An Arabidopsis berberine bridge enzyme-like protein specifically oxidizes cellulose oligomers and plays a role in immunity

Between Host and Invaders: The Subcellular Cell Wall Dynamics at the Plant-Pathogen Interface

Plant-pathogen interactions have profound ecological implications and are crucial for food security. Usually studied at the two extreme scales of plant organ symptomatology and host-microbe molecules, they are a cell-cell event mainly occurring at the subcellular level of the plant apoplast. Here, the cell walls of both organisms suffer an intense alteration as a consequence of active degradation by the opponent and self-protection mechanisms to survive and continue growing. The plant cell wall modifications and their role in defense as danger signals and activators of signaling cascades have been studied for a few decades, mainly at the organ plane. Still, much remains unknown about this process, including cellular and subcellular minority decorations, proteins, and mechanical cues. Comparatively, the microbial cell wall changes in planta are virtually unexplored. By investigating the interface between plant and microbial cell walls biochemically, structurally, and mechanically, we aim to highlight the dynamic interplay in these subcellular areas and its significance for the host-invader interaction.

How it all begins: molecular players of the early graviresponse in the non-elongating part of flax stem

Plants have developed two major strategies to adjust their position in response to gravity: differential cell growth on opposing sides of elongating regions and complex processes in non-elongating stem parts, such as the development of reaction wood. Gravistimulation of flax plants induces gravitropic curvature in non-elongating stem parts, largely associated with modifications in phloem and xylem fibers. To gain insight into the key "triggers" and "forward players" that induce negative gravitropic reactions, transcriptome profiling of phloem fibers and xylem tissues from the pulling and opposite stem sides was conducted 1 and 8 h after gravistimulation. The first observed reaction was the activation of processes associated with RNA synthesis and protein folding in both tissues and stem sides, followed by the activation of kinases and transferases. Transcriptomic data revealed rapid and substantial shifts in chloroplast metabolism across all analyzed tissues, including the temporal activation of the branched-chain amino acid pathway, adjustments to light-harvesting complexes, and jasmonic acid biosynthesis. Notably, auxin transporter genes were activated only in the xylem, while other auxin-related genes showed minimal upregulation 1 h after stem inclination in any analyzed sample. Asymmetric changes between stem sides included the sharp activation of ethylene-related genes in the phloem fibers of the opposite stem side, as well as tertiary cell wall deposition in both the phloem and xylem fibers of the pulling stem side during the later stages of the graviresponse. These results provide valuable insights into the mechanisms underlying plant response to gravity.

Berberine bridge enzyme-like oxidases orchestrate homeostasis and signaling of oligogalacturonides in defense and upon mechanical damage

Plant immunity is triggered by endogenous elicitors known as damage-associated molecular patterns (DAMPs). Oligogalacturonides (OGs) are DAMPs released from the cell wall (CW) demethylated homogalacturonan during microbial colonization, mechanical or pest-provoked mechanical damage, and physiological CW remodeling. Berberine bridge enzyme-like (BBE-l) proteins named OG oxidases (OGOXs) oxidize and inactivate OGs to avoid deleterious growth-affecting hyper-immunity and possible cell death. Using OGOX1 over-expressing lines and ogox1/2 double mutants, we show that these enzymes determine the levels of active OGs vs. inactive oxidized products (ox-OGs). The ogox1/2-deficient plants have elevated levels of OGs, while plants overexpressing OGOX1 accumulate ox-OGs. The balance between OGs and ox-OGs affects disease resistance against Pseudomonas syringae pv. tomato, Pectobacterium carotovorum, and Botrytis cinerea depending on the microbial capacity to respond to OGs and metabolize ox-OGs. Gene expression upon plant infiltration with OGs reveals that OGOXs orchestrate OG signaling in defense as well as upon mechanical damage, pointing to these enzymes as apoplastic players in immunity and tissue repair.

Cell wall bricks of defence: the case study of oligogalacturonides

The plant cell wall (CW) is more than a structural barrier; it serves as the first line of defence against pathogens and environmental stresses. During pathogen attacks or physical damage, fragments of the CW, known as CW-derived Damage-Associated Molecular Patterns (CW-DAMPs), are released. These molecular signals play a critical role in activating the plant's immune responses. Among CW-DAMPs, oligogalacturonides (OGs), fragments derived from the breakdown of pectin, are some of the most well-studied. This review highlights recent advances in understanding the functional and signalling roles of OGs, beginning with their formation through enzymatic CW degradation during pathogen invasion or mechanical injury. We discuss how OGs perception triggers intracellular signalling pathways that enhance plant defence and regulate interactions with microbes. Given that excessive OG levels can negatively impact growth and development, we also examine the regulatory mechanisms plants use to fine-tune their responses, avoiding immune overactivation or hyper- immunity. As natural immune modulators, OGs (and more generally CW-DAMPs), offer a promising, sustainable alternative to chemical pesticides by enhancing crop resilience without harming the environment. By strengthening plant defences and supporting eco-friendly agricultural practices, OGs hold great potential for advancing resilient and sustainable farming systems.

The reducing end of cell wall oligosaccharides is critical for DAMP activity in Arabidopsis thaliana and can be exploited by oligosaccharide oxidases in the reduction of oxidized phenolics

The enzymatic hydrolysis of cell wall polysaccharides results in the production of oligosaccharides with nature of damage-associated molecular patterns (DAMPs) that are perceived by plants as danger signals. The in vitro oxidation of oligogalacturonides and cellodextrins by plant FAD-dependent oligosaccharide-oxidases (OSOXs) suppresses their elicitor activity in vivo, suggesting a protective role of OSOXs against a prolonged activation of defense responses potentially deleterious for plant health. However, OSOXs are also produced by phytopathogens and saprotrophs, complicating the understanding of their role in plant-microbe interactions. Here, we demonstrate the oxidation catalyzed by specific fungal OSOXs also converts the elicitor-active cello-tetraose and xylo-tetraose into elicitor-inactive forms, indicating that the oxidation state of cell wall oligosaccharides is crucial for their DAMP function, irrespective of whether the OSOX originates from fungi or plants. In addition, we also found that certain OSOXs can transfer the electrons from the reducing end of these oligosaccharides to oxidized phenolics (bi-phenoquinones) instead of molecular O2, highlighting an unexpected sub-functionalization of these enzymes. The activity of OSOXs may be crucial for a thorough understanding of cell wall metabolism since these enzymes can redirect the reducing power from sugars to phenolic components of the plant cell wall, an insight with relevant implications for plant physiology and biotechnology.

Cell walls, a comparative view of the composition of cell surfaces of plants, algae and microorganisms

While evolutionary studies indicate that the most ancient groups of organisms on Earth likely descended from a common wall-less ancestor, contemporary organisms lacking a carbohydrate-rich cell surface are exceedingly rare. By developing a cell wall to cover the plasma membrane, cells were able to withstand higher osmotic pressures, colonise new habitats and develop complex multicellular structures. This way, the cells of plants, algae and microorganisms are covered by a cell wall, which can generally be defined as a highly complex structure whose main framework is usually composed of carbohydrates. Rather than static structures, they are highly dynamic and serve a multitude of functions that modulate vital cellular processes, such as growth and interactions with neighbouring cells or the surrounding environment. Thus, despite its vital importance for many groups of life, it is striking that there are few comprehensive documents comparing the cell wall composition of these groups. Thus, the aim of this review was to compare the cell walls of plants with those of algae and microorganisms, paying particular attention to their polysaccharide components. It should be highlighted that, despite the important differences in composition, we have also found numerous common aspects and functionalities.

Linear β-1,2-glucans trigger immune hallmarks and enhance disease resistance in plants

Immune responses in plants are triggered by molecular patterns or elicitors, recognized by plant pattern recognition receptors. Such molecular patterns are the consequence of host-pathogen interactions, and the response cascade activated after their perception is known as pattern-triggered immunity (PTI). Glucans have emerged as key players in PTI, but the ability of certain glucans to stimulate defensive responses in plants remains understudied. This work focused on identifying novel glucan oligosaccharides as molecular patterns. The ability of various microorganism-derived glucans to trigger PTI responses was tested, revealing that specific microbial-derived molecules, such as short linear β-1,2-glucans, trigger this response in plants by increasing the production of reactive oxygen species (ROS), mitogen-activated protein kinase phosphorylation, and differential expression of defence-related genes in Arabidopsis thaliana. Pre-treatments with β-1,2-glucan trisaccharide (B2G3) improved Arabidopsis defence against bacterial and fungal infections in a hypersusceptible genotype. The knowledge generated was then transferred to the monocotyledonous model species maize and wheat, demonstrating that these plants also respond to β-1,2-glucans, with increased ROS production and improved protection against fungal infections following B2G3 pre-treatments. In summary, as with other β-glucans, plants perceive β-1,2-glucans as warning signals which stimulate defence responses against phytopathogens.

Plant cell walls: source of carbohydrate-based signals in plant-pathogen interactions

Plant cell walls are essential elements for disease resistance that pathogens need to overcome to colonise the host. Certain pathogens secrete a large battery of enzymes to hydrolyse plant cell wall polysaccharides, which leads to the release of carbohydrate-based molecules (glycans) that are perceived by plant pattern recognition receptors and activate pattern-triggered immunity and disease resistance. These released glycans are used by colonizing microorganisms as carbon source, chemoattractants to locate entry points at plant surface, and as signals triggering gene expression reprogramming. The release of wall glycans and their perception by plants and microorganisms determines plant-microbial interaction outcome. Here, we summarise and discuss the most recent advances in these less explored aspects of plant-microbe interaction.

Enzymatic Routes to Designer Hemicelluloses for Use in Biobased Materials

Various enzymes can be used to modify the structure of hemicelluloses directly in vivo or following extraction from biomass sources, such as wood and agricultural residues. Generally, these enzymes can contribute to designer hemicelluloses through four main strategies: (1) enzymatic hydrolysis such as selective removal of side groups by glycoside hydrolases (GH) and carbohydrate esterases (CE), (2) enzymatic cross-linking, for instance, the selective addition of side groups by glycosyltransferases (GT) with activated sugars, (3) enzymatic polymerization by glycosynthases (GS) with activated glycosyl donors or transglycosylation, and (4) enzymatic functionalization, particularly via oxidation by carbohydrate oxidoreductases and via amination by amine transaminases. Thus, this Perspective will first highlight enzymes that play a role in regulating the degree of polymerization and side group composition of hemicelluloses, and subsequently, it will explore enzymes that enhance cross-linking capabilities and incorporate novel chemical functionalities into saccharide structures. These enzymatic routes offer a precise way to tailor the properties of hemicelluloses for specific applications in biobased materials, contributing to the development of renewable alternatives to conventional materials derived from fossil fuels.

The Vacuolar H+-ATPase subunit C is involved in oligogalacturonide (OG) internalization and OG-triggered immunity

In plants, the perception of cell wall fragments initiates signal transduction cascades that activate the immune response. Previous research on early protein dynamics induced by oligogalacturonides (OGs), pectin fragments acting as damage-associated molecular patterns (DAMPs), revealed significant phosphorylation changes in several proteins. Among them, the subunit C of the vacuolar H+-ATPase, known as DE-ETIOLATED 3 (DET3), was selected to elucidate its role in the OG-triggered immune response. The Arabidopsis det3 knockdown mutant exhibited defects in H2O2 accumulation, mitogen-activated protein kinases (MAPKs) activation, and induction of defense marker genes in response to OG treatment. Interestingly, the det3 mutant showed a higher basal resistance to the fungal pathogen Botrytis cinerea that, in turn, was completely reversed by the pre-treatment with OGs. Our results suggest a compromised ability of the det3 mutant to maintain a primed state over time, leading to a weaker defense response when the plant is later exposed to the fungal pathogen. Using fluorescently labelled OGs, we demonstrated that endocytosis of OGs was less efficient in the det3 mutant, implicating DET3 in the internalization process of OGs. This impairment aligns with the observed defect in the priming response in the det3 mutant, underscoring that proper internalization and signaling of OGs are crucial for initiating and maintaining a primed state in plant defense responses.

Extracellular vesicles of Norway spruce contain precursors and enzymes for lignin formation and salicylic acid

Lignin is a phenolic polymer in plants that rigidifies the cell walls of water-conducting tracheary elements and support-providing fibers and stone cells. Different mechanisms have been suggested for the transport of lignin precursors to the site of lignification in the cell wall. Extracellular vesicle (EV)-enriched samples isolated from a lignin-forming cell suspension culture of Norway spruce (Picea abies L. Karst.) contained both phenolic metabolites and enzymes related to lignin biosynthesis. Metabolomic analysis revealed mono-, di-, and oligolignols in the EV isolates, as well as carbohydrates and amino acids. In addition, salicylic acid (SA) and some proteins involved in SA signaling were detected in the EV-enriched samples. A proteomic analysis detected several laccases, peroxidases, β-glucosidases, putative dirigent proteins, and cell wall-modifying enzymes, such as glycosyl hydrolases, transglucosylase/hydrolases, and expansins in EVs. Our findings suggest that EVs are involved in transporting enzymes required for lignin polymerization in Norway spruce, and radical coupling of monolignols can occur in these vesicles.

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.

Metabolic Aspects of Lentil-Fusarium Interactions

Fusarium oxysporum f. sp. lentis (Fol) is considered the most destructive disease for lentil (Lens culinaris Medik.) worldwide. Despite the extensive studies elucidating plants' metabolic response to fungal agents, there is a knowledge gap in the biochemical mechanisms governing Fol-resistance in lentil. Τhis study aimed at comparatively evaluating the metabolic response of two lentil genotypes, with contrasting phenotypes for Fol-resistance, to Fol-inoculation. Apart from gaining insights into the metabolic reprogramming in response to Fol-inoculation, the study focused on discovering novel biomarkers to improve early selection for Fol-resistance. GC-MS-mediated metabolic profiling of leaves and roots was employed to monitor changes across genotypes and treatments as well as their interaction. In total, the analysis yielded 178 quantifiable compounds, of which the vast majority belonged to the groups of carbohydrates, amino acids, polyols and organic acids. Despite the magnitude of metabolic fluctuations in response to Fol-inoculation in both genotypes under study, significant alterations were noted in the content of 18 compounds, of which 10 and 8 compounds referred to roots and shoots, respectively. Overall data underline the crucial contribution of palatinitol and L-proline in the metabolic response of roots and shoots, respectively, thus offering possibilities for their exploitation as metabolic biomarkers for Fol-resistance in lentil. To the best of our knowledge, this is the first metabolomics-based approach to unraveling the effects of Fol-inoculation on lentil's metabolome, thus providing crucial information related to key aspects of lentil-Fol interaction. Future investigations in metabolic aspects of lentil-Fol interactions will undoubtedly revolutionize the search for metabolites underlying Fol-resistance, thus paving the way towards upgrading breeding efforts to combat fusarium wilt in lentil.

Oligogalacturonide application increases resistance to Fusarium head blight in durum wheat

Fusariosis causes substantial yield losses in the wheat crop worldwide and compromises food safety because of the presence of toxins associated with the fungal disease. Among the current approaches to crop protection, the use of elicitors able to activate natural defense mechanisms in plants is a strategy gaining increasing attention. Several studies indicate that applications of plant cell-wall-derived elicitors, such as oligogalacturonides (OGs) derived from partial degradation of pectin, induce local and systemic resistance against plant pathogens. The aim of this study was to establish the efficacy of OGs in protecting durum wheat (Triticum turgidum subsp. durum), which is characterized by an extreme susceptibility to Fusarium graminearum. To evaluate the functionality of OGs, spikes and seedlings of cv. Svevo were inoculated with OGs, F. graminearum spores, and a co-treatment of both. Results demonstrated that OGs are active elicitors of wheat defenses, triggering typical immune marker genes and determining regulation of fungal genes. Moreover, bioassays on spikes and transcriptomic analyses on seedlings showed that OGs can regulate relevant physiological processes in Svevo with dose-dependent specificity. Thus, the OG sensing system plays an important role in fine tuning immune signaling pathways in durum wheat.

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.

Pattern recognition receptors as potential therapeutic targets for developing immunological engineered plants

There is an urgent need to combat pathogen infestations in crop plants to ensure food security worldwide. To counter this, plants have developed innate immunity mediated by Pattern Recognition Receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and damage- associated molecular patterns (DAMPs). PRRs activate Pattern-Triggered Immunity (PTI), a defence mechanism involving intricate cell-surface and intracellular receptors. The diverse ligand-binding ectodomains of PRRs, including leucine-rich repeats (LRRs) and lectin domains, facilitate the recognition of MAMPs and DAMPs. Pathogen resistance is mediated by a variety of PTI responses, including membrane depolarization, ROS production, and the induction of defence genes. An integral part of intracellular immunity is the Nucleotide-binding Oligomerization Domain, Leucine-rich Repeat proteins (NLRs) which recognize and respond to effectors in a potent manner. Enhanced understanding of PRRs, their ligands, and downstream signalling pathways has contributed to the identification of potential targets for genetically modified plants. By transferring PRRs across plant species, it is possible to create broad-spectrum resistance, potentially offering innovative solutions for plant protection and global food security. The purpose of this chapter is to provide an update on PRRs involved in disease resistance, clarify the mechanisms by which PRRs recognize ligands to form active receptor complexes and present various applications of PRRs and PTI in disease resistance management for plants.

Cacao pod transcriptome profiling of seven genotypes identifies features associated with post-penetration resistance to Phytophthora palmivora

The oomycete Phytophthora palmivora infects the fruit of cacao trees (Theobroma cacao) causing black pod rot and reducing yields. Cacao genotypes vary in their resistance levels to P. palmivora, yet our understanding of how cacao fruit respond to the pathogen at the molecular level during disease establishment is limited. To address this issue, disease development and RNA-Seq studies were conducted on pods of seven cacao genotypes (ICS1, WFT, Gu133, Spa9, CCN51, Sca6 and Pound7) to better understand their reactions to the post-penetration stage of P. palmivora infection. The pod tissue-P. palmivora pathogen assay resulted in the genotypes being classified as susceptible (ICS1, WFT, Gu133 and Spa9) or resistant (CCN51, Sca6 and Pound7). The number of differentially expressed genes (DEGs) ranged from 1625 to 6957 depending on genotype. A custom gene correlation approach identified 34 correlation groups. De novo motif analysis was conducted on upstream promoter sequences of differentially expressed genes, identifying 76 novel motifs, 31 of which were over-represented in the upstream sequences of correlation groups and associated with gene ontology terms related to oxidative stress response, defense against fungal pathogens, general metabolism and cell function. Genes in one correlation group (Group 6) were strongly induced in all genotypes and enriched in genes annotated with defense-responsive terms. Expression pattern profiling revealed that genes in Group 6 were induced to higher levels in the resistant genotypes. An additional analysis allowed the identification of 17 candidate cis-regulatory modules likely to be involved in cacao defense against P. palmivora. This study is a comprehensive exploration of the cacao pod transcriptional response to P. palmivora spread after infection. We identified cacao genes, promoter motifs, and promoter motif combinations associated with post-penetration resistance to P. palmivora in cacao pods and provide this information as a resource to support future and ongoing efforts to breed P. palmivora-resistant cacao.

A combination of conserved and diverged responses underlies Theobroma cacao's defense response to Phytophthora palmivora

BACKGROUND

Plants have complex and dynamic immune systems that have evolved to resist pathogens. Humans have worked to enhance these defenses in crops through breeding. However, many crops harbor only a fraction of the genetic diversity present in wild relatives. Increased utilization of diverse germplasm to search for desirable traits, such as disease resistance, is therefore a valuable step towards breeding crops that are adapted to both current and emerging threats. Here, we examine diversity of defense responses across four populations of the long-generation tree crop Theobroma cacao L., as well as four non-cacao Theobroma species, with the goal of identifying genetic elements essential for protection against the oomycete pathogen Phytophthora palmivora.

RESULTS

We began by creating a new, highly contiguous genome assembly for the P. palmivora-resistant genotype SCA 6 (Additional file 1: Tables S1-S5), deposited in GenBank under accessions CP139290-CP139299. We then used this high-quality assembly to combine RNA and whole-genome sequencing data to discover several genes and pathways associated with resistance. Many of these are unique, i.e., differentially regulated in only one of the four populations (diverged 40 k-900 k generations). Among the pathways shared across all populations is phenylpropanoid biosynthesis, a metabolic pathway with well-documented roles in plant defense. One gene in this pathway, caffeoyl shikimate esterase (CSE), was upregulated across all four populations following pathogen treatment, indicating its broad importance for cacao's defense response. Further experimental evidence suggests this gene hydrolyzes caffeoyl shikimate to create caffeic acid, an antimicrobial compound and known inhibitor of Phytophthora spp.

CONCLUSIONS

Our results indicate most expression variation associated with resistance is unique to populations. Moreover, our findings demonstrate the value of using a broad sample of evolutionarily diverged populations for revealing the genetic bases of cacao resistance to P. palmivora. This approach has promise for further revealing and harnessing valuable genetic resources in this and other long-generation plants.

The damage-associated molecular pattern cellotriose alters the phosphorylation pattern of proteins involved in cellulose synthesis and trans-Golgi trafficking in Arabidopsis thaliana

We have recently demonstrated that the cellulose breakdown product cellotriose is a damage-associated molecular pattern (DAMP) which induces responses related to the integrity of the cell wall. Activation of downstream responses requires the Arabidopsis malectin domain-containing CELLOOLIGOMER RECEPTOR KINASE1 (CORK1)¹. The cellotriose/CORK1 pathway induces immune responses, including NADPH oxidase-mediated reactive oxygen species production, mitogen-activated protein kinase 3/6 phosphorylation-dependent defense gene activation, and the biosynthesis of defense hormones. However, apoplastic accumulation of cell wall breakdown products should also activate cell wall repair mechanisms. We demonstrate that the phosphorylation pattern of numerous proteins involved in the accumulation of an active cellulose synthase complex in the plasma membrane and those for protein trafficking to and within the trans-Golgi network (TGN) are altered within minutes after cellotriose application to Arabidopsis roots. The phosphorylation pattern of enzymes involved in hemicellulose or pectin biosynthesis and the transcript levels for polysaccharide-synthesizing enzymes responded barely to cellotriose treatments. Our data show that the phosphorylation pattern of proteins involved in cellulose biosynthesis and trans-Golgi trafficking is an early target of the cellotriose/CORK1 pathway.

Cuscuta campestris fine-tunes gene expression during haustoriogenesis as an adaptation to different hosts

The Cuscuta genus comprises obligate parasitic plants that have an unusually wide host range. Whether Cuscuta uses different infection strategies for different hosts or whether the infection strategy is mechanistically and enzymatically conserved remains unknown. To address this, we investigated molecular events during the interaction between field dodder (Cuscuta campestris) and two host species of the Solanum genus that are known to react differently to parasitic infection. We found that host gene induction, particularly of cell wall fortifying genes, coincided with a differential induction of genes for cell wall degradation in the parasite in the cultivated tomato (Solanum lycopersicum) but not in a wild relative (Solanum pennellii). This indicates that the parasite can adjust its gene expression in response to its host. This idea was supported by the increased expression of C. campestris genes encoding an endo-β-1,4-mannanase in response to exposure of the parasite to purified mono- and polysaccharides in a host-independent infection system. Our results suggest multiple key roles of the host cell wall in determining the outcome of an infection attempt.

Berberine bridge enzyme-like oxidases of cellodextrins and mixed-linked β-glucans control seed coat formation

Plants have evolved various resistance mechanisms to cope with biotic stresses that threaten their survival. The BBE23 member (At5g44360/BBE23) of the Arabidopsis berberine bridge enzyme-like (BBE-l) protein family (Arabidopsis thaliana) has been characterized in this paper in parallel with the closely related and previously described CELLOX (At4g20860/BBE22). In addition to cellodextrins, both enzymes, renamed here as CELLODEXTRIN OXIDASE 2 and 1 (CELLOX2 and CELLOX1), respectively, oxidize the mixed-linked β-1→3/β-1→4-glucans (MLGs), recently described as capable of activating plant immunity, reinforcing the view that the BBE-l family includes members that are devoted to the control of the homeostasis of potential cell wall-derived damage-associated molecular patterns (DAMPs). The 2 putatively paralogous genes display different expression profiles. Unlike CELLOX1, CELLOX2 is not expressed in seedlings or adult plants and is not involved in immunity against Botrytis cinerea. Both are instead expressed in a concerted manner in the seed coat during development. Whereas CELLOX2 is expressed mainly during the heart stage, CELLOX1 is expressed at the immediately later stage, when the expression of CELLOX2 decreases. Analysis of seeds of cellox1 and cellox2 knockout mutants shows alterations in the coat structure: the columella area is smaller in cellox1, radial cell walls are thicker in both cellox1 and cellox2, and the mucilage halo is reduced in cellox2. However, the coat monosaccharide composition is not significantly altered, suggesting an alteration of the organization of the cell wall, thus reinforcing the notion that the architecture of the cell wall in specific organs is determined not only by the dynamics of the synthesis/degradation of the main polysaccharides but also by its enzymatic oxidation.

Genome-Wide Characterization of Berberine Bridge Enzyme Gene Family in Wheat (Triticum aestivum L.) and the Positive Regulatory Role of TaBBE64 in Response to Wheat Stripe Rust

Berberine bridge enzymes (BBEs), functioning as carbonate oxidases, enhance disease resistance in Arabidopsis and tobacco. However, the understanding of BBEs' role in monocots against pathogens remains limited. This study identified 81 TaBBEs with FAD_binding_4 and BBE domains. Phylogenetic analysis revealed a separation of the BBE gene family between monocots and dicots. Notably, RNA-seq showed TaBBE64's significant induction by both pathogen-associated molecular pattern treatment and Puccinia striiformis f. sp. tritici (Pst) infection at early stages. Subcellular localization revealed TaBBE64 at the cytoplasmic membrane. Knocking down TaBBE64 compromised wheat's Pst resistance, reducing reactive oxygen species and promoting fungal growth, confirming its positive role. Molecular docking and enzyme activity assays confirmed TaBBE64's glucose oxidation to produce H2O2. Since Pst relies on living wheat cells for carbohydrate absorption, TaBBE64's promotion of glucose oxidation limits fungal growth and resists pathogen infection. This study sheds light on BBEs' role in wheat resistance against biotrophic fungi.

A GH81-type β-glucan-binding protein enhances colonization by mutualistic fungi in barley

Cell walls are important interfaces of plant-fungal interactions, acting as robust physical and chemical barriers against invaders. Upon fungal colonization, plants deposit phenolics and callose at the sites of fungal penetration to prevent further fungal progression. Alterations in the composition of plant cell walls significantly impact host susceptibility. Furthermore, plants and fungi secrete glycan hydrolases acting on each other's cell walls. These enzymes release various sugar oligomers into the apoplast, some of which activate host immunity via surface receptors. Recent characterization of cell walls from plant-colonizing fungi has emphasized the abundance of β-glucans in different cell wall layers, which makes them suitable targets for recognition. To characterize host components involved in immunity against fungi, we performed a protein pull-down with the biotinylated β-glucan laminarin. Thereby, we identified a plant glycoside hydrolase family 81-type glucan-binding protein (GBP) as a β-glucan interactor. Mutation of GBP1 and its only paralog, GBP2, in barley led to decreased colonization by the beneficial root endophytes Serendipita indica and S. vermifera, as well as the arbuscular mycorrhizal fungus Rhizophagus irregularis. The reduction of colonization was accompanied by enhanced responses at the host cell wall, including an extension of callose-containing cell wall appositions. Moreover, GBP mutation in barley also reduced fungal biomass in roots by the hemibiotrophic pathogen Bipolaris sorokiniana and inhibited the penetration success of the obligate biotrophic leaf pathogen Blumeria hordei. These results indicate that GBP1 is involved in the establishment of symbiotic associations with beneficial fungi-a role that has potentially been appropriated by barley-adapted pathogens.

Subcritical water extraction of Equisetum arvense biomass withdraws cell wall fractions that trigger plant immune responses and disease resistance

Plant cell walls are complex structures mainly made up of carbohydrate and phenolic polymers. In addition to their structural roles, cell walls function as external barriers against pathogens and are also reservoirs of glycan structures that can be perceived by plant receptors, activating Pattern-Triggered Immunity (PTI). Since these PTI-active glycans are usually released upon plant cell wall degradation, they are classified as Damage Associated Molecular Patterns (DAMPs). Identification of DAMPs imply their extraction from plant cell walls by using multistep methodologies and hazardous chemicals. Subcritical water extraction (SWE) has been shown to be an environmentally sustainable alternative and a simplified methodology for the generation of glycan-enriched fractions from different cell wall sources, since it only involves the use of water. Starting from Equisetum arvense cell walls, we have explored two different SWE sequential extractions (isothermal at 160 ºC and using a ramp of temperature from 100 to 160 ºC) to obtain glycans-enriched fractions, and we have compared them with those generated with a standard chemical-based wall extraction. We obtained SWE fractions enriched in pectins that triggered PTI hallmarks in Arabidopsis thaliana such as calcium influxes, reactive oxygen species production, phosphorylation of mitogen activated protein kinases and overexpression of immune-related genes. Notably, application of selected SWE fractions to pepper plants enhanced their disease resistance against the fungal pathogen Sclerotinia sclerotiorum. These data support the potential of SWE technology in extracting PTI-active fractions from plant cell wall biomass containing DAMPs and the use of SWE fractions in sustainable crop production.

Structural insights of cell wall integrity signaling during development and immunity

A communication system between plant cells and their surrounding cell wall is required to coordinate development, immunity, and the integration of environmental cues. This communication network is facilitated by a large pool of membrane- and cell-wall-anchored proteins that can potentially interact with the matrix or its fragments, promoting cell wall patterning or eliciting cellular responses that may lead to changes in the architecture and chemistry of the wall. A mechanistic understanding of how these receptors and cell wall proteins recognize and interact with cell wall epitopes would be key to a better understanding of all plant processes that require cell wall remodeling such as expansion, morphogenesis, and defense responses. This review focuses on the latest developments in structurally and biochemically characterized receptors and protein complexes implicated in reading and regulating cell wall integrity and immunity.

The power of patterns: new insights into pattern-triggered immunity

The plant immune system features numerous immune receptors localized on the cell surface to monitor the apoplastic space for danger signals from a broad range of plant colonizers. Recent discoveries shed light on the enormous complexity of molecular signals sensed by these receptors, how they are generated and removed to maintain cellular homeostasis and immunocompetence, and how they are shaped by host-imposed evolutionary constraints. Fine-tuning receptor sensing mechanisms at the molecular, cellular and physiological level is critical for maintaining a robust but adaptive host barrier to commensal, pathogenic, and symbiotic colonizers alike. These receptors are at the core of any plant-colonizer interaction and hold great potential for engineering disease resistance and harnessing beneficial microbiota to keep crops healthy.

Mapping of prehaustorial resistance against wheat leaf rust in einkorn (Triticum monococcum), a progenitor of wheat

Wheat leaf rust (Puccinia triticina) is one of the most significant fungal diseases of wheat, causing substantial yield losses worldwide. Infestation is currently being reduced by fungicide treatments and mostly vertical resistance. However, these measures often break down when the fungal virulence pattern changes, resulting in a breakdown of vertical resistances. In contrast, the prehaustorial resistance (phr) that occurs in the einkorn-wheat leaf rust interaction is race-independent, characterized by an early defense response of plants during the prehaustorial phase of infestation. Einkorn (Triticum monococcum) is closely related to Triticum urartu as a progenitor of wheat and generally shows a high level of resistance against leaf rust of wheat. Hence, einkorn can serve as a valuable source to improve the level of resistance to the pathogen in future wheat lines. In particular, einkorn accession PI272560 is known to exhibit a hypersensitive prehaustorial effector triggered immune reaction, preventing the infection of P. triticina. Remarkably, this effector-triggered immune reaction turned out to be atypical as it is non-race-specific (horizontal). To genetically dissect the prehaustorial resistance (phr) in PI272560, a biparental F2 population of 182 plants was established after crossing PI272560 with the susceptible T. boeoticum accession 36554. Three genetic maps comprising 2,465 DArT-seq markers were constructed, and a major QTL was detected on chromosome 5A. To locate underlying candidate genes, marker sequences flanking the respective QTL were aligned to the T. urartu reference genome and transcriptome data available from the parental accessions were used. Within the QTL interval of approximately 16.13 million base pairs, the expression of genes under inoculated and non-inoculated conditions was analyzed via a massive analysis of cDNA (MACE). Remarkably, a single gene located 3.4 Mbp from the peak marker within the major QTL was upregulated (20- to 95-fold) after the inoculation in the resistant accession in comparison to the susceptible T. boeoticum accession. This gene belongs to a berberine bridge enzyme-like protein that is suspected to interact on the plant surface with glycoside hydrolases (GH) secreted by the fungus and to induce a hypersensitive defense reaction in the plant after fungal infections.

Signals and Their Perception for Remodelling, Adjustment and Repair of the Plant Cell Wall

The integrity of the cell wall is important for plant cells. Mechanical or chemical distortions, tension, pH changes in the apoplast, disturbance of the ion homeostasis, leakage of cell compounds into the apoplastic space or breakdown of cell wall polysaccharides activate cellular responses which often occur via plasma membrane-localized receptors. Breakdown products of the cell wall polysaccharides function as damage-associated molecular patterns and derive from cellulose (cello-oligomers), hemicelluloses (mainly xyloglucans and mixed-linkage glucans as well as glucuronoarabinoglucans in Poaceae) and pectins (oligogalacturonides). In addition, several types of channels participate in mechanosensing and convert physical into chemical signals. To establish a proper response, the cell has to integrate information about apoplastic alterations and disturbance of its wall with cell-internal programs which require modifications in the wall architecture due to growth, differentiation or cell division. We summarize recent progress in pattern recognition receptors for plant-derived oligosaccharides, with a focus on malectin domain-containing receptor kinases and their crosstalk with other perception systems and intracellular signaling events.

Radical cation scavenging activity of berberine bridge enzyme-like oligosaccharide oxidases acting on short cell wall fragments

Oligogalacturonide-oxidases (OGOXs) and cellodextrin-oxidase (CELLOX) are plant berberine bridge enzyme-like oligosaccharide-oxidases (OSOXs) that oxidize, respectively, oligogalacturonides (OGs) and cellodextrins (CDs), thereby inactivating their elicitor nature and concomitantly releasing H₂O₂. Little is known about the physiological role of OSOX activity. By using an ABTS^·+-reduction assay, we identified a novel reaction mechanism through which the activity of OSOXs on cell wall oligosaccharides scavenged the radical cation ABTS^·+ with an efficiency dependent on the type and length of the oxidized oligosaccharide. In contrast to the oxidation of longer oligomers such as OGs (degree of polymerization from 10 to 15), the activity of OSOXs on short galacturonan- and cellulose-oligomers (degree of polymerization ≤ 4) successfully counteracted the radical cation-generating activity of a fungal laccase, suggesting that OSOXs can generate radical cation scavenging activity in the apoplast with a power proportional to the extent of degradation of the plant cell wall, with possible implications for redox homeostasis and defense against oxidative stress.

OliveAtlas: A Gene Expression Atlas Tool for Olea europaea

The olive (Olea europaea L.) is an ancient crop of great importance in the Mediterranean basin due to the production of olive oil and table olives, which are important sources of fat and have benefits for human health. This crop is expanding and increasing its production worldwide and five olive genomes have recently been sequenced, representing a wild olive and important cultivars in terms of olive oil production, intensive agriculture, and adaptation to the East Asian climate. However, few bioinformatic and genomic resources are available to assist olive research and breeding, and there are no platforms to query olive gene expression data. Here, we present OliveAtlas, an interactive gene expression atlas for olive with multiple bioinformatics tools and visualization methods, enabling multiple gene comparison, replicate inspection, gene set enrichment, and data downloading. It contains 70 RNA-seq experiments, organized in 10 data sets representing the main olive plant organs, the pollen germination and pollen tube elongation process, and the response to a collection of biotic and abiotic stresses, among other experimental conditions. OliveAtlas is a web tool based on easyGDB with expression data based on the 'Picual' genome reference and gene annotation.

Cellobiose elicits immunity in lettuce conferring resistance to Botrytis cinerea

Cellobiose is the primary product of cellulose hydrolysis and is expected to function as a type of pathogen/damage-associated molecular pattern in evoking plant innate immunity. In this study, cellobiose was demonstrated to be a positive regulator in the immune response of lettuce, but halted autoimmunity when lettuce was exposed to concentrations of cellobiose >60 mg l-1. When lettuce plants were infected by Botrytis cinerea, cellobiose endowed plants with enhanced pre-invasion resistance by activating high β-1,3-glucanase and antioxidative enzyme activities at the initial stage of pathogen infection. Cellobiose-activated core regulatory factors such as EDS1, PTI6, and WRKY70, as well as salicylic acid signaling, played an indispensable role in modulating plant growth-defense trade-offs. Transcriptomics data further suggested that the cellobiose-activated plant-pathogen pathways are involved in microbe/pathogen-associated molecular pattern-triggered immune responses. Genes encoding receptor-like kinases, transcription factors, and redox homeostasis, phytohormone signal transduction, and pathogenesis-related proteins were also up- or down-regulated by cellobiose. Taken together, the findings of this study demonstrated that cellobiose serves as an elicitor to directly activate disease-resistance-related cellular functions. In addition, multiple genes have been identified as potential modulators of the cellobiose-induced immune response, which could aid understanding of underlying molecular events.

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 β-1,3/β-1,4-glucans (MLGs; e.g. β-1,4-D-(Glc)₂ -β-1,3-D-Glc, MLG43) and cellulose (e.g. β-1,4-D-(Glc)₃ , CEL3). The mechanisms behind carbohydrate perception in plants are poorly characterized except for fungal chitin oligosaccharides (e.g. β-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 K_D  = 1.19 ± 0.03 μm) and cellopentaose (K_D  = 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.

Selective Synthesis of Oligosaccharides by Mechanochemical Hydrolysis of Chitin over a Carbon-Based Catalyst

Oligosaccharides possess fascinating functions that are applicable in a variety of fields, such as agriculture. However, the selective synthesis of oligosaccharides, especially chitin-oligosaccharides, has remained a challenge. Chitin-oligosaccharides activate the plant immune system, enabling crops to withstand pathogens without harmful agrichemicals. Here, we demonstrate the conversion of chitin to chitin-oligosaccharides using a carbon catalyst with weak acid sites and mechanical milling. The catalyst produces chitin-oligosaccharides with up to 94 % selectivity in good yields. Monte-Carlo simulations indicate that our system preferentially hydrolyzes larger chitin molecules over oligomers, thus providing the desired high selectivity. This unique kinetics is in contrast to the fact that typical catalytic systems rapidly hydrolyze oligomers to monomers. Unlike other materials carbons more strongly adsorb large polysaccharides than small oligomers, which is suitable for the selective synthesis of small oligosaccharides.

Plant Cell Wall Integrity Perturbations and Priming for Defense

A plant cell wall is a highly complex structure consisting of networks of polysaccharides, proteins, and polyphenols that dynamically change during growth and development in various tissues. The cell wall not only acts as a physical barrier but also dynamically responds to disturbances caused by biotic and abiotic stresses. Plants have well-established surveillance mechanisms to detect any cell wall perturbations. Specific immune signaling pathways are triggered to contrast biotic or abiotic forces, including cascades dedicated to reinforcing the cell wall structure. This review summarizes the recent developments in molecular mechanisms underlying maintenance of cell wall integrity in plant-pathogen and parasitic interactions. Subjects such as the effect of altered expression of endogenous plant cell-wall-related genes or apoplastic expression of microbial cell-wall-modifying enzymes on cell wall integrity are covered. Targeted genetic modifications as a tool to study the potential of cell wall elicitors, priming of signaling pathways, and the outcome of disease resistance phenotypes are also discussed. The prime importance of understanding the intricate details and complete picture of plant immunity emerges, ultimately to engineer new strategies to improve crop productivity and sustainability.

The function of the plant cell wall in plant-microbe interactions

The plant cell wall is an interface of plant-microbe interactions. The ability of microbes to decompose cell wall polysaccharides contributes to microbial pathogenicity. Plants have evolved mechanisms to prevent cell wall degradation. However, the role of the cell wall in plant-microbe interactions is not well understood. Here, we discuss four functions of the plant cell wall-physical defence, storage of antimicrobial compounds, production of cell wall-derived elicitors, and provision of carbon sources-in the context of plant-microbe interactions. In addition, we discuss the four families of cell surface receptors associated with plant cell walls (malectin-like receptor kinase family, wall-associated kinase family, leucine-rich repeat receptor-like kinase family, and lysin motif receptor-like kinase family) that have been the subject of several important studies in recent years. This review summarises the findings on both plant cell wall and plant immunity, improving our understanding and may provide impetus to various researchers.

Berberine Bridge Enzyme-like Oligosaccharide Oxidases Act as Enzymatic Transducers Between Microbial Glycoside Hydrolases and Plant Peroxidases

Oligogalacturonide (OG)-oxidase 1 (OGOX1) and cellodextrin (CD)-oxidase (CELLOX) are plant berberine bridge enzyme-like oligosaccharide oxidases that oxidize OGs and CDs, cell-wall fragments with the nature of damage-associated molecular patterns. The oxidation of OGs and CDs attenuates their elicitor activity and concomitantly releases H₂O₂. By using a multiple enzyme-based assay, we demonstrate that the H₂O₂ generated downstream of the combined action between a fungal polygalacturonase and OGOX1 or an endoglucanase and CELLOX can be directed by plant peroxidases (PODs) either towards a reaction possibly involved in plant defense, such as the oxidation of monolignol or a reaction possibly involved in a developmental event, such as the oxidation of auxin (indole-3-acetic acid), pointing to OGOX1 and CELLOX as enzymatic transducers between microbial glycoside hydrolases and plant PODs. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A Berberine Bridge Enzyme-Like Protein, GmBBE-like43, Confers Soybean's Coordinated Adaptation to Aluminum Toxicity and Phosphorus Deficiency

Phosphorus (P) deficiency and aluminum (Al) toxicity often coexist and are two major limiting factors for crop production in acid soils. The purpose of this study was to characterize the function of GmBBE-like43, a berberine bridge enzyme-like protein-encoding gene, in soybean (Glycine max) adaptation to Al and low P stresses. Present quantitative real-time PCR (qRT-PCR) assays confirmed the phosphate (Pi)-starvation enhanced and Al-stress up-regulated expression pattern of GmBBE-like43 in soybean roots. Meanwhile, the expression of a GmBBE-like43-GFP chimera in both common bean hairy roots and tobacco leaves demonstrated its cell wall localization. Moreover, both transgenic Arabidopsis and soybean hairy roots revealed the function of GmBBE-like43 in promoting root growth under both Al and low P stresses. GmBBE-like43-overexpression also resulted in more H₂O₂ production on transgenic soybean hairy root surface with oligogalacturonides (OGs) application and antagonized the effects of Al on the expression of two SAUR-like genes. Taken together, our results suggest that GmBBE-like43 might be involved in the soybean's coordinated adaptation to Al toxicity and Pi starvation through modulation of OGs-oxidation in the cell wall.

Transcriptome analysis of peach fruit under 1-MCP treatment provides insights into regulation network in melting peach softening

Peach (Prunus persica L.) displays distinguish texture phenotype during postharvest, which could be classified into three types, including melting flesh (MF), non-melting flesh (NMF) and stony-hard (SH). Of that MF peach would soften rapidly with an outbreak of ethylene production, which cause a huge waste during fruit transportation and storage. 1-methylcyclopropene (1-MCP) was used to alleviate fruit softening. In this study, we performed RNA-sequencing on two MF peach cultivars (‘YuLu’ and ‘Yanhong’) after 1-MCP treatment to identify the candidate genes participating in peach fruit softening. 167 genes were identified by WGCNA and correlation analysis, which could respond to 1-MCP treatment and might be related to softening. Among them, 5 auxin related genes including 2 IAAs, 1 ARF and 2 SAURs, and 4 cell wall modifying genes (PpPG1, PpPG2, PpPG24 and PpPMEI) were characterized as key genes participating in MF peach softening. Furthermore, 2 transcription factors, which belong to HD-ZIP and MYB were predicted as candidates regulating softening process by constructing transcriptional network of these 4 cell wall modifying genes combined with expression pattern analysis, of that the HD-ZIP could trans-activate promoter of PpPG1.

Upregulation of protein N-glycosylation plays crucial roles in the response of Camellia sinensis leaves to fluoride

The tea plant (Camellia sinensis) is one of the three major beverage crops in the world with its leaves consumption as tea. However, it can hyperaccumulate fluoride with about 98% fluoride deposition in the leaves. Our previously studies found that cell wall proteins (CWPs) might play a central role in fluoride accumulation/detoxification in C. sinensis. CWP is known to be glycosylated, however the response of CWP N-glycosylation to fluoride remains unknown in C. sinensis. In this study, a comparative N-glycoproteomic analysis was performed through HILIC enrichment coupled with UPLC-MS/MS based on TMT-labeling approach in C. sinensis leaves. Totally, 237 N-glycoproteins containing 326 unique N-glycosites were identified. 73.4%, 18.6%, 6.3% and 1.7% of these proteins possess 1, 2, 3, and ≥4 modification site, respectively. 93.2% of these proteins were predicted to be localized in the secretory pathway and 78.9% of them were targeted to the cell wall and the plasma membrane. 133 differentially accumulated N-glycosites (DNGSs) on 100 N-glycoproteins (DNGPs) were detected and 85.0% of them exhibited upregulated expression after fluoride treatment. 78.0% DNGPs were extracellular DNGPs, which belonged to CWPs, and 53.0% of them were grouped into protein acting on cell wall polysaccharides, proteases and oxido-reductases, whereas the majority of the remaining DNGPs were mainly related to N-glycoprotein biosynthesis, trafficking and quality control. Our study shed new light on the N-glycoproteome study, and revealed that increased N-glycosylation abundance of CWPs might contribute to fluoride accumulation/detoxification in C. sinensis leave.

Cell wall integrity regulation across plant species

Plant cell walls are highly dynamic and chemically complex structures surrounding all plant cells. They provide structural support, protection from both abiotic and biotic stress as well as ensure containment of turgor. Recently evidence has accumulated that a dedicated mechanism exists in plants, which is monitoring the functional integrity of cell walls and initiates adaptive responses to maintain integrity in case it is impaired during growth, development or exposure to biotic and abiotic stress. The available evidence indicates that detection of impairment involves mechano-perception, while reactive oxygen species and phytohormone-based signaling processes play key roles in translating signals generated and regulating adaptive responses. More recently it has also become obvious that the mechanisms mediating cell wall integrity maintenance and pattern triggered immunity are interacting with each other to modulate the adaptive responses to biotic stress and cell wall integrity impairment. Here we will review initially our current knowledge regarding the mode of action of the maintenance mechanism, discuss mechanisms mediating responses to biotic stresses and highlight how both mechanisms may modulate adaptive responses. This first part will be focused on Arabidopsis thaliana since most of the relevant knowledge derives from this model organism. We will then proceed to provide perspective to what extent the relevant molecular mechanisms are conserved in other plant species and close by discussing current knowledge of the transcriptional machinery responsible for controlling the adaptive responses using selected examples.

Plant immunity by damage-associated molecular patterns (DAMPs)

Recognition by plant receptors of microbe-associated molecular patterns (MAMPs) and pathogenicity effectors activates immunity. However, before evolving the capacity of perceiving and responding to MAMPs and pathogenicity factors, plants, like animals, must have faced the necessity to protect and repair the mechanical wounds used by pathogens as an easy passage into their tissue. Consequently, plants evolved the capacity to react to damage-associated molecular patterns (DAMPs) with responses capable of functioning also in the absence of pathogens. DAMPs include not only primarily cell wall (CW) fragments but also extracellular peptides, nucleotides and amino acids that activate both local and long-distance systemic responses and, in some cases, prime the subsequent responses to MAMPs. It is conceivable that DAMPs and MAMPs act in synergy to activate a stronger plant immunity and that MAMPs exploit the mechanisms and transduction pathways traced by DAMPs. The interest for the biology and mechanism of action of DAMPs, either in the plant or animal kingdom, is expected to substantially increase in the next future. This review focuses on the most recent advances in DAMPs biology, particularly in the field of CW-derived DAMPs.

Cell Wall Signaling in Plant Development and Defense

Plant architecture fundamentally differs from that of other multicellular organisms in that individual cells serve as osmotic bricks, defined by the equilibrium between the internal turgor pressure and the mechanical resistance of the surrounding cell wall, which constitutes the interface between plant cells and their environment. The state and integrity of the cell wall are constantly monitored by cell wall surveillance pathways, which relay information to the cell interior. A recent surge of discoveries has led to significant advances in both mechanistic and conceptual insights into a multitude of cell wall response pathways that play diverse roles in the development, defense, stress response, and maintenance of structural integrity of the cell. However, these advances have also revealed the complexity of cell wall sensing, and many more questions remain to be answered, for example, regarding the mechanisms of cell wall perception, the molecular players in this process, and how cell wall-related signals are transduced and integrated into cellular behavior. This review provides an overview of the mechanistic and conceptual insights obtained so far and highlights areas for future discoveries in this exciting area of plant biology.

The wheat secreted root proteome: Implications for phosphorus mobilisation and biotic interactions

Root secreted acid phosphatases and organic anions are widely perceived as major players of plant phosphorus (P) mobilisation from the rhizosphere under P limiting growth conditions. Previous research indicated that other mechanisms play a role, especially in species with fine roots, such as wheat. In this study we characterised the plant-derived extracellular proteome of wheat roots by profiling root tip mucilage, soluble root secreted and root tip proteomes. Extracellular acid phosphatases and enzymes of the central carbon metabolism were targeted using selected reaction monitoring. More than 140 proteins with extracellular localisation prediction were identified in mucilage. P starvation induced proteins predicted to be localised to the apoplast which are related to cell wall modification and defence in both, root tip and soluble root-secreted proteomes. Glycolytic enzymes were strongly increased in abundance by P limitation in root tips, as were PEPC and plastidial MDH. Soluble acid phosphatases were not identified in extracellular protein samples. Our results indicate that root tip mucilage contains proteins with the functional potential to actively shape their immediate environment by modification of plant structural components and biotic interactions. Wheat acid phosphatases appear to play a minor role in P mobilisation beyond the immediate root surface. SIGNIFICANCE: Phosphorus (P) is a plant growth limiting nutrient in many agricultural situations and the development of phosphorus efficient crops is of paramount importance for future agricultural management practices. As P is relatively immobile in soils, processes occurring at the root-soil interface, the rhizosphere, are suspected to play a key role in plant-induced P mobilisation. According to the current view, the secretion of extracellular acid phosphatases and organic anions enhances P mobilisation within several millimetres beyond the root surface, either directly or indirectly through the selection and appropriate soil microbes. However, the mechanisms of P mobilisation in species with fine roots, such as wheat, and the role of other secreted root proteins are poorly understood. Here, we carried out the profiling of wheat root tip mucilage, soluble root secreted and root tip proteomes. We analysed proteome changes in response to P starvation. We found that proteins with a predicted localisation to the apoplast made up a major proportion of stress-responsive proteins. Acid phosphatases were not identified within extracellular protein samples, which were enriched in proteins with predicted extracellular localisation. The absence of extracellular APases was further validated by multiple reaction monitoring. Our data indicates that wheat acid phosphatases play a minor role in P mobilisation beyond the immediate root surface and provides a resource for breeding strategies and further investigations of the functional roles of root tip-released proteins in the rhizosphere under P limitation.

Mechanisms of plant cell wall surveillance in response to pathogens, cell wall-derived ligands and the effect of expansins to infection resistance or susceptibility

Cell wall integrity is tightly regulated and maintained given that non-physiological modification of cell walls could render plants vulnerable to biotic and/or abiotic stresses. Expansins are plant cell wall-modifying proteins active during many developmental and physiological processes, but they can also be produced by bacteria and fungi during interaction with plant hosts. Cell wall alteration brought about by ectopic expression, overexpression, or exogenous addition of expansins from either eukaryote or prokaryote origin can in some instances provide resistance to pathogens, while in other cases plants become more susceptible to infection. In these circumstances altered cell wall mechanical properties might be directly responsible for pathogen resistance or susceptibility outcomes. Simultaneously, through membrane receptors for enzymatically released cell wall fragments or by sensing modified cell wall barrier properties, plants trigger intracellular signaling cascades inducing defense responses and reinforcement of the cell wall, contributing to various infection phenotypes, in which expansins might also be involved. Here, we review the plant immune response activated by cell wall surveillance mechanisms, cell wall fragments identified as responsible for immune responses, and expansin's roles in resistance and susceptibility of plants to pathogen attack.

Poaceae-specific cell wall-derived oligosaccharides activate plant immunity via OsCERK1 during Magnaporthe oryzae infection in rice

Many phytopathogens secrete cell wall degradation enzymes (CWDEs) to damage host cells and facilitate colonization. As the major components of the plant cell wall, cellulose and hemicellulose are the targets of CWDEs. Damaged plant cells often release damage-associated molecular patterns (DAMPs) to trigger plant immune responses. Here, we establish that the fungal pathogen Magnaporthe oryzae secretes the endoglucanases MoCel12A and MoCel12B during infection of rice (Oryza sativa). These endoglucanases target hemicellulose of the rice cell wall and release two specific oligosaccharides, namely the trisaccharide 31-β-D-Cellobiosyl-glucose and the tetrasaccharide 31-β-D-Cellotriosyl-glucose. 31-β-D-Cellobiosyl-glucose and 31-β-D-Cellotriosyl-glucose bind the immune receptor OsCERK1 but not the chitin binding protein OsCEBiP. However, they induce the dimerization of OsCERK1 and OsCEBiP. In addition, these Poaceae cell wall-specific oligosaccharides trigger a burst of reactive oxygen species (ROS) that is largely compromised in oscerk1 and oscebip mutants. We conclude that 31-β-D-Cellobiosyl-glucose and 31-β-D-Cellotriosyl-glucose are specific DAMPs released from the hemicellulose of rice cell wall, which are perceived by an OsCERK1 and OsCEBiP immune complex during M. oryzae infection in rice.

A molecular dynamics-guided mutagenesis identifies two aspartic acid residues involved in the pH-dependent activity of OG-OXIDASE 1

During the infection, plant cells secrete different OG-oxidase (OGOX) paralogs, defense flavoproteins that oxidize the oligogalacturonides (OGs), homogalacturonan fragments released from the plant cell wall that act as Damage Associated Molecular Patterns. OGOX-mediated oxidation inactivates their elicitor nature, but on the other hand makes OGs less hydrolysable by microbial endo-polygalacturonases (PGs). Among the different plant defense responses, apoplastic alkalinization can further reduce the degrading potential of PGs by boosting the oxidizing activity of OGOXs. Accordingly, the different OGOXs so far characterized showed an optimal activity at pH values greater than 8. Here, an approach of molecular dynamics (MD)-guided mutagenesis succeeded in identifying the amino acids responsible for the pH dependent activity of OGOX1 from Arabidopsis thaliana. MD simulations indicated that in alkaline conditions (pH 8.5), the residues Asp325 and Asp344 are engaged in the formation of two salt bridges with Arg327 and Lys415, respectively, at the rim of enzyme active site. According to MD analysis, the presence of such ionic bonds modulates the size and flexibility of the cavity used to accommodate the OGs, in turn affecting the activity of OGOX1. Based on functional properties of the site-directed mutants OGOX1.D325A and OGOX.D344A, we demonstrated that Asp325 and Asp344 are major determinants of the alkaline-dependent activity of OGOX1.

The scope of flavin-dependent reactions and processes in the model plant Arabidopsis thaliana

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are utilized as coenzymes in many biochemical reduction-oxidation reactions owing to the ability of the tricyclic isoalloxazine ring system to employ the oxidized, radical and reduced state. We have analyzed the genome of Arabidopsis thaliana to establish an inventory of genes encoding flavin-dependent enzymes (flavoenzymes) as a basis to explore the range of flavin-dependent biochemical reactions that occur in this model plant. Expectedly, flavoenzymes catalyze many pivotal reactions in primary catabolism, which are connected to the degradation of basic metabolites, such as fatty and amino acids as well as carbohydrates and purines. On the other hand, flavoenzymes play diverse roles in anabolic reactions most notably the biosynthesis of amino acids as well as the biosynthesis of pyrimidines and sterols. Importantly, the role of flavoenzymes goes much beyond these basic reactions and extends into pathways that are equally crucial for plant life, for example the production of natural products. In this context, we outline the participation of flavoenzymes in the biosynthesis and maintenance of cofactors, coenzymes and accessory plant pigments (e. g. carotenoids) as well as phytohormones. Moreover, several multigene families have emerged as important components of plant immunity, for example the family of berberine bridge enzyme-like enzymes, flavin-dependent monooxygenases and NADPH oxidases. Furthermore, the versatility of flavoenzymes is highlighted by their role in reactions leading to tRNA-modifications, chromatin regulation and cellular redox homeostasis. The favorable photochemical properties of the flavin chromophore are exploited by photoreceptors to govern crucial processes of plant adaptation and development. Finally, a sequence- and structure-based approach was undertaken to gain insight into the catalytic role of uncharacterized flavoenzymes indicating their involvement in unknown biochemical reactions and pathways in A. thaliana.

Recent Advances in Understanding the Roles of Pectin as an Active Participant in Plant Signaling Networks

Pectin is an abundant cell wall polysaccharide with essential roles in various biological processes. The structural diversity of pectins, along with the numerous combinations of the enzymes responsible for pectin biosynthesis and modification, plays key roles in ensuring the specificity and plasticity of cell wall remodeling in different cell types and under different environmental conditions. This review focuses on recent progress in understanding various aspects of pectin, from its biosynthetic and modification processes to its biological roles in different cell types. In particular, we describe recent findings that cell wall modifications serve not only as final outputs of internally determined pathways, but also as key components of intercellular communication, with pectin as a major contributor to this process. The comprehensive view of the diverse roles of pectin presented here provides an important basis for understanding how cell wall-enclosed plant cells develop, differentiate, and interact.

Cell wall associated immunity in plants

The plant cell wall is the first physical and defensive barrier against pathogens. The plant cell wall usually undergoes dynamic remodeling as an immune response to prevent infection by pathogens. In this review, we summarize advances on relationship between cell wall and immunity in plants. In particular, we outline current progresses regarding the regulation of the cell wall components, including cellulose, hemicellulose, pectin and lignin, on plant disease resistance. We also discuss the impacts of cell wall-derived cellodextrin, oligogalacturonic acid and xyloglucan/xylan oligosaccharides as potent elicitors or signal molecules to trigger plant immune response. We further propose future studies on dissecting the molecular regulation of cell wall on plant immunity, which have potentials in practical application of crop breeding aiming at improvement of plant disease resistance.

Large-scale whole-genome resequencing unravels the domestication history of Cannabis sativa

Cannabis sativa has long been an important source of fiber extracted from hemp and both medicinal and recreational drugs based on cannabinoid compounds. Here, we investigated its poorly known domestication history using whole-genome resequencing of 110 accessions from worldwide origins. We show that C. sativa was first domesticated in early Neolithic times in East Asia and that all current hemp and drug cultivars diverged from an ancestral gene pool currently represented by feral plants and landraces in China. We identified candidate genes associated with traits differentiating hemp and drug cultivars, including branching pattern and cellulose/lignin biosynthesis. We also found evidence for loss of function of genes involved in the synthesis of the two major biochemically competing cannabinoids during selection for increased fiber production or psychoactive properties. Our results provide a unique global view of the domestication of C. sativa and offer valuable genomic resources for ongoing functional and molecular breeding research.