Being sessile, plants do not have an immune system as advanced as animals; they rely on innate immunity to respond rapidly and accurately to changing environment. In the event of pathogen infection, plants recognize molecules produced by microorganisms or derived from insect predation named microbe-associated molecular patterns (MAMPs) and damage-associated molecular patterns (DAMPs), respectively, via plasma membrane (PM)-bound pattern recognition receptors (PRRs). Concomitant with receptor activation, ligand binding also triggers internalization of the signaling receptors from the PM into early endosomes, where receptors are sorted for degradation to late endosomes or recycled back to the PM to be reused. Receptor-mediated endocytosis (RME) plays an essential role in cell signaling as it protect plants from over-activation of signaling by way of receptor removal from the PM. In addition to signaling attenuation, RME might also be required for signaling as endosomes can carry active receptor complexes or signaling components and they can be used as signaling platforms in the cytoplasm. In mammals, endosomal signaling was demonstrated for many receptor families including receptor tyrosine kinases, G-protein‐ coupled receptors and toll‐like receptors. Unlike mammals, the mechanisms of endosomal signaling in plants are unknown.
In plants, most of PM receptor kinases (RKs) undergo Clathrin mediated endocytosis (CME). In addition to maintaining basic cellular functions, CME has a role in growth and development, hormone signaling and interaction with the environment, nutrient uptake, restrain toxin and pathogen defense. The very well -known immune receptors FLAGELLIN SENSING2 (FLS2, receptor for flg22) or PEPR1 (a receptor for AtPep1) both requires clathrin for their downstream responses. Interference with clathrin function in Arabidopsis completely blocked the endocytosis of PEPR1 and impaired MAPK activation. While, slight decrease in FLS2 endocytosis only affected the activation of a subset of flg22-triggered signaling.
Recent study of the immune receptor PEPR1 that perceives the endogenous peptide AtPep1, showed that endocytosis is required for mitogen-activated protein kinase (MAPK) activation after elicitation with AtPep1. However, because MAPK activation occurs faster than the endocytosis of the main receptor it raised the question if endocytosis of so far unknown signaling components is required for AtPep1-elicited immune responses.
The overall objective of ENDOLOGISTIC is to decode signaling components involved in early MAPK activation and late endocytosis of PEPR1 upon AtPep1 perception using Arabidopsis thaliana as a model plant. The goal of the study was addressed by combining mass spectrometry analysis on clathrin coated vesicles (CCVs) and TurboID based proximity labelling (PL) approach using PEPR1 as bait upon AtPep1 elicitation.
The study of Identification of unknown signals and endocytic components will be major advance in understanding the regulation of immunity and will fill the gap in our understanding of how endocytosis controls immunity in plants.
