CS-1.4 XANTHOMONAS EFFECTOR AVRRXO1 SUPPRESSES PLANT IMMUNITY BY REGULATING THE PLANT STOMATAL APERTURE SIZES Bingyu Zhao Department of Horticulture, Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061, U.S.A. Rice bacterial leaf streak disease caused by Xanthomonas oryzae pv. oryzicola (Xoc) is one of most important rice bacterial diseases in Asia. A type III effector gene avrRxo1 was isolated from the genome of Xoc. AvrRxo1 triggers defense responses in maize, rice and tobacco plants that carry the Rxo1 resistance gene. Ectopic expression of avrRxo1 in Arabidopsis suppresses plant immunity. To characterize the biochemical functions of AvrRxo1, we expressed and purified AvrRxo1 from E. coli and determined its three dimensional structure. Structural and functional analysis of AvrRxo1 revealed a conserved ATP binding site that is critical for its virulence and avirulence functions. Through yeast twohybrid screening, we identified an Arabidopsis transcription factor AIN2 that interacts with AvrRxo1. Gene expression profiling of transgenic Arabidopsis plants expressing avrRxo1 leads us to identify a collection of AvrRxo1 specifically induced (Aig) genes. The induction Aig1 gene is depending on the function of AIN2. We demonstrated that AvrRxo1 interacts with AIN2 and binds to the promoter of Aig1. Transgenic Arabidopsis plants with overexpression of Aig1 have increased stomatal aperture sizes and are more susceptible to Pseudomonas syringae pv. tomato DC3000 than that of the wild type plants. Therefore, we conclude AvrRxo1 interacts with a transcription factor to up-regulate the expression of Aig1, which can suppresses Arabidopsis immunity.