Xanthomonas euvesicatoria effector XeAvrRxo1 triggers a Rxo1-mediated defense response in Nicotiana benthamiana and its chaperone Xe4429 functions as an antitoxin, an expression repressor, and an enhancer of XeAvrRxo1 secretion

Xanthomonas euvesicatoria ( X. euvesicatoria ) is the causal agent of bacterial spot disease that threatens pepper and tomato production around the globe. X. euvesicatoria gene Xe4428 encodes a type III effector (T3E) that shares 89.67% amino acid identity with Xanthomonas oryzae pv. oryzicola ( Xoc ) T3E AvrRxo1. Deletion of Xe4428 in the genome of X. euvesicatoria (strain Xcv85-10) compromised its virulence to infect pepper and Nicotiana benthamiana plants. Transient co-expression of Xe4428 and Rxo1 on pepper and N. benthamiana plant leaves results in a robust hypersensitive reaction. Thus, Xe4428, renamed as Xe AvrRxo1, is a bona fide orthologue of Xoc AvrRxo1 that possesses both virulence and avirulence functions. Expression of Xe AvrRxo1 in E. coli and X. euvesicatoria is toxic to both bacterial cells. Another X. euvesicatoria gene Xe4429 , encodes a putative chaperone of Xe AvrRxo1, which can interact with XeAvrRxo1 to suppress its toxicity in X. euvesicatoria and E. coli bacterial cells. Xe4429 also binds to the promoter region of XeavrRxo1 and represses its transcription/translation in X. euvesicatoria bacterial cells. In addition, expression of Xe4429 can enhance the secretion and translocation of Xe AvrRxo1 into plant cells. Therefore, Xe4429 functions as an antitoxin, a transcription repressor, and a type III chaperone that is capable of enhancing the secretion and translocation of Xe AvrRxo1 during pathogenesis. ### Competing Interest Statement The authors have declared no competing interest.