Molecular modeling of protein-protein interaction to decipher the structural mechanism of nonhost resistance in rice.

Nonhost resistance (NHR) is the most common and durable form of plant resistance to disease-causing organisms. A successful example of NHR is the cloning of a maize R gene Rxo1 in rice and validating its function in conferring bacterial streak resistance in transgenic rice lines. In order to understand the structural mechanism of NHR in rice, we built the model of the protein-protein interaction between the encoded Rxo1 (RXO1) and AvrRXO1 (avirulence protein of rice pathogen, Xanthomonas oryzae pv. oryzicola). Interestingly, although a RXO1 homolog in rice (RHR) is present, it does not interact with AvrRXO1 in nature. We have confirmed that the specificity of RXO1-AvrRXO1 interaction originates from the structured leucine rich repeat (LRR) domain of RXO1, facilitating the recognition process, while the absence of such ordered LRR region makes RHR unfavorable to recognize AvrRXO1. We postulate that the RXO1-AvrRXO1 complex formation is a three step process where electrostatic interactions, shape complementarity and short-range interactions play an important role. The presence of the structural and physicochemical properties essential for the protein-protein recognition process empowers RXO1 to mediate NHR, which the host protein RHR lacks and consequently loses its specificity to bind with AvrRXO1. To the best of our knowledge, this is the first report on the understanding of NHR in rice from the structural perspective of protein-protein interaction.