Insights into stress responses in mandarins triggered by Bacillus subtilis cyclic lipopeptides and exogenous plant hormones upon Penicillium digitatum infection

Sanxiapeptin is an ideal preservative with a dual effect of controlling green mold and inducing systemic defense in postharvest citrus

Green mold is a common postharvest disease infected by Penicillium digitatum that causes citrus fruit decay, and severely affects fruit storage quality. This work aimed to investigate the antifungal activity of Sanxiapeptin against P. digitatum, and elucidate the possible mechanisms involved. Sanxiapeptin was capable of inhibiting spore germination, germ tube length and mycelial growth. The SYTOX green staining assay revealed that Sanxiapeptin targeted the fungal membrane, and changed the membrane permeability, leading to the leakage of cell constituents. Meanwhile, Sanxiapeptin could influence the cell wall permeability and integrity by increasing the activities of chitinase and glucanase, resulting in abnormal chitin consumption and the decrease of glucan. Intriguingly, Sanxiapeptin could effectively control postharvest decay in citrus fruits, and activate the host resistance responses by regulating the phenylpropanoid pathway. In conclusion, Sanxiapeptin exhibits multiphasic antifungal mechanisms of action to control green mold in citrus fruits, shows great potential as novel food preservatives.

Identification and genomic insights into a strain of Bacillus velezensis with phytopathogen-inhibiting and plant growth-promoting properties

The use of biological agents offers a sustainable alternative to chemical control in managing plant diseases. In this study, Bacillus velezensis IFST-221 was isolated from the rhizosphere of a healthy maize plant amidst a population showing severe disease symptoms. The investigation demonstrated a broad-spectrum antagonistic activity of IFST-221 against eight species of pathogenic ascomycetes and oomycetes, suggesting its potential utility in combating plant diseases like maize ear rot and cotton Verticillium wilt. Additionally, our study unveiled that IFST-221 has demonstrated significant plant growth-promoting properties, particularly in maize, cotton, tomato, and broccoli seedlings. This growth promotion was linked to its ability to produce indole-3-acetic acid, nitrogen fixation, phosphate and potassium solubilization, and biofilm formation in laboratory conditions. A complete genome sequencing of IFST-221 yielded a genome size of 3.858 M bp and a GC content of 46.71%. The genome analysis identified 3659 protein-coding genes, among which were nine secondary metabolite clusters with known antimicrobial properties. Additionally, three unknown compounds with potentially novel properties were also predicted from the genomic data. Genome mining also identified several key genes associated with plant growth regulation, colonization, and biofilm formation. These findings provide a compelling case for the application of B. velezensis IFST-221 in agricultural practices. The isolate's combined capabilities of plant growth promotion and antagonistic activity against common plant pathogens suggest its promise as an integrated biological agent in disease management and plant productivity enhancement.

A comprehensive investigation of the regulatory roles of OsERF096, an AP2/ERF transcription factor, in rice cold stress response

KEY MESSAGE

OsERF096 negatively regulates rice cold tolerance and mediates IAA biosynthesis and signaling under cold stress. The APETALA2/ethylene-responsive factor (AP2/ERF) transcription factors play important roles in regulating plant tolerance to abiotic stress. OsERF096 was previously identified as a direct target of miR1320, and was suggested to negatively regulate rice cold tolerance. In this study, we performed RNA-sequencing and targeted metabolomics assays to reveal the regulatory roles of OsERF096 in cold stress response. GO and KEGG analysis of differentially expressed genes showed that the starch and sucrose metabolism, plant-pathogen interaction, and plant hormone signal transduction pathways were significantly enriched. Quantification analysis confirmed a significant difference in sugar contents among WT and OsERF096 transgenic lines under cold treatment. Targeted metabolomics analysis uncovered that IAA accumulation and signaling were modified by OsERF096 in response to cold stress. Expectedly, qRT-PCR assays confirmed significant OsIAAs and OsARFs expression changes in OsERF096 transgenic lines. Finally, we identified three targets of OsERF096 based on RNA-seq, qRT-PCR, and dual-LUC assays. In summary, these results revealed the multiple regulatory roles of OsERF096 in cold stress response.

Transcriptomic analysis reveals that Bacillomycin D-C16 induces multiple pathways of disease resistance in cherry tomato

BACKGROUND

Bacillomycin D-C16 can induce resistance in cherry tomato against pathogens; however, the underlying molecular mechanism is poorly understood. Here, the effect of Bacillomycin D-C16 on induction of disease resistance in cherry tomato was investigated using a transcriptomic analysis.

RESULTS

Transcriptomic analysis revealed a series of obvious enrichment pathways. Bacillomycin D-C16 induced phenylpropanoid biosynthesis pathways and activated the synthesis of defense-related metabolites including phenolic acids and lignin. Moreover, Bacillomycin D-C16 triggered a defense response through both hormone signal transduction and plant-pathogen interactions pathways, and increased the transcription of several transcription factors (e.g., AP2/ERF, WRKY and MYB). These transcription factors might contribute to the further activated the expression of defense-related genes (PR1, PR10 and CHI) and stimulated the accumulation of H2O2.

CONCLUSION

Bacillomycin D-C16 can induce resistance in cherry tomato by activating the phenylpropanoid biosynthesis pathway, hormone signal transduction pathway and plant-pathogen interactions pathway, thus activating comprehensive defense reaction against pathogen invasion. These results provided a new insight into the bio-preservation of cherry tomato by the Bacillomycin D-C16.

Microbial Lipopeptides: Properties, Mechanics and Engineering for Novel Lipopeptides

Microorganisms produce active surface agents called lipopeptides (LPs) which are amphiphilic in nature. They are cyclic or linear compounds and are predominantly isolated from Bacillus and Pseudomonas species. LPs show antimicrobial activity towards various plant pathogens and act by inhibiting the growth of these organisms. Several mechanisms are exhibited by LPs, such as cell membrane disruption, biofilm production, induced systematic resistance, improving plant growth, inhibition of spores, etc., making them suitable as biocontrol agents and highly advantageous for industrial utilization. The biosynthesis of lipopeptides involves large multimodular enzymes referred to as non-ribosomal peptide synthases. These enzymes unveil a broad range of engineering approaches through which lipopeptides can be overproduced and new LPs can be generated asserting high efficacy. Such approaches involve several synthetic biology systems and metabolic engineering techniques such as promotor engineering, enhanced precursor availability, condensation domain engineering, and adenylation domain engineering. Finally, this review provides an update of the applications of lipopeptides in various fields.

Pepper growth promotion and biocontrol against Xanthomonas euvesicatoria by Bacillus cereus and Bacillus thuringiensis formulations

Background

Bacillus genus has been used in horticultural crops as a biocontrol agent against insect pests, microbial phytopathogens, and plant growth-promoting bacteria (PGPB), representing an alternative to agrochemicals. In particular, B. cereus (Bc) and B. thuringiensis (Bt) have been studied for their fungicidal and insecticidal activities. However, their use as biofertilizer formulations and biocontrol agents against phytopathogenic bacteria is limited.

Objective

To evaluate Bc and Bt formulations as PGPB and biocontrol agents against the bacterial spot agent Xanthomonas euvesicatoria (Xe) in greenhouse-grown chili peppers.

Methods

Bc and Bt isolates obtained from soil samples were identified and characterized using conventional biochemical and multiplex PCR identification methods. Bioassays to determine Bc and Bt isolates potential as PGPB were evaluated on chili pepper seedlings in seedbeds. In addition, formulations based on Bc (F-BC26 and F-BC08) and Bt (F-BT24) strains were assessed as biofertilizers on pepper, under controlled conditions. Furthermore, in vitro antagonism assays were performed by confronting Bc and Bt isolate formulations against Xe isolates in direct (foliage) and indirect (resistance induction) phytopathogen biocontrol assays on pepper plants, which were grown under controlled conditions for 15 d after formulations treatment.

Results

Isolates were identified as Bc and Bt. Formulations significantly improved pepper growth in seedbeds and pots, whereas in vitro bioassays demonstrated the bactericidal effect of Bc and Bt strains against Xe isolates. Furthermore, assays showed significant plant protection by F-BC26, F-BC08, and F-BT24 formulated strains against Xe.

Conclusion

Results indicated that F-BT24 and F-BC26 isolates formulations promoted pepper growth and protected it against Xanthomonas euvesicatoria.

Metabolites from Bacillus subtilis J-15 Affect Seedling Growth of Arabidopsis thaliana and Cotton Plants

Bacillus subtilis J-15 is a plant growth-promoting rhizobacteria isolated from the soil rhizosphere of cotton and is resistant to cotton verticillium wilt. This study evaluated the effects of metabolites of J-15 (J-15-Ms), including mycosubtilin, on plant growth using Arabidopsis and cotton plants. The results showed that J-15-Ms promoted Arabidopsis seeding growth at lower concentrations of 0.2 μg/mL but inhibited the growth at higher concentrations, such as 20 μg/mL. Similar results were obtained in cotton. Thus, J-15-Ms-treated plants showed low-concentration-induced growth promotion and high-concentration-induced growth inhibition. The J-15-Ms components were analyzed by liquid chromatography-mass spectrometry. Correlation analysis using the J-15 genomic databases suggested that J-15 may synthesize indoleacetic acid via the indole-3-pymvate pathway and indole-3-acetamide pathway. Treatment with mycosubtilin, a purified peptide from J-15-Ms, showed that the peptide promoted Arabidopsis growth at a low concentration (0.1 μg/mL) and inhibited plant growth at high concentrations (higher than 1 μg/mL), which also significantly increased plant lateral root number. Transcriptomic analysis showed that mycosubtilin might promote lateral root development and inhibit plant primary root growth by regulating the expression of the plant hormone signaling pathway. This study reveals the mechanism of Bacillus subtilis J-15 in affecting plant growth.

Genomic and Metabolomic Insights into Secondary Metabolites of the Novel Bacillus halotolerans Hil4, an Endophyte with Promising Antagonistic Activity against Gray Mold and Plant Growth Promoting Potential

The endophytic bacterial strain Hil4 was isolated from leaves of the medicinal plant Hypericum hircinum. It exhibited antifungal activity against Botrytis cinerea and a plethora of plant growth promoting traits in vitro. Whole genome sequencing revealed that it belongs to Bacillus halotolerans and possesses numerous secondary metabolite biosynthetic gene clusters and genes involved in plant growth promotion, colonization, and plant defense elicitation. The Mojavensin cluster was present in the genome, making this strain novel among plant-associated B. halotolerans strains. Extracts of secreted agar-diffusible compounds from single culture secretome extracts and dual cultures with B. cinerea were bioactive and had the same antifungal pattern on TLC plates after bioautography. UHPLC-HRMS analysis of the single culture secretome extract putatively annotated the consecutively produced antimicrobial substances and ISR elicitors. The isolate also proved efficient in minimizing the severity of gray mold post-harvest disease on table grape berries, as well as cherry tomatoes. Finally, it positively influenced the growth of Arabidopsis thaliana Col-0 and Solanum lycopersicum var. Chondrokatsari Messinias after seed biopriming in vitro. Overall, these results indicate that the B. halotolerans strain Hil4 is a promising novel plant growth promoting and biocontrol agent, and can be used in future research for the development of biostimulants and/or biological control agents.

Combined transcriptome and metabolome analyses reveal the potential mechanism for the inhibition of Penicillium digitatum by X33 antimicrobial oligopeptide

Penicillium digitatum is the primary spoilage fungus that causes green mold during postharvest in citrus. To reduce economic losses, developing more efficient and less toxic natural antimicrobial agents is urgently required. We previously found that the X33 antimicrobial oligopeptide (X33 AMOP), produced by Streptomyces lavendulae X33, exhibited a sterilization effect on P. digitatum. In this study, the effects, and physiological mechanisms of X33 AMOP as an inhibitor of P. digitatum were investigated. The transcriptional and metabolome profiling of P. digitatum exposed to X33 AMOP revealed 3648 genes and 190 metabolites that were prominently changed. The omics analyses suggested that X33 AMOP mainly inhibited P. digitatum growth by affecting cell integrity, genetic information delivery, oxidative stress tolerance, and energy metabolism. These findings provide helpful information regarding the antimicrobial mechanism of X33 AMOP against P. digitatum at the molecular level and indicate that X33 AMOP is a potential candidate to control P. digitatum.

A Genome-Wide Analysis of Pathogenesis-Related Protein-1 (PR-1) Genes from Piper nigrum Reveals Its Critical Role during Phytophthora capsici Infection

Black pepper (Piper nigrum L.) is a prominent spice that is an indispensable ingredient in cuisine and traditional medicine. Phytophthora capsici, the causative agent of footrot disease, causes a drastic constraint in P. nigrum cultivation and productivity. To counterattack various biotic and abiotic stresses, plants employ a broad array of mechanisms that includes the accumulation of pathogenesis-related (PR) proteins. Through a genome-wide survey, eleven PR-1 genes that belong to a CAP superfamily protein with a caveolin-binding motif (CBM) and a CAP-derived peptide (CAPE) were identified from P. nigrum. Despite the critical functional domains, PnPR-1 homologs differ in their signal peptide motifs and core amino acid composition in the functional protein domains. The conserved motifs of PnPR-1 proteins were identified using MEME. Most of the PnPR-1 proteins were basic in nature. Secondary and 3D structure analyses of the PnPR-1 proteins were also predicted, which may be linked to a functional role in P. nigrum. The GO and KEGG functional annotations predicted their function in the defense responses of plant-pathogen interactions. Furthermore, a transcriptome-assisted FPKM analysis revealed PnPR-1 genes mapped to the P. nigrum-P. capsici interaction pathway. An altered expression pattern was detected for PnPR-1 transcripts among which a significant upregulation was noted for basic PnPR-1 genes such as CL10113.C1 and Unigene17664. The drastic variation in the transcript levels of CL10113.C1 was further validated through qRT-PCR and it showed a significant upregulation in infected leaf samples compared with the control. A subsequent analysis revealed the structural details, phylogenetic relationships, conserved sequence motifs and critical cis-regulatory elements of PnPR-1 genes. This is the first genome-wide study that identified the role of PR-1 genes during P. nigrum-P. capsici interactions. The detailed in silico experimental analysis revealed the vital role of PnPR-1 genes in regulating the first layer of defense towards a P. capsici infection in Panniyur-1 plants.

Antibacterial Potential of Bacillus amyloliquefaciens GJ1 against Citrus Huanglongbing

Citrus huanglongbing (HLB) is a destructive disease caused by Candidatus Liberibacter species and is a serious global concern for the citrus industry. To date, there is no established strategy for control of this disease. Previously, Bacillus amyloliquefaciens GJ1 was screened as the biocontrol agent against HLB. In this study, two-year-old citrus infected by Ca. L. asiaticus were treated with B. amyloliquefaciens GJ1 solution via root irrigation. In these plants, after seven irrigation treatments, the results indicated that the photosynthetic parameters, chlorophyll content, resistance-associated enzyme content and the expression of defense-related genes were significantly higher than for the plants treated with the same volume water. The content of starch and soluble sugar were significantly lower, compared to the control treatment. The parallel reaction monitoring (PRM) results revealed that treatment with B. amyloliquefaciens GJ1 solution, the expression levels of 3 proteins with photosynthetic function were upregulated in citrus leaves. The accumulation of reactive oxygen species (ROS) in citrus leaves treated with B. amyloliquefaciens GJ1 flag22 was significantly higher than untreated plants and induced the defense-related gene expression in citrus. Finally, surfactin was identified from the fermentation broth of B. amyloliquefaciens GJ1 by high-performance liquid chromatography. These results indicate that B. amyloliquefaciens GJ1 may improve the immunity of citrus by increasing the photosynthesis and enhancing the expression of the resistance-related genes.

Targeted transcriptional and proteomic studies explicate specific roles of Bacillus subtilis iturin A, fengycin, and surfactin on elicitation of defensive systems in mandarin fruit during stress

Application of Bacillus cyclic lipopeptides (CLPs); fengycin, iturin A and surfactin has shown a great potential in controlling the spread of green mold pathogen invasion (Penicillium digitatum) in wounded mandarin fruit during postharvest period. The limited defensive protein profiles followed specific expression of pivotal genes relating to plant hormone mediating signaling pathways of the CLPs’ action on stimulating host plant resistance have been exhibited. The present study aimed to elucidate the specific effect of individual CLP obtained from Bacillus subtilis ABS-S14 as elicitor role on activation of plant defensive system at transcriptional and proteomic levels with and without P. digitatum co-application in mandarin fruit. Fengycin and iturin A elevated the gene expression of PAL, ACS1, ACO, CHI, and GLU while significantly stimulating plant POD transcription was only detected in the treatments of surfactin both with and without following P. digitatum. An increase of LOX and PR1 gene transcripts was determined in the treatments of individual CLP with fungal pathogen co-application. Fengycin activated production of unique defensive proteins such as protein involved in ubiquinone biosynthetic process in treated flavedo without P. digitatum infection. Proteins involved in the auxin modulating pathway were present in the iturin A and surfactin treatments. CLP-protein binding assay following proteome analysis reveals that iturin A attached to 12-oxophytodienoate reductase 2 involved in the oxylipin biosynthetic process required for jasmonic acid production which is implicated in induced systemic resistance (ISR). This study suggests specific elicitor action of individual CLP, particularly iturin A showed the most powerful in stimulating the ISR system in response to stresses in postharvest mandarins.