The Type III Effector AvrBs2 in Xanthomonas oryzae pv. oryzicola Suppresses Rice Immunity and Promotes Disease Development

Abstract

Xanthomonas oryzae pv. oryzicola, the causal agent of bacterial leaf streak, is one of the most important bacterial pathogens in rice. However, little is known about the functions of individual type III effectors in virulence and pathogenicity of X. oryzae pv. oryzicola. Here, we examined the effect of the mutations of 23 putative nontranscription activator-like effector genes on X. oryzae pv. oryzicola virulence. The avrBs2 knock-out mutant was significantly attenuated in virulence to rice. In contrast, the xopAA deletion caused enhanced virulence to a certain rice cultivar. It was also demonstrated that six putative effectors, including XopN, XopX, XopA, XopY, XopF1, and AvrBs2, caused the hypersensitive response on nonhost Nicotiana benthamiana leaves. Virulence function of AvrBs2 was further confirmed by transgenic technology. Pathogen-associated molecular pattern-triggered immune responses including the generation of reactive oxygen species and expression of pathogenesis-related genes were strongly suppressed in the AvrBs2-expressing transgenic rice lines. Although not inhibiting flg22-induced activation of mitogen-activated protein kinases, heterologous expression of AvrBs2 greatly promotes disease progression in rice caused by two important bacterial pathogens X. oryzae pvs. oryzae and oryzicola. Collectively, these results indicate that AvrBs2 is an essential virulence factor that contributes to X. oryzae pv. oryzicola virulence through inhibiting defense responses and promoting bacterial multiplication in monocot rice.

Quantitative, image-based phenotyping methods provide insight into spatial and temporal dimensions of plant disease

“…Thus, it is possible this protein is associated with host cell membranes following secretion. Our observations for XopK are contrasted with the roles of AvrBs2 and XopX, which exhibit reduced virulence phenotypes, consistent with previous studies (Kearney and Staskawicz, 1990;Metz et al, 2005;Zhao et al, 2011;Sinha et al, 2013;Li et al, 2015b;Stork et al, 2015). Our results further illustrate that T3E mutants may impact certain aspects of host-pathogen interactions more than others.…”

“…AvrBs2 contains a glycerol phosphodiesterase domain that is required for its virulence functions in other Xanthomonas pathovars (Kearney and Staskawicz, 1990;Tai et al, 1999;Zhao et al, 2011;Li et al, 2015b). XopX is involved in suppressing pathogen-triggered immunity (Metz et al, 2005;Sinha et al, 2013;Stork et al, 2015).…”

Comparative genomics of a cannabis pathogen reveals insight into the evolution of pathogenicity in Xanthomonas

“…This is an interesting finding for several reasons. First, how do the cannabis pathogens suppress plant defense, a function that became more and more linked to the activity of T3-effectors (e.g., Canonne et al, 2011; Schulze et al, 2012; Sinha et al, 2013; Li et al, 2015; Stork et al, 2015)? Second, which molecular entities are triggering the HR in Nicotiana tabacum if not type III effectors?…”

The role of type III effectors from Xanthomonas axonopodis pv. manihotis in virulence and suppression of plant immunity

“…For example, XopR from Xoo has previously been shown to be important in the suppression of PTI in Arabidopsis (Akimoto‐Tomiyama et al ., ), and AvrBs2 from Xanthomonas oryzae pv. oryzicola suppresses the expression of genes related to PTI in rice (Li et al ., ). However, this is the first report of XopAO1 in Xanthomonas as a PTI suppressor, and it would be interesting to determine whether this is the case in X. gardneri or X. arboricola pv.…”