Bacteria-derived Peptidoglycans Constitute Pathogen-associated Molecular Patterns Triggering Innate Immunity in Arabidopsis

TLDR

Pathogen‑associated molecular pattern (PAMP)‑triggered immunity is the primary plant immune response that evolved to recognize invariant microbial surface structures. The authors propose that PGN perception systems evolved independently in plants and animals through convergent evolution. PGN from Gram‑positive bacteria acts as a novel PAMP in Arabidopsis, eliciting rapid immune responses—including alkalinization, calcium influx, nitric oxide, camalexin, and MAPK activation—and inducing 236 genes, while its perception relies on the PGN sugar backbone, differs from chitin signaling, and parallels PAMP recognition in animals.

Abstract

Pathogen-associated molecular pattern (PAMP)-triggered immunity constitutes the primary plant immune response that has evolved to recognize invariant structures of microbial surfaces. Here we show that Gram-positive bacteria-derived peptidoglycan (PGN) constitutes a novel PAMP of immune responses in <i>Arabidopsis thaliana</i>. Treatment with PGN from <i>Staphylococcus aureus</i> results in the activation of plant responses, such as medium alkalinization, elevation of cytoplasmic calcium concentrations, nitric oxide, and camalexin production and the post-translational induction of MAPK activities. Microarray analysis performed with RNA prepared from PGN-treated <i>Arabidopsis</i> leaves revealed enhanced transcript levels for 236 genes, many of which are also altered upon administration of flagellin. Comparison of cellular responses after treatment with bacteria-derived PGN and structurally related fungal chitin indicated that both PAMPs are perceived via different perception systems. PGN-mediated immune stimulation in <i>Arabidopsis</i> is based upon recognition of the PGN sugar backbone, while muramyl dipeptide, which is inactive in this plant, triggers immunity-associated responses in animals. PGN adds to the list of PAMPs that induce innate immune programs in both plants and animals. However, we propose that PGN perception systems arose independently in both lineages and are the result of convergent evolution.