Altered glycosylation of exported proteins, including surface immune receptors, compromises calcium and downstream signaling responses to microbe-associated molecular patterns in Arabidopsis thaliana

Abstract

BackgroundCalcium, as a second messenger, transduces extracellular signals into cellular reactions. A rise in cytosolic calcium concentration is one of the first plant responses after exposure to microbe-associated molecular patterns (MAMPs). We reported previously the isolation of Arabidopsis thaliana mutants with a “changed calcium elevation” (cce) response to flg22, a 22-amino-acid MAMP derived from bacterial flagellin.ResultsHere, we characterized the cce2 mutant and its weaker allelic mutant, cce3. Besides flg22, the mutants respond with a reduced calcium elevation to several other MAMPs and a plant endogenous peptide that is proteolytically processed from pre-pro-proteins during wounding. Downstream defense-related events such flg22-induced mitogen-activated protein kinase activation, accumulation of reactive oxygen species and growth arrest are also attenuated in cce2/cce3. By genetic mapping, next-generation sequencing and allelism assay, CCE2/CCE3 was identified to be ALG3 (Asparagine-linked glycosylation 3). This encodes the α-1,3-mannosyltransferase responsible for the first step of core oligosaccharide Glc3Man9GlcNAc2 glycan assembly on the endoplasmic reticulum (ER) luminal side. Complementation assays and glycan analysis in yeast alg3 mutant confirmed the reduced enzymatic function of the proteins encoded by the cce2/cce3 alleles – leading to accumulation of M5ER, the immature five mannose-containing oligosaccharide structure found in the ER. Proper protein glycosylation is required for ER/Golgi processing and trafficking of membrane proteins to the plasma membrane. Endoglycosidase H-insensitivity of flg22 receptor, FLS2, in the cce2/cce3 mutants suggests altered glycan structures in the receptor.ConclusionProper glycosylation of MAMP receptors (or other exported proteins) is required for optimal responses to MAMPs and is important for immune signaling of host plants.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0718-3) contains supplementary material, which is available to authorized users.

Antagonistic bacteria disrupt calcium homeostasis and immobilize algal cells

“…Ca 2+ signaling has been implicated in early stages of higher plant–microbe interactions of both symbiotic and antagonistic nature 36 . Likewise, in marine diatoms, environmental signals and intraspecies interactions can activate Ca 2+ signaling 37 , 38 .…”

At the intersection of exocytosis and endocytosis in plants

“…Proper protein folding of newly synthesized PRRs at the ER is a key step in ensuring their arrival at their final destinations via the secretory pathway, and hence the plant defense response. Asparagine‐linked glycosylation 3, responsible for adding the first mannose to flipped precursor glycans in the ER, is required for delivery of FLS2 to the PM and downstream defense‐related events (Trempel et al ., ). In addition, the ER‐resident proteins RTNLB1 and RTNLB2 regulate ER trafficking of FLS2 to the PM.…”

N‐glycan maturation mutants in Lotus japonicus for basic and applied glycoprotein research

“…In conclusion, the generated N-glycan maturation mutants presented here form a set of non-genetically modified organisms (GMO) with a deconstructed and more simplified N-glycan pattern. There is no limitation to the number of different plant mutants that can be generated using this technology and this tool provides a valuable resource to systematically investigate the biological importance of plant N-glycosylation in plant development and growth, in food allergy and in pathogen-host relationships (Trempel et al, 2016). Furthermore, plant-specific N-glycosylation can also be completely eliminated and replaced by mammalian N-glycan glycosyltransferase genes to generate stable Lotus japonicus plant lines for the pharmaceutical production of more efficacious glycoproteins with fewer side effects.…”