Antibacterial activity of peptaibols from Trichoderma longibrachiatum SMF2 against gram-negative Xanthomonas oryzae pv. oryzae, the causal agent of bacterial leaf blight on rice

Antimicrobial Peptides: Classification, Mechanism, and Application in Plant Disease Resistance

Antimicrobial peptides (AMPs) are a class of alkaline, small molecules found widely in nature. This article surveys the classifications of AMPs, delving into their modes of action and their role in controlling significant plant diseases caused by bacteria, viruses, and fungi. It also explores the prospects and challenges in this field, aiming to provide insights for enhancing crop disease resistance, ensuring food security, deepening the understanding of pathogen mechanisms, and protecting ecological balance.

Functional disparities of malonyl-ACP decarboxylase between Xanthomonas campestris and Xanthomonas oryzae

Xanthomonas campestris pv. campestris (Xcc) and X. oryzae pv. oryzae (Xoo) are crucial plant pathogenic bacteria, causing crucifer black rot and rice leaf blight, respectively. Both bacterial species encode a protein containing the YiiD_C domain, designated MadB, which exhibits an 87.5% sequence identity between their MadBs. The madB genes from either Xoo or Xcc successfully restored the growth defect in Ralstonia solanacearum and Escherichia coli fabH mutants in vivo. In vitro assays demonstrated that MadB proteins possess malonyl-ACP decarboxylase activity, although Xcc MadB exhibited lower activity compared with Xoo MadB. Mutation of madB in both Xoo and Xcc strains led to decreased pathogenicity in their respective host plants. Interestingly, the Xoo madB mutant exhibited a significant increase in branched-chain fatty acid production, whereas the Xcc madB mutant showed only minor changes in fatty acid composition. Despite the reduction in exopolysaccharide (EPS) synthesis due to madB mutation in both Xoo and Xcc, EPS production in the Xoo madB mutant could be restored by exogenous sodium acetate supplementation. In contrast, sodium acetate failed to restore EPS synthesis in the Xcc madB mutant. Biochemical and genetic analyses indicated that these divergent physiological roles arise from the distinct biochemical functions of MadB in the two bacteria. In Xoo, the fatty acid synthesis (FAS) pathway mediated by MadB operates independently of the FAS pathway mediated by FabH. Conversely, in Xcc, the FAS pathway mediated by FabH is the primary route, with MadB's pathway serving a supplementary and regulatory role. Further analysis of gene organization and expression regulation of madB in both bacteria corroborates these distinctions.

IMPORTANCE

Despite the high conservation of the mad gene within the Proteobacteria, the physiological roles of the Mad protein remain largely unclear. Xoo and Xcc are bacteria with very close phylogenetic relationships, both encoding malonyl-ACP decarboxylase (MadB). However, MadB demonstrates substantial physiological function variations between these two species. This study demonstrates that even in closely related bacteria, homologous genes have adopted different evolutionary pathways to adapt to diverse living environments, forming unique gene expression regulation mechanisms. This has led to the biochemical functional divergence of homologous proteins within their respective species, ultimately resulting in distinct physiological functions.

Harnessing Fungal Bioagents Rich in Volatile Metabolites for Sustainable Crop Protection: A Critical Review

Pests and diseases have a significant impact on crop health and yields, posing a serious threat to global agriculture. Effective management strategies, such as integrated pest management (IPM), including crop rotation, use of synthetic pesticides, biological control, and resistant/tolerant crop varieties, are essential to mitigate these risks and ensure sustainable agricultural practices. Fungal bioagents play an important role in managing phytopathogens and insect pests by acting as biological agents. They promote healthy plant growth by enhancing the uptake of nutrients and combating systemic resistance in plants. Furthermore, fungal bioagents are environmentally friendly, reducing application of fungicides and insecticides and minimizing their negative impact on the crops and environment. Their use in IPM promotes sustainable agriculture and ensures high-quality crops while maintaining soil health and microbial biodiversity. These fungal bioagents are rich sources of volatile organic compounds (VOCs), which play an important role in biological communication during interaction with insect pests and phytopathogens. In pest management, VOC production by beneficial fungi is accountable for their efficacy against pests and pathogens. Thus, this review discusses the important fungal bioagents producing VOCs, extraction methods of VOC, and the use of VOC-producing fungi in pest and disease management, knowledge gaps, and future research areas.

Biological control of bacterial leaf blight (BLB) in rice-A sustainable approach

Bacterial leaf blight (BLB) in rice, caused by the pathogen Xanthomonas oryzae pv. oryzae, is a significant agricultural problem managed through chemical control and cultivating rice varieties with inherent resistance to the bacterial pathogen. Research has highlighted the potential of using antagonistic microbes which can suppress the BLB pathogen through the production of secondary metabolites like siderophores, rhamnolipids, and hydroxy-alkylquinolines offering a sustainable alternative for BLB management. Additionally, the induction of plant immunity and defense-related enzymes in rice further enhances the resistance against the disease. Therefore, implementation of biological controls can complement chemical treatments in contributing towards the sustainability of rice production systems by aiming at host immunity improvement and killing of pathogen. It is crucial to continue exploring and understanding the complex interactions between various beneficial microbes, the rice plants, and the BLB pathogen to optimize and implement effective biocontrol strategies in future.

Synergistic Effect of Two Peptaibols from Biocontrol Fungus Trichoderma longibrachiatum Strain 40418 on CO-Induced Plant Resistance

Trichoderma longibrachiatum is a filamentous fungus used as a biological control agent against different plant diseases. The multifunctional secondary metabolites synthesized by Trichoderma, called peptaibols, have emerged as key elicitors in plant innate immunity. This study obtained a high-quality genome sequence for the T. longibrachiatum strain 40418 and identified two peptaibol biosynthetic gene clusters using knockout techniques. The two gene cluster products were confirmed as trilongin AIV a (11-residue) and trilongin BI (20-residue) using liquid chromatography coupled with tandem mass spectrometry. Further investigations revealed that these peptaibols induce plant resistance to Pseudomonas syringae pv tomato (Pst) DC3000 infection while triggering plant immunity and cell death. Notably, the two peptaibols exhibit synergistic effects in plant-microbe signaling interactions, with trilongin BI having a predominant role. Moreover, the induction of tomato resistance against Meloidogyne incognita showed similarly promising results.

Cytotoxic Peptaibols from Trichoderma strigosum

Five new lipopeptaibols (1-5) and eight new 19-residue peptaibols (8-15) along with two known lipopeptaibols, lipovelutibols C (6) and D (7) were isolated from Trichoderma strigosum. The planar structures of the newly discovered peptaibols (1-5, 8-15) were elucidated using 1D and 2D NMR, and UPLC-MS/MS data. The absolute configurations for new peptaibols (1-5, 8-15) were elucidated using the advanced Marfey's method and GITC (2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl isothiocyanate) derivatization. Through analysis of CD spectra, these peptabols were found to have right-handed helical conformations. While most of the new compounds were significantly more active than the positive control, 9, 10, 12, and 15 containing Ser and Leu at positions 10 and 11, respectively, were the most cytotoxic against MDA-MB-231, SNU449, SKOV3, DU145, and HCT116 cancer cell lines, and the 19-residue peptaibols were generally more potent than lipopeptaibols.

Cytotoxic Peptaibols from Trichoderma guizhouense, a Fungus Isolated from an Urban Soil Sample

Soil sustains human life by nourishing crops, storing food sources, and housing microbes, which may affect the nutrition and biosynthesis of secondary metabolites, some of which are used as drugs. To identify lead compounds for a new class of drugs, we collected soil-derived fungal strains from various environments, including urban areas. As various human pathogens are assumed to influence the biosynthetic pathways of metabolites in soil fungi, leading to the production of novel scaffolds, we focused our work on densely populated urban areas and tourist attractions. A soil-derived fungal extract library was screened against MDA-MB-231 cells to derive their cytotoxic activity. Notably, 10 μg/mL of the extract of Trichoderma guizhouense (DS9-1) was found to exhibit an inhibitory effect of 71%. Fractionation, isolation, and structure elucidation efforts led to the identification of nine new peptaibols, trichoguizaibols A-I (1-9), comprising 14 amino acid residues (14-AA peptaibols), and three new peptaibols, trichoguizaibols J-L (10-12), comprising 18 amino acid residues (18-AA peptaibols). The chemical structures of 1-12 were determined based on their 1D and 2D NMR spectra, HRESIMS, electronic circular dichroism data, and results of the advanced Marfey's method. The 18-AA peptaibols were found to exhibit cytotoxicity against MDA-MB-231, SK-Hep1, SKOV3, DU145, and HCT116 cells greater than that of the 14-AA peptaibols. Among these compounds, 10-12 exhibited potent sub-micromolar IC50 values. These results are expected to shed light on a new direction for developing novel scaffolds as anticancer agents.

An insight into pathogenicity and virulence gene content of Xanthomonas spp. and its biocontrol strategies

The genus Xanthomonas primarily serves as a plant pathogen, targeting a diverse range of economically significant crops on a global scale. Xanthomonas spp. utilizes a collection of toxins, adhesins, and protein effectors as part of their toolkit to thrive in their surroundings, and establish themselves within plant hosts. The bacterial secretion systems (Type 1 to Type 6) assist in delivering the effector proteins to their intended destinations. These secretion systems are specialized multi-protein complexes responsible for transporting proteins into the extracellular milieu or directly into host cells. The potent virulence and systematic infection system result in rapid dissemination of the bacteria, posing significant challenges in management due to complexities and substantial loss incurred. Consequently, there has been a notable increase in the utilization of chemical pesticides, leading to bioaccumulation and raising concerns about adverse health effects. Biological control mechanisms through beneficial microorganism (Bacillus, Pseudomonas, Trichoderma, Burkholderia, AMF, etc.) have proven to be an appropriate alternative in integrative pest management system. This review details the pathogenicity and virulence factors of Xanthomonas, as well as its control strategies. It also encourages the use of biological control agents, which promotes sustainable and environmentally friendly agricultural practices.

Peptaibiotics: Harnessing the potential of microbial secondary metabolites for mitigation of plant pathogens

Agricultural systems are in need of low-cost, safe antibiotics to protect crops from pests and diseases. Peptaibiotics, a family of linear, membrane-active, amphipathic polypeptides, have been shown to exhibit antibacterial, antifungal, and antiviral activity, and to be inducers of plant resistance against a wide range of phytopathogens. Peptaibiotics belong to the new generation of alternatives to agrochemicals, aligned with the United Nations Sustainable Development Goals and the One Health approach toward ensuring global food security and safety. Despite that, these fungi-derived, non-ribosomal peptides remain surprisingly understudied, especially in agriculture, where only a small number has been tested against a reduced number of phytopathogens. This lack of adoption stems from peptaibiotics' poor water solubility and the difficulty to synthesize and purify them in vitro, which compromises their delivery and inclusion in formulations. In this review, we offer a comprehensive analysis of peptaibiotics' classification, biosynthesis, relevance to plant protection, and mode of action against phytopathogens, along with the techniques enabling researchers to extract, purify, and elucidate their structure, and the databases holding such valuable data. It is also discussed how chemical synthesis and ionic liquids could increase their solubility, how genetic engineering and epigenetics could boost in vitro production, and how omics can reduce screenings' workload through in silico selection of the best candidates. These strategies could turn peptaibiotics into effective, ultra-specific, biodegradable tools for phytopathogen control.

Chitosan: Properties and Its Application in Agriculture in Context of Molecular Weight

Chitosan is a naturally occurring compound that can be obtained from deacetylated chitin, which is obtained from various sources such as fungi, crustaceans, and insects. Commercially, chitosan is produced from crustaceans. Based on the range of its molecular weight, chitosan can be classified into three different types, namely, high molecular weight chitosan (HMWC, >700 kDa), medium molecular weight chitosan (MMWC, 150-700 kDa), and low molecular weight chitosan (LMWC, less than 150 kDa). Chitosan shows several properties that can be applied in horticultural crops, such as plant root growth enhancer, antimicrobial, antifungal, and antiviral activities. Nevertheless, these properties depend on its molecular weight (MW) and acetylation degree (DD). Therefore, this article seeks to extensively review the properties of chitosan applied in the agricultural sector, classifying them in relation to chitosan's MW, and its use as a material for sustainable agriculture.

The Importance of Microorganisms for Sustainable Agriculture-A Review

In the face of climate change, progressive degradation of the environment, including agricultural land negatively affecting plant growth and development, endangers plant productivity. Seeking efficient and sustainable agricultural techniques to replace agricultural chemicals is one of the most important challenges nowadays. The use of plant growth-promoting microorganisms is among the most promising approaches; however, molecular mechanisms underneath plant-microbe interactions are still poorly understood. In this review, we summarized the knowledge on plant-microbe interactions, highlighting the role of microbial and plant proteins and metabolites in the formation of symbiotic relationships. This review covers rhizosphere and phyllosphere microbiomes, the role of root exudates in plant-microorganism interactions, the functioning of the plant's immune system during the plant-microorganism interactions. We also emphasized the possible role of the stringent response and the evolutionarily conserved mechanism during the established interaction between plants and microorganisms. As a case study, we discussed fungi belonging to the genus Trichoderma. Our review aims to summarize the existing knowledge about plant-microorganism interactions and to highlight molecular pathways that need further investigation.