Molecular functions of Xanthomonas type III effector AvrBsT and its plant interactors in cell death and defense signaling

Abstract

Xanthomonas effector AvrBsT interacts with plant defense proteins and triggers cell death and defense response. This review highlights our current understanding of the molecular functions of AvrBsT and its host interactor proteins. The AvrBsT protein is a member of a growing family of effector proteins in both plant and animal pathogens. Xanthomonas type III effector AvrBsT, a member of the YopJ/AvrRxv family, suppresses plant defense responses in susceptible hosts, but triggers cell death signaling leading to hypersensitive response (HR) and defense responses in resistant plants. AvrBsT interacts with host defense-related proteins to trigger the HR cell death and defense responses in plants. Here, we review and discuss recent progress in understanding the molecular functions of AvrBsT and its host interactor proteins in pepper (Capsicum annuum). Pepper arginine decarboxylase1 (CaADC1), pepper aldehyde dehydrogenase1 (CaALDH1), pepper heat shock protein 70a (CaHSP70a), pepper suppressor of the G2 allele of skp1 (CaSGT1), pepper SNF1-related kinase1 (SnRK1), and Arabidopsis acetylated interacting protein1 (ACIP1) have been identified as AvrBsT interactors in pepper and Arabidopsis. Gene expression profiling, virus-induced gene silencing, and transient transgenic overexpression approaches have advanced the functional characterization of AvrBsT-interacting proteins in plants. AvrBsT is localized in the cytoplasm and forms protein-protein complexes with host interactors. All identified AvrBsT interactors regulate HR cell death and defense responses in plants. Notably, CaSGT1 physically binds to both AvrBsT and pepper receptor-like cytoplasmic kinase1 (CaPIK1) in the cytoplasm. During infection with Xanthomonas campestris pv. vesicatoria strain Ds1 (avrBsT), AvrBsT is phosphorylated by CaPIK1 and forms the active AvrBsT-CaSGT1-CaPIK1 complex, which ultimately triggers HR cell death and defense responses. Collectively, the AvrBsT interactor proteins are involved in plant cell death and immunity signaling.

Iron- and Reactive Oxygen Species-Dependent Ferroptotic Cell Death in Rice-Magnaporthe oryzae Interactions

“…Plant pathogens have undergone convergent evolution to suppress the plant immune system, including the ROS burst (Jwa and Hwang, 2017). By contrast, the recognition of nucleotide binding Leu-rich repeat receptors of host plants by specific pathogen effectors leads to robust resistance responses, such as the strong ROS burst and HR cell death (Jones and Dangl, 2006;Dangl et al, 2013;Han and Hwang, 2017). The experimental evidence of iron-and ROSdependent ferroptotic cell death in incompatible rice-M. oryzae interactions may provide important clues to understand the fundamental functions of HR cell death in the plant immune system.…”

“…Several pattern recognition receptors that recognize pathogen-associated molecular patterns have been identified in plant cell membranes (Zipfel, 2014). In incompatible plant-pathogen interactions, intracellular nucleotide binding Leu-rich repeat receptors of resistant host genotypes recognize specific pathogen effectors to induce the ROS burst and rapid HR cell death in plants (McHale et al, 2006;van der Hoorn and Kamoun, 2008;Spoel and Dong, 2012;Cesari et al, 2014;Han and Hwang, 2017).…”

Functional roles of the pepper leucine-rich repeat protein and its interactions with pathogenesis-related and hypersensitive-induced proteins in plant cell death and immunity

“…3). Xcv effector AvrBsT triggers HR cell death in pepper and N. benthamiana leaves (Orth et al 2000;Escolar et al 2001;Kim et al 2010;Han and Hwang 2017). CaLRR1 alone does not function as a regulator of cell death and disease resistance, but coordinates defense responses with its interacting partner proteins CaPR10, CaPR4b, and CaHIR1 in the different subcellular locations (Fig.…”

“…LRR proteins function in an array of developmental and immune signaling pathways and are involved in receptor/coreceptor complex formation (Kobe and Deisenhofer 1994;Jaillaisa et al 2011). Some NLR proteins directly bind to pathogen-associated proteins, primarily pathogen effector molecules, to induce basal defense responses and effector-triggered immunity (ETI) in resistant host genotypes (DeYoung and Innes 2006;Jones and Dangl 2006;Han and Hwang 2017).…”

“…vesicatoria (Xcv) pathosystem is useful for exploring the molecular and cellular bases of plant cell death and defense responses to microbial pathogens (Han and Hwang 2017). Infection with the avirulent (incompatible) Xcv strain Bv5-4a carrying the effector AvrBsT activates rapid and distinct expression of a repertoire of cell death-and defense-related genes in pepper plants (Hong et al 2008;Hwang 2011, 2015;Kim et al 2014;Kim and Hwang 2015a, b;Han and Hwang 2017). The pepper LRR1 (CaLRR1) gene is induced by pathogens and encodes a small extracellular protein containing a single LRR domain with five tandem repeats of a 24 amino acid LRR motif (Jung et al 2004).…”