A Truncated NLR Protein, TIR-NBS2, Is Required for Activated Defense Responses in the exo70B1 Mutant

Abstract

During exocytosis, the evolutionarily conserved exocyst complex tethers Golgi-derived vesicles to the target plasma membrane, a critical function for secretory pathways. Here we show that exo70B1 loss-of-function mutants express activated defense responses upon infection and express enhanced resistance to fungal, oomycete and bacterial pathogens. In a screen for mutants that suppress exo70B1 resistance, we identified nine alleles of TIR-NBS2 (TN2), suggesting that loss-of-function of EXO70B1 leads to activation of this nucleotide binding domain and leucine-rich repeat-containing (NLR)-like disease resistance protein. This NLR-like protein is atypical because it lacks the LRR domain common in typical NLR receptors. In addition, we show that TN2 interacts with EXO70B1 in yeast and in planta. Our study thus provides a link between the exocyst complex and the function of a ‘TIR-NBS only’ immune receptor like protein. Our data are consistent with a speculative model wherein pathogen effectors could evolve to target EXO70B1 to manipulate plant secretion machinery. TN2 could monitor EXO70B1 integrity as part of an immune receptor complex.

Exocyst Subunit EXO70H4 Has a Specific Role in Callose Synthase Secretion and Silica Accumulation

“…In this study we demonstrate that there is functional divergence between the paralogs. This is 262 supported also by other studies showing specific EXO70 roles (Kulich et al, 2013;Zhao et al, 2015) 263 and recently also the specific PM domain localizations of NtEXO70A1 and NtEXO70B1 in the tobacco 264 pollen tubes (Sekereš et al, 2017). 265…”

“…Besides secretion, the EXO70B1 subunit functions in autophagy (Kulich et al, 2013), immune 77 responses (Stegmann et al, 2013;Zhao et al, 2015) and stomatal opening (Hong et al, 2016;Seo et 78 al., 2016). EXO70H1 and B2 were shown to facilitate the defence papilla buildup (Pecenková et al, 79 2011).…”

Phytophthora infestans RXLR effector AVR1 interacts with exocyst component Sec5 to manipulate plant immunity

“…Such an expansion is often associated with a differentiation in function, and indeed, for EXO70B1 in Arabidopsis, it was shown that this subtype adopted a specific role in autophagic transport (Kulich et al, 2013). Besides defects in autophagy, exo70B1 mutants show small spontaneous lesions and changes in susceptibility to pathogens (Stegmann et al, 2012(Stegmann et al, , 2013Zhao et al, 2015).…”

“…More recent reports point to roles in plant defense, including cell wall apposition formation, PTI, and suppression of spontaneous HR mediated by salicylic acid (SA; Pecenková et al, 2011;Stegmann et al, 2012;Kulich et al, 2013). An anticipated role of vesicle trafficking and the exocyst in defense is further supported by the findings of Zhao et al (2015), who showed physical interaction between a truncated NLR protein and one of the exocyst subunits, and by the observation that several pathogen effectors target the vesicle-trafficking machinery in the host to facilitate colonization. For example, HopM1 from Pseudomonas syringae destabilizes Arabidopsis (Arabidopsis thaliana) AtMIN7, an ADP ribosylation factor guanine nucleotideexchange factor and a key regulator of vesicle formation, and this destabilization leads to enhanced pathogen colonization (Nomura et al, 2006(Nomura et al, , 2011.…”

TIR-only protein RBA1 recognizes a pathogen effector to regulate cell death in Arabidopsis

“…Like RBA1 or TIR domains truncated from full-length TNLs, overexpression of TIR-X and TIR-NB proteins can trigger EDS1-dependent cell death (28). Loss-of-function mutations in TN2, a TIR-NB protein lacking LRRs, were isolated as genetic suppressors of cell death triggered by loss-of-function alleles of EXO70B1, a component of the exocyst vesicle trafficking complex (51). These phenotypes are consistent with a model wherein EXO70B1 is a guardee, monitored by the immune receptor TN2 and targeted by as yet unknown pathogen effectors.…”