Effects of the Endophytic Bacteria Bacillus cereus BCM2 on Tomato Root Exudates and Meloidogyne incognita Infection

Unearthing the soil-bacteria nexus to enhance potassium bioavailability for global sustainable agriculture: A mechanistic preview

Established as a plant macronutrient, potassium (K) substantially bestows plant growth and thus, global food production. It is absorbed by plants as potassium cation (K+) from soil solution, which is enriched through slow-release from soil minerals or addition of soluble fertilizers. Contribution of bioavailable K+ from soil is usually insignificant (< 2 %), although the earth's crust is rich in K-bearing minerals. However, K is fixed largely in interlayer spaces of K-bearing minerals, which can be released by K-solubilizing bacteria (KSB) such as Bacillus, Pseudomonas, Enterobacter, and Acidithiobacillus. The underlying mechanisms of K dissolution by KSB include acidolysis, ion exchange reactions, chelation, complexolysis, and release of various organic and inorganic acids such as citric, oxalic, acetic, gluconic, and tartaric acids. These acids cause disintegration of K-bearing minerals and bring K+ into soil solution that becomes available to the plants. Current literature review updates the scientific information about microbial species, factors, and mechanisms governing the bio-intrusion of K-bearing minerals. Moreover, it explores the potential of KSB not only for K-solubilization but also to enhance bioavailability of phosphorus, nitrogen, and micronutrients, as well as its other beneficial impact on plant growth. Thus, in the context of sustainable agricultural production and global food security, utilization of KSB may facilitate plant nutrient availability, conserve natural resources, and reduce environmental impacts caused by chemical fertilizers.

Biocontrol activity and potential mechanism of Bacillus cereus G5 against Meloidogyne graminicola

Root-knot nematodes (Meloidogyne spp.) are highly destructive pests that cause significant yield losses annually. Biological control of nematodes has emerged as a potential alternative in sustainable agriculture. In this study, we originally isolated Bacillus cereus G5 from the rhizosphere soil of rice (Oryza sativa). Treatment with the fermentation supernatant of G5 in vitro demonstrated high toxicity to second-stage juveniles (J2) of Meloidogyne graminicola and remarkably inhibited egg hatching. Moreover, G5 steadily colonized rhizosphere soil and rice seedlings, and exhibited excellent biocontrol efficacy against M. graminicola under greenhouse conditions. Notably, the volatile organic compounds (VOCs) produced by G5 displayed high fumigant activity against M. graminicola. The G5 VOCs efficiently reduced the gall index and nematode population in rice roots, while also promoting rice growth in double-layered pot tests. Additionally, the expression of defense genes involved in the salicylic acid (OsNPR1, OsWRKY45, OsPAL1), jasmonic acid (OsJaMYB, OsAOS2) and ethylene (OsACS1) signalling pathways was significantly upregulated in rice seedlings treated with G5 VOCs. This suggests that G5 VOCs contribute to eliciting plant defense responses. Furthermore, we identified 14 major VOCs produced by G5 using solid-phase micro-extraction gas chromatography and mass spectrometry (SPEM-GC-MS). Notably, allomatrine, morantel, 1-octen-3-ol and 3-methyl-2-butanol displayed strong contact nematicidal activity. Among these, only 1-octen-3-ol demonstrated fumigant activity against J2s of M. graminicola, with an LC50 value of 758.95 mg/L at 24 h. Overall, these results indicated that the B. cereus G5 and its synthetic VOCs possess high potential as biocontrol agents for managing root-knot nematodes.

Biocontrol of plant parasitic nematodes by bacteria and fungi: a multi-omics approach for the exploration of novel nematicides in sustainable agriculture

Plant parasitic nematodes (PPNs) pose a significant threat to global crop productivity, causing an estimated annual loss of US $157 billion in the agriculture industry. While synthetic chemical nematicides can effectively control PPNs, their overuse has detrimental effects on human health and the environment. Biocontrol agents (BCAs), such as bacteria and fungi in the rhizosphere, are safe and promising alternatives for PPNs control. These BCAs interact with plant roots and produce extracellular enzymes, secondary metabolites, toxins, and volatile organic compounds (VOCs) to suppress nematodes. Plant root exudates also play a crucial role in attracting beneficial microbes toward infested roots. The complex interaction between plants and microbes in the rhizosphere against PPNs is mostly untapped which opens new avenues for discovering novel nematicides through multi-omics techniques. Advanced omics approaches, including metagenomics, transcriptomics, proteomics, and metabolomics, have led to the discovery of nematicidal compounds. This review summarizes the status of bacterial and fungal biocontrol strategies and their mechanisms for PPNs control. The importance of omics-based approaches for the exploration of novel nematicides and future directions in the biocontrol of PPNs are also addressed. The review highlighted the potential significance of multi-omics techniques in biocontrol of PPNs to ensure sustainable agriculture.

Hydroalcoholic Extracts from Pleurotus ostreatus Spent Substrate with Nematocidal Activity against Nacobbus aberrans Phytonematode and the Non-Target Species Panagrellus redivivus

Pleurotus ostreatus, an edible mushroom widely consumed worldwide, generates a by-product known as spent mushroom substrate (SMS). This material has demonstrated biological activity against agricultural crop pathogens. In this study, we evaluated the nematocidal effectiveness of hydroalcoholic extracts (T5, T2, AT5, and AT2) derived from SMS of P. ostreatus against (J2) of the phytonematode Nacobbus aberrans and assessed their potential toxicity towards the non-target nematode Panagrellus redivivus. Among these extracts, AT5 exhibited the highest efficacy against N. aberrans and was the least toxic against P. redivivus. Liquid-liquid partitioning yielded the AQU fraction, which showed significant nematocidal activity against J2 (75.69% ± 8.99 mortality), comparable to chitosan. The GC-MS analysis revealed the presence of several compounds, including palmitic acid, linoleic acid, and 2,4-Di-tert-butylphenol. These findings are consistent with studies confirming the antagonistic effectiveness of these compounds against phytonematodes. Additionally, all extracts exhibited toxicity against P. redivivus, with T2 being the most toxic. Our findings demonstrate that while the AT5 extract displays antagonistic effectiveness against both N. aberrans and P. redivivus, it was the least toxic among the extracts tested. Thus, SMS of P. ostreatus holds potential as a source of nematocidal compounds, which could offer significant benefits for agricultural pest control.

Unraveling the enigma of root-knot nematodes: from origins to advanced management strategies in agriculture

MAIN CONCLUSION

Integrated management strategies, including novel nematicides and resilient cultivars, offer sustainable solutions to combat root-knot nematodes, crucial for safeguarding global agriculture against persistent threats. Root-knot nematodes (RKN) pose a significant threat to a diverse range of host plants, with their obligatory endoparasitic nature leading to substantial agricultural losses. RKN spend much of their lives inside or in contact by secreting plant cell wall-modifying enzymes resulting in the giant cell development for establishing host-parasite relationships. Additionally, inflicting physical harm to host plants, RKN also contributes to disease complexes creation with fungi and bacteria. This review comprehensively explores the origin, history, distribution, and physiological races of RKN, emphasizing their economic impact on plants through gall formation. Management strategies, ranging from cultural and physical to biological and chemical controls, along with resistance mechanisms and marker-assisted selection, are explored. While recognizing the limitations of traditional nematicides, recent breakthroughs in non-fumigant alternatives like fluensulfone, spirotetramat, and fluopyram offer promising avenues for sustainable RKN management. Despite the success of resistance mechanisms like the Mi gene, challenges persist, prompting the need for integrative approaches to tackle Mi-virulent isolates. In conclusion, the review stresses the importance of innovative and resilient control measures for sustainable agriculture, emphasizing ongoing research to address evolving challenges posed by RKN. The integration of botanicals, resistant cultivars, and biological controls, alongside advancements in non-fumigant nematicides, contributes novel insights to the field, laying the ground work for future research directions to ensure the long-term sustainability of agriculture in the face of persistent RKN threats.

Understanding the dynamic interactions of root-knot nematodes and their host: role of plant growth promoting bacteria and abiotic factors

Root-knot nematodes (Meloidogyne spp., RKN) are among the most destructive endoparasitic nematodes worldwide, often leading to a reduction of crop growth and yield. Insights into the dynamics of host-RKN interactions, especially in varied biotic and abiotic environments, could be pivotal in devising novel RKN mitigation measures. Plant growth-promoting bacteria (PGPB) involves different plant growth-enhancing activities such as biofertilization, pathogen suppression, and induction of systemic resistance. We summarized the up-to-date knowledge on the role of PGPB and abiotic factors such as soil pH, texture, structure, moisture, etc. in modulating RKN-host interactions. RKN are directly or indirectly affected by different PGPB, abiotic factors interplay in the interactions, and host responses to RKN infection. We highlighted the tripartite (host-RKN-PGPB) phenomenon with respect to (i) PGPB direct and indirect effect on RKN-host interactions; (ii) host influence in the selection and enrichment of PGPB in the rhizosphere; (iii) how soil microbes enhance RKN parasitism; (iv) influence of host in RKN-PGPB interactions, and (v) the role of abiotic factors in modulating the tripartite interactions. Furthermore, we discussed how different agricultural practices alter the interactions. Finally, we emphasized the importance of incorporating the knowledge of tripartite interactions in the integrated RKN management strategies.

Counter-attack of biocontrol agents: Environmentally benign Approaches against Root-knot nematodes (Meloidogyne spp.) on Agricultural crops

Root-knot nematodes (Meloidogyne spp.) are obligate sedentary endoparasites, considered severe crop-damaging taxa among all plant-parasitic nematodes globally. Their attacks through parasitic proteins alter the physiology and machinery of the host cells to favour parasitism and reduction in crop yield. Currently, the use of excessive pesticides as a fast remedy to manage this pest is hazardous for both the environment and humans. Keeping this view in mind, there is an urgent need for developing efficient eco-friendly strategies. Bio-control as an eco-friendly is considered the best approach to manage nematodes without disturbing non-target microbes. In bio-control, living agents such as fungi and bacteria are the natural enemies of nematodes and the best substitute for pesticides. Fungi, including nematode-trapping fungi, can sense host signals and produce special trapping devices viz., constricting rings and adhesive knobs/loops, to capture nematodes and kill them. Whereas, endo-parasitic fungi kill nematodes by enzymatic secretions and spore adhesion through their hyphae. Bacteria can also control nematodes by producing antibiotic compounds, competing for nutrients and rhizosphere, production of hydrolytic enzymes viz., chitinases, proteases, lipases, and induction of systemic resistance (ISR) in host plants. Scientists throughout the world are trying to evolve environmentally benign methods that sustain agricultural production and keep nematodes below a threshold level. Whatever methods evolve, in the future the focus should be on important aspects like green approaches for managing nematodes without disturbing human health and the environment.

Volatile Organic Compounds of Bacillus pumilus Strain S1-10 Exhibit Fumigant Activity Against Meloidogyne incognita

Root-knot nematodes (RKNs) are highly specialized parasites that cause significant yield losses worldwide. In this study, we isolated Bacillus pumilus strain S1-10 from the rhizosphere soil of Zingiber officinale Rosc. plants and evaluated its fumigant activity against Meloidogyne incognita. S1-10 exhibited a strong repellent effect on second-stage juveniles (J2s) of M. incognita, and in vitro assays indicated that S1-10 volatile organic compounds (VOCs) suppressed J2 activity and egg hatching. Under greenhouse conditions, 71 and 79% reductions of nematodes and eggs were detected on plants treated with S-10 VOCs compared with controls. Ten VOCs were identified through gas chromatography and mass spectrometry (GC-MS), of which 2-(methylamino)-ethanol (2-ME) had strong fumigant activity against J2s of M. incognita, with an LC50 value of 1.5 mM at 12 h. These results indicate that S1-10 represents a potential novel biocontrol agent for RKNs.

Plant Growth Promotion Using Bacillus cereus

Plant growth-promoting bacteria (PGPB) appear to be a sensible competitor to conventional fertilization, including mineral fertilizers and chemical plant protection products. Undoubtedly, one of the most interesting bacteria exhibiting plant-stimulating traits is, more widely known as a pathogen, Bacillus cereus. To date, several environmentally safe strains of B. cereus have been isolated and described, including B. cereus WSE01, MEN8, YL6, SA1, ALT1, ERBP, GGBSTD1, AK1, AR156, C1L, and T4S. These strains have been studied under growth chamber, greenhouse, and field conditions and have shown many significant traits, including indole-3-acetic acid (IAA) and aminocyclopropane-1-carboxylic acid (ACC) deaminase production or phosphate solubilization, which allows direct plant growth promotion. It includes an increase in biometrics traits, chemical element content (e.g., N, P, and K), and biologically active substances content or activity, e.g., antioxidant enzymes and total soluble sugar. Hence, B. cereus has supported the growth of plant species such as soybean, maize, rice, and wheat. Importantly, some B. cereus strains can also promote plant growth under abiotic stresses, including drought, salinity, and heavy metal pollution. In addition, B. cereus strains produced extracellular enzymes and antibiotic lipopeptides or triggered induced systemic resistance, which allows indirect stimulation of plant growth. As far as biocontrol is concerned, these PGPB can suppress the development of agriculturally important phytopathogens, including bacterial phytopathogens (e.g., Pseudomonas syringae, Pectobacterium carotovorum, and Ralstonia solanacearum), fungal phytopathogens (e.g., Fusarium oxysporum, Botrytis cinerea, and Rhizoctonia solani), and other phytopathogenic organisms (e.g., Meloidogyne incognita (Nematoda) and Plasmodiophora brassicae (Protozoa)). In conclusion, it should be noted that there are still few studies on the effectiveness of B. cereus under field conditions, particularly, there is a lack of comprehensive analyses comparing the PGP effects of B. cereus and mineral fertilizers, which should be reduced in favor of decreasing the use of mineral fertilizers. It is also worth mentioning that there are still very few studies on the impact of B. cereus on the indigenous microbiota and its persistence after application to soil. Further studies would help to understand the interactions between B. cereus and indigenous microbiota, subsequently contributing to increasing its effectiveness in promoting plant growth.

Biocontrol of Meloidogyne spp. in Solanum lycopersicum using a dual combination of Bacillus strains

Root-knot nematodes (RKNs, Meloidogyne spp.) are obligate plant parasites that constitute a significant pest for agriculture worldwide. They penetrate the plant roots, reducing the uptake of water and nutrients, causing a significant impact on crop yield. One alternative on focus now for nematode management is biological control. Rhizobacteria within the Bacillus genus show multiple modes of action against plant-parasitic nematodes (PPNs) that can act alone or in combination. In this context, we evaluated a dual-strain bacteria combination (B. paralicheniformi FMCH001 and B. subtilis FMCH002) to reduce nematode infection in tomato plants. We evaluated mortality of larvae from Meloidogyne javanica in vitro, as well as eggs hatching after the treatment. Atraction, penetration, establishment, and reproduction assays in vitro or in pots in tomato plants infected with M. javanica and treated/ untreated with the dual-strain bacteria combination were also performed. Additionally, morphometric parameters comparing giant cells size from galls of treated and untreated plants by using confocal microscopy were also measured. The results showed that this combination of strains has nematicidal properties in the pre-infection phase by decreasing the egg-hatching, juvenile survival, and attractiveness to the roots. Furthermore, nematode establishment, gall formation, and, remarkably, giant cell development was severely impaired after the bacterial treatment, suggesting interference with morphogenetic mechanisms induced by the nematode during GCs development within the plant. Nematode reproduction in tomato plants was reduced independently of the application mode in soil, before or after bacterial treatment. The dual-strain combination was also effective against other PPNs (i.e. Pratylenchus spp.) and in different crops (soybean). Therefore, combining B. paralicheniformis FMCH001 and B. subtilis FMCH002 is an efficient agent for the biological control of Meloidogyne spp. by interfering with different stages of the nematode cycle as a result of multiple modes of action.

Endophytic Paenibacillus polymyxa LMG27872 inhibits Meloidogyne incognita parasitism, promoting tomato growth through a dose-dependent effect

The root-knot nematode, Meloidogyne incognita, is a major pest in tomato production. Paenibacillus polymyxa, which is primarily found in soil and colonizing roots, is considered a successful biocontrol organism against many pathogens. To evaluate the biocontrol capacity of P. polymyxa LMG27872 against M. incognita in tomato, experiments were conducted both in vitro and in vivo. A dose-response effect [30, 50, and 100% (108 CFU/mL)] of bacterial suspensions (BSs) on growth and tomato susceptibility to M. incognita with soil drenching as a mode of application was first evaluated. The results show that the biological efficacy of P. polymyxa LMG27872 against M. incognita parasitism in tomato was dose-dependent. A significantly reduced number of galls, egg-laying females (ELF), and second-stage juveniles (J2) were observed in BS-treated plants, in a dose-dependent manner. The effect of P. polymyxa on tomato growth was also dose-dependent. A high dose of BSs had a negative effect on growth; however, this negative effect was not observed when the BS-treated plants were challenged with M. incognita, indicating tolerance or a defense priming mechanism. In subsequent in vivo experiments, the direct effect of BSs was evaluated on J2 mortality and egg hatching of M. incognita. The effect of BS on J2 mortality was observed from 12 to 24 h, whereby M. incognita J2 was significantly inhibited by the BS treatment. The effect of P. polymyxa on M. incognita egg hatching was also dependent on the BS dose. The results show a potential of P. polymyxa LMG27872 to protect plants from nematode parasitism and its implementation in integrated nematode management suitable for organic productions.

Plant Growth-Promoting Microorganism Pseudarthrobacter sp. NIBRBAC000502770 Enhances the Growth and Flavonoid Content of Geum aleppicum

Plant growth-promoting rhizobacteria are known to enhance the growth and antioxidant activity of several plants. However, the effects of such rhizobacteria on Geum aleppicum, a plant with pharmacological potential in Korea are unknown. In this study, we investigated the effects of Pseudarthrobacter sp. NIBRBAC000502770 treatment (100 mL/pot, every two weeks for 55 days), in the form of culture medium, 100-fold diluted culture, culture supernatant, and pelleted cells resuspended in water, on the growth, antibacterial activity and flavonoid content of G. aleppicum. The NIBRBAC000502770 strain showed high indole-3-acetic acid (IAA) content of 461.81 μg∙mL^-1. The dry weight of the roots was significantly higher in the supernatant, diluted culture, and pellet-treated plants compared to that in the control plants. Additionally, the plant height, root length, leaf length, leaf width, chlorophyll content, biomass, and dry weight of the shoot were highest in the pellet-treated plants. Further, methanol extracts of pellet-treated plants showed significantly high flavonoid content compared to that in the control plants (28 mg∙g^-1 vs. 7.5 mg∙g^-1) and exhibited strong antibacterial activity against Gram-positive and negative bacteria. These results demonstrate the beneficial effects of Pseudarthrobacter sp. NIBRBAC000502770 on the growth and flavonoid content of G. aleppicum.

Bacillus licheniformis JF-22 to Control Meloidogyne incognita and Its Effect on Tomato Rhizosphere Microbial Community

Meloidogyne incognita is one of the most destructive soil pests, causing serious economic losses in tomato production. Here, in vitro experiments demonstrated that the Bacillus licheniformis strain JF-22 has the potential to prevent M. incognita infection. A pot experiment confirmed that B. licheniformis strain JF-22 isolated from the tomato rhizosphere soil and planted in the tomato root-knot nematode disease area effectively prevented and controlled M. incognita, reducing its negative effect on tomato growth. Additionally, the composition of volatile substances secreted by B. licheniformis strain JF-22 was analyzed using solid-phase microextraction and gas chromatography–mass spectrometry. We detected acetoin, 2,3-Butanediol, [R-(R*,R*) ]-, and hexamethyl cyclotrisiloxane as the main components among these volatiles. Using MiSeq sequencing technology and bioinformatics, we analyzed the influence of B. licheniformis strain JF-22 on the microbial community of the tomato rhizosphere. B. licheniformis strain JF-22 changed the composition of the microbial community; particularly, it significantly reduced the diversity of the fungal community. Furthermore, using the FUNGuild and PICRUSt databases, we predicted the effect of JF-22 on microbial community function. In conclusion, B. licheniformis strain JF-22 may be considered as a potential biocontrol agent against M. incognita.

Phytostimulating Potential of Endophytic Bacteria from Ethnomedicinal Plants of North-East Indian Himalayan Region

North-East Indian Himalayan Region has a humid subtropical climate having diverse ecosystems. The majority of the population of the region depends on agriculture for sustainable livelihood. However, it can produce only 1.5% of the country’s food grains, thereby importing from other parts of the country for consumption. To feed the increase in the population of the region, there is an urgent need to augment the agricultural and allied products to sustain the population and uplift the economic conditions. Plant beneficial endophytes isolated from ethnomedicinal plants of North-East India play an important role as a plant growth promoter by the production of phytohormones, solubilization and mobilization of mineral nutrients. It also indirectly promotes growth by protecting the plants from diseases through the production of antibiotics, enzymes and volatile compounds. The bacteria also have the potential to induce systemic resistance against various abiotic stresses. Since the region has various agro-climatic conditions, the plants are continuously affected by abiotic stress particularly, acidity, drought and waterlogging, there is a need to explore the indigenous endophytes that can mitigate the stress and enhance the sustainable development of agricultural products.

Bacillusvelezensis Strains for Protecting Cucumber Plants from Root-Knot Nematode Meloidogyne incognita in a Greenhouse

Meloidogyne incognita Kofoid et White is one of the most dangerous root-knot nematodes in greenhouses. In this study, we evaluated two Bacillus strains (Bacillus velezensis BZR 86 and Bacillus velezensis BZR 277) as promising microbiological agents for protecting cucumber plants from the root-knot nematode M. incognita Kof. The morphological and cultural characteristics and enzymatic activity of the strains have been studied and the optimal conditions for its cultivation have been developed. We have shown the nematicidal activity of these strains against M. incognita. Experiments with the cucumber variety Courage were conducted under greenhouse conditions in 2016-2018. We determined the effect of plant damage with M. incognita to plants on the biometric parameters of underground and aboveground parts of cucumber plants, as well as on the gall formation index and yield. It was found that the treatment of plants with Bacillus strains contributed to an increase in the height of cucumber plants by 7.4-43.1%, an increase in leaf area by 2.7-17.8%, and an increase in root mass by 3.2-16.1% compared with the control plants without treatment. The application of these strains was proved to contribute to an increase in yield by 4.6-45.8% compared to control. Our experiments suggest that the treatment of cucumber plants with two Bacillus strains improved plant health and crop productivity in the greenhouse. B. velezensis BZR 86 and B. velezensis BZR 277 may form the basis for bionematicides to protect cucumber plants from the root-knot nematode M. incognita.

Optimizing Safe Approaches to Manage Plant-Parasitic Nematodes

Plant-parasitic nematodes (PPNs) infect and cause substantial yield losses of many foods, feed, and fiber crops. Increasing concern over chemical nematicides has increased interest in safe alternative methods to minimize these losses. This review focuses on the use and potential of current methods such as biologicals, botanicals, non-host crops, and related rotations, as well as modern techniques against PPNs in sustainable agroecosystems. To evaluate their potential for control, this review offers overviews of their interactions with other biotic and abiotic factors from the standpoint of PPN management. The positive or negative roles of specific production practices are assessed in the context of integrated pest management. Examples are given to reinforce PPN control and increase crop yields via dual-purpose, sequential, and co-application of agricultural inputs. The involved PPN control mechanisms were reviewed with suggestions to optimize their gains. Using the biologicals would preferably be backed by agricultural conservation practices to face issues related to their reliability, inconsistency, and slow activity against PPNs. These practices may comprise offering supplementary resources, such as adequate organic matter, enhancing their habitat quality via specific soil amendments, and reducing or avoiding negative influences of pesticides. Soil microbiome and planted genotypes should be manipulated in specific nematode-suppressive soils to conserve native biologicals that serve to control PPNs. Culture-dependent techniques may be expanded to use promising microbial groups of the suppressive soils to recycle in their host populations. Other modern techniques for PPN control are discussed to maximize their efficient use.

Plant-Microbiome Crosstalk: Dawning from Composition and Assembly of Microbial Community to Improvement of Disease Resilience in Plants

Plants host diverse but taxonomically structured communities of microorganisms, called microbiome, which colonize various parts of host plants. Plant-associated microbial communities have been shown to confer multiple beneficial advantages to their host plants, such as nutrient acquisition, growth promotion, pathogen resistance, and environmental stress tolerance. Systematic studies have provided new insights into the economically and ecologically important microbial communities as hubs of core microbiota and revealed their beneficial impacts on the host plants. Microbiome engineering, which can improve the functional capabilities of native microbial species under challenging agricultural ambiance, is an emerging biotechnological strategy to improve crop yield and resilience against variety of environmental constraints of both biotic and abiotic nature. This review highlights the importance of indigenous microbial communities in improving plant health under pathogen-induced stress. Moreover, the potential solutions leading towards commercialization of proficient bioformulations for sustainable and improved crop production are also described.

The Use of Plant Growth-Promoting Bacteria to Prevent Nematode Damage to Plants

Simple Summary

It has been estimated that 100 g of bulk soil can host about 2000–4000 nematodes and this amount is increased 5-fold in the rhizosphere. A certain number of these nematodes are pathogenic for plants and cause yield and economic losses. Application of chemical nematicides is the most common method used to reduce nematode populations, but these chemicals can have a negative impact on both the environment and human health. Therefore, other more environmentally friendly methods of suppression of plant-parasitic nematodes have been proposed. Among them, the use of plant beneficial soil bacteria, behaving as biocontrol agents against nematodes, represent a potential alternative to chemicals.

Abstract

Plant-parasitic nematodes have been estimated to annually cause around US $173 billion in damage to plant crops worldwide. Moreover, with global climate change, it has been suggested that the damage to crops from nematodes is likely to increase in the future. Currently, a variety of potentially dangerous and toxic chemical agents are used to limit the damage to crops by plant-parasitic nematodes. As an alternative to chemicals and a more environmentally friendly means of decreasing nematode damage to plants, researchers have begun to examine the possible use of various soil bacteria, including plant growth-promoting bacteria (PGPB). Here, the current literature on some of the major mechanisms employed by these soil bacteria is examined. It is expected that within the next 5–10 years, as scientists continue to elaborate the mechanisms used by these bacteria, biocontrol soil bacteria will gradually replace the use of chemicals as nematicides.

Recent Advances in the Development of Environmentally Benign Treatments to Control Root-Knot Nematodes

Root-knot nematodes (RKNs), Meloidogyne spp., are sedentary endoparasites that negatively affect almost every crop in the world. Current management practices are not enough to completely control RKN. Application of certain chemicals is also being further limited in recent years. It is therefore crucial to develop additional control strategies through the application of environmentally benign methods. There has been much research performed around the world on the topic, leading to useful outcomes and interesting findings capable of improving farmers' income. It is important to have dependable resources gathering the data produced to facilitate future research. This review discusses recent findings on the application of environmentally benign treatments to control RKN between 2015 and April 2020. A variety of biological control strategies, natural compounds, soil amendments and other emerging strategies have been included, among which, many showed promising results in RKN control in vitro and/or in vivo. Development of these methods continues to be an area of active research, and new information on their efficacy will continuously become available. We have discussed some of the control mechanisms involved and suggestions were given on maximizing the outcome of the future efforts.

Impact of Plant Growth Promoting Rhizobacteria in the Orchestration of Lycopersicon esculentum Mill. Resistance to Plant Parasitic Nematodes: A Metabolomic Approach to Evaluate Defense Responses Under Field Conditions

The present study deals with biological control of Meloidogyne incognita in 45-days old Lycopersicon esculentum, inoculated with Pseudomonas aeruginosa(M1) and Burkholderia gladioli (M2). The improved plant growth and biomass of nematode infested Plant growth promoting rhizobacteria (PGPR) inoculated plants was observed. Remarkable reduction in the numbers of second stage juvenile (J2s), root galls was recorded after treatment of microbes relative to experimental controls. Moreover, the lowered activities of oxidative stress markers (H2O2 (hydrogen peroxide), O2- (superoxide anion), malondialdehyde (MDA)) was estimated in plants after rhizobacterial supplementation. Higher activities of enzymatic (SOD (Superoxide dismutase), POD (Guaiacol peroxidase), CAT (Catalase), GPOX (Glutathione peroxidase), APOX (Ascorbate peroxidase), GST (Glutathione-S-transferase), GR (Glutathione reductase), DHAR (Dehydroascorbate reductase), PPO (Polyphenol oxidase)) and non-enzymatic (glutathione, ascorbic acid, tocopherol) antioxidants were further determined in nematode infected plants following the addition of bacterial strains. The upregulation of photosynthetic activities were depicted by evaluating plant pigments and gas exchange attributes. An increase in the levels of phenolic compounds (total phenols, flavonoids, anthocyanins), osmoprotectants (total osmolytes, carbohydrates, reducing sugars, trehalose, proline, glycine betaine, free amino acids) and organic acids (fumaric, succinic, citric, malic acid) were reflected in infected plants, showing further enhancement after application of biocontrol agents. The study revealed the understanding of plant metabolism, along with the initiative to commercially exploit the biocontrol agents as an alternative to chemical nematicides in infected fields for sustainable agriculture.