In their natural environment, plants live in a close association with a large variety of microorganisms. A number of these microorganisms can be detrimental to plants and are considered as potential pathogens. In order to ward off these pathogens, plants have developed a highly effective and dynamic immune system.
As a first line of defense, plants recognize the presence of microbes through the perception of molecular structures typical of a microbial class, termed microbe-associated molecular patterns (MAMPs). In Arabidopsis, the Leu-rich repeat receptor-like kinases FLS2 and EFR recognize the bacterial MAMPs flagellin and EF-Tu (and their bioactive epitopes flg22 and elf18), respectively. Perception of these MAMPs triggers defense responses that restrict microbial invasion and growth. However, the molecular basis of MAMP-triggered immunity (MTI) is still largely unknown. As MTI functionally links to and provides an evolutionary basis for different branches of plant immunity, it is instrumental for the understanding of plant-microbe interactions.
The work presented here aimed at the identification of molecular components of MTI. A forward-genetic screen revealed priority in sweet life (psl) mutants that show de-repressed anthocyanin accumulation in the presence of elf18 or flg22. PSL2 was identified as the single-copy Arabidopsis UDP-glucose:glycoprotein glucosyltranseferase (UGGT), whereas PSL25 most likely identifies the Arabidopsis endoplasmic reticulum (ER) Glucosidase I. These are components of an ER protein quality control (ERQC) pathway that ensures proper folding and maturation of membrane-resident and secreted proteins. These and other ERQC components are required for the generation of functional EFR.
PSL36 was identified as a novel allele of EIN2 (ETHYLENE INSENSITIVE2), a central regulator of the ethylene(ET) pathway. Loss of EIN2 function results in pronounced defects in FLS2 and EFR signaling outputs. Whereas ET signaling is crucial for FLS2 expression, EFR steady-state levels are unaltered in ein2 plants. These data point to a role for ET in post-recognition signaling by EFR. The identification of a set of EFR-triggered genes that depend on ET-signaling for their full activation reveals possible mechanisms of signal integration during MTI
