Solution structure of the Magnaporthe oryzae avirulence protein AvrPiz-t

Abstract

Plants have evolved two major layers of defense mechanisms against invasion by diverse pathogens. When host membranes are exposed to pathogens, plants use extracellular surface receptors to identify pathogen-associated molecular patterns (PAMPs) and initiate so-called PAMPtriggered immunity (PTI) (Chisholm et al. 2006). Once pathogens gain hold in plants by suppressing this primary defense, plants express cytoplasmic resistance proteins that recognize pathogen-derived effector proteins and mount a more specialized defense mechanism referred to as effector-triggered immunity (ETI, Chisholm et al. 2006). To enable parasitism, pathogens usually use an arsenal of effector proteins that are delivered into plant cells to exert their primary function of interfering with the host immunity (Abramovitch et al. 2006; Feng et al. 2012; Wilton et al. 2010). However, effector proteins in some cases act as traitors by inducing effector-triggered immunity; these proteins are often referred to as avirulence proteins (Jones and Dangl 2006). Despite unparalleled progress in the identification of a vast repertoire of putative effectors in diverse pathogens in the last decade, the biochemical basis for the vast majority of those effectors remains unknown. The interaction between rice and the fungal pathogen Magnaporthe oryzae, the causal agent of the devastating rice blast disease, observes the gene-for-gene model. The rice blast resistance gene Piz-t and the M. oryzae avirulence gene AvrPiz-t were successively isolated by mapbased cloning methods (Zhou et al. 2006; Li et al. 2009). Co-expression of the pair of Piz-t/AvrPiz-t mounts effective immunity in rice (Li et al. 2009). The AvrPiz-t gene encodes a small protein predicted to be secreted that shows no homology to known proteins and is 108 amino acids in length (Li et al. 2009). AvrPiz-t can suppress programmed cell death (PCD) induced by BAX in tobacco, suggesting that it might contribute to the pathogenicity of M. oryzae (Li et al. 2009). Recently, Park et al. (2012) demonstrated that ectopic expression of AvrPiz-t gene in rice leads to reduced response of ROS triggered by flg22 and chitin, and enhanced susceptibility to the virulent rice blast strain. These data suggested that AvrPiz-t functions primarily as a Zhi-Min Zhang and Xu Zhang contributed equally to the present work.