No abstract

Plant innate immunity relies on a set of pattern recognition receptors (PRRs) that respond to ligands known as pathogen-associated molecular patterns (PAMPs). To overcome such immunity, phytopathogenic bacteria deliver virulence molecules called effector proteins into the plant cell that collectively promote pathogenesis. The vast majority of PRRs controlling PAMP-triggered immunity (PTI) and the mechanisms used by specific effectors to suppress these pathways are mostly unknown. Here, we show that the Arabidopsis LysM receptor kinase CERK1, which is critical for chitin elicitor signaling and resistance to fungal pathogens, plays an essential role in restricting bacterial growth on plants. This is supported by the fact that CERK1 is a target of the bacterial type III effector protein AvrPtoB, which blocks all defense responses through this receptor. AvrPtoB ubiquitinates the CERK1 kinase domain in vitro and targets CERK1 for degradation in vivo. We show that CERK1 is a determinant of bacterial immunity, but its contribution is overcome by bacteria expressing AvrPtoB. Our results reveal a new pathway for plant immunity against bacteria and a role for AvrPtoB E3-ligase activity in suppressing PTI.

Our current understanding of pathogen-associated molecular pattern (PAMP)-triggered immunity signaling pathways in plants is limited due to the redundancy of several components or the lethality of mutants in Arabidopsis (Arabidopsis thaliana).To overcome this, we used a virus-induced gene silencing-based approach in combination with pharmacological studies to decipher links between early PAMP-triggered immunity events and their roles in immunity following PAMP perception in Nicotiana benthamiana. Two different calcium influx inhibitors suppressed the reactive oxygen species (ROS) burst: activation of the mitogen-activated protein kinases (MAPKs) and PAMP-induced gene expression. The calcium burst was unaffected in plants specifically silenced for components involved in ROS generation or for MAPKs activated by PAMP treatment. Importantly, the ROS burst still occurred in plants silenced for the two major defense-associated MAPK genes NbSIPK (for salicylic acid-induced protein kinase) and NbWIPK (for wound-induced protein kinase) or for both genes simultaneously, demonstrating that these MAPKs are dispensable for ROS production. We further show that NbSIPK silencing is sufficient to prevent PAMP-induced gene expression but that both MAPKs are required for bacterial immunity against two virulent strains of Pseudomonas syringae and their respective nonpathogenic mutants. These results suggest that the PAMP-triggered calcium burst is upstream of separate signaling branches, one leading to MAPK activation and then gene expression and the other to ROS production. In addition, this study highlights the essential roles of NbSIPK and NbWIPK in antibacterial immunity. Unexpectedly, negative regulatory mechanisms controlling the intensity of the PAMP-triggered calcium and ROS bursts were also revealed by this work.

SummaryConserved microbial molecules known as PAMPs (pathogen-associated molecular patterns) elicit defence responses in plants through extracellular receptor proteins. One important PAMP is the flagellin protein derived from motile bacteria. We show here that the solanaceous species Nicotiana benthamiana perceives the flagellin proteins of both pathogenic and non-host species of Pseudomonas syringae. The response to flagellin required a gene closely related to that encoding the Arabidopsis thaliana flagellin receptor that we designated NbFls2. In addition, silencing of NbFls2 led to increased growth of compatible, non-host and non-pathogenic strains of P. syringae. Thus, flagellin perception restricts growth of P. syringae strains on N. benthamiana. Pathogenic bacteria secrete effector proteins into the plant cell to enhance virulence. We tested the ability of several unrelated effectors to suppress PAMP-mediated defences. The effector proteins AvrPto and AvrPtoB, but not AvrRps4, suppressed all responses tested including the hypersensitive response induced by non-host flagellins and the oomycete elicitor INF1. Strikingly, transient expression of avrPto or avrPtoB stimulated the growth of non-pathogenic Agrobacterium tumefaciens in planta, suggesting that multiplication of this species is also restricted by PAMP perception. Unexpectedly, AvrPtoB but not AvrPto required the defence-associated genes Rar1, Sgt1 and Eds1 for suppression. This observation separates the respective mechanisms of the two effectors, and suggests that AvrPtoB may target the defence machinery directly for its suppressive effect.