Dissecting virulence function from recognition: cell death suppression in Nicotiana benthamiana by XopQ/HopQ1‐family effectors relies on EDS1‐dependent immunity

Abstract

Many Gram-negative plant pathogenic bacteria express effector proteins of the XopQ/HopQ1 family which are translocated into plant cells via the type III secretion system during infection. In Nicotiana benthamiana, recognition of XopQ/HopQ1 proteins induces an effector-triggered immunity (ETI) reaction which is not associated with strong cell death but renders plants immune against Pseudomonas syringae and Xanthomonas campestris pv. vesicatoria strains. Additionally, XopQ suppresses cell death in N. benthamiana when transiently co-expressed with cell death inducers. Here, we show that representative XopQ/HopQ1 proteins are recognized similarly, likely by a single resistance protein of the TIR-NB-LRR class. Extensive analysis of XopQ derivatives indicates the recognition of structural features. We performed Agrobacterium-mediated protein expression experiments in wild-type and EDS1-deficient (eds1) N. benthamiana leaves, not recognizing XopQ/HopQ1. XopQ recognition limits multiplication of Agrobacterium and attenuates levels of transiently expressed proteins. Remarkably, XopQ fails to suppress cell death reactions induced by different effectors in eds1 plants. We conclude that XopQ-mediated cell death suppression in N. benthamiana is due to the attenuation of Agrobacterium-mediated protein expression rather than the cause of the genuine XopQ virulence activity. Thus, our study expands our understanding of XopQ recognition and function, and also challenges the commonly used co-expression assays for elucidation of in planta effector activities, at least under conditions of ETI induction.

The Xanthomonas effector XopL uncovers the role of microtubules in stromule extension and dynamics in Nicotiana benthamiana

“…It is known that N . benthamiana recognizes XopQ, via the R protein Roq1 (Schultink et al, ), and initiates ETI responses (Schwartz et al ., ; Adlung et al ., ; Adlung and Bonas, ). Although the XopQ‐triggered stromule induction is impressive it is not surprising given the known link between ETI and increased stromule frequencies (Caplan et al ., ).…”

“…XopQ induced an increase in stromule frequency to 56%, whereas XopL caused a dramatic reduction to 2% stromule frequency (n = 2 plants). It is known that N. benthamiana recognizes XopQ, via the R protein Roq1 (Schultink et al, 2017), and initiates ETI responses (Schwartz et al, 2015;Adlung et al, 2016;Adlung and Bonas, 2017). Although the XopQ-triggered stromule induction is impressive it is not surprising given the known link between ETI and increased stromule frequencies (Caplan et al, 2015).…”

The type III effector RipB from Ralstonia solanacearum RS1000 acts as a major avirulence factor in Nicotiana benthamiana and other Nicotiana species

“…a,b) might be explained by differences in signalling pathways between HR cell death and other ETI responses. From another viewpoint, it has recently been suggested that the ETI response induced by the RipB homologues XopQ and HopQ1 has severe negative effects on the multiplication of A. tumefaciens as well as on Agrobacterium ‐mediated transient protein expression in N. benthamiana (Adlung and Bonas, ). Therefore, the chlorosis induced by the transient expression of RipB might be caused by the attenuated transient expression of RipB.…”

Bacterial injection machines: Evolutionary diverse but functionally convergent

“…The repertoire of T3 effectors of X. campestris pv. vesicatoria comprises more than 35 proteins, among them the AvrBs3 transcription factor, cell death inducers, plant immunity suppressors (Adlung & Bonas, ), or the XopL ubiquitine ligase. The characterisation of the latter reveals an unprecedented role of microtubules in stromule extension and dynamics in Nicotiana benthamiana (Erickson, Adlung, Lampe, Bonas, & Schattat, ).…”