STER1, a novel receptor-like kinase, functions in MAMP signalling in Arabidopsis

Abstract

S. epidermidis has long been recognized as an important opportunistic pathogen accounting for the majority of nosocomial infections alongside S. aureus. However, in spite of this, our understanding of the S. epidermidis virulence mechanisms is still limited. Previous studies have emphasized various analogies in innate immunity against pathogens in plant, invertebrate and mammalian hosts. When compared to in vivo animal models, plant models are an attractive alternative and experiments using the S.aureus-Arabidopsis pathosystem have shown potential for this approach with S. epidermidis. In this study, an Arabidopsis−S.epidermidis system was established aiming to identify possible bacterial virulence traits. As S. epidermidis is not a true plant pathogen it fails to multiply in planta; however, most S. epidermidis strains tested generated a salycilic acid (SA)-dependent necrotic phenotype in Arabidopsis 5 days−post inoculation. Additionally, inoculation with boiled bacteria generated the same visual response as live cells, suggesting a pre−existent, heat stable molecule underlies the plant visual response. Taken together, this data suggests the necrotic response is a visual expression of MAMP perception. Subsequent exploitation of the Arabidopsis natural variation through QTL analysis, resulted in the isolation of the STER1 (Staphylococcus elicitor response 1) gene. This gene is essential for the visual response to S. epidermidis 18888 and encodes a membrane localized DUF26−containing receptor−like kinase. The ster1-1 mutant remained asymptomatic following inoculation with Gram−positive S. epidermidis 18888 and Gram−negative B. ambifaria, suggesting STER1’s likely role in the recognition of a common molecule. Peptidoglycan (PGN), an essential bacterial membrane component was considered a likely candidate and isolated from both species. Inoculation with B. ambifaria PGN generates a plant response that mirrores the one seen with bacterial suspensions of the same organism. By contrast, pure S. epidermidis 18888 PGN does not trigger a visual response in Arabidopsis. Instead, an S. epidermidis 18888 membrane fraction (MF), consisting of PGN, teichoic acid (TA) and an uncharacterized capsular polysaccharide (CPS), was found to generate a necrotic response similar to live cells. Treatment with S. epidermidis MF and B. ambifaria PGN triggered stereotypical defence responses, such as PR1 up−regulation and cell death in wild−type plants, but not in the ster1-1 mutant. Additionally, pre-treatment with S. epidermidis MF and B. ambifaria PGN also restricted Pst DC3000 growth in wild−type plants only, thus emphasizing a likely role for STER1 in basal resistance and PGN perception. In conclusion, the data obtained in this study implicate STER1 in PGN and possibly specialized CPS recognition, either as a receptor, co-receptor or essential signalling component.