Induction of plant disease resistance by mixed oligosaccharide elicitors prepared from plant cell wall and crustacean shells

Carbohydrate elicitor-induced plant immunity: Advances and prospects

The perceived negative impacts of synthetic agrochemicals gave way to alternative, biological plant protection strategies. The deployment of induced resistance, comprising boosting the natural defense responses of plants, is one of those. Plants developed multi-component defense mechanisms to defend themselves against biotic and abiotic stresses. These are activated upon recognition of stress signatures via membrane-localized receptors. The induced immune responses enable plants to tolerate and limit the impact of stresses. A systemic cascade of signals enables plants to prime un-damaged tissues, which is crucial during secondary encounters with stress. Comparable stress tolerance mechanisms can be induced in plants by the application of carbohydrate elicitors such as chitin/chitosan, β-1,3-glucans, oligogalacturonides, cellodextrins, xyloglucans, alginates, ulvans, and carrageenans. Treating plants with carbohydrate-derived elicitors enable the plants to develop resistance appliances against diverse stresses. Some carbohydrates are also known to have been involved in promoting symbiotic signaling. Here, we review recent progresses on plant resistance elicitation effect of various carbohydrate elicitors and the molecular mechanisms of plant cell perception, cascade signals, and responses to cascaded cues. Besides, the molecular mechanisms used by plants to distinguish carbohydrate-induced immunity signals from symbiotic signals are discussed. The structure-activity relationships of the carbohydrate elicitors are also described. Furthermore, we forwarded future research outlooks that might increase the utilization of carbohydrate elicitors in agriculture in order to improve the efficacy of plant protection strategies.

Two structurally different oomycete lipophilic microbe-associated molecular patterns induce distinctive plant immune responses

Plants recognize a variety of external signals and induce appropriate mechanisms to increase their tolerance to biotic and abiotic stresses. Precise recognition of attacking pathogens and induction of effective resistance mechanisms are critical functions for plant survival. Some molecular patterns unique to a certain group of microbes, microbe-associated molecular patterns (MAMPs), are sensed by plant cells as nonself molecules via pattern recognition receptors. While MAMPs of bacterial and fungal origin have been identified, reports on oomycete MAMPs are relatively limited. This study aimed to identify MAMPs from an oomycete pathogen Phytophthora infestans, the causal agent of potato late blight. Using reactive oxygen species (ROS) production and phytoalexin production in potato (Solanum tuberosum) as markers, two structurally different groups of elicitors, namely ceramides and diacylglycerols, were identified. P. infestans ceramides (Pi-Cer A, B, and D) induced ROS production, while diacylglycerol (Pi-DAG A and B), containing eicosapentaenoic acid (EPA) as a substructure, induced phytoalexins production in potato. The molecular patterns in Pi-Cers and Pi-DAGs essential for defense induction were identified as 9-methyl-4,8-sphingadienine (9Me-Spd) and 5,8,11,14-tetraene-type fatty acid (5,8,11,14-TEFA), respectively. These structures are not found in plants, but in oomycetes and fungi, indicating that they are microbe molecular patterns recognized by plants. When Arabidopsis (Arabidopsis thaliana) was treated with Pi-Cer D and EPA, partially overlapping but different sets of genes were induced. Furthermore, expression of some genes is upregulated only after the simultaneous treatment with Pi-Cer D and EPA, indicating that plants combine the signals from simultaneously recognized MAMPs to adapt their defense response to pathogens.

Leucine rich repeat-malectin receptor kinases IGP1/CORK1, IGP3 and IGP4 are required for arabidopsis immune responses triggered by β-1,4-D-Xylo-oligosaccharides from plant cell walls

Pattern-Triggered Immunity (PTI) in plants is activated upon recognition by Pattern Recognition Receptors (PRRs) of Damage- and Microbe-Associated Molecular Patterns (DAMPs and MAMPs) from plants or microorganisms, respectively. An increasing number of identified DAMPs/MAMPs are carbohydrates from plant cell walls and microbial extracellular layers, which are perceived by plant PRRs, such as LysM and Leucine Rich Repeat-Malectin (LRR-MAL) receptor kinases (RKs). LysM-RKs (e.g. CERK1, LYK4 and LYK5) are needed for recognition of fungal MAMP chitohexaose (β-1,4-D-(GlcNAc)6, CHI6), whereas IGP1/CORK1, IGP3 and IGP4 LRR-MAL RKs are required for perception of β-glucans, like cellotriose (β-1,4-D-(Glc)3, CEL3) and mixed-linked glucans. We have explored the diversity of carbohydrates perceived by Arabidopsis thaliana seedlings by determining PTI responses upon treatment with different oligosaccharides and polysaccharides. These analyses revealed that plant oligosaccharides from xylans [β-1,4-D-(xylose)4 (XYL4)], glucuronoxylans and α-1,4-glucans, and polysaccharides from plants and seaweeds activate PTI. Cross-elicitation experiments of XYL4 with other glycans showed that the mechanism of recognition of XYL4 and the DAMP 33-α-L-arabinofuranosyl-xylotetraose (XA3XX) shares some features with that of CEL3 but differs from that of CHI6. Notably, XYL4 and XA3XX perception is impaired in igp1/cork1, igp3 and igp4 mutants, and almost not affected in cerk1 lyk4 lyk5 triple mutant. XYL4 perception is conserved in different plant species since XYL4 pre-treatment triggers enhanced disease resistance in tomato to Pseudomonas syringae pv tomato DC3000 and PTI responses in wheat. These results expand the number of glycans triggering plant immunity and support IGP1/CORK1, IGP3 and IGP4 relevance in Arabidopsis thaliana glycans perception and PTI activation.

Significance Statement

The characterization of plant immune mechanisms involved in the perception of carbohydrate-based structures recognized as DAMPs/MAMPs is needed to further understand plant disease resistance modulation. We show here that IGP1/CORK1, IGP3 and IGP4 LRR-MAL RKs are required for the perception of carbohydrate-based DAMPs β-1,4-D-(xylose)4 (XYL4) and 33-α-L-arabinofuranosyl-xylotetraose (XA3XX), further expanding the function of these LRR-MAL RKs in plant glycan perception and immune activation.

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.

Characterization of carboxylated cellulose nanofibrils and oligosaccharides from Kraft pulp fibers and their potential elicitor effect on the gene expression of Capsicum annuum

Biomass-derived oligo- and polysaccharides may act as elicitors, i.e., bioactive molecules that trigger plant immune responses. This is particularly important to increase the resistance of plants to abiotic and biotic stresses. In this study, cellulose nanofibrils (CNF) gels were obtained by TEMPO-mediated oxidation of unbleached and bleached kraft pulps. The molecular structures were characterized with ESI and MALDI MS. Analysis of the fine sequences was achieved by MS and MS/MS of the water-soluble oligosaccharides obtained by acid hydrolysis of the CNF gels. The analysis revealed the presence of two families: one corresponding to homoglucuronic acid sequences and the other composed by alternating glucose and glucuronic acid units. The CNF gels, alone or with the addition of the water-soluble oligosaccharides, were tested on Chili pepper (Capsicum annuum). Based on the characterization of the gene expression with Next Generation Sequencing (NGS) of the C. annuum's total messenger RNA, the differences in growth of the C. annuum seeds correlated well with the downregulation of the pathways regulating photosynthesis. A downregulation of the response to abiotic factors was detected, suggesting that these gels would improve the resistance of the C. annuum plants to abiotic stress due to, e.g., water deprivation and cold temperatures.

Mechanochemical Hydrolysis of Polysaccharide Biomass: Scope and Mechanistic Insights

Mechanical forces can affect chemical reactions in a way that thermal reactions cannot do, which may have a variety of applications. In biomass conversion, the selective conversion of cellulose and chitin is a grand challenge because they are the top two most abundant resources and recalcitrant materials that are insoluble in common solvents. However, recent works have clarified that mechanical forces enable the depolymerization of these polysaccharides, leading to the selective production of corresponding monomers and oligomers. This article reviews the mechanochemical hydrolysis of cellulose and chitin, particularly focusing on the scope and mechanisms to show a landscape of this research field and future subjects. We introduce the background of mechanochemistry and biomass conversion, followed by recent progress on the mechanochemical hydrolysis of the polysaccharides. Afterwards, a considerable space is devoted to the mechanistic consideration on the mechanochemical reactions.