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 the hypersensitive response (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.

Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

病原体利用效应子库来促进发病机理，但是当宿主识别出其中之一时，就会引起效应子触发的免疫力。的假单胞菌III型效应AvrRps4是在植物中加工成的功能性133个氨基酸的N末端（AvrRps4-N）和88个氨基酸的C-末端（AvrRps4-C）的二分效应。先前的研究发现，AvrRps4-C足以触发萝卜中的超敏反应（HR）。相反，我们最近的工作发现，AvrRps4-N而不是AvrRps4-C触发了生菜中的HR，而拟南芥中的这两种诱导抗性都需要。在这里，我们最初使用瞬态表达比较了芜菁和生菜对AvrRps4的识别。通过连续截短，我们确定了由37个氨基酸组成的中央保守区是AvrRps4-N识别必不可少的，而假定的III型分泌信号肽或C末端的13个氨基酸是可有可无的。出乎意料的是，通过莴苣识别AvrRps4-N，还需要全长AvrRps4处理所需的位置112（R112）处的保守精氨酸。将R112突变为疏水性亮氨酸或带负电荷的谷氨酸消除了AvrRps4-N的HR诱导能力，而在此位置带正电荷的赖氨酸导致HR缓慢而微弱。总之，我们的结果表明R112在生菜中具有AvrRps4-N识别特异性。

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