A Xanthomonas uridine 5′-monophosphate transferase inhibits plant immune kinases

Abstract

Plant innate immunity is activated on the detection of pathogen-associated molecular patterns (PAMPs) at the cell surface, or of pathogen effector proteins inside the plant cell. Together, PAMP-triggered immunity and effector-triggered immunity constitute powerful defences against various phytopathogens. Pathogenic bacteria inject a variety of effector proteins into the host cell to assist infection or propagation. A number of effector proteins have been shown to inhibit plant immunity, but the biochemical basis remains unknown for the vast majority of these effectors. Here we show that the Xanthomonas campestris pathovar campestris type III effector AvrAC enhances virulence and inhibits plant immunity by specifically targeting Arabidopsis BIK1 and RIPK, two receptor-like cytoplasmic kinases known to mediate immune signalling. AvrAC is a uridylyl transferase that adds uridine 5'-monophosphate to and conceals conserved phosphorylation sites in the activation loop of BIK1 and RIPK, reducing their kinase activity and consequently inhibiting downstream signalling.

Two redundant receptor-like cytoplasmic kinases function downstream of pattern recognition receptors to regulate activation of SA biosynthesis in Arabidopsis

“…In addition to Pto and PBS1, RLCKs such as BIK1, PBL1, RIPK, and OsRLCK185 are also targeted by pathogen effector proteins (J. Liu et al, 2011;Feng et al, 2012;Yamaguchi et al, 2013), further supporting the importance of RLCKs in plant immunity.…”

Pseudomonas syringae type III effector HopAF1 suppresses plant immunity by targeting methionine recycling to block ethylene induction

“…T3Es commonly contain sequences addressing specific eukaryotic subcellular localizations (10,11) and enzymatic activities (12)(13)(14)(15)(16)(17) that disrupt and/or suppress PTI. Known targets of T3Es include plasma membrane-localized receptor complexes (13,(18)(19)(20)(21)(22)(23), downstream MAPK cascades (24,25), the stability of defenserelated transcripts (26), phytoalexin biosynthesis (27), and vesicle trafficking (28).…”

PBL13 is a serine/threonine protein kinase that negatively regulates Arabidopsis immune responses.

“…For example, although BIK1 and PBL1 are dispensable for MAPK3/6 activation, transgenic plants expressing AvrAC are unable to fully inhibit MAPK3/6, suggesting that other RLCKs fulfill the role for MAPK3/6 activation (Feng et al, 2012). These additional RLCK members likely also play key roles in immune signaling.…”

“…Single Arabidopsis bik1 and pbl1 knockout (KO) lines are compromised in MTI, and the bik1/pbl1 double KO exhibits weaker MTI responses . Furthermore, transgenic plants expressing either AvrPphB or AvrAC are compromised in MTI, indicating that multiple RLCKs are critical for immune signaling Feng et al, 2012).…”

“…In addition to PBS1, AvrPphB also cleaves BIK1, PBL1, and six other RLCK VIIs . The Xanthomonas campestris pv campestris effector AvrAC is a UMP transferase that uridylates BIK1, PBL1, and RIPK at highly conserved Ser and Thr residues within the kinase activation loop, thereby attenuating RLCK kinase activity (Feng et al, 2012). Single Arabidopsis bik1 and pbl1 knockout (KO) lines are compromised in MTI, and the bik1/pbl1 double KO exhibits weaker MTI responses .…”