Identification of a Potential ISR Determinant from Pseudomonas aeruginosa PM12 against Fusarium Wilt in Tomato

Progress in the Study of Natural Antimicrobial Active Substances in Pseudomonas aeruginosa

The prevalence of antimicrobial resistance reduces the effectiveness of antimicrobial drugs in the prevention and treatment of infectious diseases caused by pathogens such as bacteria, fungi, and viruses. Microbial secondary metabolites have been recognized as important sources for new drug discovery and development, yielding a wide range of structurally novel and functionally diverse antimicrobial drugs for the treatment of a variety of diseases that are considered good producers of novel antimicrobial drugs. Bacteria produce a wide variety of antimicrobial compounds, and thus, antibiotics derived from natural products still dominate over purely synthetic antibiotics among the antimicrobial drugs developed and introduced over the last four decades. Among them, Pseudomonas aeruginosa secondary metabolites constitute a richly diverse source of antimicrobial substances with good antimicrobial activity. Therefore, they are regarded as an outstanding resource for finding novel bioactive compounds. The exploration of antimicrobial compounds among Pseudomonas aeruginosa metabolites plays an important role in drug development and biomedical research. Reports on the secondary metabolites of Pseudomonas aeruginosa, many of which are of pharmacological importance, hold great promise for the development of effective antimicrobial drugs against microbial infections by drug-resistant pathogens. In this review, we attempt to summarize published articles from the last twenty-five years (2000-2024) on antimicrobial secondary metabolites from Pseudomonas aeruginosa.

Soybean microbiome composition and the impact of host plant resistance

Microbial communities play an important role in the growth and development of plants, including plant immunity and the decomposition of complex substances into absorbable nutrients. Hence, utilizing beneficial microbes becomes a promising strategy for the optimization of plant growth. The objective of this research was to explore the root bacterial profile across different soybean genotypes and the change in the microbial community under soybean cyst nematode (SCN) infection in greenhouse conditions using 16S rRNA sequencing. Soybean genotypes with soybean cyst nematode (SCN) susceptible and resistant phenotypes were grown under field and greenhouse conditions. Bulked soil, rhizosphere, and root samples were collected from each replicate. Sequencing of the bacterial 16S gene indicated that the bacterial profile of soybean root and soil samples partially overlapped but also contained different communities. The bacterial phyla Proteobacteria, Actinobacteria, and Bacteroidetes dominate the soybean root-enriched microbiota. The structure of bacteria was significantly affected by sample year (field) or time point (greenhouse). In addition, the host genotype had a small but significant effect on the diversity of the root microbiome under SCN pressure in the greenhouse test. These differences may potentially represent beneficial bacteria or secondary effects related to SCN resistance.

Controlling Tomato Fusarium Wilt Disease through Bacillus thuringiensis-Mediated Defense Primining

Background

Fusarium wilt caused by the fungus Fusarium oxysporum f. sp. lycopersici (Fol) (Sacc.) W.C. Snyder and H.N. Hans is one of the most prevalent and devastating diseases of tomato plants (Solanum lycoprsicum L.) that leads to a severe reduction in crop yield almost worldwide.

Objective

Evaluation of biocontrol potential of Bacillus thuringiensis (Bt) isolate IBRC-M11096, against Fol in tomato through primin.

Materials and Methods

qRT-PCR technique was applied to analyze the effect of the strain on the hormonal defensive pathways; transcriptional responses of jasmonic acid (COI1, Pin2) and salicylic acid (NRP1 and PR1) pathway genes in Bt-treated plants following inoculation of Fol as compared to the plants only challenged with Fol. Also, the potential of the bacterial strain as a biocontrol agent was studied by evaluating growth indices and area under disease progress curve (AUDPC).

Results

The transcription of both defensive hormonal pathway genes (COI1, Pin2, NPR1, PR1) increased due to bacterial priming. The bacterial priming reduced the AUDPC compared to the inoculation with only Fol. The strain reduced the disease symptoms, and compared to the plants only challenged with the fungus, the bacterial strain significantly raised shoot dry and fresh weights and root dry weight.

Conclusion

Priming with the Bt strain led to improved shoot and root growth indices, reduced AUDPC, and fortified responses of both JA and SA hormonal pathways. However, further full-span studies are required to judge the efficacy of the bacterial strain in the biological control of tomato fusarium wilt under field conditions.

Microbial diversity in soils suppressive to Fusarium diseases

Fusarium species are cosmopolitan soil phytopathogens from the division Ascomycota, which produce mycotoxins and cause significant economic losses of crop plants. However, soils suppressive to Fusarium diseases are known to occur, and recent knowledge on microbial diversity in these soils has shed new lights on phytoprotection effects. In this review, we synthesize current knowledge on soils suppressive to Fusarium diseases and the role of their rhizosphere microbiota in phytoprotection. This is an important issue, as disease does not develop significantly in suppressive soils even though pathogenic Fusarium and susceptible host plant are present, and weather conditions are suitable for disease. Soils suppressive to Fusarium diseases are documented in different regions of the world. They contain biocontrol microorganisms, which act by inducing plants' resistance to the pathogen, competing with or inhibiting the pathogen, or parasitizing the pathogen. In particular, some of the Bacillus, Pseudomonas, Paenibacillus and Streptomyces species are involved in plant protection from Fusarium diseases. Besides specific bacterial populations involved in disease suppression, next-generation sequencing and ecological networks have largely contributed to the understanding of microbial communities in soils suppressive or not to Fusarium diseases, revealing different microbial community patterns and differences for a notable number of taxa, according to the Fusarium pathosystem, the host plant and the origin of the soil. Agricultural practices can significantly influence soil suppressiveness to Fusarium diseases by influencing soil microbiota ecology. Research on microbial modes of action and diversity in suppressive soils should help guide the development of effective farming practices for Fusarium disease management in sustainable agriculture.

The Cultured Microbiome of Pollinated Maize Silks Shifts after Infection with Fusarium graminearum and Varies by Distance from the Site of Pathogen Inoculation

Styles transmit pollen-derived sperm nuclei from pollen to ovules, but also transmit environmental pathogens. The microbiomes of styles are likely important for reproduction/disease, yet few studies exist. Whether style microbiome compositions are spatially responsive to pathogens is unknown. The maize pathogen Fusarium graminearum enters developing grain through the style (silk). We hypothesized that F. graminearum treatment shifts the cultured transmitting silk microbiome (TSM) compared to healthy silks in a distance-dependent manner. Another objective of the study was to culture microbes for future application. Bacteria were cultured from husk-covered silks of 14 F. graminearum-treated diverse maize genotypes, proximal (tip) and distal (base) to the F. graminearum inoculation site. Long-read 16S sequences from 398 isolates spanned 35 genera, 71 species, and 238 OTUs. More bacteria were cultured from F. graminearum-inoculated tips (271 isolates) versus base (127 isolates); healthy silks were balanced. F. graminearum caused a collapse in diversity of ~20-25% across multiple taxonomic levels. Some species were cultured exclusively or, more often, from F. graminearum-treated silks (e.g., Delftia acidovorans, Klebsiella aerogenes, K. grimontii, Pantoea ananatis, Stenotrophomonas pavanii). Overall, the results suggest that F. graminearum alters the TSM in a distance-dependent manner. Many isolates matched taxa that were previously identified using V4-MiSeq (core and F. graminearum-induced), but long-read sequencing clarified the taxonomy and uncovered greater diversity than was initially predicted (e.g., within Pantoea). These isolates represent the first comprehensive cultured collection from pathogen-treated maize silks to facilitate biocontrol efforts and microbial marker-assisted breeding.

Evaluation and Genome Mining of Bacillus stercoris Isolate B.PNR1 as Potential Agent for Fusarium Wilt Control and Growth Promotion of Tomato

Recently, strategies for controlling Fusarium oxysporum f. sp. lycopersici (Fol), the causal agent of Fusarium wilt of tomato, focus on using effective biocontrol agents. In this study, an analysis of the biocontrol and plant growth promoting (PGP) attributes of 11 isolates of loamy soil Bacillus spp. has been conducted. Among them, the isolates B.PNR1 and B.PNR2 inhibited the mycelial growth of Fol by inducing abnormal fungal cell wall structures and cell wall collapse. Moreover, broad-spectrum activity against four other plant pathogenic fungi, F. oxysporum f. sp. cubense race 1 (Foc), Sclerotium rolfsii, Colletotrichum musae, and C. gloeosporioides were noted for these isolates. These two Bacillus isolates produced indole acetic acid, phosphate solubilization enzymes, and amylolytic and cellulolytic enzymes. In the pot experiment, the culture filtrate from B.PNR1 showed greater inhibition of the fungal pathogens and significantly promoted the growth of tomato plants more than those of the other treatments. Isolate B.PNR1, the best biocontrol and PGP, was identified as Bacillus stercoris by its 16S rRNA gene sequence and whole genome sequencing analysis (WGS). The WGS, through genome mining, confirmed that the B.PNR1 genome contained genes/gene cluster of a nonribosomal peptide synthetase/polyketide synthase, such as fengycin, surfactin, bacillaene, subtilosin A, bacilysin, and bacillibactin, which are involved in antagonistic and PGP activities. Therefore, our finding demonstrates the effectiveness of B. stercoris strain B.PNR1 as an antagonist and for plant growth promotion, highlighting the use of this microorganism as a biocontrol agent against the Fusarium wilt pathogen and PGP abilities in tomatoes.

Bacterial volatile organic compounds as biopesticides, growth promoters and plant-defense elicitors: Current understanding and future scope

Bacteria emit a large number of volatile organic compounds (VOCs) into the environment. VOCs are species-specific and their emission depends on environmental conditions, such as growth medium, pH, temperature, incubation time and interaction with other microorganisms. These VOCs can enhance plant growth, suppress pathogens and act as signaling molecules during plant-microorganism interactions. Some bacterial VOCs have been reported to show strong antimicrobial, nematicidal, pesticidal, plant defense, induced tolerance and plant-growth-promoting activities under controlled conditions. Commonly produced antifungal VOCs include dimethyl trisulfide, dimethyl disulfide, benzothiazole, nonane, decanone and 1-butanol. Species of Bacillus, Pseudomonas, Arthrobacter, Enterobacter and Burkholderia produce plant growth promoting VOCs, such as acetoin and 2,3-butenediol. These VOCs affect expression of genes involved in defense and development in plant species (i.e., Arabidopsis, tobacco, tomato, potato, millet and maize). VOCs are also implicated in altering pathogenesis-related genes, inducing systemic resistance, modulating plant metabolic pathways and acquiring nutrients. However, detailed mechanisms of action of VOCs need to be further explored. This review summarizes the bioactive VOCs produced by diverse bacterial species as an alternative to agrochemicals, their mechanism of action and challenges for employment of bacterial VOCs for sustainable agricultural practices. Future studies on technological improvements for bacterial VOCs application under greenhouse and open field conditions are warranted.

Diversity and Exploration of Endophytic Bacilli for the Management of Head Scab (Fusarium graminearum) of Wheat

Fusarium graminearum causing head scab (HS) or head blight (HB) disease in wheat is one of the nasty fungi reported to cause significant grain quality and yield loss. Biological control using endophytic bacteria has emerged as a prospective option for containing fungal diseases in an environmentally benevolent, durable, and sustainable manner. In this regard, 112 endophytic bacilli were isolated from the anthesis stage (Zadok’s growth stage 65) from five different wheat genotypes with an aim to identify prospective antagonistic strains against F. graminearum. The molecular identity of the strains was confirmed by matching 16S rRNA sequences of bacterial strains with the gene sequences of type strains available in the National Center for Biotechnology Information database and reported 38 different species of Bacillus in all the five wheat cultivars. Further, it has been observed that only fourteen strains (B. clarus NOK09, B. mojavensis NOK16, B. subtilis NOK33, B. rugosus NOK47, B. mojavensis NOK52, B. clarus NOK59, B. coahuilensis NOK72, B. cabrialesii NOK78, B. cabrialesii NOK82, B. rugosus NOK85, B. amyloliquefaciens NOK89, B. australimaris NOK95, B. pumilus NOK103, and B. amyloliquefaciens NOK109) displayed in-vitro antagonistic effect against Fusarium graminearum fungus. Furthermore, the three endophytic Bacillus strains showing the strongest antagonistic effect (>70% of growth inhibition of fungal mycelium) under in-vitro antagonistic assay were selected for field experiments. In a two-year consecutive field study, a combination of three strains (B. clarus NOK09 + B. subtilis NOK33 + B. amyloliquefaciens NOK109) displayed a remarkable reduction in HS disease index by 81.47% and 77.85%, respectively. Polymerase chain reaction assay detected three genes (ituD, bmyC, and srfA) involved in antibiotic biosynthesis pathways. Additional attributes such as potassium solubilization, siderophore release, and hydrolytic enzyme (protease, lipase, amylase, chitinase, and pectinase) synthesis have been observed in these strains. Overall, the present study was successful in profiling endophytic bacilli and selecting the combination of effective antagonistic endophytic Bacillus strains that could be the best alternative for the sustainable and ecological sound management of HS disease in wheat under field conditions.

A Meta-Analysis Approach to Defining the Culturable Core of Plant Endophytic Bacterial Communities

Endophytic bacteria are key members of the plant microbiome, which phylogenetic diversity has been widely described through next-generation sequencing technologies in the last decades. On the other side, a synopsis of culturable plant endophytic bacteria is still lacking in the literature. However, culturability is necessary for biotechnology innovations related to sustainable agriculture, such as biofertilizer and biostimulant agents' development. In this review, 148 scientific papers were analyzed to establish a large data set of cultured endophytic bacteria, reported at the genus level, inhabiting different compartments of wild and farmed plants, sampled around the world from different soil types and isolated using various growth media. To the best of our knowledge, this work provides the first overview of the current repertoire of cultured plant endophytic bacteria. Results indicate the presence of a recurrent set of culturable bacterial genera regardless of factors known to influence the plant bacterial community composition and the growth media used for the bacterial isolation. Moreover, a wide variety of bacterial genera that are currently rarely isolated from the plant endosphere was identified, demonstrating that culturomics can catch previously uncultured bacteria from the plant microbiome, widening the panorama of strains exploitable to support plant holobiont health and production.

Biofertilizer microorganisms accompanying pathogenic attributes: a potential threat

Application of biofertilizers containing living or dormant plant growth promoting bacterial cells is considered to be an ecofriendly alternative of chemical fertilizers for improved crop production. Biofertilizers opened myriad doors towards sustainable agriculture as they effectively reduce heavy use of chemical fertilizers and pesticides by keeping soils profuse in micro and macronutrients, regulating plant hormones and restraining infections caused by the pests present in soil without inflicting environmental damage. Generally, pathogenicity and biosafety testing of potential plant growth promoting bacteria (PGPB) are not performed, and the bacteria are reported to be beneficial solely on testing plant growth promoting characteristics. Unfortunately, some rhizosphere and endophytic PGPB are reported to be involved in various diseases. Such PGPB can also spread virulence and multidrug resistance genes carried by them through horizontal gene transfer to other bacteria in the environment. Therefore, deployment of such microbial populations in open fields could lead to disastrous side effects on human health and environment. Careless declaration of bacteria as PGPB is more pronounced in research publications. Here, we present a comprehensive report of declared PGPB which are reported to be pathogenic in other studies. This review also suggests the employment of some additional safety assessment protocols before reporting a bacteria as beneficial and product development.

The Biocontrol and Plant Growth-Promoting Properties of Streptomyces alfalfae XN-04 Revealed by Functional and Genomic Analysis

Fusarium wilt of cotton, caused by the pathogenic fungal Fusarium oxysporum f. sp. vasinfectum (Fov), is a devastating disease of cotton, dramatically affecting cotton production and quality. With the increase of pathogen resistance, controlling Fusarium wilt disease has become a significant challenge. Biocontrol agents (BCAs) can be used as an additional solution to traditional crop breeding and chemical control. In this study, an actinomycete with high inhibitory activity against Fov was isolated from rhizosphere soil and identified as Streptomyces alfalfae based on phylogenetic analyses. Next, an integrative approach combining genome mining and metabolites detection was applied to decipher the significant biocontrol and plant growth-promoting properties of XN-04. Bioinformatic analysis and bioassays revealed that the antagonistic activity of XN-04 against Fov was associated with the production of various extracellular hydrolytic enzymes and diffusible antifungal metabolites. Genome analysis revealed that XN-04 harbors 34 secondary metabolite biosynthesis gene clusters. The ability of XN-04 to promote plant growth was correlated with an extensive set of genes involved in indoleacetic acid biosynthesis, 1-aminocyclopropane-1-carboxylic acid deaminase activity, phosphate solubilization, and iron metabolism. Colonization experiments indicated that EGFP-labeled XN-04 had accumulated on the maturation zones of cotton roots. These results suggest that S. alfalfae XN-04 could be a multifunctional BCA and biofertilizer used in agriculture.

Pseudomonas palmensis sp. nov., a Novel Bacterium Isolated From Nicotiana glauca Microbiome: Draft Genome Analysis and Biological Potential for Agriculture

A novel Pseudomonas, designated strain BBB001T, an aerobic, rod-shaped bacterium, was isolated from the rhizosphere of Nicotiana glauca in Las Palmas Gran Canaria, Spain. Genomic analysis revealed that it could not be assigned to any known species of Pseudomonas, so the name Pseudomonas palmensis sp. nov. was proposed. A 16S rRNA gene phylogenetic analysis suggested affiliation to the Pseudomonas brassicae group, being P. brassicae MAFF212427 T the closest related type strain. Upon genomic comparisons of both strains, all values were below thresholds established for differentiation: average nucleotide identity (ANI, 88.29%), average amino acid identity (AAI, 84.53%), digital DNA-DNA hybridization (dDDH, 35.4%), and TETRA values (0.98). When comparing complete genomes, a total of 96 genes present exclusively in BBB001T were identified, 80 of which appear associated with specific subsystems. Phenotypic analysis has shown its ability to assimilate glucose, potassium gluconate, capric acid malate, trisodium citrate, and phenylacetic acid; it was oxidase positive. It is able to produce auxins and siderophores in vitro; its metabolic profile based on BIOLOG Eco has shown a high catabolic capacity. The major fatty acids accounting for 81.17% of the total fatty acids were as follows: C16:0 (33.29%), summed feature 3 (22.80%) comprising C16:1 ω7c and C16:1 ω6c, summed feature 8 (13.66%) comprising C18:1 ω7c, and C18:1ω6c and C17:0 cyclo (11.42%). The ability of this strain to improve plant fitness was tested on tomato and olive trees, demonstrating a great potential for agriculture as it is able to trigger herbaceous and woody species. First, it was able to improve iron nutrition and growth on iron-starved tomatoes, demonstrating its nutrient mobilization capacity; this effect is related to its unique genes related to iron metabolism. Second, it increased olive and oil yield up to 30% on intensive olive orchards under water-limiting conditions, demonstrating its capacity to improve adaptation to adverse conditions. Results from genomic analysis together with differences in phenotypic features and chemotaxonomic analysis support the proposal of strain BBB001T (=LMG 31775T = NCTC 14418T) as the type strain of a novel species for which the name P. palmensis sp. nov is proposed.

Biological control: a novel strategy for the control of the plant parasitic nematodes

Plant parasitic nematodes (Root-knot nematodes, Meloidogyne spp.) are rounded worms, microscopic, and cause many agricultural economic losses. Their attacks have a direct impact on the productivity of cultivated crops by reducing their fruit quantity. Chemical control is widespread all over the world, but biological control is the most effective way to reduce the number of pests that infect crops, particularly by the use of microorganisms like fungi and bacteria. Biological control is rapidly evolving, and more products are being sold worldwide over time. They can be produced by fungi, bacteria, or actinomycetes that can destruct plant parasite nematodes and feed on them. Nematophagous microorganisms as the natural enemies of nematodes have a promising way of controlling nematodes. Some of them create net-like substances and traps to take the worms from outside and finally kill them. Other parasites serve as internal parasites in order to produce toxins and to produce virulence to kill nematodes. Comprehension of the molecular basis for microbial nematode interactions gives important insights into how successful biological nematode control agents can be created. We discuss recent advances in our understanding of nematodes and nematophagous microorganisms, with an emphasis on molecular mechanisms that infect nematodes with nematophagous microorganisms and on nematode safety from pathogenic attacks. Finally, we addressed numerous key areas for future research and development, including possible approaches to the application of our recent expertise in the development of successful biocontrol strategies.

Bioeffectors as Biotechnological Tools to Boost Plant Innate Immunity: Signal Transduction Pathways Involved

The use of beneficial rhizobacteria (bioeffectors) and their derived metabolic elicitors are efficient biotechnological alternatives in plant immune system elicitation. This work aimed to check the ability of 25 bacterial strains isolated from the rhizosphere of Nicotiana glauca, and selected for their biochemical traits from a group of 175, to trigger the innate immune system of Arabidopsis thaliana seedlings against the pathogen Pseudomonas syringae pv. tomato DC3000. The five strains more effective in preventing pathogen infection were used to elucidate signal transduction pathways involved in the plant immune response by studying the differential expression of Salicylic acid and Jasmonic acid/Ethylene pathway marker genes. Some strains stimulated both pathways, while others stimulated either one or the other. The metabolic elicitors of two strains, chosen for the differential expression results of the genes studied, were extracted using n-hexane, ethyl acetate, and n-butanol, and their capacity to mimic bacterial effect to trigger the plant immune system was studied. N-hexane and ethyl acetate were the most effective fractions against the pathogen in both strains, achieving similar protection rates although gene expression responses were different from that obtained by the bacteria. These results open an amount of biotechnological possibilities to develop biological products for agriculture.

Current Utility of Plant Growth-Promoting Rhizobacteria as Biological Control Agents towards Plant-Parasitic Nematodes

Plant-parasitic nematodes (PPN) are among the most economically and ecologically damaging pests, causing severe losses of crop production worldwide. Chemical-based nematicides have been widely used, but these may have adverse effects on human health and the environment. Hence, biological control agents (BCAs) have become an alternative option for controlling PPN, since they are environmentally friendly and cost effective. Lately, a major effort has been made to evaluate the potential of a commercial grade strain of plant growth-promoting rhizobacteria (PGPR) as BCAs, because emerging evidence has shown that PGPR can reduce PPN in infested plants through direct and/or indirect antagonistic mechanisms. Direct antagonism occurs by predation, release of antinematicidal metabolites and semiochemicals, competition for nutrients, and niche exclusion. However, the results of direct antagonism may be inconsistent due to unknown endogenous and exogenous factors that may prevent PGPR from colonizing plant's roots. On the other hand, indirect antagonism may occur from the induced systemic resistance (ISR) that primes whole plants to better fight against various biotic and abiotic constraints, actuating faster and/or stronger defense responses (adaption), enhancing their promise as BCAs. Hence, this review will briefly revisit (i) two modes of PGPR in managing PPN, and (ii) the current working models and many benefits of ISR, in the aim of reassessing current progresses and future directions for isolating more effective BCAs and/or developing better PPN management strategy.

Plant growth-promoting activity and quorum quenching-mediated biocontrol of bacterial phytopathogens by Pseudomonas segetis strain P6

Given the major threat of phytopathogenic bacteria to food production and ecosystem stability worldwide, novel alternatives to conventional chemicals-based agricultural practices are needed to combat these bacteria. The objective of this study is to evaluate the ability of Pseudomonas segetis strain P6, which was isolated from the Salicornia europaea rhizosphere, to act as a potential biocontrol agent given its plant growth-promoting (PGP) and quorum quenching (QQ) activities. Seed biopriming and in vivo assays of tomato plants inoculated with strain P6 resulted in an increase in seedling height and weight. We detected QQ activity, involving enzymatic degradation of signal molecules in quorum sensing communication systems, against a broad range of N-acylhomoserine lactones (AHLs). HPLC-MRM data and phylogenetic analysis indicated that the QQ enzyme was an acylase. The QQ activity of strain P6 reduced soft rot symptoms caused by Dickeya solani, Pectobacterium atrosepticum and P. carotovorum on potato and carrot. In vivo assays showed that the PGP and QQ activities of strain P6 protect tomato plants against Pseudomonas syringae pv. tomato, indicating that strain P6 could have biotechnological applications. To our knowledge, this is the first report to show PGP and QQ activities in an indigenous Pseudomonas strain from Salicornia plants.

The impact of microbes in the orchestration of plants' resistance to biotic stress: a disease management approach

The struggle for survival is a natural and a continuous process. Microbes are struggling to survive by depending on plants for their nutrition while plants on the other hand are resisting the attack of microbes in order to survive. This interaction is a tug of war and the knowledge of microbe-plant relationship will enable farmers/agriculturists improve crop health, yield, sustain regular food supply, and minimize the use of agrochemicals such as fungicides and pesticides in the fight against plant pathogens. Although, these chemicals are capable of inhibiting pathogens, they also constitute an environmental hazard. However, certain microbes known as plant growth-promoting microbes (PGPM) aid in the sensitization and priming of the plant immune defense arsenal for it to conquer invading pathogens. PGPM perform this function by the production of elicitors such as volatile organic compounds, antimicrobials, and/or through competition. These elicitors are capable of inducing the expression of pathogenesis-related genes in plants through induced systemic resistance or acquired systemic resistance channels. This review discusses the current findings on the influence and participation of microbes in plants' resistance to biotic stress and to suggest integrative approach as a better practice in disease management and control for the achievement of sustainable environment, agriculture, and increasing food production.

Rhizosphere pseudomonads as probiotics improving plant health

Many root-colonizing microbes are multifaceted in traits that improve plant health. Although isolates designated as biological control agents directly reduce pathogen growth, many exert additional beneficial features that parallel changes induced in animal and other hosts by health-promoting microbes termed probiotics. Both animal and plant probiotics cause direct antagonism of pathogens and induce systemic immunity in the host to pathogens and other stresses. They also alter host development and improve host nutrition. The probiotic root-colonizing pseudomonads are generalists in terms of plant hosts, soil habitats and the array of stress responses that are ameliorated in the plant. This article illustrates how the probiotic pseudomonads, nurtured by the carbon (C) and nitrogen (N) sources released by the plant in root exudates, form protective biofilms on the root surface and produce the metabolites or enzymes to boost plant health. The findings reveal the multifunctional nature of many of the microbial metabolites in the plant-probiotic interplay. The beneficial effects of probiotics on plant function can contribute to sustainable yield and quality in agricultural production.

2, 3-Butanediol activated disease-resistance of creeping bentgrass by inducing phytohormone and antioxidant responses

Brown patch, caused by Rhizoctonia solani, is a serious disease in Agrostis stolonifera. 2, 3-butanediol (2, 3-BD) is the major component of volatile organic compounds and was found to initiate induced systemic resistance (ISR). To investigate the induced resistance mechanism of 2, 3-BD, we examined the effects of resistance by area affected, along with changes in the content of phytohormones (Zeatin (ZT), Abscisic Acid (ABA) and Indole-3-Acetic Acid (IAA)), the activities of three phenylpropanoid metabolic enzymes (Phenylalaninammo-Nialyase (PAL), Chalcone Isomerase (CHI) and 4-Coumarate:Coenzyme A Ligase (4CL)) and the level of secondary metabolites (total phenols, flavonoid and lignin). The result showed that 2, 3-BD treatment at 250 μmoL/L had the best induction effect with the area affected decreased from 95% of the control to 55%. Compared to the controls, treatment with 250 μmoL/L 2, 3-BD induced higher levels of PAL, CHI and 4CL activity and increased total phenols, flavonoid and lignin levels. While 2, 3-BD treatment decreased the content of ZT and ABA but increased the content of IAA compared to controls. This study provides a basis for elucidating the mechanism of 2, 3-BD as a new plant disease control agent.

Potential of Finger Millet Indigenous Rhizobacterium Pseudomonas sp. MSSRFD41 in Blast Disease Management-Growth Promotion and Compatibility With the Resident Rhizomicrobiome

Finger millet [Eleusine coracona (L). Gaertner] "Ragi" is a nutri-cereal with potential health benefits, and is utilized solely for human consumption in semi-arid regions of Asia and Africa. It is highly vulnerable to blast disease caused by Pyricularia grisea, resulting in 50-100% yield loss. Chemical fungicides are used for the management of blast disease, but with great safety concern. Alternatively, bioinoculants are widely used in promoting seedling efficiency, plant biomass, and disease control. Little is known about the impact of introduced indigenous beneficial rhizobacteria on the rhizosphere microbiota and growth promotion in finger millet. Strain MSSRFD41 exhibited a 22.35 mm zone of inhibition against P. grisea, produces antifungal metabolites, siderophores, hydrolytic enzymes, and IAA, and solubilizes phosphate. Environmental SEM analysis indicated the potential of MSSRFD41 to inhibit the growth of P. grisea by affecting cellular functions, which caused deformation in fungal hyphae. Bioprimed finger millet seeds exhibited significantly higher levels of germination, seedling vigor index, and enhanced shoot and root length compared to control seeds. Cross streaking and RAPD analysis showed that MSSRFD41 is compatible with different groups of rhizobacteria and survived in the rhizosphere. In addition, PLFA analysis revealed no significant difference in microbial biomass between the treated and control rhizosphere samples. Field trials showed that MSSRFD41 treatment significantly reduced blast infestation and enhanced plant growth compared to other treatments. A liquid formulated MSSRFD41 product maintained shelf life at an average of 10⁸ CFU ml^-1 over 150 days of storage at 25°C. Overall, results from this study demonstrated that Pseudomonas sp. MSSRFD41, an indigenous rhizobacterial strain, is an alternative, effective, and sustainable resource for the management of P. grisea infestation and growth promotion of finger millet.

Complete genome sequence of Bacillus velezensis 157 isolated from Eucommia ulmoides with pathogenic bacteria inhibiting and lignocellulolytic enzymes production by SSF

Bacillus velezensis 157 was isolated from the bark of Eucommia ulmoides, and exhibited antagonistic activity against a broad spectrum of pathogenic bacteria and fungi. Moreover, B. velezensis 157 also showed various lignocellulolytic activities including cellulase, xylanase, α-amylase, and pectinase, which had the ability of using the agro-industrial waste (soybean meal, wheat bran, sugarcane bagasse, wheat straw, rice husk, maize flour and maize straw) under solid-state fermentation and obtained several industrially valuable enzymes. Soybean meal appeared to be the most efficient substrate for the single fermentation of B. velezensis 157. Highest yield of pectinase (19.15 ± 2.66 U g-1), cellulase (46.69 ± 1.19 U g-1) and amylase (2097.18 ± 15.28 U g-1) was achieved on untreated soybean meal. Highest yield of xylanase (22.35 ± 2.24 U g-1) was obtained on untreated wheat bran. Here, we report the complete genome sequence of the B. velezensis 157, composed of a circular 4,013,317 bp chromosome with 3789 coding genes and a G + C content of 46.41%, one circular 8439 bp plasmid and a G + C content of 40.32%. The genome contained a total of 8 candidate gene clusters (bacillaene, difficidin, macrolactin, butirosin, bacillibactin, bacilysin, fengycin and surfactin), and dedicates over 15.8% of the whole genome to synthesize secondary metabolite biosynthesis. In addition, the genes encoding enzymes involved in degradation of cellulose, xylan, lignin, starch, mannan, galactoside and arabinan were found in the B. velezensis 157 genome. Thus, the study of B. velezensis 157 broadened that B. velezensis can not only be used as biocontrol agents, but also has potentially a wide range of applications in lignocellulosic biomass conversion.

Modulation of plant chemistry by beneficial root microbiota

Beneficial root microbiota modulate plant chemistry and represent an untapped potential to discover new pathways involved in the biosynthesis of high value natural plant products.