Citrus reticulata CrRAP2.2 Transcriptional Factor Shares Similar Functions to the Arabidopsis Homolog and Increases Resistance to Xylella fastidiosa

Xylella fastidiosa is a worldwide multihost pathogen that causes diseases in different crops. It is considered a new global threat and substantial efforts have been made in order to identify sources of resistance. Indeed, many genes have been associated with resistance to X. fastidiosa, but without functional validation. Here, we describe a C. reticulata gene homologous to the transcriptional factor RAP2.2 from Arabidopsis thaliana that increases resistance to citrus variegated chlorosis (CVC). This gene was previously detected in C. reticulata challenged with X. fastidiosa. Bioinformatics analysis together with subcellular localization and auto-activation assays indicated that RAP2.2 from C. reticulata (CrRAP2.2) is a transcriptional factor orthologous to AtRAP2.2. Thus, we used A. thaliana as a model host to evaluate the functional role of CrRAP2.2 in X. fastidiosa resistance. The inoculation of X. fastidiosa in the A. thaliana rap2.2 mutant resulted in a larger bacterial population, which was complemented by CrRAP2.2. In addition, symptoms of anthocyanin accumulation were higher in the mutant, whose phenotype was restored by CrRAP2.2, indicating that they have conserved functions in plant defense response. We therefore transformed C. sinensis with CrRAP2.2 and verified a positive correlation between CVC resistance and gene expression in transgenic lines. This is the first study using A. thaliana as model host that characterizes the function of a gene related to X. fastidiosa defense response and its application in genetic engineering to obtain citrus resistance to CVC.