease that affects most species of the genus Citrus, occurs in almost all continents and stands as a threat to the Brazilian citrus industry. The molecular mechanism by which Xac causes canker is not entirely known, however, the bacterium translocates proteins into the host cell, including members of the AvrBs3/PthA family, also known as TAL effectors (transcriptional activator-like), to suppress defenses and cause disease. TAL effectors have been extensively studied and shown to modulate transcription in the host by directly binding to plant promoters and transactivating genes associated with symptom development. However, how exactly TAL effectors activate transcription is presently unknown. To address this question, we performed a series of yeast two-hybrid screenings and identified a number of citrus proteins as targets of the PthAs from X. citri, including MAF1, a negative regulator of RNA polymerase III. Previously, we showed that MAF1 interacts specifically with PthA4, it complements the yeast maf1 mutant phenotype and its silencing in sweet orange plants resulted in enhanced canker development. Therefore, in this follow up proposal, we intend to functionally characterize CsMAF1. For instance, how exactly CsMAF1 is regulated by phosphorylation, whether or not it interacts with RNA Pol III in a monomeric or dimeric form and how PthA4 interferes with this interaction is presently unknown. Thus, in addition to answering these questions we expect to find novel ways to control citrus canker disease.
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