Physiological and biochemical changes in tomato cultivar PT-3 with dual inoculation of mycorrhiza and PGPR against root-knot nematode

Bacillus velezensis A-27 as a potential biocontrol agent against Meloidogyne incognita and effects on rhizosphere communities of celery in field

Meloidogyne incognita, a highly destructive plant-parasitic nematode, poses a significant threat to crop production. The reliance on chemical nematicides for nematode control has been crucial; however, the banning of many effective nematicides due to their adverse effects has necessitated the exploration of alternative solutions. Rhizosphere biocontrol bacteria, particularly strains of Bacillus, have demonstrated promising results in managing plant-parasitic nematodes. In this study, strain A-27 was identified as Bacillus velezensis based on its morphological, physiological, and molecular characteristics, including 16 S rRNA and gyrA sequencing. Strain A-27 exhibited high larvicidal and ovicidal efficacy in vitro, with estimated LC50 values of 4.0570 × 108 CFU/mL for larvicidal efficacy and 3.6464 × 108 CFU/mL for ovicidal efficacy. In a pot experiment, B. velezensis A-27 significantly reduced the root gall index, achieving a control efficacy of 85.36%. Field experiments further indicated that A-27 reduced the root gall index with a control efficacy of 67.31%, while also decreasing the J2 population density of M. incognita and significantly enhancing the growth of celery plants. Additionally, high-throughput sequencing analysis revealed that B. velezensis A-27 significantly increased the relative abundances of Bacillus and Sphingomonas, while markedly reducing the relative abundances of Fusarium, Mortierella, and Cephaliophora in the celery rhizosphere. These findings suggest that B. velezensis A-27 has potential as an effective biocontrol agent against M. incognita, offering a promising alternative to chemical nematicides in sustainable agriculture.

Arbuscular mycorrhizal fungus and Pseudomonas bacteria affect tomato response to Tuta absoluta (Lepidoptera: Gelechiidae) herbivory

Tuta absoluta (Lepidoptera: Gelechiidae) is one of the most significant invasive and destructive pests worldwide, causing serious economic losses to the tomato industry. Rhizosphere microorganism, such as arbuscular mycorrhizal fungi (AMF) and Pseudomonas bacteria, can interact with plants individually or collectively to improve plant growth and resistance to pests and disease. However, the effects of AMF, Pseudomonas, and their interactions on plant responses to insect herbivores remain unclear. A pot experiment was conducted to investigate the effects of single/dual inoculation with AMF (Funneliformis mosseae, M) and Pseudomonas putida (P) on the growth and defense of tomato variety Dafen (Solanum lycopersicum L.) in response to infestation by T. absoluta, as well as the growth, development, and enzyme activity of insect. The results showed that M, P, and MP promoted tomato growth by increasing nutrient concentrations, with the growth-promoting effect of dual-inoculation significantly surpassing that of single inoculation. M, P, and MP still improved tomato growth in T. absoluta infestation, with biomass increases of 57.34%, 54.46%, and 255.49%. M, P, and MP significantly increased the defense ability of tomato, with jasmonic acid concentrations increasing by 42.15%, 60.87% and 90.02%, and phenylalanine ammonia-lyase activity increasing by 47.40%, 47.68%, and 59.97%. The inoculation treatments inhibited the growth and development of T. absoluta, reduced its feeding, prolonged its growth and development, decreased egg weight, and increased the activity of protective and detoxifying enzymes. Overall, our results indicated that AMF and bacteria can stimulate each other, positively influence tomato growth and enhance resistance to T. absoluta. These findings indicate the feasibility of AMF and bacteria in combinations as potential biocontrol agents for the management of T. absoluta.

Growth and protein response of rice plant with plant growth-promoting rhizobacteria inoculations under salt stress conditions

Soil salinity has been one of the significant barriers to improving rice production and quality. According to reports, Bacillus spp. can be utilized to boost plant development in saline soil, although the molecular mechanisms behind the interaction of microbes towards salt stress are not fully known. Variations in rice plant protein expression in response to salt stress and plant growth-promoting rhizobacteria (PGPR) inoculations were investigated using a proteomic method and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Findings revealed that 54 salt-responsive proteins were identified by mass spectrometry analysis (LC-MS/MS) with the Bacillus spp. interaction, and the proteins were functionally classified as gene ontology. The initial study showed that all proteins were labeled by mass spectrometry analysis (LC-MS/MS) with Bacillus spp. interaction; the proteins were functionally classified into six groups. Approximately 18 identified proteins (up-regulated, 13; down-regulated, 5) were involved in the photosynthetic process. An increase in the expression of eight up-regulated and two down-regulated proteins in protein synthesis known as chaperones, such as the 60 kDa chaperonin, the 70 kDa heat shock protein BIP, and calreticulin, was involved in rice plant stress tolerance. Several proteins involved in protein metabolism and signaling pathways also experienced significant changes in their expression. The results revealed that phytohormones regulated the manifestation of various chaperones and protein abundance and that protein synthesis played a significant role in regulating salt stress. This study also described how chaperones regulate rice salt stress, their different subcellular localizations, and the activity of chaperones.

Exploiting fly ash as an ecofriendly pesticide/nematicide on Abesmoschus esculuntus: Insights into soil amendment-induced antioxidant fight against nematode mediated ROS

Conventional pest control measures, such as chemical pesticides and nematicides, have limited efficacy and raise environmental concerns, necessitating sustainable and eco-friendly alternatives for pest management. Therefore, to find a complementary eco-friendly pesticide/nematicide, this study investigated the role of fly ash (FA) in managing a notorious pest, Meloidogyne javanica and its impact on the growth and physiology of Abelmoschus esculentus. Molecular characterization using SSU and LSU rDNA gene markers confirmed the identity of Indian M. javanica as belonging to the same species. Biotic stress induced by nematode infection was significantly alleviated (P < 0.05) by FA application at a 20% w/v, regulating of ROS accumulation (44.1% reduction in superoxide anions and 39.7% reduction in hydrogen peroxide content) in the host plant. Moreover, FA enhanced antioxidant defence enzymes like superoxide dismutase (46.6%) and catalase (112%) to combat nematode induced ROS. Furthermore, the application of FA at a 20% concentration significantly improved the biomass and biochemical attributes of okra. Fly ash also upregulated the activity of the important osmo-protectant proline (11.5 μmol/g FW) to mitigate nematode stress in host cells. Suppression of disease indices like gall index and reproduction factor, combined with in-vitro experiments, revealed that FA exhibits strong nematode mortality capacity and thus can be used as a sustainable and eco-friendly control agent against root-knot nematodes.

Plant growth promoting Bacillus species elicit defense against Meloidogyne incognita infecting tomato in polyhouse

The effects of four nematicidal rhizobacterial isolates; Bacillus subtilis, Bacillus pumilus, Bacillus megaterium, and Bacillus cereus on infection and multiplication of root-knot nematode, Meloidogyne incognita on tomato were compared with the application of a chemical nematicide, fluopyram 34.48% SC (Velum Prime). The bio-efficacy trial conducted in pots preinoculated with the above isolates followed by M. incognita inoculation resulted in a significant reduction in percent root galling viz. 91.95 in B. subtilis, 84.21 in B. pumilus, 83.70 in B. megaterium, and 81.8 in B. cereus, at 75 days after inoculation (DAI). The reproduction factor of the nematode was the lowest (15.83) in B. subtilis, followed by B. pumilus (21.00), compared with 48.16 in control, with enhanced photosynthetic and transpiration rates. The mechanism of induced resistance was assessed using quantitative reverse-transcription polymerase chain reaction (qRT-PCR) for quantification of three key defense genes (PR-1b, JERF3, and CAT) at 0,2,4,8 and16 days DAI. The defence genes, PR-1b, JERF3, and CAT were expressed at 2.5-7.5-folds in rhizobacterialtreated plants, but not in nematicide treatment. The defense enzymes viz., super oxide dismutase (SOD), polyphenol oxidase (PPO), peroxidase (PO), and phenylalanine ammonia lyase (PAL) when quantified (μmol/mg protein) showed an increase from 1.5 to 17.5 for SOD, 2.1 to 7.8 in PPO, 1.8 to 10.2 in PO, and 1.8 to 8.7 in PAL during 0 to 16 DAI, in rhizobacteria-treated plants.

From soil to plant: strengthening carrot defenses against Meloidogyne incognita with vermicompost and arbuscular mycorrhizal fungi biofertilizers

Introduction

Sustainable agricultural practices for controlling crop pests are urgently needed to reduce the reliance on chemical pesticides, which have long-term detrimental effects on ecosystems. In this study, we assessed the effectiveness of arbuscular mycorrhizal fungi (AMF) and vermicompost (Vc) supplementation, alone and in combination, in mitigating the negative impacts of Meloidogyne incognita infestation on carrot (Daucus carota L.) growth, development, and physiology.

Methods

We measured different plant growth parameters such as plant height and biomass accumulation, several plant physiological parameters such as the levels of photosynthetic pigments, phenolics, and the activity of defense enzymes such as peroxidases and polyphenol oxidases, and evaluated the severity of Meloidogyne incognita nematode infestation on plants treated or not treated with vermicompost (Vc) and/or arbuscular mycorrhizal fungi (AMF).

Results

Our findings show that M. incognita significantly affects plant growth, biomass accumulation, and photosynthetic pigment and carotenoid content. The incorporation of Vc and AMF into the soil, either individually or in combination, significantly alleviates the negative effects of nematode infestation on carrot plants. This was accompanied by the induction of phenolic compounds and defense enzymes such as peroxidases (+15.65%) and polyphenol oxidases (29.78%), and by a reduction in the severity of nematode infestation on Vc and AMF-treated plants compared to nematode-infested plants. Principal component analysis (PCA) shows significant correlations between various of the studied parameters. In particular, we observed negative correlations between the application of AMF and Vc alone and in combination and disease severity, and positive correlations between plant growth, photosynthetic pigments phenol content, and activity of defense enzymes.

Discussion

Our study highlights the relevance of cultural practices and beneficial microorganisms for the sustainable and environmentally friendly management of agricultural pests.

Inducing Systemic Acquired Resistance (SAR) Against Root-knot Nematode Meloidogyne Javanica and Evaluation of Biochemical Changes in Cucumber Root

For inducing systemic acquired resistance (SAR), Salicylic acid (SA), Ascorbic acid (AA), and silicon (Si) were applied on shoots, roots, and both of them simultaneously. Results showed that all treatments reduced the number of galls, root gall index, number of egg mass/root system, number of nematodes/root system, number of eggs/root system, number of nematodes/pot soil, the final population density of nematodes, and rate of reproduction. Treatments also increased growth criteria, including chlorophyll, shoot fresh weight, root fresh weight, shoots dry weight, root dry weight, shoots length, and root length. SA foliar and root application decreased infection criteria and increased total phenol, peroxidase, and phenol oxidase activities. Ascorbic acid and silicon increased total phenol, peroxidase, and phenol oxidase activities.

Biocontrol potential of Pseudomonas rhodesiae GC-7 against the root-knot nematode Meloidogyne graminicola through both antagonistic effects and induced plant resistance

Plant-parasitic nematodes (PPNs) cause serious damage to agricultural production worldwide. Currently, because of a lack of effective and environmental-friendly chemical nematicides, the use of microbial nematicides has been proposed as an eco-friendly management strategy to control PPNs. A nematicidal bacterium GC-7 was originally isolated from the rice rhizosphere, and was identified as Pseudomonas rhodesiae. Treatment with the fermentation supernatant of GC-7 in vitro showed a highly lethal effect on second-stage juveniles of Meloidogyne graminicola, with the mortality rate increasing to 95.82% at 24 h and egg hatching significantly inhibited, with a hatch inhibition rate of 60.65% at 96 h. The bacterium significantly reduced the level of damage caused by M. graminicola infestations to rice (Oryza sativa) in greenhouse and field experiments. Under greenhouse conditions, the GC-7 culture efficiently reduced the gall index and nematode population in rice roots and soils, as well as inhibited nematode development compared to the control. Under field conditions, application of the GC-7 consistently showed a high biocontrol efficacy against M. graminicola (with a control efficiency of 58.85%) and promoted plant growth. In addition, the inoculation of GC-7 in M. graminicola-infested rice plant fields significantly suppressed final nematode populations in soil under natural conditions. Furthermore, activities of plant defense-related enzymes, peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase were remarkably increased in plant roots treated with GC-7 compared with roots that were challenge to M. graminicola. Moreover, quantitative real-time PCR analysis showed that GC-7 significantly enhanced the expression of defense genes (PR1a, WRKY45, JaMYB, AOS2, ERF1, and ACS1) related to salicylic acid, jasmonic acid, and ethylene signaling pathways in rice roots after inoculation with GC-7 at different levels. The results indicated that GC-7 could be an effective biological component in the integrated management of M. graminicola infecting rice.

Potential of vermicompost extract in enhancing the biomass and bioactive components along with mitigation of Meloidogyne incognita-induced stress in tomato

Increasing inorganic fertilizer and pesticide use has been linked to increased health risks for humans and cattle, as well as substantial water and soil contamination. In recent years, vermicomposting has shown to be a viable alternative to chemical pesticides. Vermicompost and vermicompost products such as extract and leachate assist plants in a number of ways. According to recent studies, vermicompost extract (VCE), when used as a supplement, is thought to work as a growth and stress tolerance booster for plants. These liquid supplements also help to suppress a range of pests, such as root knot nematodes. In the present study, neem- and cattle dung-based vermicompost extracts of different concentrations (0, 20, 40, 60, 80 and 100%) were prepared and used for their application against nematode infection in tomato seedlings under laboratory conditions. Apart from its antagonistic action against Meloidogyne incognita, the influence of VCE on plant growth was investigated by analyzing its morphological characteristics in tomato seedlings infected and uninfected with M. incognita. Seeds were pre-soaked in VCE for the seed priming process before being allowed for germination. After 10 days of nematode inoculation, biochemical parameters like protein content, activity of antioxidative enzymes, non-enzymatic antioxidants, stress indices, photosynthetic pigments, proline content and secondary metabolites were also analyzed. The results revealed that neem-based VCE was fatal to second-stage juveniles, with an 82% mortality rate following exposure to the highest dose. When eggs were exposed to 100% VCE, 33.8% of hatching was suppressed, indicating that VCE had an antagonistic effect on nematode egg hatching. Further, all the morphological and biochemical parameters were significantly enhanced in VCE-treated tomato seedlings as compared to untreated seedlings. Stress indices were also found to be significantly lowered by the VCE treatments in the infected plants. The effect of VCE on seedling growth and physiology was shown to be concentration dependent. As a result, the current findings show that VCE has the potential to be used as a plant growth accelerator as well as an environmentally friendly biocontrol agent against nematode pathogenesis in tomato plants.

Nematicidal Activity of Burkholderia arboris J211 Against Meloidogyne incognita on Tobacco

Root-knot nematode (Meloidogyne incognita) is the most widespread nematode affecting Solanaceae crops. Due to the lack of effective measures to control this nematode, its management can be achieved, using biocontrol agents. This study investigated in vitro efficacy of the antagonistic bacterial strain J211 isolated from tobacco rhizosphere soil against M. incognita, and further assessed its role in controlling nematodes, both in pot and field trials. Phylogenetic analysis of the 16S rRNA gene sequence of strain J211 assigned to Burkholderia arboris. Culture filtrates B. arboris J211 exhibited anematicidal activity against the second-stage juveniles (J2s) of M. incognita, with a 96.6% mortality after 24 h exposure. Inoculation of J211 in tobacco roots significantly reduced the root galling caused by M. incognita, both in pot and field trials. Meanwhile, plant growth-promoting (PGP) traits results showed that J211 had outstanding IAA-producing activity, and the IAA production reached 66.60 mg L⁻¹. In the field study, B. arboris J211 also promoted tobacco growth and increase flue-cured tobacco yield by 8.7–24.3%. Overall, B. arboris J211 as a high-yielding IAA nematicidal strain effectively controlled M. incognita and improved tobacco yield making it a promising alternative bionematocide.

Effect of Inoculation with Arbuscular Mycorrhizal Fungi and Fungicide Application on the Secondary Metabolism of Solanum tuberosum Leaves

In potato (Solanum tuberosum) crops, the use of fungicides to control some diseases is widespread; however, it has been reported that this practice can modify the potato polyphenolic content, and new strategies oriented to the potato defense system are necessary. One alternative is the use of arbuscular mycorrhizal fungi (AMF) to improve the defense mechanisms of plants. In this study, phenolic profiles and antioxidant activities in leaves of three potato genotypes (CB2011-509, CB2011-104, and VR808) were evaluated in crops inoculated with three AMF strains (Claroideoglomus claroideum, Claroideoglomus lamellosum, and Fumneliformis mosseae) and with AMF in combination with the use of two commercial fungicides (MONCUT [M] and ReflectXtra [R]). Eight phenolic compounds were detected, mainly hydroxycinnamic acids (HCAD) and flavonols, in samples where the highest concentrations of HCAD were obtained, 5-caffeoylquinic acid was the most abundant phenolic. The antioxidant activity was higher using the cupric reducing antioxidant capacity (CUPRAC) and ferric reducing antioxidant power (FRAP) methods. The association of AMF with plants had benefits on the secondary metabolism; however, the response differed according to genotype. The different combinations of potato genotypes, AMF strain, and fungicide modified the content of phenolic compounds in leaves in different ways; the treatment using C. lamellosum and ReflectXtra was the ideal combination for the genotypes analyzed here, with the higher antioxidant response, which supports the further technological evaluation of efficient AMF strains and fungicides in potato crops.

Halotolerant Rhizobacteria for Salinity-Stress Mitigation: Diversity, Mechanisms and Molecular Approaches

Agriculture is the best foundation for human livelihoods, and, in this respect, crop production has been forced to adopt sustainable farming practices. However, soil salinity severely affects crop growth, the degradation of soil quality, and fertility in many countries of the world. This results in the loss of profitability, the growth of agricultural yields, and the step-by-step decline of the soil nutrient content. Thus, researchers have focused on searching for halotolerant and plant growth-promoting bacteria (PGPB) to increase soil fertility and productivity. The beneficial bacteria are frequently connected with the plant rhizosphere and can alleviate plant growth under salinity stress through direct or indirect mechanisms. In this context, PGPB have attained a unique position. The responses include an increased rate of photosynthesis, high production of antioxidants, osmolyte accumulation, decreased Na+ ions, maintenance of the water balance, a high germination rate, and well-developed root and shoot elongation under salt-stress conditions. Therefore, the use of PGPB as bioformulations under salinity stress has been an emerging research avenue for the last few years, and applications of biopesticides and biofertilizers are being considered as alternative tools for sustainable agriculture, as they are ecofriendly and minimize all kinds of stresses. Halotolerant PGPB possess greater potential for use in salinity-affected soil as sustainable bioinoculants and for the bioremediation of salt-affected soil.

Bacterial strains integrated with surfactin molecules of Bacillus subtilis MTCC441 enrich nematocidal activity against Meloidogyne incognita

Bacteria secrete lipopeptide (surfactin) molecules, which are known to act as natural antibiotics. Recently, research on lipopeptide molecules has grown because of their role in increasing resistance to plant pathogens. Isolated surfactin molecules at several concentrations, viz. 35, 25, 15 and 5 ppm, were used in an in vitro study for analysis of egg hatching inhibition and second-stage juvenile (J2) mortality of the nematode Meloidogyne incognita. Cell suspensions (1.2 × 10⁸  cfu·ml^-1 ) of both Bacillus subtilis (MTCC-441) and Pseudomonas putida (MTCC-102) were used in J2 inoculated tomato plants. Root-dip treatment of tomato seedlings with the crude lipopeptide (35 ppm) was also performed to analyse biocontrol potential. MALDI-TOF MS analysis was conducted to determine specific lipopeptide molecules. Data showed nematode egg mortality of 85% in the 35 ppm surfactin concentration 96 h after exposure. The maximum ovicidal activity was 83.97% after incubation with 35 ppm surfactin for the same exposure period. Plant growth attributes and biochemical parameters were significantly improved when bacterial cultures were applied before J2 inoculation of tomato seedlings. We also recorded a reduction in egg masses, nematode population and root galling. The J2 penetration into tomato roots was effective in the root-dip experiments. Surfactin mass peak was determined at m/z 1058 [M+Na]⁺ using MALDI-TOF MS. These results indicate that bacterial cell suspensions can be used as a potent and versatile source to deal with nematode infection and provide a rich source of bioactive compounds with antinematode activity.

Pre-inoculation with arbuscular mycorrhizal fungi affects essential oil quality and the reproduction of root lesion nematode in Cymbopogon citratus

Cymbopogon citratus (lemongrass) is an important medicinal and aromatic plant containing citral-rich essential oil, of which the quality and quantity may be affected by nematode infection. Research has shown that arbuscular mycorrhizal fungi (AMF) may act as nematode biocontrol agents and improve the chemical composition of plants. Three experiments were conducted to assess the effects of AMF inoculation on vegetative growth, essential oil composition, induction of defense-related proteins, and control of Pratylenchus brachyurus in C. citratus. Seedlings were transplanted into pots inoculated with one of two AMF species (Rhizophagus clarus or Claroideoglomus etunicatum). At 30 days after AMF inoculation, plants were inoculated with P. brachyurus. Evaluations were performed at 75 days after nematode inoculation. Although both AMF treatments led to effective root colonization (> 84%), fungus inoculation was not effective in reducing P. brachyurus population density. Nevertheless, C. etunicatum promoted an increase in shoot weight, and AMF treatments contributed to preserving essential oil composition in nematode-infected plants. In addition, both AMF treatments enhanced polyphenol oxidase activity and R. clarus increased peroxidase activity after nematode inoculation.

Bioprospection: in vitro antimicrobial potential of the leaf extract of mycorrhizal guava infected by Meloidogyne enterolobii on Klebsiella pneumoniae

Guava (Psidium guajava L.), is adapted to tropical and subtropical climates, and, in addition to its nutritional value, has great medicinal potential. One of the medicinal effects is antibacterial, and this can be identified from the phytochemicals present in its various parts, especially the leaf, which contains flavonoids, phenols, and tannins, as well as phytocomposites with antibacterial action. Therefore, the interaction of this plant with arbuscular mycorrhizal fungi and Meloidogyne enterolobii is a biotechnological resource that can increase the production of secondary metabolites so that the guava ethanolic extract is effective against multidrug-resistant bacterial strains. Therefore, the objective of this study was to test the inhibitory action of mycorrhizal guava leaf extract and Meloidogyne enterolobii on strains of Klebsiella pneumoniae carbapenemase. Guava seedlings from cuttings were inoculated with Acaulospora longula, and later with Meloidogyne enterolobii; the leaves were harvested at two maturation times of the plant and placed in an oven. Next, a leaf extract was prepared using ethanol as a solvent. The extract was tested in multidrug-resistant strains of K. pneumoniae carbapenemase from operative wounds using disc diffusion methodology. The plant-AMF-phytonematode interaction positively potentialize the inhibitory action of guava leaf ethanolic extract on multidrug-resistant bacterial strains.

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.

Bacillus thuringiensis and Silicon Modulate Antioxidant Metabolism and Improve the Physiological Traits to Confer Salt Tolerance in Lettuce

We investigated the impact of Bacillus thuringiensis as seed treatment and application with silicon on lettuce plants exposed to salinity levels (4 dS m⁻¹ and 8 dS m⁻¹). Results revealed that leaves number, head weight, total yield, relative water content (RWC), and chlorophyll a and b declined considerably due to two salinity levels. Oxidative stress markers, i.e., hydrogen peroxide (H₂O₂), superoxide (O₂⁻), and lipid peroxidation (MDA) dramatically augmented in stressed plants. On the other hand, leaves number, total yield, RWC, and chlorophyll a, b in stressed lettuce plants were considerably enhanced because of the application of Si or B. thuringiensis. In contrast, EL%, MDA, and H₂O₂ were considerably reduced in treated lettuce plants with Si and B. thuringiensis. In addition, the treatment with Si and B. thuringiensis increased head weight (g) and total yield (ton hectare-1), and caused up-regulation of proline and catalase, superoxide dismutase, peroxidase, and polyphenol oxidase activity in lettuce leaves under salinity conditions.

Mycorrhizal-Bacterial Amelioration of Plant Abiotic and Biotic Stress

Soil microbiota plays an important role in the sustainable production of the different types of agrosystems. Among the members of the plant microbiota, mycorrhizal fungi (MF) and plant growth-promoting bacteria (PGPB) interact in rhizospheric environments leading to additive and/or synergistic effects on plant growth and heath. In this manuscript, the main mechanisms used by MF and PGPB to facilitate plant growth are reviewed, including the improvement of nutrient uptake, and the reduction of ethylene levels or biocontrol of potential pathogens, under both normal and stressful conditions due to abiotic or biotic factors. Finally, it is necessary to expand both research and field use of bioinoculants based on these components and take advantage of their beneficial interactions with plants to alleviate plant stress and improve plant growth and production to satisfy the demand for food for an ever-increasing human population.

Ménage à Trois: Unraveling the Mechanisms Regulating Plant-Microbe-Arthropod Interactions

Plant-microbe-arthropod (PMA) three-way interactions have important implications for plant health. However, our poor understanding of the underlying regulatory mechanisms hampers their biotechnological applications. To this end, we searched for potential common patterns in plant responses regarding taxonomic groups or lifestyles. We found that most signaling modules regulating two-way interactions also operate in three-way interactions. Furthermore, the relative contribution of signaling modules to the final plant response cannot be directly inferred from two-way interactions. Moreover, our analyses show that three-way interactions often result in the activation of additional pathways, as well as in changes in the speed or intensity of defense activation. Thus, detailed, basic knowledge of plant-microbe-arthropod regulation will be essential for the design of environmentally friendly crop management strategies.

Exploiting Potential of <i>Trichoderma harzianum</i> and <i>Glomus versiforme</i> in Mitigating <i>Cercospora</i> Leaf Spot Disease and Improving Cowpea Growth

BACKGROUND AND OBJECTIVE

Trichoderma species are of utmost importance in agro-biotechnological applications because, in their interactions with plant hosts, they out-compete most pathogenic microorganisms. This study aimed at exploiting the potential of Trichoderma harzianum together with Glomus versiforme and its mutants, in inhibiting cowpea leaf spot rot induced due to Cercospora canescens infestation and improving agronomic growth parameter in a screen house experiment.

MATERIALS AND METHODS

The experiment was designed using single and co-inoculation of the bioagents: in all, eleven treatments were applied, consisting of Glom_verwild, Glom_ver30, Glom_ver60, Glom_ver90, Trich_h, Glom_verwild+Trich_h, Glom_ver30+Trich_h, Glom_ver60+Trich_h, Glom_ver90+Trich_h, Pathogen alone and control. Cowpea growth yield parameters and disease severity were assessed after 7 weeks.

RESULTS

The deployed treatments improved agronomic growth parameters substantially (p<0.05) relative to control. Glom_ver 60+Trich_h treatment exerted the highest agronomic growth improvement yield. In addition, the best reduction in the incidence and severity of cowpea leaf spot disease was obtained using Glom_ver 60+Trich_h. A significantly higher germination rate in seeding, confirms both inhibitory and growth improvement potency of the bio inoculants treatment.

CONCLUSION

This study's findings confirmed the beneficial impacts of the treatment of seed and soil with dual T. harzianum and G. versiforme, in improving the immunity of cowpea to Cercospora canescens leaf spot infection and improve cowpea growth.

Insights into the Role of Streptomyces hydrogenans as the Plant Growth Promoter, Photosynthetic Pigment Enhancer and Biocontrol Agent against Meloidogyne incognita in Solanum lycopersicum Seedlings

Root-knot nematodes (RKN), Meloidogyne sp. hinders functioning of crops and causes global losses in terms of productivity and yield. Meloidogyne sp. are microscopic, obligatory endoparasites with ubiquitous distribution in different parts of the world. Taking into consideration these aspects, the present study was conducted to explore nematicidal activity of the Streptomyces hydrogenans strain DH-16 against M. incognita to regulate its pathogenicity in plants. In-vitro experimentation revealed that pretreated seeds with solvent and culture supernatant lowered root galls in infested plants and promoted growth of Solanum lycopersicum seedlings, revealed through the morphological analysis. Additionally, antioxidative defense responses were induced with microbes. However, oxidative stress markers were considerably reduced after microbial inoculations. Apart from this, secondary metabolites were assessed and modulated in RKN infested plants on microbial supplementations. Confocal studies evaluated glutathione accumulation within root apices and its enhancement was directly proportional to defense responses. Therefore, the current study concluded the role of S. hydrogenans in stimulating antioxidant potential against RKN along with growth promoting aids. Thus, the outcome of the current study endorses that metabolites produced by S. hydrogenans can be used as safe biocontrol agents against M. incognita and also as plant growth promoting agents.

Arbuscular mycorrhizal fungi decrease Meloidogyne enterolobii infection of Guava seedlings

Guava (Psidium guajava L.) production is prominent in the irrigated fruit growing area of Brazil. However, the parasite Meloidogyne enterolobii (a phytonematode) has caused a decrease in guava production. Arbuscular mycorrhizal fungi (AMF) are known to be beneficial to plants; however, their ability to protect plants against nematodes such as M. enterolobii remains poorly known. This study aimed to monitor M. enterolobii infection in guava seedlings inoculated with three AMF species. After AMF inoculation, the seedlings were grown in sterile soil for 60 days before inoculation with 2000 M. enterolobii eggs. Plant growth parameters, mycorrhizal colonization and the number of Meloidogyne in the roots were determined over time (30 and 60 days after Meloidogyne inoculation). The AMF enhanced guava seedling growth, and reduced the amount of Meloidogyne in the roots at 30 and 60 days after nematode inoculation, indicating that these AMF species could serve as biocontrol agents of M. enterolobii in guava cultivation.

Chlorophyll Fluorescence Parameters and Antioxidant Defense System Can Display Salt Tolerance of Salt Acclimated Sweet Pepper Plants Treated with Chitosan and Plant Growth Promoting Rhizobacteria

Salinity stress deleteriously affects the growth and yield of many plants. Plant growth promoting rhizobacteria (PGPR) and chitosan both play an important role in combating salinity stress and improving plant growth under adverse environmental conditions. The present study aimed to evaluate the impacts of PGPR and chitosan on the growth of sweet pepper plant grown under different salinity regimes. For this purpose, two pot experiments were conducted in 2019 and 2020 to evaluate the role of PGPR (Bacillus thuringiensis MH161336 106–8 CFU/cm3) applied as seed treatment and foliar application of chitosan (30 mg dm−3) on sweet pepper plants (cv. Yolo Wonder) under two salinity concentrations (34 and 68 mM). Our findings revealed that, the chlorophyll fluorescence parameter (Fv/Fm ratio), chlorophyll a and b concentrations, relative water content (RWC), and fruit yield characters were negatively affected and significantly reduced under salinity conditions. The higher concentration was more harmful. Nevertheless, electrolyte leakage, lipid peroxidation, hydrogen peroxide (H2O2), and superoxide (O2−) significantly increased in stressed plants. However, the application of B. thuringiensis and chitosan led to improved plant growth and resulted in a significant increase in RWC, chlorophyll content, chlorophyll fluorescence parameter (Fv/Fm ratio), and fruit yield. Conversely, lipid peroxidation, electrolyte leakage, O2−, and H2O2 were significantly reduced in stressed plants. Also, B. thuringiensis and chitosan application regulated the proline accumulation and enzyme activity, as well as increased the number of fruit plant−1, fruit fresh weight plant−1, and total fruit yield of sweet pepper grown under saline conditions.

Bacterial Community Structure Dynamics in Meloidogyne incognita-Infected Roots and Its Role in Worm-Microbiome Interactions

Plant parasitic nematodes such as Meloidogyne incognita have a complex life cycle, occurring sequentially in various niches of the root and rhizosphere. They are known to form a range of interactions with bacteria and other microorganisms that can affect their densities and virulence. High-throughput sequencing can reveal these interactions in high temporal and geographic resolutions, although thus far we have only scratched the surface. In this study, we have carried out a longitudinal sampling scheme, repeatedly collecting rhizosphere soil, roots, galls, and second-stage juveniles from 20 plants to provide a high-resolution view of bacterial succession in these niches, using 16S rRNA metabarcoding. Our findings indicate that a structured community develops in the root, in which gall communities diverge from root segments lacking a gall, and that this structure is maintained throughout the crop season. We describe the successional process leading toward this structure, which is driven by interactions with the nematode and later by an increase in bacteria often found in hypoxic and anaerobic environments. We present evidence that this structure may play a role in the nematode's chemotaxis toward uninfected root segments. Finally, we describe the J2 epibiotic microenvironment as ecologically deterministic, in part, due to the active bacterial attraction of second-stage juveniles.IMPORTANCE The study of high-resolution successional processes within tightly linked microniches is rare. Using the power and relatively low cost of metabarcoding, we describe the bacterial succession and community structure in roots infected with root-knot nematodes and in the nematodes themselves. We reveal separate successional processes in galls and adjacent non-gall root sections, which are driven by the nematode's life cycle and the progression of the crop season. With their relatively low genetic diversity, large geographic range, spatially complex life cycle, and the simplified agricultural ecosystems they occupy, root-knot nematodes can serve as a model organism for terrestrial holobiont ecology. This perspective can improve our understanding of the temporal and spatial aspects of biological control efficacy.

Biological Control of Plant-Parasitic Nematodes by Filamentous Fungi Inducers of Resistance: Trichoderma, Mycorrhizal and Endophytic Fungi

Plant-parasitic-nematodes represent a major threat to the agricultural production of different crops worldwide. Due to the high toxicity of chemical nematicides, it is necessary to develop new control strategies against nematodes. In this respect, filamentous fungi can be an interesting biocontrol alternative. The genus Trichoderma, mycorrhizal and endophytic fungi are the main groups of filamentous fungi studied and used as biological control agents (BCAs) against nematodes as resistance inducers. They are able to reduce the damage caused by plant-parasitic nematodes directly by parasitism, antibiosis, paralysis and by the production of lytic enzymes. But they also minimize harm by space and resource-competition, by providing higher nutrient and water uptake to the plant, or by modifying the root morphology, and/or rhizosphere interactions, that constitutes an advantage for the plant-growth. Besides, filamentous fungi are able to induce resistance against nematodes by activating hormone-mediated (salicylic and jasmonic acid, strigolactones among others) plant-defense mechanisms. Additionally, the alteration of the transport of chemical defense components through the plant or the synthesis of plant secondary metabolites and different enzymes can also contribute to enhancing plant defenses. Therefore, the use of filamentous fungi of the mentioned groups as BCAs is a promising durable biocontrol strategy in agriculture against plant-parasitic nematodes.

Variations in Growth, Physiology, and Antioxidative Defense Responses of Two Tomato (Solanum lycopersicum L.) Cultivars after Co-Infection of Fusarium oxysporum and Meloidogyne incognita

The soil-borne fungus Fusarium oxysporum (Fo) and the nematode Meloidogyne incognita (Mi) are destructive pathogens that cause substantial yield losses to tomato (Solanum lycopersicum L.) crops worldwide. The present study sought to elucidate the physiological, biochemical, and cytological responses of tomato cultivars (Gailing maofen 802 and Zhongza 09) by root invasion of Fo (1 × 105 CFUmL−1) and Mi (1500 second-stage juveniles (J2) alone and in combination after 14 days. Results revealed that combined inoculation of Fo and Mi significantly increased disease intensity, electrolyte leakage, and hydrogen peroxide and malondialdehyde contents; and decreased photosynthetic capacity and enzyme activity in both cultivars as compared to their solo inoculation. Increasing the disease intensity reduced the maximum morphological traits, such as shoot length, total dry weight, and total chlorophyll contents, in G. maofen 802 (by 32%, 54.2%, and 52.3%, respectively) and Zhongza 09 (by 18%, 32%, and 21%, respectively) as compared to the control. Others factors were also reduced in G. maofen 802 and Zhongza 09, such as photosynthetic capacity (by 70% and 57%, respectively), stomatal conductance (by 86% and 70%, respectively), photochemical quantum yield of photosystem II (YII) (by 36.6% and 29%, respectively), and electron transport rate (by 17.7% and 10%, respectively), after combined inoculation of Fo and Mi. Furthermore, the combined infestation of Fo and Mi resulted in reduced activity of plant-defense-related antioxidants in G. maofen 802 compared with their single application or control. However, these antioxidants were highly up-regulated in Zhongza 09 (by 59%–93%), revealing the induction of tolerance against studied pathogens. The transmission electron microscopy (TEM) results further demonstrated that root cells of Zhongza 09 had unique tetrahedral crystal-like structures in the membrane close to mitochondria under all treatments except control. Therefore, it is concluded that Mi caused severe root damage, suppressed plant growth, depleted antioxidants, and caused high generation of ROS in the presence of Fo as compared to its solo inoculation. Tolerant cultivars adopted different mechanistic strategies at the structural and cellular levels to tolerate the Mi and Fo stresses.

Plant growth promoting bacteria as an alternative strategy for salt tolerance in plants: A review

Approximately 5.2 billion hectare agriculture land are affected by erosion, salinity and soil degradation. Salinity stress has significantly affecting the fertile lands, and therefore possesses a huge impact on the agriculture and economy of a country. Salt stress has severe effects on the growth and development of plants as well as reducing its yield. Plants are inherently equipped with stress tolerance ability to responds the specific type of stress. Plants retained specific mechanisms for salt stress mitigation, such as hormonal stimulation, ion exchange, antioxidant enzymes and activation of signaling cascades on their metabolic and genetic frontiers that sooth the stressed condition. Additional to the plant inherent mechanisms, certain plant growth promoting bacteria (PGPB) also have specialized mechanism that play key role for salt stress tolerance and plant growth promotion. These bacteria triggers plants to produce different plant growth hormones like auxin, cytokinine and gibberellin as well as volatile organic compounds. These bacteria also produces growth regulators like siderophore, which fix nitrogen, solubilize organic and inorganic phosphate. Considering the importance of PGPB in compensation of salt tolerance in plants, the present study has reviewed the different aspect and mechanism of bacteria that play key role in promoting plants growth and yield. It can be concluded that PGPB can be used as a cost effective and economical tool for salinity tolerance and growth promotion in plants.