The Conserved Arginine Required for AvrRps4 Processing Is Also Required for Recognition of Its N-Terminal Fragment in Lettuce

Abstract

Pathogens utilize a repertoire of effectors to facilitate pathogenesis, but when the host recognizes one of them it causes effector-triggered immunity. The Pseudomonas type III effector AvrRps4 is a bipartite effector that is processed in planta into a functional 133 amino acid N-terminus (AvrRps4-N) and 88 amino acid C-terminus (AvrRps4-C). Previous studies found AvrRps4-C to be sufficient to trigger HR in turnip. In contrast, our recent work found that AvrRps4-N, but not AvrRps4-C, triggered HR in lettuce whereas both were required for resistance induction in Arabidopsis. Here, we initially compared AvrRps4 recognition by turnip and lettuce using transient expression. By serial truncation, we identified the central conserved region consisting of 37 amino acids as essential for AvrRps4-N recognition, whereas the putative type III secretion signal peptide or the C-terminal 13 amino acids were dispensable. Surprisingly, the conserved arginine at position 112 (R112) that is required for full-length AvrRps4 processing is also required for the recognition of AvrRps4-N by lettuce. Mutating R112 to hydrophobic leucine or negatively charged glutamate abolished the HR-inducing capacity of AvrRps4-N, while a positively charged lysine at this position resulted in a slow and weak HR. Together, our results suggest an AvrRps4-N recognition-specific role of R112 in lettuce.

Perception of structurally distinct effectors by the integrated WRKY domain of a plant immune receptor

“…Previous studies have highlighted the role of AvrRps4 C in triggering RRS1/RPS4dependent immune responses (26,27). AvrRps4 N has been reported to potentiate immune signaling from AvrRps4 C (28,29). PopP2 is sequence and structurally distinct from AvrRps4 and has an acetyltransferase activity that is likely related to its role in virulence.…”

Perception of structurally distinct effectors by the integrated WRKY domain of a plant immune receptor

“…Previous studies have highlighted the role of AvrRps4 C in triggering RRS1/RPS4-dependent immune responses ( 25 , 26 ). AvrRps4 N has been reported to potentiate immune signaling from AvrRps4 C ( 27 , 28 ). PopP2 is sequence and structurally distinct from AvrRps4 and has an acetyltransferase activity that is likely related to its role in virulence.…”

AvrRps4 effector family processing and recognition in lettuce

“…In addition, not only AvrRps4 C but also AvrRps4 N interacts with EDS1, and both termini are required to trigger immunity in Arabidopsis when delivered by bacterial pathogens at natural protein levels (Bhattacharjee et al, 2011 ; Halane et al, 2018 ; Heidrich et al, 2011 ). These findings suggest the function of AvrRps4 N , which was formerly proposed to only contain a type III secretion system and a chloroplast targeting signal, to be a bona fide effector domain that acts beyond being a signal peptide (Halane et al, 2018 , Su et al, 2021 ).…”

“…In addition, transient expression of AvrRps4 C is sufficient to trigger an HR in turnip (Sohn et al, 2009 ). While the C‐terminus of AvrRps4 elicits ETI in Arabidopsis and turnip, its N‐terminus can be recognized to induce HR in lettuce ( Lactuca sativa ‘Kordaat’) (Halane et al, 2018 ; Su et al, 2021 ). In addition, not only AvrRps4 C but also AvrRps4 N interacts with EDS1, and both termini are required to trigger immunity in Arabidopsis when delivered by bacterial pathogens at natural protein levels (Bhattacharjee et al, 2011 ; Halane et al, 2018 ; Heidrich et al, 2011 ).…”

“…tomato HopK1 and Xanthomonas campestris pv. vesicatoria XopO (Halane et al, 2018 ; Li et al, 2014 ; Sohn et al, 2009 ; Su et al, 2021 ), which are together called the AvrRps4 effector family. These three effectors bear a striking similarity in amino acid sequences at the N‐terminal domain, while their C‐terminal domains are unrelated.…”