Plant cell wall-mediated disease resistance: Current understanding and future perspectives.

Beyond their function as structural barriers, plant cell walls are essential elements for the adaptation of plants to environmental conditions. Cell walls are dynamic structures whose composition and integrity can be altered in response to environmental challenges and developmental cues. These wall changes are perceived by plant sensors/receptors to trigger adaptative responses during development and upon stress perception. Plant cell wall damage caused by pathogen infection, wounding, or other stresses leads to the release of wall molecules, such as carbohydrates (glycans), that function as damage-associated molecular patterns (DAMPs). DAMPs are perceived by the extracellular ectodomains (ECDs) of pattern recognition receptors (PRRs) to activate pattern-triggered immunity (PTI) and disease resistance. Similarly, glycans released from the walls and extracellular layers of microorganisms interacting with plants are recognized as microbe-associated molecular patterns (MAMPs) by specific ECD-PRRs triggering PTI responses. The number of oligosaccharides DAMPs/MAMPs identified that are perceived by plants has increased in recent years. However, the structural mechanisms underlying glycan recognition by plant PRRs remain limited. Currently, this knowledge is mainly focused on receptors of the LysM-PRR family, which are involved in the perception of various molecules, such as chitooligosaccharides from fungi and lipo-chitooligosaccharides (i.e., Nod/MYC factors from bacteria and mycorrhiza, respectively) that trigger differential physiological responses. Nevertheless, additional families of plant PRRs have recently been implicated in oligosaccharide/polysaccharide recognition. These include receptor kinases (RKs) with leucine-rich repeat and Malectin domains in their ECDs (LRR-MAL RKs), Catharanthus roseus RECEPTOR-LIKE KINASE 1-LIKE group (CrRLK1L) with Malectin-like domains in their ECDs, as well as wall-associated kinases, lectin-RKs, and LRR-extensins. The characterization of structural basis of glycans recognition by these new plant receptors will shed light on their similarities with those of mammalians involved in glycan perception. The gained knowledge holds the potential to facilitate the development of sustainable, glycan-based crop protection solutions.

Mitogen-activated protein kinase phosphatase 1 controls broad spectrum disease resistance in Arabidopsis thaliana through diverse mechanisms of immune activation.

Arabidopsis thaliana Mitogen-activated protein Kinase Phosphatase 1 (MKP1) negatively balances production of reactive oxygen species (ROS) triggered by Microbe-Associated Molecular Patterns (MAMPs) through uncharacterized mechanisms. Accordingly, ROS production is enhanced in mkp1 mutant after MAMP treatment. Moreover, mkp1 plants show a constitutive activation of immune responses and enhanced disease resistance to pathogens with distinct colonization styles, like the bacterium Pseudomonas syringae pv. tomato DC3000, the oomycete Hyaloperonospora arabidopsidis Noco2 and the necrotrophic fungus Plectosphaerella cucumerina BMM. The molecular basis of this ROS production and broad-spectrum disease resistance controlled by MKP1 have not been determined. Here, we show that the enhanced ROS production in mkp1 is not due to a direct interaction of MKP1 with the NADPH oxidase RBOHD, nor is it the result of the catalytic activity of MKP1 on RBHOD phosphorylation sites targeted by BOTRYTIS INDUCED KINASE 1 (BIK1) protein, a positive regulator of RBOHD-dependent ROS production. The analysis of bik1 mkp1 double mutant phenotypes suggested that MKP1 and BIK1 targets are different. Additionally, we showed that phosphorylation residues stabilizing MKP1 are essential for its functionality in immunity. To further decipher the molecular basis of disease resistance responses controlled by MKP1, we generated combinatory lines of mkp1-1 with plants impaired in defensive pathways required for disease resistance to pathogen: cyp79B2 cyp79B3 double mutant defective in synthesis of tryptophan-derived metabolites, NahG transgenic plant that does not accumulate salicylic acid, aba1-6 mutant impaired in abscisic acid (ABA) biosynthesis, and abi1 abi2 hab1 triple mutant impaired in proteins described as ROS sensors and that is hypersensitive to ABA. The analysis of these lines revealed that the enhanced resistance displayed by mkp1-1 is altered in distinct mutant combinations: mkp1-1 cyp79B2 cyp79B3 fully blocked mkp1-1 resistance to P. cucumerina, whereas mkp1-1 NahG displays partial susceptibility to H. arabidopsidis, and mkp1-1 NahG, mkp1-1 aba1-6 and mkp1-1 cyp79B2 cyp79B3 showed compromised resistance to P. syringae. These results suggest that MKP1 is a component of immune responses that does not directly interact with RBOHD but rather regulates the status of distinct defensive pathways required for disease resistance to pathogens with different lifestyles.

Arabidopsis immune responses triggered by cellulose- and mixed-linked glucan-derived oligosaccharides require a group of leucine-rich repeat malectin receptor kinases.

The plant immune system perceives a diversity of carbohydrate ligands from plant and microbial cell walls through the extracellular ectodomains (ECDs) of pattern recognition receptors (PRRs), which activate pattern-triggered immunity (PTI). Among these ligands are oligosaccharides derived from mixed-linked ß-1,3/ß-1,4-glucans (MLGs; e.g. ß-1,4-D-(Glc)2 -ß-1,3-D-Glc, MLG43) and cellulose (e.g. ß-1,4-D-(Glc)3 , CEL3). The mechanisms behind carbohydrate perception in plants are poorly characterized except for fungal chitin oligosaccharides (e.g. ß-1,4-d-(GlcNAc)6 , CHI6), which involve several receptor kinase proteins (RKs) with LysM-ECDs. Here, we describe the isolation and characterization of Arabidopsis thaliana mutants impaired in glycan perception (igp) that are defective in PTI activation mediated by MLG43 and CEL3, but not by CHI6. igp1-igp4 are altered in three RKs - AT1G56145 (IGP1), AT1G56130 (IGP2/IGP3) and AT1G56140 (IGP4) - with leucine-rich-repeat (LRR) and malectin (MAL) domains in their ECDs. igp1 harbors point mutation E906K and igp2 and igp3 harbor point mutation G773E in their kinase domains, whereas igp4 is a T-DNA insertional loss-of-function mutant. Notably, isothermal titration calorimetry (ITC) assays with purified ECD-RKs of IGP1 and IGP3 showed that IGP1 binds with high affinity to CEL3 (with dissociation constant KD  = 1.19 ± 0.03 µm) and cellopentaose (KD  = 1.40 ± 0.01 µM), but not to MLG43, supporting its function as a plant PRR for cellulose-derived oligosaccharides. Our data suggest that these LRR-MAL RKs are components of a recognition mechanism for both cellulose- and MLG-derived oligosaccharide perception and downstream PTI activation in Arabidopsis.