The recognition of oligosaccharide elicitors, which originate from both pathogens and hosts, by membrane-localized receptors is of fundamental importance for triggering host immunity and disease resistance. It is therefore a topic of utmost significance for agricultural and botanical research.
Introduction
By 2100, addressing global food production will become imperative for satisfying the demands of a burgeoning population that is projected to reach -10.9 billion. Sustainable crop management hinges upon a comprehensive understanding of plant defense mechanisms. Plant diseases account for approximately 30% of annual losses in crop production, highlighting the importance of these mechanisms. To protect themselves against potentially harmful microbes, plants have evolved a complex and multi-faceted innate immune system. These molecular patterns can be released through the action of cell-wall-degrading enzymes (CWDEs) that are secreted by the invading pathogens or by the plants themselves as part of their defense response.
The plant cell wall, composed of polysaccharides like cellulose, hemicellulose, and pectin, serves as an effective physical barrier against microbial pathogens. When microbial CWDEs act on glycans of the plant cell wall, hydrolysis occurs, releasing various oligosaccharides. These oligosaccharides, derived from larger polysaccharides, exhibit linear or branched structures and are composed of D-monosaccharides (e.g., glucose [Glc], xylose [Xyl], and arabinose [Ara]), typically linked by β-1,4, β-1,3, or α-1,4 bonds. Notably, the parent polysaccharides have often undergone biochemical modifications (e.g., acetylation, methylation, and esterification) that influence oligosaccharide structure and function. For example, acetylated or methylesterified residues in pectin can alter how CWDEs hydrolyze polysaccharides, shaping the composition of the released oligosaccharides. This complexity, rooted in both glycosidic linkages and chemical modifications, renders plant cell-wall-derived oligosaccharides diverse in structure and function. Unbranched mixed-linked glucans (MLGs) such as β-1,4-D-(Glc)2-β-1,3-D-Glc (MLG43) are found not only in plant cell walls but also in those of oomycetes and microbes. Furthermore, certain plant storage oligosaccharides that are absent from cell walls, such as fructooligosaccharides derived from fructans, can be perceived by plant cells and initiate immune responses. In addition to those from terrestrial plants, oligosaccharides from marine plant cell-wall polysaccharides, such as alginate and fucoidan, have been shown to activate defensive responses in some plant species.
During plant-pathogen interactions, the cell walls and extracellular outer layers of microorganisms serve as significant sources of oligosaccharide elicitors that are perceived as MAMPs. For instance, chitin oligosaccharides (CTOS), linear β-1,3-glucans, β-1,3-glucans with β-1,6-glucan branches, and β-1,6-glucan oligosaccharides are released from the cell walls of fungi/oomycetes, and peptidoglycans (PGNs) and extracellular polysaccharides are released from bacteria. These biocompatible and biodegradable oligosaccharides can trigger a complex series of signaling events within plants, including reactive oxygen species (ROS) bursts, calcium (Ca2+) influx, mitogen-activated protein kinase (MAPK) activation, callose deposition, and upregulation of defense-related genes.
Sources of oligosaccharides as immune elicitors
Chitin oligosaccharides
Chitin, the second-most abundant natural biopolymer after cellulose, is an insoluble β-1,4-linked homopolymer of N-acetyl-D-glucosamine (GlcNAc). It is a fundamental structural component of fungal cell walls and the exoskeletons of insects, nematodes, and crustaceans. During plant-pathogen interactions, fungal attack triggers plants to secrete degrading proteases (e.g., chitinases and EC3.2.1.14) into the apoplastic space. These enzymes degrade chitin into bioactive fragments such as CTOS, which act as MAMPs to initiate plant immune responses.
CTOS are recognized by plant PRRs, initiating PTI. In rice suspension cells, CTOS induce dose- and polymerization-dependent cell death and defense gene expression, as well as MAPK activation and ROS production. Recent years have seen a growing agricultural interest in CTOS, driven by their biocompatibility, biodegradability, non-toxicity, and broad-spectrum biological activities with no reported environmental risks. Treatment with CTOS enhances resistance against fungi, oomycetes, bacteria, viruses, and nematode pathogens in diverse plant species, including crops. Notably, CTOS can induce plant systemic immunity: root perception of CTOS primes leaf PTI, enabling plants to mount more robust defenses against subsequent pathogen attacks. Soil amendment with CTOS has been shown to protect lettuce, tomato, and Arabidopsis against Pseudomonas syringae pv. tomato DC3000 (Pst) and wheat against Blumeria graminis (powdery mildew).
Chitosan oligosaccharides
Chitosan, the deacetylated product of chitin, is a linear polysaccharide composed of β-(1,4)-linked D-glucosamine. Chitosan oligosaccharides (CSOS), the primary degradation products of chitosan generated by chemical hydrolysis or enzymatic degradation, are fully water soluble owing to their shorter chain lengths and the free amino groups in their D-glucosamine residues. CSOS have been used extensively across diverse fields because of their low molecular weights, non-toxicity, rapid absorption, and stable biocompatibility. In agriculture, CSOS serve as potent plant immune activators for the eco-friendly control of plant diseases. For instance, CSOS induce dose-dependent cell death in tobacco cells, accompanied by H2O2 accumulation through CSOS-mediated apoptosis-like cell death independent of the H2O2 signaling pathway. In Arabidopsis and tobacco, CSOS activate the salicylic acid (SA) and Ca2+ signaling pathways, respectively, to enhance resistance against tobacco mosaic virus (TMV). In addition, CSOS enhance the resistance of carrot to Sclerotinia sclerotiorum, potato to Phytophthora infestans, tomato to Phytophthora nicotianae, and camellia to Colletotrichum camelliae. CSOS can mitigate abiotic as well as biotic stresses. For example, CSOS enhance the tolerance of edible rape (Brassica rapa L.) to cadmium (Cd) by promoting antioxidant enzyme activities and altering the subcellular distribution of Cd.
CSOS can also synergize with other compounds to amplify the induction effect. Binary mixtures of CSOS-propolis and CSOS-silver nanoparticles exhibit potent antifungal activity against Fusarium circinatum and Diplodia pinea, respectively, reducing their mycelial growth by approximately 80%. Combined biofungicides like ε-poly-L-lysine + CSOS combine direct antifungal activity against Botrytis cinerea with the induction of plant resistance, positioning them as promising biocontrol agents. The cytosinpeptidemycin-CSOS complex triggers ROS production and upregulates defense-responsive genes (PR1, PR5, FLS2, and Hsp70) while inducing TMV resistance in Nicotiana glutinosa. Collectively, these findings demonstrate that CSOS-based complexes can serve as versatile plant immune activators and eco-friendly fungicides for integrated disease management.
Alginate oligosaccharides
Alginate is a linear copolymer of (1,4)-linked β-D-mannuronate (M) and its C-5 epimer, α-L-guluronate (G). These monomers are arranged in diverse sequences, forming homopolymeric M-blocks (PolyM), homopolymeric G-blocks (PolyG), or heteropolymeric MG-blocks (PolyMG). As the most abundant polysaccharide in brown algae, comprising up to 40% of algal dry matter, alginate is harvested primarily for applications in medicine, food science, and agriculture. Alginate lyases are key enzymes for the production of alginate oligosaccharides (AOSs), which are functional oligosaccharides composed of 2-20 M/G units and low-molecular-weight alginates. These enzymes degrade alginate by β-elimination at the non-reducing end. For example, alginate lyase Alg7A derived from Vibrio sp. W1, which was characterized by thin-layer chromatography and ESI-MS, preferentially released trisaccharides from alginate, highlighting its potential for AOS production.
The biological activity of AOS is tightly linked to their DP and structure. For example, the disaccharide ΔG strongly induces glyceollin biosynthesis in soybean seeds. Low-molecular-weight alginates trigger defense responses in diverse plant species through SA-mediated signaling pathways, upregulating defense marker genes such as PAL (phenylalanine ammonia-lyase), PR1 (pathogenesis-related protein 1), SOD (superoxide dismutase), and DHN (dehydrin). Although AOSs show promise in biotechnological applications, their structural characterization remains challenging owing to their compositional complexity and heterogeneity.
Fucoidans from brown marine algae are polysaccharides composed predominantly of sulfated L-fucoses. A. thaliana can perceive glycan structures containing diverse monosaccharides, including L-fucose in fucoidans and D-mannuronic acid and L-guluronic acid in alginates. The unbranched β-1,3-glucans laminarin and laminarihexaose (β-1,3-(Glc)6) trigger robust immune responses in H. vulgare (barley) and Brachypodium distachyon. In the dicot Nicotiana benthamiana, laminarin elicits strong immune responses, whereas laminarihexaose induces a weaker Ca2+ influx and ROS burst.
Carrageenans are gel-forming linear sulfated galactans extracted from red marine algae, composed of D-galactose residues with alternating α-1,3- and β-1,4-linkages. They are classified into three main types based on sulfate substitution patterns and the 3,6-anhydro bridge in α-l,4-linked galactose: κ-carrageenan (3,6-anhydro-α-D-galactopyranosyl-1,4-4-sulfate-β-D-galactose), ι-carrageenan (2-sulfate-3,6-anhydro-α-D-galactopyranosyl-1,4-4-sulfate-β-D-galactose), and λ-carrageenan (2,6-sulfate-α-D-galactopyranosyl-1,4-2-sulfate-β-D-galactose). Among these, the κ-carrageenase Car19 hydrolysate protects cucumber plants against cucumber mosaic virus by suppressing virus replication and enhancing antioxidant enzyme activity in infected tissues.
Alginate Oligosaccharide (AOSs) are the degradation products of agar, a red algae cell-wall component, classified on the basis of their cleavage sites into agaroligosaccharides (reducing ends: β-D-galactose) and neoagarooligosaccharides (3,6-endo-α-L-galactose). According to their cleavage sites, AOSs induce disease resistance in peach fruit by activating antioxidant and phenylpropanoid metabolism pathways.
Other oligosaccharides
Glucans have emerged as key players in PTI, and in addition to cellooligomers and hemicellulose oligosaccharides, other glucans can also activate plant immunity. For example, β-1,2-glucan trisaccharide (B2G3) can trigger ROS production, MAPK phosphorylation, and differential expression of defense-related genes in Arabidopsis, maize, and wheat. Pretreatments with B2G3 improved these plants’ defense against fungal infections. Curdlan, a linear water-insoluble β-1,3-glucan produced by Agrobacterium sp. fermentation, is approved as a safe food additive. Pretreatment of potato plants with curdlan oligosaccharide 1 day before P. infestans infection significantly reduced the lesion area on potato leaves. Interestingly, the perception of short and long β-1,3-glucans differs among plant species. The leaves of N. benthamiana activate immunity in response to long β-1,3-glucans, whereas A. thaliana and Capsella rubella perceive short β-1,3-glucans, suggesting that the structural complexity of β-glucans contributes to differences in their recognition by plant species. One insufficiently studied type of β-glucan is pustulan (β-1,6-glucan), which can trigger a weak Ca2+ burst and MAPK activity in A. thaliana. In addition, A. thaliana can perceive α-1,4-glucan-derived oligosaccharides such as α-D-maltotetraose (MAL4) to trigger plant immune responses, and bacteria can differentially recognize and respond to α-1,4-D-glucans and cellooligomers by triggering distinct genome-wide transcriptional responses that have antagonistic effects on bacterial motility.
Pullulan is produced by Aureobasidium pullulans as an amorphous slime material consisting of maltotriose repeating units joined by α-1,6 linkages. Application of pullulan shows potential for the control of postharvest soft rot in kiwifruit, offering an effective strategy for enhancing storage stability and shelf life of this economically important fruit.
Burdock fructooligosaccharide (BFO), a natural elicitor isolated from Arcitum lappa root, is a linear chain of 12 β-(2,1)-linked fructofuranose residues with a single terminal α-(1,2)-linked glucopyranose unit. It shows significant potential as an elicitor for control of postharvest fruit disease. BFO induces the upregulation of NPR1, PR1, PAL, and STS genes; inhibits declines in total phenol content; and activates chitinase and β-1,3-glucanase enzymes. By triggering the SA-dependent signaling pathway, BFO suppresses postharvest browning in Kyoho grapes. It also enhances the resistance of tomatoes to B. cinerea, cucumbers to Colletotrichum orbiculare, and tobacco to TMV. Mechanistically, BFO-induced stomatal closure is mediated by ROS and ROS-dependent NO production.
Conclusion
These oligosaccharides, with their unique structures and properties, play a crucial role in modulating plant defense mechanisms. It is import to focus on the recognition receptors and complex immune signal transduction pathways associated with reported oligosaccharide activators. Understanding these molecular components and the signaling cascades they initiate is fundamental to deciphering how plants perceive and respond to oligosaccharide-mediated immune stimuli. Understanding the complex interactions between oligosaccharides and immune responses during plant-microbe interactions can facilitate sustainable agricultural development, crop improvement, and the prevention and control of plant diseases. This information can serve as a cornerstone for the development of innovative and eco-friendly agricultural practices that enhance crop productivity while minimizing the use of chemicals.
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