A chitin deacetylase from the endophytic fungus Pestalotiopsis sp. efficiently inactivates the elicitor activity of chitin oligomers in rice cells

Exploring the modern approaches to enhance fungal endophyte-derived bioactive secondary metabolites

Over the past few decades, microbial-derived bioactive compounds have been tested for antiviral, antimicrobial, and anticancer properties. In addition, fungal-derived bioactive secondary metabolites (SMs) are increasingly being suggested as suitable alternative sources of potent bioactive compounds. The development of suitable, precise in vitro and in vivo screening techniques may contribute to identifying the biochemical and physiological effects of compounds. This advancement in bioassay evaluation techniques helps identify potential bioactive microbes rapidly. The main obstacles, however, have been the production of insufficient amounts of chemicals, endophytes' attenuation or loss of ability to produce the molecule of interest when grown in cultures, and fungal endophytes' failure to exhibit their full biosynthetic potential in lab conditions. These have led to the use of small chemical elicitors that activate the silent biosynthetic gene clusters (BGCs) in fungi, causing epigenetic alterations that increase the amount of desired metabolites or trigger the synthesis of hitherto unknown compounds. The silent BGCs were activated to maximize production of bioactive secondary metabolites, thereby increasing the yield of desired compounds or triggering the synthesis of novel metabolites. Other strategies include gene knocking, inducing mutations, heterologous expression, one strain-many compounds (OSMAC), epigenetic modifications, etc. This review is focused on the mechanism of plant-microbe interaction in enhancing the biosynthesis of fungal metabolites along with the BGCs for the biosynthesis of the bioactive fungal metabolites. Furthermore, we also discuss the genomic mining approaches for BGCs, the role of ribosomal engineering, precursor feeding, and various elicitors to explore the structural diversity of novel bioactive compounds.

Exoskeletal cuticle proteins enable Drosophila locomotion

Exo- and Endoskeleton function enables muscle-mediated locomotion in animals. In mammals, the defective protein matrix of bones found in systematic skeletal disorders such as osteoporosis causes fractures and severe skeletal deformations under high muscle tension. We identified an analogous mechanism for integrating muscle-mediated tension into the apical extracellular matrix (aECM) of the invertebrate body wall exoskeleton. Obstructor chitin-binding proteins, the chitin deacetylases, Chitinases, and the matrix-protecting proteins Knickkopf and Retroactive are epidermally expressed during late embryogenesis. Their control of forming epidermal chitinous structures protects the exoskeletal aECM from collapsing when embryos start moving and hatch as larvae. In a larval locomotion assay we tested the function of these cuticle related genes. Gene mutations and knockdowns caused changes in normal movement behavior and lower the speed of larvae. Moreover, we found that the transmembrane Zona Pellucida domain protein Piopio provides the adhesion between the epidermal apical membrane and the overlaying chitinous aECM in a matriptase-dependent manner. A failure of Pio and chitin-associated proteins leads to exoskeletal deformations and detachment from the epidermal membrane, destabilizing muscle forces and impairing larval mobility. Our data identifies a protein network that transforms the chitinous aECM into a stable exoskeleton that directly resists muscle impact at epidermal tendon cells, thereby serving locomotion. Demonstrating the importance of these proteins in producing aECM as a three-dimensional cuticular scaffold for exoskeletal function opens up opportunities for the development of biomimetic applications of synthetic materials. STATEMENT OF SIGNIFICANCE: Chitin-based materials include hydrogels, microcapsules, membranous films, sponges, tubes, and various porous structures. In nature, chitin structures form cuticles, which serves as the exoskeleton of arthropods. Using Drosophila melanogaster, we have performed systematic analyses to identify the proteins and enzymes that organize chitin polymers in 3D structures of the cuticle exoskeleton. Three-dimensional laser-scanning and ultrastructural electron microscopy revealed deformations of the cuticle structure, lack of cellular cuticle adhesion, and overall changes in the flexibility of the chitin-based material, leading to insufficient function of the exoskeleton. Components such as the identified proteins and enzymes, which play a unique role in the organization of the chitin fibers and the formation of the exoskeleton, offer suitable materials for tissue engineering for biomimetic applications.

Discovery of Novel Inhibitors Targeting Fungal Chitin Deacetylase via Virtual Screening for Plant Disease Control

Fungal chitin deacetylase (CDA) plays a crucial role in pathogen-plant interactions, which is regarded as an innovative and promising target for fungicides. In this study, a pharmacophore-based virtual screening strategy was employed to identify compounds VS-24 and VS-25 as potent inhibitors against Puccinia striiformis f. sp. tritici CDA (PstCDA). Further bioassays demonstrated that VS-24 exhibited a protective effect of 61.2% against rice blast at 100 μg/mL, while VS-25 showed a superior protective effect of 45.5% against corn rust at 5 μg/mL, both superior to the reported CDA inhibitor benzohydroxamic acid (BHA). Molecular dynamics simulations revealed that multiple key interactions involving the Zn2+ ion and residues His207 and Tyr152 of PstCDA are critical for the binding of VS-24 or VS-25, with electrostatic interactions contributing most significantly to the binding free energy. Finally, toxicity predictions confirmed the potential biosafety of VS-24 and VS-25. Overall, this study identified two promising lead compounds targeting fungal CDA to control plant diseases.

Hydrolysis of chitin and chitosans by the human chitinolytic enzymes: chitotriosidase, acidic mammalian chitinase, and lysozyme

Human chitinolytic enzymes trigger growing interest, not only because a wide range of diseases and allergic responses are linked to chitinous components of pathogens, including their interplay with human enzymes, but also due to the increasing use of chitosans in biomedical applications. Here, we present a detailed side-by-side analysis of the only two human chitinases, chitotriosidase and acidic mammalian chitinase, as well as human lysozyme. By analyzing the cleavage of well-characterized chitosan polymers and defined chitin and chitosan oligomers, we report mild processivity and a quantitative subsite preference typical for GH18 chitinases for chitotriosidase and acidic mammalian chitinase. In contrast, lysozyme is negligibly processive and preferentially binds acetylated units at subsites -2, -1, and +1, thus exhibiting an even higher overall preference for acetylated units. A common feature of all three enzymes is their endo-chitinase behavior. For efficient hydrolysis, chitotriosidase or lysozyme require substrates of ≥4 or ≥5 units, respectively, and we identified defined chitosan oligomers which can competitively inhibit chitotriosidase. Knowledge about the enzymes' actions provides insight into the metabolic fate of chitin and chitosans in the human body, which is crucial to develop and approve chitosan applications, and to elucidate molecular mechanisms in chitin-associated diseases.

Dissecting and optimizing bioactivities of chitosans by enzymatic modification

Chitosans are versatile biopolymers with antimicrobial and plant-strengthening properties relevant to agriculture. However, a limited understanding of molecular structure-function relationships and cellular modes of action of chitosans hampers the development of effective chitosan-based agro-biologics. We partially hydrolyzed a chitosan polymer (degree of polymerization DP 800, fraction of acetylation FA 0.2) using acetic acid, a GH18 chitinase, or a GH8 chitosanase. All hydrolysates contained mixtures of chitosan oligomers and small polymers, but their composition in terms of DP, FA, and pattern of acetylation (PA) differed greatly. Importantly, chitinase products had mostly deacetylated residues at their ends, flanking mostly deacetylated residues, and vice versa for chitosanase products, while the products of acid hydrolysis had random PA. Acid hydrolysis did not significantly change antifungal and antibacterial activities. In contrast, chitinase hydrolysis slightly increased antibacterial, and chitosanase almost abolished antifungal activity. Elicitor and priming activities in the plant Arabidopsis were unchanged by acid, destroyed by chitinase, and increased by chitosanase hydrolysis. Transcriptomic analysis revealed that the chitosan polymer strongly induced genes involved in photosynthesis, while the chitosanase hydrolysate strongly induced genes involved in disease resistance. Clearly, different bioactivities require different chitosans, and enzymatic modification can fine-tune these activities as required for different agricultural products.

Chitins and chitosans-A tale of discovery and disguise, of attachment and attainment

Chitin polymers are an essential structural component of fungal cell walls, but host chitinases can weaken them, contributing to disease resistance in fungal pathogens. Chitin oligomers thus produced are immunogenic signal molecules eliciting additional disease resistance mechanisms. Fungi may counteract these, e.g. by partial deacetylation of chitin, converting it into chitosans, protecting the cell walls against chitinase attack, and inactivating elicitor active oligomers. This molecular stealth hypothesis for fungal pathogenicity has repeatedly been tested by mutating single or multiple chitin deacetylase genes, supporting the hypothesis but simultaneously suggesting additional roles for chitin deacetylation in virulence, such as surface attachment and sensing, host tissue penetration and colonization, as well as spore formation, stabilization, and germination. Interestingly, recent evidence suggests that host plants have evolved counter strategies by inhibiting fungal chitin deacetylases, lending further credibility to the suggested action of these enzymes as pathogenicity/virulence factors, and possibly offering leads toward novel functional fungicides.

Plant Immunity Modulation in Arbuscular Mycorrhizal Symbiosis and Its Impact on Pathogens and Pests

Arbuscular mycorrhizal (AM) symbiosis is the oldest and most widespread mutualistic association on Earth and involves plants and soil fungi belonging to Glomeromycotina. A complex molecular, cellular, and genetic developmental program enables partner recognition, fungal accommodation in plant tissues, and activation of symbiotic functions such as transfer of phosphorus in exchange for carbohydrates and lipids. AM fungi, as ancient obligate biotrophs, have evolved strategies to circumvent plant defense responses to guarantee an intimate and long-lasting mutualism. They are among those root-associated microorganisms able to boost plants' ability to cope with biotic stresses leading to mycorrhiza-induced resistance (MIR), which can be effective across diverse hosts and against different attackers. Here, we examine the molecular mechanisms underlying the modulation of plant immunity during colonization by AM fungi and at the onset and display of MIR against belowground and aboveground pests and pathogens. Understanding the MIR efficiency spectrum and its regulation is of great importance to optimizing the biotechnological application of these beneficial microbes for sustainable crop protection.

Low microbial diversity, yeast prevalence, and nematode-trapping fungal presence in fungal colonization and leaf microbiome of Serjania erecta

Medicinal plant microbiomes undergo selection due to secondary metabolite presence. Resident endophytic/epiphytic microorganisms directly influence plant's bioactive compound synthesis. Hypothesizing low microbial diversity in Serjania erecta leaves, we assessed leaf colonization by epiphytic and endophytic fungi. Given its traditional medicinal importance, we estimated diversity in the endophytic fungal microbiome. Analyses included scanning electron microscopy (SEM), isolation of cultivable species, and metagenomics. Epiphytic fungi interacted with S. erecta leaf tissues, horizontally transmitted via stomata/trichome bases, expressing traits for nematode trapping. Cultivable endophytic fungi, known for phytopathogenic habits, didn't induce dysbiosis symptoms. This study confirms low leaf microbiome diversity in S. erecta, with a tendency towards more fungal species, likely due to antibacterial secondary metabolite selection. The classification of Halicephalobus sp. sequence corroborated the presence of nematode eggs on the epidermal surface of S. erecta by SEM. In addition, we confirmed the presence of methanogenic archaea and a considerable number of methanotrophs of the genus Methylobacterium. The metagenomic study of endophytic fungi highlighted plant growth-promoting yeasts, mainly Malassezia, Leucosporidium, Meyerozyma, and Hannaella. Studying endophytic fungi and S. erecta microbiomes can elucidate their impact on beneficial bioactive compound production, on the other hand, it is possible that the bioactive compounds produced by this plant can recruit specific microorganisms, impacting the biological system.

Unveiling the inverse antimicrobial impact of a hetero-chitooligosaccharide on Candida tropicalis growth and biofilm formation

Chitosan and chitooligosaccharide (COS) are renowned for their potent antimicrobial prowess, yet the precise antimicrobial efficacy of COS remains elusive due to scant structural information about the utilized saccharides. This study delves into the antimicrobial potential of COS, spotlighting a distinct hetero-chitooligosaccharide dubbed DACOS. In contrast to other COS, DACOS remarkably fosters the growth of Candida tropicalis planktonic cells and fungal biofilms. Employing gradient alcohol precipitation, DACOS was fractionated, unveiling diverse structural characteristics and differential impacts on C. tropicalis. Notably, in a murine model of systemic candidiasis, DACOS, particularly its 70 % alcohol precipitates, manifests a promotive effect on Candida infection. This research unveils a new pathway for exploring the intricate nexus between the structural attributes of chitosan oligosaccharides and their physiological repercussions, underscoring the imperative of crafting chitosan and COS with meticulously defined structural configurations.

Alternative processes to produce chitin, chitosan, and their oligomers

Sustainable recovery of chitin and its derivatives from shellfish waste will be achieved when the industrial production of these polymers is achieved with a high control of their molecular structure, low costs, and acceptable levels of pollution. Therefore, the conventional chemical method for obtaining these biopolymers needs to be replaced or optimized. The goal of the present review is to ascertain what alternative methods are viable for the industrial-scale production of chitin, chitosan, and their oligomers. Therefore, a detailed review of recent literature was undertaken, focusing on the advantages and disadvantages of each method. The analysis of the existing data allows suggesting that combining conventional, biological, and alternative methods is the most efficient strategy to achieve sustainable production, preventing negative impacts and allowing for the recovery of high added-value compounds from shellfish waste. In conclusion, a new process for obtaining chitinous materials is suggested, with the potential of reducing the consumption of reagents, energy, and water by at least 1/10, 1/4, and 1/3 part with respect to the conventional process, respectively.

Chitosan nanocomposites as a nano-bio tool in phytopathogen control

Chitosan, an economically viable and versatile biopolymer, exhibits a wide array of advantageous physicochemical and biological properties. Chitosan nanocomposites, formed by the amalgamation of chitosan or chitosan nanoparticles with other nanoparticles or materials, have garnered extensive attention across agricultural, pharmaceutical, and biomedical domains. These nanocomposites have been rigorously investigated due to their diverse applications, notably in combatting plant pathogens. Their remarkable efficacy against phytopathogens has positioned them as a promising alternative to conventional chemical-based methods in phytopathogen control, thus exploring interest in sustainable agricultural practices with reduced reliance on chemical interventions. This review aims to highlight the anti-phytopathogenic activity of chitosan nanocomposites, emphasizing their potential in mitigating plant diseases. Additionally, it explores various synthesis methods for chitosan nanoparticles to enhance readers' understanding. Furthermore, the analysis delves into elucidating the intricate mechanisms governing the antimicrobial effectiveness of these composites against bacterial and fungal phytopathogens.

Current Advances in the Functional Diversity and Mechanisms Underlying Endophyte-Plant Interactions

Plant phenotype is a complex entity largely controlled by the genotype and various environmental factors. Importantly, co-evolution has allowed plants to coexist with the biotic factors in their surroundings. Recently, plant endophytes as an external plant phenotype, forming part of the complex plethora of the plant microbial assemblage, have gained immense attention from plant scientists. Functionally, endophytes impact the plant in many ways, including increasing nutrient availability, enhancing the ability of plants to cope with both abiotic and biotic stress, and enhancing the accumulation of important plant secondary metabolites. The current state of research has been devoted to evaluating the phenotypic impacts of endophytes on host plants, including their direct influence on plant metabolite accumulation and stress response. However, there is a knowledge gap in how genetic factors influence the interaction of endophytes with host plants, pathogens, and other plant microbial communities, eventually controlling the extended microbial plant phenotype. This review will summarize how host genetic factors can impact the abundance and functional diversity of the endophytic microbial community, how endophytes influence host gene expression, and the host-endophyte-pathogen disease triangle. This information will provide novel insights into how breeders could specifically target the plant-endophyte extended phenotype for crop improvement.

Exploring the potential of endophyte-plant interactions for improving crop sustainable yields in a changing climate

Climate change poses a major threat to global food security, significantly reducing crop yields as cause of abiotic stresses, and for boosting the spread of new and old pathogens and pests. Sustainable crop management as a route to mitigation poses the challenge of recruiting an array of solutions and tools for the new aims. Among these, the deployment of positive interactions between the micro-biotic components of agroecosystems and plants can play a highly significant role, as part of the agro-ecological revolution. Endophytic microorganisms have emerged as a promising solution to tackle this challenge. Among these, Arbuscular Mycorrhizal Fungi (AMF) and endophytic bacteria and fungi have demonstrated their potential to alleviate abiotic stresses such as drought and heat stress, as well as the impacts of biotic stresses. They can enhance crop yields in a sustainable way also by other mechanisms, such as improving the nutrient uptake, or by direct effects on plant physiology. In this review we summarize and update on the main types of endophytes, we highlight several studies that demonstrate their efficacy in improving sustainable yields and explore possible avenues for implementing crop-microbiota interactions. The mechanisms underlying these interactions are highly complex and require a comprehensive understanding. For this reason, omic technologies such as genomics, transcriptomics, proteomics, and metabolomics have been employed to unravel, by a higher level of information, the complex network of interactions between plants and microorganisms. Therefore, we also discuss the various omic approaches and techniques that have been used so far to study plant-endophyte interactions.

Plant-endophyte communication: Scaling from molecular mechanisms to ecological outcomes

Diverse communities of fungal endophytes reside in plant tissues, where they affect and are affected by plant physiology and ecology. For these intimate interactions to form and persist, endophytes and their host plants engage in intricate systems of communication. The conversation between fungal endophytes and plant hosts ultimately dictates endophyte community composition and function and has cascading effects on plant health and plant interactions. In this review, we synthesize our current knowledge on the mechanisms and strategies of communication used by endophytic fungi and their plant hosts. We discuss the molecular mechanisms of communication that lead to organ specificity of endophytic communities and distinguish endophytes, pathogens, and saprotrophs. We conclude by offering emerging perspectives on the relevance of plant-endophyte communication to microbial community ecology and plant health and function.

Cloning and Characterization of Chitin Deacetylase from Euphausia superba

Chitin deacetylase (CDA) can catalyze the deacetylation of chitin to produce chitosan. In this study, we identified and characterized a chitin deacetylase gene from Euphausia superba (EsCDA-9k), and a soluble recombinant protein chitin deacetylase from Euphausia superba of molecular weight 45 kDa was cloned, expressed, and purified. The full-length cDNA sequence of EsCDA-9k was 1068 bp long and encoded 355 amino acid residues that contained the typical domain structure of carbohydrate esterase family 4. The predicted three-dimensional structure of EsCDA-9k showed a 67.32% homology with Penaeus monodon. Recombinant chitin deacetylase had the highest activity at 40 °C and pH 8.0 in Tris-HCl buffer. The enzyme activity was enhanced by metal ions Co2+, Fe3+, Ca2+, and Na+, while it was inhibited by Zn2+, Ba2+, Mg2+, and EDTA. Molecular simulation of EsCDA-9k was conducted based on sequence alignment and homology modeling. The EsCDA-9k F18G mutant showed a 1.6-fold higher activity than the wild-type enzyme. In summary, this is the first report of the cloning and heterologous expression of the chitin deacetylase gene in Euphausia superba. The characterization and function study of EsCDA-9k will serve as an important reference point for future application.

Integrated Genome Sequencing and Transcriptome Analysis Identifies Candidate Pathogenicity Genes from Ustilago crameri

Ustilago crameri is a pathogenic basidiomycete fungus that causes foxtail millet kernel smut (FMKS), a devastating grain disease in most foxtail-millet-growing regions of the world. Here, we report an assembled high-quality genome sequence of U. crameri strain SCZ-6 isolated from the diseased grains of foxtail millet in Changzhi, Shanxi Province, China. The genome size is 19.55 Mb, consisting of 73 contigs (N50 = 840,209 bp) with a G + C content of 54.09%, and encoding 6576 predicted genes and 6486 genes supported by RNA-seq. Evolutionarily, U. crameri lies close to the barley smut U. hordei, and an obvious co-linearity was observed between these two smut fungi. We annotated the genome of U. crameri strain SCZ-6 using databases, identifying 1827 pathogen-host interaction (PHI)-associated genes, 1324 genes encoding fungal virulence factors, 259 CAZy-related genes, 80 genes encoding transporters, and 206 putative cytochrome P450 genes; their expression profiles at different inoculation time points were also detected. Additionally, 70 candidate pathogen effectors were identified according to their expression patterns and predicted functions. In summary, our results provide important insights into the pathogenic mechanisms of the pathogenesis-related genes of U. crameri and a robust foundation for further investigation.

The molecular structure, biological roles, and inhibition of plant pathogenic fungal chitin deacetylases

Chitin/polysaccharide deacetylases belong to the carbohydrate esterases family 4 (CE4 enzymes). They play a crucial role in modifying the physiochemical characteristics of structural polysaccharides and are also involved in a wide range of biological processes such as fungal autolysis, spore formation, cell wall formation and integrity, and germling adhesion. These enzymes are mostly common in fungi, marine bacteria, and a limited number of insects. They facilitate the deacetylation of chitin which is a structural biopolymer that is abundantly found in fungal cell walls and spores and also in the cuticle and peritrophic matrices of insects. The deacetylases exhibit specificity towards a substrate containing a sequence of four GlcNAc units, with one of these units being subjected to deacetylation. Chitin deacetylation results in the formation of chitosan, which is a poor substrate for host plant chitinases, therefore it can suppress the host immune response triggered by fungal pathogens and enhance pathogen virulence and colonization. This review discusses plant pathogenic fungal chitin/polysaccharide deacetylases including their structure, substrate specificity, biological roles and some recently discovered chitin deacetylase inhibitors that can help to mitigate plant fungal diseases. This review provides fundamental knowledge that will undoubtedly lead to the rational design of novel inhibitors that target pathogenic fungal chitin deacetylases, which will also aid in the management of plant diseases, thereby safeguarding global food security.

Endophytic fungi: Unravelling plant-endophyte interaction and the multifaceted role of fungal endophytes in stress amelioration

Endophytic fungi colonize interior plant tissue and mostly form mutualistic associations with their host plant. Plant-endophyte interaction is a complex mechanism and is currently a focus of research to understand the underlying mechanism of endophyte asymptomatic colonization, the process of evading plant immune response, modulation of gene expression, and establishment of a balanced mutualistic relationship. Fungal endophytes rely on plant hosts for nutrients, shelter, and transmission and improve the host plant's tolerance against biotic stresses, including -herbivores, nematodes, bacterial, fungal, viral, nematode, and other phytopathogens. Endophytic fungi have been reported to improve plant health by reducing and eradicating the harmful effect of phytopathogens through competition for space or nutrients, mycoparasitism, and through direct or indirect defense systems by producing secondary metabolites as well as by induced systemic resistance (ISR). Additionally, for efficient crop improvement, practicing them would be a fruitful step for a sustainable approach. This review article summarizes the current research progress in plant-endophyte interaction and the fungal endophyte mechanism to overcome host defense responses, their subsequent colonization, and the establishment of a balanced mutualistic interaction with host plants. This review also highlighted the potential of fungal endophytes in the amelioration of biotic stress. We have also discussed the relevance of various bioactive compounds possessing antimicrobial potential against a variety of agricultural pathogens. Furthermore, endophyte-mediated ISR is also emphasized.

Engineering of a chitin deacetylase to generate tailor-made chitosan polymers

Chitin deacetylases (CDAs) emerge as a valuable tool to produce chitosans with a nonrandom distribution of N-acetylglucosamine (GlcNAc) and glucosamine (GlcN) units. We hypothesized before that CDAs tend to bind certain sequences within the substrate matching their subsite preferences for either GlcNAc or GlcN units. Thus, they deacetylate or N-acetylate their substrates at nonrandom positions. To understand the molecular basis of these preferences, we analyzed the binding site of a CDA from Pestalotiopsis sp. (PesCDA) using a detailed activity screening of a site-saturation mutagenesis library. In addition, molecular dynamics simulations were conducted to get an in-depth view of crucial interactions along the binding site. Besides elucidating the function of several amino acids, we were able to show that only 3 residues are responsible for the highly specific binding of PesCDA to oligomeric substrates. The preference to bind a GlcNAc unit at subsite -2 and -1 can mainly be attributed to N75 and H199, respectively. Whereas an exchange of N75 at subsite -2 eliminates enzyme activity, H199 can be substituted with tyrosine to increase the GlcN acceptance at subsite -1. This change in substrate preference not only increases enzyme activity on certain substrates and changes composition of oligomeric products but also significantly changes the pattern of acetylation (PA) when N-acetylating polyglucosamine. Consequently, we could clearly show how subsite preferences influence the PA of chitosans produced with CDAs.

Small holes, big impact: Stomata in plant-pathogen-climate epic trifecta

The regulation of stomatal aperture opening and closure represents an evolutionary battle between plants and pathogens, characterized by adaptive strategies that influence both plant resistance and pathogen virulence. The ongoing climate change introduces further complexity, affecting pathogen invasion and host immunity. This review delves into recent advances on our understanding of the mechanisms governing immunity-related stomatal movement and patterning with an emphasis on the regulation of stomatal opening and closure dynamics by pathogen patterns and host phytocytokines. In addition, the review explores how climate changes impact plant-pathogen interactions by modulating stomatal behavior. In light of the pressing challenges associated with food security and the unpredictable nature of climate changes, future research in this field, which includes the investigation of spatiotemporal regulation and engineering of stomatal immunity, emerges as a promising avenue for enhancing crop resilience and contributing to climate control strategies.

Deacetylation of chitin oligomers by Fusarium graminearum polysaccharide deacetylase suppresses plant immunity

Chitin is a long-chain polymer of β-1,4-linked N-acetylglucosamine that forms rigid microfibrils to maintain the hyphal form and protect it from host attacks. Chitin oligomers are first recognized by the plant receptors in the apoplast region, priming the plant's immune system. Here, seven polysaccharide deacetylases (PDAs) were identified and their activities on chitin substrates were investigated via systematic characterization of the PDA family from Fusarium graminearum. Among these PDAs, FgPDA5 was identified as an important virulence factor and was specifically expressed during pathogenesis. ΔFgpda5 compromised the pathogen's ability to infect wheat. The polysaccharide deacetylase structure of FgPDA5 is essential for the pathogenicity of F. graminearum. FgPDA5 formed a homodimer and accumulated in the plant apoplast. In addition, FgPDA5 showed a high affinity toward chitin substrates. FgPDA5-mediated deacetylation of chitin oligomers prevented activation of plant defence responses. Overall, our results identify FgPDA5 as a polysaccharide deacetylase that can prevent chitin-triggered host immunity in plant apoplast through deacetylation of chitin oligomers.

A chitin deacetylase PsCDA2 from Puccinia striiformis f. sp. tritici confers disease pathogenicity by suppressing chitin-triggered immunity in wheat

Plants have the ability to recognize the essential chitin molecule present in the fungal cell wall, which stimulates the immune response. Phytopathogenic fungi have developed various strategies to inhibit the chitin-triggered immune response. Here, we identified a chitin deacetylase of Puccinia striiformis f. sp. tritici (Pst), known as PsCDA2, that was induced during the initial invasion of wheat and acted as an inhibitor of plant cell death. Knockdown of PsCDA2 in wheat enhanced its resistance against Pst, highlighting the significance of PsCDA2 in the host-pathogen interaction. Moreover, PsCDA2 can protect Pst urediniospores from being damaged by host chitinase in vitro. PsCDA2 also suppressed the basal chitin-induced plant immune response, including the accumulation of callose and the expression of defence genes. Overall, our results demonstrate that Pst secretes PsCDA2 as a chitin deacetylase involved in establishing infection and modifying the acetyl group to prevent the breakdown of chitin in the cell wall by host endogenous chitinases. Our research unveils a mechanism by which the fungus suppresses plant immunity, further contributing to the understanding of wheat stripe rust control. This information could have significant implications for the development of suitable strategies for protecting crops against the devastating effects of this disease.

The source, activity influencing factors and biological activities for future development of chitin deacetylase

Chitin deacetylase (CDA), a prominent member of the carbohydrate esterase enzyme family 4 (CE4), is found ubiquitously in bacteria, fungi, insects, and crustaceans. This metalloenzyme plays a pivotal role in recognizing and selectively removing acetyl groups from chitin, thus offering an environmentally friendly and biologically-driven preparation method for chitosan with immense industrial potential. Due to its diverse origins, CDAs sourced from different organisms exhibit unique functions, optimal pH ranges, and temperature preferences. Furthermore, certain organic reagents can induce structural changes in CDAs, influencing their catalytic activity. Leveraging CDA's capabilities extends beyond chitosan biocatalysis, as it demonstrates promising application value in agricultural pest control. In this paper, the source, reaction mechanism, influencing factors, the fermentation methods and applications of CDA are reviewed, which provides theoretical help for the research and application of CDA.

Deep discovery informs difficult deployment in plant microbiome science

Plant-associated microbiota can extend plant immune system function, improve nutrient acquisition and availability, and alleviate abiotic stresses. Thus, naturally beneficial microbial therapeutics are enticing tools to improve plant productivity. The basic definition of plant microbiota across species and ecosystems, combined with the development of reductionist experimental models and the manipulation of plant phenotypes with microbes, has fueled interest in its translation to agriculture. However, the great majority of microbes exhibiting plant-productivity traits in the lab and greenhouse fail in the field. Therapeutic microbes must reach détente, the establishment of uneasy homeostasis, with the plant immune system, invade heterogeneous pre-established plant-associated communities, and persist in a new and potentially remodeled community. Environmental conditions can alter community structure and thus impact the engraftment of therapeutic microbes. We survey recent breakthroughs, challenges, and opportunities in translating beneficial microbes from the lab to the field.

Suppression of Chitin-Triggered Immunity by Plant Fungal Pathogens: A Case Study of the Cucurbit Powdery Mildew Fungus Podosphaera xanthii

Fungal pathogens are significant plant-destroying microorganisms that present an increasing threat to the world's crop production. Chitin is a crucial component of fungal cell walls and a conserved MAMP (microbe-associated molecular pattern) that can be recognized by specific plant receptors, activating chitin-triggered immunity. The molecular mechanisms underlying the perception of chitin by specific receptors are well known in plants such as rice and Arabidopsis thaliana and are believed to function similarly in many other plants. To become a plant pathogen, fungi have to suppress the activation of chitin-triggered immunity. Therefore, fungal pathogens have evolved various strategies, such as prevention of chitin digestion or interference with plant chitin receptors or chitin signaling, which involve the secretion of fungal proteins in most cases. Since chitin immunity is a very effective defensive response, these fungal mechanisms are believed to work in close coordination. In this review, we first provide an overview of the current understanding of chitin-triggered immune signaling and the fungal proteins developed for its suppression. Second, as an example, we discuss the mechanisms operating in fungal biotrophs such as powdery mildew fungi, particularly in the model species Podosphaera xanthii, the main causal agent of powdery mildew in cucurbits. The key role of fungal effector proteins involved in the modification, degradation, or sequestration of immunogenic chitin oligomers is discussed in the context of fungal pathogenesis and the promotion of powdery mildew disease. Finally, the use of this fundamental knowledge for the development of intervention strategies against powdery mildew fungi is also discussed.

Endophytic fungi: hidden treasure chest of antimicrobial metabolites interrelationship of endophytes and metabolites

Endophytic fungi comprise host-associated fungal communities which thrive within the tissues of host plants and produce a diverse range of secondary metabolites with various bioactive attributes. The metabolites such as phenols, polyketides, saponins, alkaloids help to mitigate biotic and abiotic stresses, fight against pathogen attacks and enhance the plant immune system. We present an overview of the association of endophytic fungal communities with a plant host and discuss molecular mechanisms induced during their symbiotic interaction. The overview focuses on the secondary metabolites (especially those of terpenoid nature) secreted by endophytic fungi and their respective function. The recent advancement in multi-omics approaches paved the way for identification of these metabolites and their characterization via comparative analysis of extensive omics datasets. This study also elaborates on the role of diverse endophytic fungi associated with key agricultural crops and hence important for sustainability of agriculture.

Inhibition of chitin deacetylases to attenuate plant fungal diseases

Phytopathogenic fungi secrete chitin deacetylase (CDA) to escape the host's immunological defense during infection. Here, we showed that the deacetylation activity of CDA toward chitin is essential for fungal virulence. Five crystal structures of two representative and phylogenetically distant phytopathogenic fungal CDAs, VdPDA1 from Verticillium dahliae and Pst_13661 from Puccinia striiformis f. sp. tritici, were obtained in ligand-free and inhibitor-bound forms. These structures suggested that both CDAs have an identical substrate-binding pocket and an Asp-His-His triad for coordinating a transition metal ion. Based on the structural identities, four compounds with a benzohydroxamic acid (BHA) moiety were obtained as phytopathogenic fungal CDA inhibitors. BHA exhibited high effectiveness in attenuating fungal diseases in wheat, soybean, and cotton. Our findings revealed that phytopathogenic fungal CDAs share common structural features, and provided BHA as a lead compound for the design of CDA inhibitors aimed at attenuating crop fungal diseases.

Cell Wall Carbohydrate Dynamics during the Differentiation of Infection Structures by the Apple Scab Fungus, Venturia inaequalis

Scab, caused by the biotrophic fungal pathogen Venturia inaequalis, is the most economically important disease of apples. During infection, V. inaequalis colonizes the subcuticular host environment, where it develops specialized infection structures called runner hyphae and stromata. These structures are thought to be involved in nutrient acquisition and effector (virulence factor) delivery, but also give rise to conidia that further the infection cycle. Despite their importance, very little is known about how these structures are differentiated. Likewise, nothing is known about how these structures are protected from host defenses or recognition by the host immune system. To better understand these processes, we first performed a glycosidic linkage analysis of sporulating tubular hyphae from V. inaequalis developed in culture. This analysis revealed that the V. inaequalis cell wall is mostly composed of glucans (44%) and mannans (37%), whereas chitin represents a much smaller proportion (4%). Next, we used transcriptomics and confocal laser scanning microscopy to provide insights into the cell wall carbohydrate composition of runner hyphae and stromata. These analyses revealed that, during subcuticular host colonization, genes of V. inaequalis putatively associated with the biosynthesis of immunogenic carbohydrates, such as chitin and β-1,6-glucan, are downregulated relative to growth in culture, while on the surface of runner hyphae and stromata, chitin is deacetylated to the less-immunogenic carbohydrate chitosan. These changes are anticipated to enable the subcuticular differentiation of runner hyphae and stromata by V. inaequalis, as well as to protect these structures from host defenses and recognition by the host immune system. IMPORTANCE Plant-pathogenic fungi are a major threat to food security. Among these are subcuticular pathogens, which often cause latent asymptomatic infections, making them difficult to control. A key feature of these pathogens is their ability to differentiate specialized subcuticular infection structures that, to date, remain largely understudied. This is typified by Venturia inaequalis, which causes scab, the most economically important disease of apples. In this study, we show that, during subcuticular host colonization, V. inaequalis downregulates genes associated with the biosynthesis of two immunogenic cell wall carbohydrates, chitin and β-1,6-glucan, and coats its subcuticular infection structures with a less-immunogenic carbohydrate, chitosan. These changes are anticipated to enable host colonization by V. inaequalis and provide a foundation for understanding subcuticular host colonization by other plant-pathogenic fungi. Such an understanding is important, as it may inform the development of novel control strategies against subcuticular plant-pathogenic fungi.

Plant-Fungi Interactions: Where It Goes?

Fungi live different lifestyles-including pathogenic and symbiotic-by interacting with living plants. Recently, there has been a substantial increase in the study of phytopathogenic fungi and their interactions with plants. Symbiotic relationships with plants appear to be lagging behind, although progressive. Phytopathogenic fungi cause diseases in plants and put pressure on survival. Plants fight back against such pathogens through complicated self-defense mechanisms. However, phytopathogenic fungi develop virulent responses to overcome plant defense reactions, thus continuing their deteriorative impacts. Symbiotic relationships positively influence both plants and fungi. More interestingly, they also help plants protect themselves from pathogens. In light of the nonstop discovery of novel fungi and their strains, it is imperative to pay more attention to plant-fungi interactions. Both plants and fungi are responsive to environmental changes, therefore construction of their interaction effects has emerged as a new field of study. In this review, we first attempt to highlight the evolutionary aspect of plant-fungi interactions, then the mechanism of plants to avoid the negative impact of pathogenic fungi, and fungal strategies to overcome the plant defensive responses once they have been invaded, and finally the changes of such interactions under the different environmental conditions.

Characterization of Arbuscular Mycorrhizal Effector Proteins

Plants are colonized by various fungi with both pathogenic and beneficial lifestyles. One type of colonization strategy is through the secretion of effector proteins that alter the plant's physiology to accommodate the fungus. The oldest plant symbionts, the arbuscular mycorrhizal fungi (AMF), may exploit effectors to their benefit. Genome analysis coupled with transcriptomic studies in different AMFs has intensified research on the effector function, evolution, and diversification of AMF. However, of the current 338 predicted effector proteins from the AM fungus Rhizophagus irregularis, only five have been characterized, of which merely two have been studied in detail to understand which plant proteins they associate with to affect the host physiology. Here, we review the most recent findings in AMF effector research and discuss the techniques used for the functional characterization of effector proteins, from their in silico prediction to their mode of action, with an emphasis on high-throughput approaches for the identification of plant targets of the effectors through which they manipulate their hosts.

Secretome analysis of the phytopathogen Macrophomina phaseolina cultivated in liquid medium supplemented with and without soybean leaf infusion

Macrophomina phaseolina (Tassi) Goid. is a fungal pathogen that causes root and stem rot in several economically important crops. However, most of disease control strategies have shown limited effectiveness. Despite its impact on agriculture, molecular mechanisms involved in the interaction with host plant remains poorly understood. Nevertheless, it has been proven that fungal pathogens secrete a variety of proteins and metabolites to successfully infect their host plants. In this study, a proteomic analysis of proteins secreted by M. phaseolina in culture media supplemented with soybean leaf infusion was performed. A total of 250 proteins were identified with a predominance of hydrolytic enzymes. Plant cell wall degrading enzymes together peptidases were found, probably involved in the infection process. Predicted effector proteins were also found that could induce plant cell death or suppress plant immune response. Some of the putative effectors presented similarities to known fungal virulence factors. Expression analysis of ten selected protein-coding genes showed that these genes are induced during host tissue infection and suggested their participation in the infection process. The identification of secreted proteins of M. phaseolina could be used to improve the understanding of the biology and pathogenesis of this fungus. Although leaf infusion was able to induce changes at the proteome level, it is necessary to study the changes induced under conditions that mimic the natural infection process of the soil-borne pathogen M. phaseolina to identify virulence factors.

Novel Chitin Deacetylase from Thalassiosira weissflogii Highlights the Potential for Chitin Derivative Production

β-Chitin is an important carbon fixation product of diatoms, and is the most abundant nitrogen-containing polysaccharide in the ocean. It has potential for widespread application, but the characterization of chitin-related enzymes from β-chitin producers has rarely been reported. In this study, a chitin deacetylase (TwCDA) was retrieved from the Marine Microbial Eukaryote Transcriptome Sequencing Project (MMETSP) database and was heterologously expressed in vitro for functional analysis. The results showed that both the full-length sequence (TwCDA) and the N-terminal truncated sequence (TwCDA-S) had chitin deacetylase and chitinolytic activities after expression in Escherichia coli. High-performance liquid chromatography (HPLC) and gas chromatography–mass spectrometry (GC-MS) indicated that TwCDA and TwCDA-S could catalyze the deacetylation of oligosaccharide (GlcNAc)5. TwCDA had higher deacetylase activity, and also catalyzed the deacetylation of the β-chitin polymer. A dinitrosalicylic acid (DNS) assay showed that TwCDA-S had high chitinolytic activity for (GlcNAc)5, and the optimal reaction temperature was 35 °C. Liquid chromatography combined with time-of-flight mass spectrometry (LC-coTOF-MS) detected the formation of a N-acetylglucosamine monomer (C8H15NO6) in the reaction mixture. Altogether, we isolated a chitin deacetylase from a marine diatom, which can catalyze the deacetylation and degradation of chitin and chitin oligosaccharides. The relevant results lay a foundation for the internal regulation mechanism of chitin metabolism in diatoms and provide a candidate enzyme for the green industrial preparation of chitosan and chitin oligosaccharides.

Three Chitin Deacetylase Family Members of Beauveria bassiana Modulate Asexual Reproduction and Virulence of Fungi by Mediating Chitin Metabolism and Affect Fungal Parasitism and Saprophytic Life

As an important chitin-modifying enzyme, chitin deacetylase (CDA) has been characterized in many fungi, but its function in the entomopathogenic fungus Beauveria bassiana remains unclear. Three CDAs with conserved domains of the carbohydrate esterase 4 (CE-4) family were identified in B. bassiana. Disruption of CDA1 resulted in growth restriction of the fungus on medium with chitin as a carbon source or without a carbon source. Deletion of CDA1 and CDA2 led to defects in fungal conidial formation and conidial vitality compared with those of the wild type (WT), and the conidial yield decreased by 25.81% to 47.68%. Inactivation of three CDA genes resulted in a decrease of 20.23% to 27% in the blastospore yield. ΔCDA1 and ΔCDA3 showed 29.33% and 23.34% reductions in cuticular infection virulence, respectively. However, the CDA family may not contribute to hemocoel infection virulence. Additionally, the sporulation of the insect carcass showed that the three gene deletion mutants were 68.45%, 63.84%, and 56.65% less than WT. Penetration experiments with cicada wings and enzyme activity assays were used to further explore the effect of the fungus on chitin metabolism after gene deletion. Although the three gene deletion mutants penetrated the cicada wings successfully and continued to grow on the underlying medium, their colony sizes were reduced by 29.12% to 47.76%. The CDA enzyme activity of ΔCDA1 and ΔCDA3 decreased by 84.76% and 83.04%, respectively. These data showed that members of the CDA family play a different role in fungal growth, conidial quality, and virulence. IMPORTANCE In this study, we report the roles of CDA family in entomopathogenic fungus B. bassiana. Our results indicated that CDA modulates asexual development and regulates fungal virulence by altering chitin deacetylation and metabolic capacity. CDA affected the biological control potential and life history of B. bassiana by affecting its parasitic and saprophytic life. These findings provide novel insights into the roles of multiple CDA paralogues existing in fungal biocontrol agents.

Combatting insects mediated biotic stress through plant associated endophytic entomopathogenic fungi in horticultural crops

Horticultural production is a vital catalyst for economic growth, yet insect infestations reduce horticultural crop yield and quality. Pesticides and other pest control methods are used during planting to eliminate pests that cause direct and indirect losses. In such situations, endophytic entomo-pathogenic fungi (EEPF) can act as a potential tools for biological control. They protect plants by boosting growth, nutrition, morpho-physiology and salt or iron tolerance. Antixenosis, antibiosis and plant tolerance change insect performance and preferences. EEPF- plant colonisation slows herbivore development, food consumption, oviposition and larval survival. EEPF changes plant physio-chemical properties like volatile emission profile and secondary metabolite production to regulate insect pest defences. EEPF produces chitinases, laccases, amylases, and cellulases for plant defence. Recent studies focused on EEPF species' significance, isolation, identification and field application. Realizing their full potential is difficult due to insufficient mass production, storage stability and formulation. Genetic-molecular and bioinformatics can help to build EEPF-based biological control systems. Metagenomics helps study microbial EEPF taxonomy and function. Multi-omics and system biology can decode EEPF interactions with host plants and microorganisms. NGS (Next Generation Sequencing), comparative genomics, proteomics, transcriptomics, metabolomics, metatranscriptomics and microarrays are used to evaluate plant-EEPF relationships. IPM requires understanding the abiotic and biotic elements that influence plant-EEPF interaction and the physiological mechanisms of EEPF colonisation. Due to restricted research, there are hundreds of unexplored EEPFs, providing an urgent need to uncover and analyse them.

Expression and Molecular Modification of Chitin Deacetylase from Streptomyces bacillaris

Chitin deacetylase can be used in the green and efficient preparation of chitosan from chitin. Herein, a novel chitin deacetylase SbCDA from Streptomyces bacillaris was heterologously expressed and comprehensively characterized. SbDNA exhibits its highest deacetylation activity at 35 °C and pH 8.0. The enzyme activity is enhanced by Mn²⁺ and prominently inhibited by Zn²⁺, SDS, and EDTA. SbCDA showed better deacetylation activity on colloidal chitin, (GlcNAc)₅, and (GlcNAc)₆ than other forms of the substrate. Molecular modification of SbCDA was conducted based on sequence alignment and homology modeling. A mutant SbCDA63G with higher activity and better temperature stability was obtained. The deacetylation activity of SbCDA63G was increased by 133% compared with the original enzyme, and the optimal reaction temperature increased from 35 to 40 °C. The half-life of SbCDA63G at 40 °C is 15 h, which was 5 h longer than that of the original enzyme. The improved characteristics of the chitin deacetylase SbCDA63G make it a potential candidate to industrially produce chitosan from chitin.

New insights into engineered plant-microbe interactions for pesticide removal

Over the past decades, rapid industrialization along with the overutilization of organic pollutants/pesticides has altered the environmental circumstances. Moreover, various anthropogenic, xenobiotics and natural activities also affected plants, soil, and human health, in both direct and indirect ways. To counter this, several conventional methods are currently practiced, but are uneconomical, noxious, and is yet inefficient for large-scale application. Plant-microbe interactions are mediated naturally in an ecosystem and are practiced in several areas. Plant growth promoting rhizobacteria (PGPR) possess certain attributes affecting plant and soil consequently performing decontamination activity via a direct and indirect mechanism. PGPR also harbors indispensable genes stimulating the mineralization of several organic and inorganic compounds. This makes microbes potential candidates for contributing to sustainably remediating the harmful pesticide contaminants. There is a limited piece of information about the plant-microbe interaction pertaining predict and understand the overall interaction concerning a sustainable environment. Therefore, this review focuses on the plant-microbe interaction in the rhizosphere and inside the plant's tissues, along with the utilization augmenting the crop productivity, reduction in plant stress along with decontamination of pesticides/organic pollutants in soil for sustainable environmental management.

Biotechnologically produced chitosans with nonrandom acetylation patterns differ from conventional chitosans in properties and activities

Chitosans are versatile biopolymers with multiple biological activities and potential applications. They are linear copolymers of glucosamine and N-acetylglucosamine defined by their degree of polymerisation (DP), fraction of acetylation (F_A), and pattern of acetylation (PA). Technical chitosans produced chemically from chitin possess defined DP and F_A but random PA, while enzymatically produced natural chitosans probably have non-random PA. This natural process has not been replicated using biotechnology because chitin de-N-acetylases do not efficiently deacetylate crystalline chitin. Here, we show that such enzymes can partially N-acetylate fully deacetylated chitosan in the presence of excess acetate, yielding chitosans with F_A up to 0.7 and an enzyme-dependent non-random PA. The biotech chitosans differ from technical chitosans both in terms of physicochemical and nanoscale solution properties and biological activities. As with synthetic block co-polymers, controlling the distribution of building blocks within the biopolymer chain will open a new dimension of chitosan research and exploitation.

Endophyte-Mediated Stress Tolerance in Plants: A Sustainable Strategy to Enhance Resilience and Assist Crop Improvement

Biotic and abiotic stresses severely affect agriculture by affecting crop productivity, soil fertility, and health. These stresses may have significant financial repercussions, necessitating a practical, cost-effective, and ecologically friendly approach to lessen their negative impacts on plants. Several agrochemicals, such as fertilizers, pesticides, and insecticides, are used to improve plant health and protection; however, these chemical supplements have serious implications for human health. Plants being sessile cannot move or escape to avoid stress. Therefore, they have evolved to develop highly beneficial interactions with endophytes. The targeted use of beneficial plant endophytes and their role in combating biotic and abiotic stresses are gaining attention. Therefore, it is important to experimentally validate these interactions and determine how they affect plant fitness. This review highlights research that sheds light on how endophytes help plants tolerate biotic and abiotic stresses through plant–symbiont and plant–microbiota interactions. There is a great need to focus research efforts on this vital area to achieve a system-level understanding of plant–microbe interactions that occur naturally.

Microbial Effectors: Key Determinants in Plant Health and Disease

Effectors are small, secreted molecules that alter host cell structure and function, thereby facilitating infection or triggering a defense response. Effectoromics studies have focused on effectors in plant-pathogen interactions, where their contributions to virulence are determined in the plant host, i.e., whether the effector induces resistance or susceptibility to plant disease. Effector molecules from plant pathogenic microorganisms such as fungi, oomycetes and bacteria are major disease determinants. Interestingly, the effectors of non-pathogenic plant organisms such as endophytes display similar functions but have different outcomes for plant health. Endophyte effectors commonly aid in the establishment of mutualistic interactions with the plant and contribute to plant health through the induction of systemic resistance against pathogens, while pathogenic effectors mainly debilitate the plant's immune response, resulting in the establishment of disease. Effectors of plant pathogens as well as plant endophytes are tools to be considered in effectoromics for the development of novel strategies for disease management. This review aims to present effectors in their roles as promotors of health or disease for the plant host.

The right microbe-associated molecular patterns for effective recognition by plants

Plants are constantly exposed to diverse microbes and thus develop a sophisticated perceive system to distinguish non-self from self and identify non-self as friends or foes. Plants can detect microbes in apoplast via recognition of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) on the cell surface to activate appropriate signaling in response to microbes. MAMPs are highly conserved but essential molecules of microbes and often buried in microbes’ complex structure. Mature MAMPs are released from microbes by invasion-induced hydrolytic enzymes in apoplast and accumulate in proximity of plasma membrane-localized PRRs to be perceived as ligands to activate downstream signaling. In response, microbes developed strategies to counteract these processing. Here, we review how the form, the concentration, and the size of mature MAMPs affect the PRR-mediated immune signaling. In particular, we describe some potential applications and explore potential open questions in the fields.

Chitosan Is Necessary for the Structure of the Cell Wall, and Full Virulence of Ustilago maydis

Smut fungi comprise a large group of biotrophic phytopathogens infecting important crops, such as wheat and corn. U. maydis is a plant pathogenic fungus responsible for common smut in maize and teocintle. Through our analysis of the transcriptome of the yeast-to-mycelium dimorphic transition at acid pH, we determined the number of genes encoding chitin deacetylases of the fungus, and observed that the gene encoding one of them (UMAG_11922; CDA1) was the only one up-regulated. The mutation of this gene and the analysis of the mutants revealed that they contained reduced amounts of chitosan, were severely affected in their virulence, and showed aberrant mycelial morphology when grown at acid pH. When the CDA1 gene was reinserted into the mutants by the use of an autonomous replication plasmid, virulence and chitosan levels were recovered in the retro mutant strains, indicating that the CDA1 gene was involved in these features. These data revealed that chitosan plays a crucial role in the structure and morphogenesis of the cell wall during mycelial development of the fungus, and that in its absence, the cell wall becomes altered and is unable to support the stress imposed by the defense mechanism mounted on by the plant host during the infection process.

Action Mechanisms of Effectors in Plant-Pathogen Interaction

Plant pathogens are one of the main factors hindering the breeding of cash crops. Pathogens, including oomycetes, fungus, and bacteria, secrete effectors as invasion weapons to successfully invade and propagate in host plants. Here, we review recent advances made in the field of plant-pathogen interaction models and the action mechanisms of phytopathogenic effectors. The review illustrates how effectors from different species use similar and distinct strategies to infect host plants. We classify the main action mechanisms of effectors in plant-pathogen interactions according to the infestation process: targeting physical barriers for disruption, creating conditions conducive to infestation, protecting or masking themselves, interfering with host cell physiological activity, and manipulating plant downstream immune responses. The investigation of the functioning of plant pathogen effectors contributes to improved understanding of the molecular mechanisms of plant-pathogen interactions. This understanding has important theoretical value and is of practical significance in plant pathology and disease resistance genetics and breeding.

CERK1, more than a co-receptor in plant-microbe interactions

CERK1 (Chitin Elicitor Receptor Kinase 1), a lysin motif-containing pattern recognition receptor (PRR), perceives chitooligosaccharides (COs) to mount immune and symbiotic responses. However, CERK1, for a relatively long time, has been regarded as a co-receptor in plant immunity, mainly due to its lack of high binding affinity to known elicitors. Recent studies demonstrated several novel carbohydrates as ligands of CERK1 in different plant species and recognized CERK1 as a key receptor in plant immunity and symbiosis. This review summarizes recent knowledge acquired on the role of CERK1 in plant-microbe interactions.

Secreted Glycoside Hydrolase Proteins as Effectors and Invasion Patterns of Plant-Associated Fungi and Oomycetes

During host colonization, plant-associated microbes, including fungi and oomycetes, deliver a collection of glycoside hydrolases (GHs) to their cell surfaces and surrounding extracellular environments. The number and type of GHs secreted by each organism is typically associated with their lifestyle or mode of nutrient acquisition. Secreted GHs of plant-associated fungi and oomycetes serve a number of different functions, with many of them acting as virulence factors (effectors) to promote microbial host colonization. Specific functions involve, for example, nutrient acquisition, the detoxification of antimicrobial compounds, the manipulation of plant microbiota, and the suppression or prevention of plant immune responses. In contrast, secreted GHs of plant-associated fungi and oomycetes can also activate the plant immune system, either by acting as microbe-associated molecular patterns (MAMPs), or through the release of damage-associated molecular patterns (DAMPs) as a consequence of their enzymatic activity. In this review, we highlight the critical roles that secreted GHs from plant-associated fungi and oomycetes play in plant-microbe interactions, provide an overview of existing knowledge gaps and summarize future directions.

Enzymatic modification of native chitin and chitin oligosaccharides by an alkaline chitin deacetylase from Microbacterium esteraromaticum MCDA02

In this study, chitin deacetylase from Microbacterium esteraromaticum MCDA02 (MeCDA) was purified by ammonium sulfate precipitation, anion exchange chromatography, and superdex column chromatography. The molecular weight of purified MeCDA was approximately 26 kDa. The optimum pH and temperature of purified MeCDA were 8.0 and 30 °C, respectively. The enzyme activity is enhanced by metal ions K⁺ and Sr⁺ and inhibited by Co²⁺, Cd²⁺, and EDTA. The degree of deacetylation through enzymatic modification of MeCDA was removed an average of 32.75% of the acetyl groups for ɑ-chitin by acid-base titration. Meanwhile, MeCDA can catalyze the hydrolytic cleavage of the acetamido bond in GlcNAc units within chitin oligomers and polymers. Hence, the MeCDA is a potent chitin decomposer to catalyze chitin and chitin oligosaccharides deacetylation to prepare chitosan and chitosan oligosaccharide. This is a value-added utilization of chitin based biological resources.

Deciphering the ChitoCode: fungal chitins and chitosans as functional biopolymers

Chitins and chitosans are among the most widespread and versatile functional biopolymers, with interesting biological activities and superior material properties. While chitins are evolutionary ancient and present in many eukaryotes except for higher plants and mammals, the natural distribution of chitosans, i.e. extensively deacetylated derivatives of chitin, is more limited. Unequivocal evidence for its presence is only available for fungi where chitosans are produced from chitin by the action of chitin deacetylases. However, neither the structural details such as fraction and pattern of acetylation nor the physiological roles of natural chitosans are known at present. We hypothesise that the chitin deacetylases are generating chitins and chitosans with specific acetylation patterns and that these provide information for the interaction with specific chitin- and chitosan-binding proteins. These may be structural proteins involved in the assembly of the complex chitin- and chitosan-containing matrices such as fungal cell walls and insect cuticles, chitin- and chitosan-modifying and -degrading enzymes such as chitin deacetylases, chitinases, and chitosanases, but also chitin- and chitosan-recognising receptors of the innate immune systems of plants, animals, and humans. The acetylation pattern, thus, may constitute a kind of 'ChitoCode', and we are convinced that new in silico, in vitro, and in situ analytical tools as well as new synthetic methods of enzyme biotechnology and organic synthesis are currently offering an unprecedented opportunity to decipher this code. We anticipate a deeper understanding of the biology of chitin- and chitosan-containing matrices, including their synthesis, assembly, mineralisation, degradation, and perception. This in turn will improve chitin and chitosan biotechnology and the development of reliable chitin- and chitosan-based products and applications, e.g. in medicine and agriculture, food and feed sciences, as well as cosmetics and material sciences.

Wheat endophytes and their potential role in managing abiotic stress under changing climate

Wheat (Triticum aestivum L.) cultivation differs considerably in respect of soil type, temperature, pH, organic matter, moisture regime, etc. Among these, rising atmospheric temperature due to global warming is most important as it affects grain yield drastically. Studies have shown that for every 1°C rise in temperature above wheat's optimal growing temperature range of 20-25°C, there is a decrease in 2.8 days and 1.5 mg in the grain filling period and kernel weight, respectively, resulting in wheat yield reduction by 4-6 quintal per hectare. Growing demand for food and multidimensional issues of global warming may further push wheat crop to heat stress environments that can substantially affect heading duration, percent grain setting, maturity duration, grain growth rate and ultimately total grain yield. Considerable genetic variation exists in wheat gene pool with respect to various attributes associated with high temperature and stress tolerance; however, only about 15% of the genetic variability could be incorporated into cultivated wheat so far. Thus, alternative strategies have to be explored and implemented for sustainable, more productive and environment friendly agriculture. One of the feasible and environment friendly option is to look at micro-organisms that reside inside the plant without adversely affecting its growth, known as 'endophytes', and these colonize virtually all plant organs such as roots, stems, leaves, flowers and grains. The relationship between plant and endophytes is vital to the plant health, productivity and overall survival under abiotic stress conditions. Thus, it becomes imperative to enlist the endophytes (bacterial and fungal) isolated till date from wheat cultivars, their mechanism of ingression and establishment inside plant organs, genes involved in ingression, the survival advantages they confer to the plant under abiotic stress conditions and the potential benefits of their use in sustainable wheat cultivation.

Cryptococcus neoformans Cda1 and Cda2 coordinate deacetylation of chitin during infection to control fungal virulence

Chitosan, a deacetylated form of chitin, is required for the virulence of Cryptococcus neoformans. There are three chitin deacetylase genes (CDA) that are essential for chitosan production, and deletion of all three genes results in the absence of chitosan, loss of virulence, and induction of a protective host response when used as a vaccine. Cda1 plays a major role in deacetylating chitin during pulmonary infection of CBA/J mice. Inoculation with the cda1Δ strain did not lead to a lethal infection. However, the infection was not cleared. The persistence of the fungus in the host suggests that chitin is still being deacetylated by Cda2 and/or Cda3. To test this hypothesis, we subjected strains deleted of two CDA genes to fungal virulence in CBA/J, C57BL/6 and BALB/c and found that cda1Δcda2Δ was avirulent in all mouse lines, as evidenced by its complete clearance. Consistent with the major role of Cda1 in CBA/J, we found that cda2Δcda3Δ was as virulent as its wild-type progenitor KN99. On the other hand, cda1Δcda3Δ displayed virulence comparable to that of cda1Δ. The virulence of each mutant correlates with the amount of chitosan produced when grown under host-mimicking culture conditions. In addition, the avirulence of cda1Δcda2Δ was followed by the induction of a protective immune response in C57BL/6 and CBA/J mice, when a live or heat-killed form of the mutant was used as a vaccine respectively. Taken together, these data imply that, in C. neoformans, coordinated activity of both Cda1 and Cda2 is essential for mediating fungal virulence.

Deciphering the Chitin Code in Plant Symbiosis, Defense, and Microbial Networks

Chitin is a structural polymer in many eukaryotes. Many organisms can degrade chitin to defend against chitinous pathogens or use chitin oligomers as food. Beneficial microorganisms like nitrogen-fixing symbiotic rhizobia and mycorrhizal fungi produce chitin-based signal molecules called lipo-chitooligosaccharides (LCOs) and short chitin oligomers to initiate a symbiotic relationship with their compatible hosts and exchange nutrients. A recent study revealed that a broad range of fungi produce LCOs and chitooligosaccharides (COs), suggesting that these signaling molecules are not limited to beneficial microbes. The fungal LCOs also affect fungal growth and development, indicating that the roles of LCOs beyond symbiosis and LCO production may predate mycorrhizal symbiosis. This review describes the diverse structures of chitin; their perception by eukaryotes and prokaryotes; and their roles in symbiotic interactions, defense, and microbe-microbe interactions. We also discuss potential strategies of fungi to synthesize LCOs and their roles in fungi with different lifestyles.