Host and pathogen factors controlling the rice-Xanthomonas oryzae interaction.

Rice (Oryza sativa) cultivation represents a world laboratory for investigation into bacterial diseases of rice, in particular, and host-parasite interactions, in general, at the molecular, genetic, and genomic levels. The crop, in its various forms, has been under intense cultivation for more than 6,000 years, resulting in the selection of a wide variety of traits and germplasm (Khush, 1997). Tremendous molecular and genetic resources have also been developed in recent years. The species can be genetically manipulated by Agrobacterium tumefaciens-mediated transformation (Krishnan et al., 2009). Against this backdrop, bacterial blight is an important disease of rice and present throughout much of the rice-growing regions (Ou, 1985; Mew et al., 1993; Fig. 1). Xanthomonas oryzae pv oryzae (Xoo) is the causal agent and a member of the g-proteobacteria and, like many other proteobacteria, depends, in part, on a type III secretion system for pathogenicity (Swings et al., 1990; Zhu et al., 2000). The bacterium invades the xylem tissue, either through wounds or water pores, leading to systemic infection. Bacterial streak of rice is caused by the closely related pathovar X. oryzae pv oryzicola (Xoc), which, in contrast to Xoo, is limited to the intervascular regions, giving the disease the characteristic leaf streaking (Niño-Liu et al., 2006). Bacterial streak can also cause severe losses, although the disease tends to be tropical and is more sporadic than blight (Niño-Liu et al., 2006). This reviewwill highlight recent advances in our knowledge of bacterial blight and, to a lesser extent, bacterial streak with regard to the host-bacteria interaction and the peculiar exploitation of a family of type III effectors. We will also provide speculation and thought as to how these findings might relate to classical analyses of disease resistance and susceptibility as well as a discussion of prospects for future research. RESISTANCE AND SUSCEPTIBILITY GENES TO XOO