Xylosylated Detoxification of the Rice Flavonoid Phytoalexin Sakuranetin by the Rice Sheath Blight Fungus Rhizoctonia solani

Potential of Streptomyces rochei G-6 for Biocontrol of Cucumber Wilt Disease and Growth Enhancement

Cucumber wilt disease, caused by Fusarium oxysporum f. sp. cucumerinum (FOC), is a major threat to cucumber production, especially in greenhouses. This study used a fermentation product derived from a new strain of Streptomyces rochei (G-6) to investigate the potential for biocontrol of cucumber wilt disease and the effect on promoting cucumber growth. In the first experiment, the inhibitory effect of S. rochei G-6 fermentation product (SGFP) on FOC growth was evaluated, then the effect of SGFP on wilt incidence and severity, as well as cucumber growth, antioxidant system, and soil nutrient conversion capacity were investigated. The results showed that SGFP inhibited FOC growth by 85.3% in the antimicrobial experiment. In the potting experiment, the incidence rate in the FOC group reached 88.7%, but it was only 56.0% in the SGFP1 group and 64.7% in the SGFP2 group, indicating the efficient inhibitory effect of SGFP on cucumber wilt, with the biocontrol effect of SGFP1 being higher than that of SGFP2. In addition, the disease index decreased significantly (p < 0.05) in both SGFP treatments, which was significantly (p < 0.05) lower in the SGFP1 group than in the SGFP2 group, indicating that pre-treatment was better than post-treatment in reducing the disease severity. In addition, SGFP promoted the growth of cucumber seedlings, as indicated by indicators related to the growth of aboveground and underground parts. Furthermore, the activities of antioxidant enzymes (superoxide dismutase, peroxidase, and catalase) in the cucumber seedlings increased after SGFP treatment and the malondialdehyde level was decreased, indicating a reduction in oxidative stress. SGFP also improved the soil nutrient conversion capacity by increasing the activities of urease, phosphatase, and sucrase, which may enhance nutrient uptake by cucumber seedling. The findings of this study suggest that SGFP is an effective biocontrol agent against cucumber wilt and also promotes cucumber growth by regulating the antioxidant system and soil environment, and its application is a promising solution to reduce wilt incidence in cucumber production.

Dissecting the role of salicylic acid in mediating stress response in mungbean cultivars concurrently exposed to Macrophomina phaseolina infection and drought stress

The combined stress studies provide fundamental knowledge that could assist in producing multiple stress resilient crops. The fungal phytopathogen, Macrophomina phaseolina is a major limiting factor in the productivity of the crop, Vigna radiata (mungbean). This fungal species tends to flourish under hot and dry conditions. Therefore, in this study the salicylic acid (SA) mediated stress responses in contrasting mungbean cultivars (Shikha and RMG-975) exposed to combined M. phaseolina infection (F) and drought stress (D) have been elucidated. The combined stress was applied to ten days seedlings in three orders i.e. drought followed by fungal infection (DF), drought followed by fungal infection with extended water deficit (DFD) and fungal infection followed by drought stress (FD). The severity of infection was analyzed using ImageJ analysis. Besides, the concentration of SA has been correlated with the phenylpropanoid pathway products, expression of pathogenesis-related proteins (β-1,3-glucanase and chitinase) and the specific activity of certain related enzymes (phenylalanine ammonia lyase, lipoxygenase and glutathione-S-transferase). The data revealed that the cultivar RMG-975 was relatively more tolerant than Shikha under individual stresses. However, the former became more susceptible to the infection under DFD treatment while the latter showed tolerance. Otherwise, the crown rot severity was reduced in both the cultivars under other combined treatments. The stress response analysis suggested that enhanced chitinase expression is vital for tolerance against both, the pathogen and drought stress. Also, it was noted that plants treat each stress combination differently and the role of SA was more prominently visible under individual stress conditions.

Biocontrol potential of volatile organic compounds produced by Streptomyces corchorusii CG-G2 to strawberry anthracnose caused by Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is a fungal disease of strawberry fruit. Biocontrol strategies holds tremendous promise in alleviating fruit decay. Here, 30 actinomycetes were isolated from rhizosphere soil of Calotropis gigantea. A strain labeled with CG-G2 exhibited the strongest antagonistic activity against C. gloeosporioides and was assigned as Streptomyces corchorusii. Compared to strain CG-G2 extracts, the volatile organic compounds (VOCs) had a high antifungal activity against anthracnose. These volatiles effectively inhibited mycelial growth and spore germination of C. gloeosporioides. The hyphal and conidial structure was severely destroyed. Metabolomics analysis revealed that VOCs inhibited C. gloeosporioides via inducing flavonoids metabolism contributing to antifungal activity. Three main antagonistic compounds in VOCs were identified as methyl 2-methyl butyrate, hexanenitrile and methyl 2-Ethyl hexanoate. Especially, methyl 2-methyl butyrate demonstrated a remarkable efficacy in inhibiting fruit decay and preserving fruit quality. Hence, S. corchorusii CG-G2 will be a potential biocontrol agent for controlling anthracnose on harvested fruits.

The OsBZR1-OsSPX1/2 module fine-tunes the growth-immunity trade-off in adaptation to phosphate availability in rice

The growth-promoting hormones brassinosteroids (BRs) and their key signaling component BZR1 play a vital role in balancing normal growth and defense reactions. Here, we discovered that BRs and OsBZR1 upregulated sakuranetin accumulation and conferred basal defense against Magnaporthe oryzae infection under normal conditions. Resource shortages, including phosphate (Pi) deficiency, potentially disrupt this growth-defense balance. OsSPX1 and OsSPX2 have been reported to sense Pi concentration and interact with the Pi signal mediator OsPHR2, thus regulating Pi starvation responses. In this study, we discovered that OsSPX1/2 interacts with OsBZR1 in both Pi-sufficient and Pi-deficient conditions, inhibiting BR-responsive genes. When Pi is sufficient, OsSPX1/2 is captured by OsPHR2, enabling most of OsBZR1 to promote plant growth and maintain basal resistance. In response to Pi starvation, more OsSPX1/2 is released from OsPHR2 to inhibit OsBZR1 activity, resulting in slower growth. Collectively, our study reveals that the OsBZR1-SPX1/2 module balances the plant growth-immunity trade-off in response to Pi availability.

Recent advancements in multifaceted roles of flavonoids in plant-rhizomicrobiome interactions

The rhizosphere consists of a plethora of microbes, interacting with each other as well as with the plants present in proximity. The root exudates consist of a variety of secondary metabolites such as strigolactones and other phenolic compounds such as coumarin that helps in facilitating communication and forming associations with beneficial microbes in the rhizosphere. Among different secondary metabolites flavonoids (natural polyphenolic compounds) continuously increasing attention in scientific fields for showing several slews of biological activities. Flavonoids possess a benzo-γ-pyrone skeleton and several classes of flavonoids have been reported on the basis of their basic structure such as flavanones, flavonols, anthocyanins, etc. The mutualistic association between plant growth-promoting rhizobacteria (PGPR) and plants have been reported to help the host plants in surviving various biotic and abiotic stresses such as low nitrogen and phosphorus, drought and salinity stress, pathogen attack, and herbivory. This review sheds light upon one such component of root exudate known as flavonoids, which is well known for nodulation in legume plants. Apart from the well-known role in inducing nodulation in legumes, this group of compounds has anti-microbial and antifungal properties helping in establishing defensive mechanisms and playing a major role in forming mycorrhizal associations for the enhanced acquisition of nutrients such as iron and phosphorus. Further, this review highlights the role of flavonoids in plants for recruiting non-mutualistic microbes under stress and other important aspects regarding recent findings on the functions of this secondary metabolite in guiding the plant-microbe interaction and how organic matter affects its functionality in soil.

Chemical Structure Diversity and Extensive Biological Functions of Specialized Metabolites in Rice

Rice (Oryza sativa L.) is thought to have been domesticated many times independently in China and India, and many modern cultivars are available. All rice tissues are rich in specialized metabolites (SPMs). To date, a total of 181 terpenoids, 199 phenolics, 41 alkaloids, and 26 other types of compounds have been detected in rice. Some volatile sesquiterpenoids released by rice are known to attract the natural enemies of rice herbivores, and play an indirect role in defense. Momilactone, phytocassane, and oryzalic acid are the most common diterpenoids found in rice, and are found at all growth stages. Indolamides, including serotonin, tryptamine, and N-benzoylserotonin, are the main rice alkaloids. The SPMs mainly exhibit defense functions with direct roles in resisting herbivory and pathogenic infections. In addition, phenolics are also important in indirect defense, and enhance wax deposition in leaves and promote the lignification of stems. Meanwhile, rice SPMs also have allelopathic effects and are crucial in the regulation of the relationships between different plants or between plants and microorganisms. In this study, we reviewed the various structures and functions of rice SPMs. This paper will provide useful information and methodological resources to inform the improvement of rice resistance and the promotion of the rice industry.

Qualitative metabolomics-based characterization of a phenolic UDP-xylosyltransferase with a broad substrate spectrum from Lentinus brumalis

Wood-decaying fungi are the major decomposers of plant litter. Heavy sequencing efforts on genomes of wood-decaying fungi have recently been made due to the interest in their lignocellulolytic enzymes; however, most parts of their proteomes remain uncharted. We hypothesized that wood-decaying fungi would possess promiscuous enzymes for detoxifying antifungal phytochemicals remaining in the dead plant bodies, which can be useful biocatalysts. We designed a computational mass spectrometry-based untargeted metabolomics pipeline for the phenotyping of biotransformation and applied it to 264 fungal cultures supplemented with antifungal plant phenolics. The analysis identified the occurrence of diverse reactivities by the tested fungal species. Among those, we focused on O-xylosylation of multiple phenolics by one of the species tested, Lentinus brumalis. By integrating the metabolic phenotyping results with publicly available genome sequences and transcriptome analysis, a UDP-glycosyltransferase designated UGT66A1 was identified and validated as an enzyme catalyzing O-xylosylation with broad substrate specificity. We anticipate that our analytical workflow will accelerate the further characterization of fungal enzymes as promising biocatalysts.

Rice Phytoalexins: Half a Century of Amazing Discoveries; Part I: Distribution, Biosynthesis, Chemical Synthesis, and Biological Activities

Cultivated rice is a staple food for more than half of the world's population, providing approximately 20% of the world's food energy needs. A broad spectrum of pathogenic microorganisms causes rice diseases leading to huge yield losses worldwide. Wild and cultivated rice species are known to possess a wide variety of antimicrobial secondary metabolites, known as phytoalexins, which are part of their active defense mechanisms. These compounds are biosynthesized transiently by rice in response to pathogens and certain abiotic stresses. Rice phytoalexins have been intensively studied for over half a century, both for their biological role and their potential application in agronomic and pharmaceutical fields. In recent decades, the growing interest of the research community, combined with advances in chemical, biological, and biomolecular investigation methods, has led to a notable acceleration in the growth of knowledge on rice phytoalexins. This review provides an overview of the knowledge gained in recent decades on the diversity, distribution, biosynthesis, chemical synthesis, and bioactivity of rice phytoalexins, with particular attention to the most recent advances in this research field.

Urate oxidase from tea microbe Colletotrichum camelliae is involved in the caffeine metabolism pathway and plays a role in fungal virulence

Tea is one of the most well-known, healthy beverages in the world. Tea plants produce caffeine as a secondary metabolite. Colletotrichum camelliae is one of the most important microbes frequently isolated from tea fields, and it causes anthracnose disease in tea plant. In the present work, we performed molecular microbiology and transcriptomic analyses of the C. camelliae - tea plant interaction to investigate the mechanism of fungal virulence and plant defense. Upon infection of tea plant with C. camelliae, we observed alterations in the expression of fungal transcripts, including those of many genes associated with caffeine metabolism, such as those encoding various transporters, xanthine dehydrogenase, and urate oxidase (UOX). In particular, the deletion of C. camelliae urate oxidase (CcUOX), which is involved in the caffeine metabolism pathway, reduced fungal tolerance to caffeine, and impaired fungal virulence. CcUOX is involved in caffeine metabolism by the degradation of uric acid contents. C. camelliaeΔCcUOX mutants impaired uric acid degradation in vivo. The CcUOX gene was cloned from C. camelliae, overexpressed in Escherichia coli, and the recombinant CcUOX protein displayed maximum activity at 30°C and a pH of 4.0. The recombinant CcUOX efficiently reduced uric acid in vitro suggesting a promising application in caffeine-contaminated environment management and in producing food with low purine contents to prevent uric acid related human diseases, such as hyperuricemia and gout.

Multifaceted roles of flavonoids mediating plant-microbe interactions

Plant-microbe interactions dynamically affect plant growth, health, and development. The mechanisms underpinning these associations are-to a large extent-mediated by specialized host-derived secondary metabolites. Flavonoids are one of the most studied classes of such metabolites, regulating both plant development and the interaction with commensal microbes. Here, we provide a comprehensive review of the multiple roles of flavonoids in mediating plant-microbe interactions. First, we briefly summarize the general aspects of flavonoid synthesis, transport, and exudation in plants. Then, we review the importance of flavonoids regulating plant-microbe interactions and dynamically influencing the overall community assembly of plant-root microbiomes. Last, we highlight potential knowledge gaps in our understanding of how flavonoids determine the interactions between plants and commensal microbes. Collectively, we advocate the importance of advancing research in this area toward innovative strategies to effectively manipulate plant-microbiome composition, in this case, via flavonoid production and exudation in plant roots. Video Abstract.

1, 2-trans-Stereoselective 7-O-Glycosylation of Flavonoids with Unprotected Pyranoses by Mitsunobu Reaction

The glycosylation of protecting-group-free pyranoses with flavonoids to generate flavonoid O-glycosides under Mitsunobu conditions was reported. The methodology allows to prepare a wide range of natural 7-flavonoid O -glycosides and their derivatives from commercially available chemicals in good to excellent yields with exclusive 1,2- trans -stereoselectivity regardless the anomeric configuration of employed pyranoses. The highly regioselective glycosylation was also achieved among different types of hydroxyl groups on the glycosyl acceptors.

Biosynthesis and antifungal activity of fungus-induced O-methylated flavonoids in maize

Fungal infection of grasses, including rice (Oryza sativa), sorghum (Sorghum bicolor), and barley (Hordeum vulgare), induces the formation and accumulation of flavonoid phytoalexins. In maize (Zea mays), however, investigators have emphasized benzoxazinoid and terpenoid phytoalexins, and comparatively little is known about flavonoid induction in response to pathogens. Here, we examined fungus-elicited flavonoid metabolism in maize and identified key biosynthetic enzymes involved in the formation of O-methylflavonoids. The predominant end products were identified as two tautomers of a 2-hydroxynaringenin-derived compound termed xilonenin, which significantly inhibited the growth of two maize pathogens, Fusarium graminearum and Fusarium verticillioides. Among the biosynthetic enzymes identified were two O-methyltransferases (OMTs), flavonoid OMT 2 (FOMT2), and FOMT4, which demonstrated distinct regiospecificity on a broad spectrum of flavonoid classes. In addition, a cytochrome P450 monooxygenase (CYP) in the CYP93G subfamily was found to serve as a flavanone 2-hydroxylase providing the substrate for FOMT2-catalyzed formation of xilonenin. In summary, maize produces a diverse blend of O-methylflavonoids with antifungal activity upon attack by a broad range of fungi.

Disarming the Host: Detoxification of Plant Defense Compounds During Fungal Necrotrophy

While fungal biotrophs are dependent on successfully suppressing/subverting host defenses during their interaction with live cells, necrotrophs, due to their lifestyle are often confronted with a suite of toxic metabolites. These include an assortment of plant defense compounds (PDCs) which can demonstrate broad antifungal activity. These PDCs can be either constitutively present in plant tissue or induced in response to infection, but are nevertheless an important obstacle which needs to be overcome for successful pathogenesis. Fungal necrotrophs have developed a number of strategies to achieve this goal, from the direct detoxification of these compounds through enzymatic catalysis and modification, to the active transport of various PDCs to achieve toxin sequestration and efflux. Studies have shown across multiple pathogens that the efficient detoxification of host PDCs is both critical for successful infection and often a determinant factor in pathogen host range. Here, we provide a broad and comparative overview of the various mechanisms for PDC detoxification which have been identified in both fungal necrotrophs and fungal pathogens which depend on detoxification during a necrotrophic phase of infection. Furthermore, the effect that these mechanisms have on fungal host range, metabolism, and disease control will be discussed.

Understanding sheath blight resistance in rice: the road behind and the road ahead

Rice sheath blight disease, caused by the basidiomycetous necrotroph Rhizoctonia solani, became one of the major threats to the rice cultivation worldwide, especially after the adoption of high-yielding varieties. The pathogen is challenging to manage because of its extensively broad host range and high genetic variability and also due to the inability to find any satisfactory level of natural resistance from the available rice germplasm. It is high time to find remedies to combat the pathogen for reducing rice yield losses and subsequently to minimize the threat to global food security. The development of genetic resistance is one of the alternative means to avoid the use of hazardous chemical fungicides. This review mainly focuses on the effort of better understanding the host-pathogen relationship, finding the gene loci/markers imparting resistance response and modifying the host genome through transgenic development. The latest development and trend in the R. solani-rice pathosystem research with gap analysis are provided.

The Evolutionary and Molecular Features of Broad Host-Range Necrotrophy in Plant Pathogenic Fungi

Necrotrophic fungal pathogens cause considerable disease on numerous economically important crops. Some of these pathogens are specialized to one or a few closely related plant species, whereas others are pathogenic on many unrelated hosts. The evolutionary and molecular bases of broad host-range necrotrophy in plant pathogens are not very well-defined and form an on-going area of research. In this review, we discuss what is known about broad host-range necrotrophic pathogens and compare them with their narrow host-range counterparts. We discuss the evolutionary processes associated with host generalism, and highlight common molecular features of the broad host-range necrotrophic lifestyle, such as fine-tuning of host pH, modulation of host reactive oxygen species and metabolic degradation of diverse host antimicrobials. We conclude that broad host-range necrotrophic plant pathogens have evolved a range of diverse and sometimes convergent responses to a similar selective regime governed by interactions with a highly heterogeneous host landscape.

Flavonoid accumulation in spontaneous cotton mutant results in red coloration and enhanced disease resistance

Cotton, the leading natural fiber, is cultivated worldwide, but its production is seriously threatened by pathogens. Accordingly, the selection of resistant cultivars has become a key priority of cotton breeding programs. In this study, a spontaneous mutant with red coloration (S156) and a control cultivar (S78) were used as experimental materials for a comparative analysis. Metabolomic analysis revealed the enrichment of flavonoids in S156 leaves compared with S78 leaves, and transcriptomic analysis revealed the upregulated expression of flavonoid biosynthesis genes in S156 leaves relative to S78 leaves. In addition, the red mutant showed a significantly increase in resistance to Verticillium dahliae, a fungal pathogen that poses a major threat to cotton production. The pathogen invasion process was suppressed in the red cotton cultivar. This study reveals the mechanism underlying the red coloration of S156 cotton and indicates the great potential of red cotton in pathogen- and insect-resistant breeding of cotton.

Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation

Rice (Oryza sativa L.) is an important food crop providing energy and nutrients for more than half of the world population. It produces vast amounts of secondary metabolites. At least 276 secondary metabolites from rice have been identified in the past 50 years. They mainly include phenolic acids, flavonoids, terpenoids, steroids, alkaloids, and their derivatives. These metabolites exhibit many physiological functions, such as regulatory effects on rice growth and development, disease-resistance promotion, anti-insect activity, and allelopathic effects, as well as various kinds of biological activities such as antimicrobial, antioxidant, cytotoxic, and anti-inflammatory properties. This review focuses on our knowledge of the structures, biological functions and activities, biosynthesis, and metabolic regulation of rice secondary metabolites. Some considerations about cheminformatics, metabolomics, genetic transformation, production, and applications related to the secondary metabolites from rice are also discussed.