Abstract
Receptor-like kinases (RLKs) play important roles in plant immunity signaling; thus, many are hijacked by pathogen effectors to promote successful pathogenesis. Xanthomonas oryzae pv. oryzae (Xoo) is the causal agent of rice leaf blight disease. The strain PXO99A has 18 non-TAL (transcription activation-like) effectors; however, their mechanisms of action and host target proteins remain largely unknown. Although the effector XopR from the Xoo strain MAFF311018 was shown to suppress PAMP-triggered immune responses in Arabidopsis, its target has not yet been identified. Here, we show that PXO99A XopR interacts with BIK1 at the plasma membrane. BIK1 is a receptor-like cytoplasmic kinase (RLCK) belonging to the RLK family of proteins and mediates PAMP-triggered stomatal immunity. In turn, BIK1 phosphorylates XopR. Furthermore, XopR suppresses PAMP-triggered stomatal closure in transgenic Arabidopsis expressing XopR. In addition, XopR is able to associate with RLCKs other than BIK1. These results suggest that XopR likely suppresses plant immunity by targeting BIK1 and other RLCKs.
Highlights
As in animals, plants possess pattern-recognition receptors (PRRs) that recognize pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) derived from microorganisms to initiate a series of basal immune responses called PAMP/MAMP-triggered immunity (PTI) (Boller and Felix, 2009; Boller and He, 2009; Cook et al, 2015)
We performed a bimolecular fluorescence complementation assay (BiFC) in Arabidopsis protoplasts, where Receptor-like kinases (RLKs) were tagged with the carboxylterminal half of YFP and XopR was tagged with the amino-terminal half of YFP (XopR-nYFP)
Recombinant His-FLAG-XopR could be pulled down by BIK1 protein fused to glutathione S-transferase (GST) in an in vitro GST pull-down assay (Figure 2D), further suggesting that XopR directly interacts with BIK1
Summary
Plants possess pattern-recognition receptors (PRRs) that recognize pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) derived from microorganisms to initiate a series of basal immune responses called PAMP/MAMP-triggered immunity (PTI) (Boller and Felix, 2009; Boller and He, 2009; Cook et al, 2015). Nase (MAPK) cascades, up-regulation of defense-related genes, callose deposition to strengthen the plant cell wall, and stomatal closure to prevent the entry of microbes (Boller and Felix, 2009; Boller and He, 2009; Cook et al, 2015). Receptor-like kinases (RLKs) play important roles in plant innate immunity signaling (Wu and Zhou, 2013). The Arabidopsis genome encodes approximately 610 RLK/ Pelle/IRAK proteins, which include receptor-like cytoplasmic kinases (RLCKs) (Shiu and Bleecker, 2001).
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