Plant Probiotic Bacteria Research Articles

Plant probiotic microbes – trending microbes for plant growth and value addition of compost- a review

New agricultural approaches, including natural and organic farming, have developed organic and microbiological goods. Beneficial microbes known as plant probiotic microorganisms (PPM) are primarily found in soil and have coevolved with plants in a symbiotic or free-living relationship. By encouraging plant growth and health, these bacteria lessen the need for artificial fertilizers, support environmentally friendly agricultural practices, and improve plant health. Agroecosystem vigour, biodiversity, biological cycles, and soil biological activity are all enhanced by using plant probiotic bacteria in biologically based agriculture. By using beneficial bacteria to support the health and function of the host organism, plant probiotics aim to lessen the demand for toxic pesticides and fertilizers. Using these probiotic microorganisms will have a synergistic and advantageous effect on crop production. Plant probiotic microorganisms improve the overall quality of compost throughout the composting process by accelerating the breakdown of organic materials and introducing helpful bacteria. This rich compost adds beneficial bacteria to the soil, creating a healthy and productive environment. This review looks at the common plant probiotic bacteria, how they work, how they complement each other to improve compost quality and plant growth, how they might be used in agriculture, and how they could change how composting is done.

Identification of synthetic consortia from a set of plant-beneficial bacteria.

The use of microbial inoculants in agriculture as biofertilisers and/or biopesticides is an appealing alternative to replace or reduce the practice of agrochemicals. Plant microbiota studies are revealing the different bacterial groups which are populating plant microbiomes re-energising the plant probiotic bacteria (PPB) translational research sector. Some single-microbial strain bioinoculants have proven valid in agriculture (e.g., based on Trichoderma, mycorrhiza or rhizobia); however, it is now recommended to consider multistrain consortia since plant-beneficial effects are often a result of community-level interactions in plant microbiomes. A limiting step is the selection of a fitting combination of microbial strains in order to accomplish the best beneficial effect upon plant inoculation. In this study, we have used a subset of 23 previously identified and characterised rice-beneficial bacterial colonisers to design and test a series of associated experiments aimed to identify potential PPB consortia which are able to co-colonise and induce plant growth promotion. Bacterial strains were co-inoculated in vitro and in planta using several different methods and their co-colonisation and co-persistence monitored. Results include the identification of two 5-strain and one 2-strain consortia which displayed plant growth-promoting features. Future practical applications of microbiome research must include experiments aimed at identifying consortia of bacteria which can be most effective as crop amendments.

Perspective on utilization of Bacillus species as plant probiotics for different crops in adverse conditions.

Plant probiotic bacteria are a versatile group of bacteria isolated from different environmental sources to improve plant productivity and immunity. The potential of plant probiotic-based formulations is successfully seen as growth enhancement in economically important plants. For instance, endophytic Bacillus species acted as plant growth-promoting bacteria, influenced crops such as cowpea and lady's finger, and increased phytochemicals in crops such as high antioxidant content in tomato fruits. The present review aims to summarize the studies of Bacillus species retaining probiotic properties and compare them with the conventional fertilizers on the market. Plant probiotics aim to take over the world since it is the time to rejuvenate and restore the soil and achieve sustainable development goals for the future. Comprehensive coverage of all the Bacillus species used to maintain plant health, promote plant growth, and fight against pathogens is crucial for establishing sustainable agriculture to face global change. Additionally, it will give the latest insight into this multifunctional agent with a detailed biocontrol mechanism and explore the antagonistic effects of Bacillus species in different crops.

Adjuvant Pluronic F68 Is Compatible with a Plant Root-Colonizing Probiotic, Pseudomonas chlororaphis O6

Plant probiotic bacteria are being increasingly used to maximize both the productivity and quality of field crops. Pseudomonas chlororaphis O6 (PcO6) is a plant root colonizer with probiotic activities. This bacterium produces an array of metabolites, including a group of phenazines that are functional in plant protection. The paper reports responses of PcO6 to a nonionic triblock copolymer surfactant, Pluronic F68. This Pluronic exhibits membrane “healing” activity and improves cryopreservation recovery in eukaryotic cells. The product is FDA-approved and is applied as an adjuvant in formulations used in agriculture, medicine, and biotechnology. Growth of PcO6 on lysogeny broth at 25 °C was unhindered by 0.1 and 1.0 g/L F68, reduced at 10 g/L, and with significant inhibition at 100 g/L F68; micelle formation could account for inhibited growth at higher doses. Phenazine production was not changed by F68, whereas the surfactant activity of F68 induced the spread of bacterial colonization on 0.5% agar. Exposure of cells to fluorescein-labeled F68 resulted in intense fluorescence, stable to washing, showing a direct association of the Pluronic with the bacterium. However, neither protection nor harm was found for PcO6 cells suspended in either 0.1% or 1% F68 after three freeze (−20 °C)/thaw cycles. These findings suggest that F68 could be compatible for use in agricultural formulations with little effect on probiotics such as PcO6.

Isolation of plant growth-promoting rhizobacteria from the agricultural fields of Tattiannaram, Telangana

BackgroundPlant probiotics bacteria are live microbes that promote soil health and plant growth and build the stress-tolerant capacity to the plants. They benefit the plants by increasing nutrient absorption and release of stress-related phytohormones. These plant probiotic bacteria serve a better purpose to the plant when compared to chemical fertilizers. Use of chemical fertilizers such as arsenic and cadmium can lead to soil acidification and even release of harmful gases such as methane which further pollutes the environment. ResultsDifferent bacterial species were isolated from the agricultural fields of Tattiannaram, Telangana, and identified as the efficient rhizosphere bacteria with the essential qualities of plant growth promotion by evaluating the nitrogen-fixing ability on a selective media and various other methods. Upon the molecular characterization of the isolates, they were identified as Corynebacterium spp., Bacillus spp., Lactobacillus spp., and Cytobacillus spp. The results were also examined using various bioinformatics tools for accuracy in their phylogenetic pattern. ConclusionThe recognized species of plant probiotics have established roles in promoting plant growth and strengthening plant immunity. This research introduces an innovative methodology for evaluating and investigating recently identified bacterial isolates, focusing on their distinctive plant probiotic attributes. Through harnessing the potential of advantageous microorganisms and comprehending their interaction with plants and soil, our objective is to formulate inventive approaches to elevate crop productivity, enhance soil richness, and foster environmentally sustainable and robust agricultural methodologies. These characteristics exhibit promising potential for future incorporation into plant systems, fortifying growth and development, and underscoring their distinctive significance within the realm of agriculture.

Foliar application of chitosan and plant probiotic bacteria influencing the growth, productivity and bulb storage life of onion

Onion having several health promoting phytochemical contents is an important spices in most of the countries of the world as well as vegetables in particular and chitosan and plant probiotic bacteria are few proven natural growth enhancing agents. Here, three bio-stimulants namely Chitosan (100 and 200 ppm), Paraburkholderia BRRh-4 bacteria (1.5 × 109 CFU mL-1) and Clybio (0. 1%) in singly as well as combining one another were applied at twelve different treatment combinations including control with the aim of investigating the below and above ground vegetative growth, bulb productivity and post-harvest storage performance of onion cv. “Taherpuri”. The experiment was set up in Randomized Complete Block Design (RCBD) maintaining three replicates and conducted during November 2019 to June 2020 where the bio-stimulant formulations were sprayed in the foliage at 20, 40 and 60 days after transplanting (DAT). Reproductive behaviors of the onion cultivar differed significantly among the treatments and those were the resultants of statistically varied vegetative performances recorded at 30, 60 and 90 DAT. Chitosan @ 100 ppm + Clybio (0.1%) + Paraburkholderia BRRh-4 (1.5 × 109 CFU mL-1) combination exhibited statistical superiority over the other treatments in terms of plant height (53.230 cm), number of leaves (10.13/plant), leaf length (41.20 cm) and diameter (10.33 mm), number of roots (32.53/plant), root length (12.23 cm), neck and bulb diameter (11.20 mm and 4.83 cm, respectively) and bulbing ratio (0.23) at 90 DAT. While, shoot and root growth didn’t vary significantly at earlier stages (at 30 DAT). Again, the highest yield (3.36 kg/plot and 10.45 t/ha) was also obtained from Chitosan @ 100 ppm + Clybio (0.1%) + Paraburkholderia BRRh-4 (1.5 × 109 CFU ml-1) treatment. The combination of chitosan and probiotic bacteria had significant influence on post-harvest storage qualities of onion where the longest shelf life/marketable life (8.17 months) with the lowest physiological weight loss and bulb rotting at 90 days after harvest was administered in Chitosan @ 100 ppm + Clybio (0.1%) + Paraburkholderia BRRh-4 (1.5 × 109 CFU mL-1) treatment. Besides the best treatment, Chitosan @ 100 ppm + Clybio (0.1%) and Chitosan @ 100 ppm + Paraburkholderia BRRh-4 (1.5 × 109 CFU mL-1) produced statistically superior results over control for onion growth and development. Therefore, chitosan and plant probiotic bacteria can be judiciously used for quality onion production in the warm and humid areas of the world

Enhance of tomato production and induction of changes on the organic profile mediated by Rhizobium biofortification.

The extensive use of chemical fertilizers has served as a response to the increasing need for crop production in recent decades. While it addresses the demand for food, it has resulted in a decline in crop productivity and a heightened negative environmental impact. In contrast, plant probiotic bacteria (PPB) offer a promising alternative to mitigate the negative consequences of chemical fertilizers. PPB can enhance nutrient availability, promote plant growth, and improve nutrient uptake efficiency, thereby reducing the reliance on chemical fertilizers. This study aimed to evaluate the impact of native Rhizobium strains, specifically Rhizobium calliandrae LBP2-1, Rhizobium mayense NSJP1-1, and Rhizobium jaguaris SJP1- 2, on the growth, quality, and rhizobacterial community of tomato crops. Various mechanisms promoting plant growth were investigated, including phosphate solubilization, siderophore production, indole acetic acid synthesis, and cellulose and cellulase production. Additionally, the study involved the assessment of biofilm formation and root colonization by GFP-tagged strains, conducted a microcosm experiment, and analyzed the microbial community using metagenomics of rhizospheric soil. The results showed that the rhizobial strains LBP2-1, NSJP1-1 and SJP1-2 had the ability to solubilize dicalcium phosphate, produce siderophores, synthesize indole acetic acid, cellulose production, biofilm production, and root colonization. Inoculation of tomato plants with native Rhizobium strains influenced growth, fruit quality, and plant microbiome composition. Metagenomic analysis showed increased Proteobacteria abundance and altered alpha diversity indices, indicating changes in rhizospheric bacterial community. Our findings demonstrate the potential that native Rhizobium strains have to be used as a plant probiotic in agricultural crops for the generation of safe food and high nutritional value.

Improvement of growth, yield and associated bacteriome of rice by the application of probiotic Paraburkholderia and Delftia.

Plant probiotic bacteria enhance growth and yield of crop plants when applied at the appropriate time and dose. Two rice probiotic bacteria, Paraburkholderia fungorum strain BRRh-4 and Delftia sp. strain BTL-M2 promote growth and yield of plants. However, no information is available on application of these two bacteria on growth, yield, and diversity and population of bacteriome in roots and rhizosphere soils of the treated rice plants. This study aimed to assess the effect of BRRh-4 and BTL-M2 application on growth, yield and bacteriome in roots and rhizosphere soil of rice under varying doses of N, P and K fertilizers. Application of BRRh-4 and BTL-M2 strains significantly (p < 0.05) increased seed germination, growth and yield of rice compared to an untreated control. Interestingly, the grain yield of rice by these bacteria with 50% less of the recommended doses of N, P, and K fertilizers were statistically similar to or better than the rice plants treated with 100% doses of these fertilizers. Targeted amplicon (16S rRNA) sequence-based analysis revealed significant differences (PERMANOVA, p = 0.00035) in alpha-diversity between the root (R) and rhizosphere soil (S) samples, showing higher diversity in the microbial ecosystem of root samples. Additionally, the bacteriome diversity in the root of rice plants that received both probiotic bacteria and chemical fertilizers were significantly higher (PERMANOVA, p = 0.0312) compared to the rice plants treated with fertilizers only. Out of 185 bacterial genera detected, Prevotella, an anaerobic and Gram-negative bacterium, was found to be the predominant genus in both rhizosphere soil and root metagenomes. However, the relative abundance of Prevotella remained two-fold higher in the rhizosphere soil metagenome (52.02%) than in the root metagenome (25.04%). The other predominant bacterial genera detected in the rice root metagenome were Bacillus (11.07%), Planctomyces (4.06%), Faecalibacterium (3.91%), Deinococcus (2.97%), Bacteroides (2.61%), and Chryseobacterium (2.30%). On the other hand, rhizosphere soil metagenome had Bacteroides (12.38%), Faecalibacterium (9.50%), Vibrio (5.94%), Roseomonas (3.40%), and Delftia (3.02%). Interestingly, we found the presence and/or abundance of specific genera of bacteria in rice associated with the application of a specific probiotic bacterium. Taken together, our results indicate that improvement of growth and yield of rice by P. fungorum strain BRRh-4 and Delftia sp. strain BTL-M2 is likely linked with modulation of diversity, structures, and signature of bacteriome in roots and rhizosphere soils. This study for the first time demonstrated that application of plant growth promoting bacteria significantly improve growth, yield and increase the diversity of bacterial community in rice.

How the plant probiotic bacteria and herbivore-induced plant volatiles (HIPVs) alter functional response of Phytoseiulus persimilis (Phytoseiidae) on the two-spotted spider mite

The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is one of the economically most important pests on different crops worldwide. Its management programs are normally conducted based on the usage of chemical acaricides. However, considering the side effects of acaricides and rapid resistance development, releasing the predator mite, Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae), is an alternative method for the control of T. urticae. In this study, the effects of plant probiotic bacteria; Bacillus pumilus INR7 (Bacillales: Bacillaceae) and Bacillus velezensis Fol (Bacillales: Bacillaceae), and herbivore-induced plant volatiles (HIPVs) (methyl jasmonate, methyl salicylate, Indole and 3-pentanol) were investigated on the functional response of P. persimilis to T. urticae eggs laid on attached leaves of kidney bean under laboratory conditions (25 ± 1 °C, 65 ± 5% RH and a photoperiod of 16:8h L:D). Densities of 2, 5, 10, 20, 30, 40, 50 and 60 prey in 10 replications were offered to the 1-day-old adult female individuals of P. persimilis during a 24-h period. The results of logistic regression analyses showed a type II functional response on all treatments. The Rogers model was used to estimate the instantaneous attack rate (a) and handling time (T&lt;sub&gt;h&lt;/sub&gt;). The instantaneous attack rate had no significant difference among treatments. The handling time on B. velezensis (0.3540 ± 0.0716 h) and methyl salicylate (0.3593 ± 0.0842 h) was significantly lower than in other treatments. These results suggest that methyl salicylate and B. velezensis may have a positive effect on the foraging behavior of P. persimilis against T. urticae.

Biofilm producing probiotic bacteria enhance productivity and bioactive compounds in tomato

The aim was to increase yield and bioactive compounds in tomatoes by application of different biofilm producing plant probiotic bacteria (BPPPB). In this study, sixteen (64%) biofilm producing bacteria (BPB) were identified using 16S rRNA gene sequencing from rhizosphere of tomato plants, comprising the genera of Pseudomonas, Bacillus and Enterobacter. Examined biofilms contained secreted biomolecules including peptidoglycans, proteins, nucleic acids, polysaccharides and lipids, and cell-associated appendages like curli fimbriae. All of the BPB produced multiple probiotic traits in vitro including indole-3-acetic acid, the solubilization of P, Fe, Zn, and K, the fixation of N, and the synthesis of NH3, catalase, acetoin, protease, and cellulase. Tomato plants that have been inoculated with P. poae ESN2, B. aryabhattai ESN3, B. megaterium ESN4, E. cloacae ESN16, P. monteilii ESN26, P. putida ESN32, P. fluorescens ESN35, P. extremorientalis ESN36, and P. chlororaphis ESN37 increased biomass production (4.3–20.7% roots and 6.0–41.7% shoots), yield (4.4–69.1%), total soluble solids (11.2–33.5%), lycopene (28.5–142.8%), β-carotene (27.2–363.6%), phenolics (6.3–52%), flavonoids (10.4–69.2%), and antioxidant capacity (13.3–66.6%) over controls. This is the first report of improvements in tomato yield and bioactive compounds achieved through the application of BPPPB.

Evaluation of Biosurfactant Property within Plant Probiotic Species

The present study focusses on proving the biosurfactant activity of the 7 isolated bacterial strains. Plants could not absorb enough nutrients as a result of contamination in the soil. This contaminants can be chemical pollutants or oil spillage. By considering the earlier potential applications of biosurfactants, this research was aimed to prove the role of a plant probiotic bacteria as oil degrading bacteria. In order to prove this approach, many screening methods were performed such as drop collapse, oil spreading (mm), emulsification stability testing by calculating emulsification index (E24%), penetration assay etc. Among the 7 selected isolates, isolate KL-015 have shown positive results with all the biosurfactant screening analysis. A unique role was proven by KL-011 isolate as it has shown a high oil degrading zone, i.e., 0.6 mm in diameter as well as the formation of more foam layer or emulsified layer with the emulsification index of 17.39% which indicates its role in having a biosurfactant molecule within it.

Characterization of three Stenotrophomonas strains isolated from different ecosystems and proposal of Stenotrophomonas mori sp. nov. and Stenotrophomonas lacuserhaii sp. nov.

Members of the genus Stenotrophomonas, and particularly Stenotrophomonas maltophilia, are opportunistic human pathogens, but not enough research has evaluated the identification of environmental Stenotrophomonas spp. However, most Stenotrophomonas spp. serves as plant-probiotic bacteria.In this study, we obtained and characterized three Stenotrophomonas strains from different ecosystems. Based on phenotypic differences, chemotaxonomic properties, ANI and dDDH identity values, and phylogenetic analyses, two novel Stenotrophomonas species are proposed for the strains identified here. The encoding genes related to plant-growth promotion in the genomes of the newly recovered Stenotrophomonas spp. were retrieved. Follow-on experiments confirmed that these strains produced the important plant hormone IAA. Thus, these Stenotrophomonas spp. could considerably contribute to shaping and maintaining ecological stability in plant-associated environments, particularly while acting as plant-probiotic microorganisms.

Application of Rhizobacteria, Paraburkholderia fungorum and Delftia sp. Confer Cadmium Tolerance in Rapeseed (Brassica campestris) through Modulating Antioxidant Defense and Glyoxalase Systems.

We investigated the role of two different plant growth-promoting probiotic bacteria in conferring cadmium (Cd) tolerance in rapeseed (Brassica campestris cv. BARI Sarisha-14) through improving reactive oxygen species scavenging, antioxidant defense, and glyoxalase system. Soil, as well as seeds of rapeseed, were separately treated with probiotic bacteria, Paraburkholderia fungorum BRRh-4 and Delftia sp. BTL-M2. Fourteen-day-old seedlings were exposed to 0.25 and 0.5 mM CdCl2 for two weeks. Cadmium-treated plants resulted in a higher accumulation of hydrogen peroxide, increased lipid peroxidation, electrolyte leakage, chlorophyll damage, and impaired antioxidant defense and glyoxalase systems. Consequently, it reduced plant growth and biomass production, and yield parameters. However, probiotic bacteria-inoculated plants significantly ameliorated the Cd toxicity by enhancing the activities of antioxidant enzymes (ascorbate peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, glutathione reductase, glutathione peroxidase, and catalase) and glyoxalase enzymes (glyoxalase I and glyoxalase II) which led to the mitigation of oxidative damage indicated by reduced hydrogen peroxide, lipid peroxidation, and electrolyte leakage that ultimately improved growth, physiology, and yield of the bacterial inoculants rapeseed plants. When taken together, our results demonstrated the potential role of the plant probiotic bacteria, BRRh-4 and BTL-M2, in mitigating the Cd-induced damages in rapeseed plants.

Effect of the plant probiotic bacteria on terpenoid indole alkaloid biosynthesis pathway gene expression profiling, vinblastine and vincristine content in the root of Catharanthus roseus.

Catharanthus roseus is the sole resource of vinblastine and vincristine, two TIAs of great interest for their powerful anticancer activities. Increasing the concentration of these alkaloids in various organs of the plant is one of the important goals in C. roseus breeding programs. Plant probiotic bacteria (PBB) act as biotic elicitors and can induce the synthesis of secondary products in plants. The purpose of this research is to study the effects of PBB on expression of the TIA biosynthetic pathway genes and the content of alkaloids in C. roseus. The individual and combined effects of P. fluorescens strains 169 and A. brasilense strains Ab-101 was studied for expression of the TIA biosynthetic pathway genes (G10H, DAT, T16H and CrPRX) using qRT-PCR and the content of vinblastine and vincristine using HPLC method in roots of C. roseus. P. fluorescens. This drastically increased the content of vinblastine and vincristine alkaloids, compared to the control in the roots, to 174 and 589 (µg/g), respectively. Molecular analysis showed bacterium significantly increased the expression of more genes in the TIA biosynthetic pathway compared to the control. P. fluorescens increased the expression of the final gene of the biosynthetic pathway (CrPRX) 47.9 times compared to the control. Our findings indicate the correlation between transcriptional and metabolic outcomes. The same was true for A. brasilense. It can be concluded that seed treatments and seedling root treatments composed of naturally occurring probiotic bacteria are likely to be widely applicable for inducing enhanced alkaloid contents in medicinal plants.

Microbial community composition and soil metabolism in the coexisting Cordyceps militaris and Ophiocordyceps highlandensis.

Cordyceps sensu lato is a complex fungus-larva symbiote, and its distribution is affected by the geography, climate, soil environment, and cohabitating microorganisms. However, despite the fact Cordyceps militaris and Ophiocordyceps highlandensis are different species, they coexist in the Pinus armandi forest of Dashao in Songming County, Yunnan, China. We explored the microbial compositions and soil metabolism inhabited by C. militaris and O. highlandensis using high-throughput sequencing and nontargeted metabonomics. The results indicated that the bacterial and fungal compositions in the soil microhabitat communities of C. militaris, O. highlandensis, and null Cordyceps group were similar. However, the community compositions in the fruiting bodies of C. militaris and O. highlandensis were different. The dominant phylum Ascomycota and dominant genus Cordyceps were detected in the fruiting body of C. militaris. Except for Ascomycota, the dominant phylum Chytridiomycota and dominant genera Ophiocordyceps, Unclassified_k__Hypocreales, Circinotrichum, Cladosporium, and Unclassified_k__chytridiomycetesg were detected in the fruiting body of O. highlandensis. The plant probiotic bacteria Phyllobacterium was detected in the fruiting body of C. militaris. The growth-promoting bacteria Pseudomonas was detected in the fruiting body of O. highlandensis. Soil metabolism analysis revealed that pathways associated with amino acid metabolism was significantly enriched in O. highlandensis. Correlation analysis of bacterial diversity and soil metabolites revealed that the relative abundances of bacterial operational taxonomic units and the relative contents of metabolites were consistent. Our results provide insights into the microbial diversity and soil metabolism of naturally coexisting C. militaris and O. highlandensis.

Novel Approaches for Encapsulation of Plant Probiotic Bacteria with Sustainable Polymer Gums: Application in the Management of Pests and Diseases

The unique attributes, biodegradability, biocompatibility, perfect accessibility, and low production costs led to the use of natural gums in a different section of our lives. Among them, we can mention gums obtained from microorganisms (xanthan gum and gellan gum), plant tissues (Arabic gum and gum tragacanth), seeds (konjac gum and guar gum), seaweeds (alginates, agar gum, and carrageenans). Gums have essential applications in the medical and pharmaceutical, food, biotechnology, and critical agricultural industries. Encapsulation is one of the new methods to increase the stability of bioactive compounds during processing and storage. Encapsulation technology using natural gums is a new way to improve the performance of microbial agents in various sciences, especially agriculture, which represents a bright future in this field.

Difficult-to-culture bacteria in the rhizosphere: The underexplored signature microbial groups

Microorganisms represent a substantial portion of the earth's biodiversity and biomass, and the plant rhizosphere is an innate reservoir teeming with heterogeneous microbes predominated by bacterial communities. Rhizospheric microbial diversity (genetic, phenotypic, and metabolic) has been extensively studied to understand the key ecological roles played by the microbial members, including plant growth promotion. The application of 16S rRNA gene sequencing and next-generation sequencing (NGS) technologies has revolutionized the discovery of novel bacterial groups that have remained undetected by traditional cultivation-based approaches. Such technological advancements have opened new vistas in our current understanding of predominant but concealed and missed bacterial diversity referred to as difficult-to-culture bacterial lineages, especially the predominant phyla Acidobacteria, Verrucomicrobia, Planctomycetes, and Gemmatimonadetes. Regardless of their ubiquity and prevalence, little is known about their ecophysiology because of the non-availability of culturable members. More recently, there has been increased interest in understanding the cosmopolitan distribution and diversity of the difficult-to-culture bacteria, focusing on their role in driving complex plant-microbial interactions and mobilizing nutrients in soil and their potential as sources of novel bioactive metabolites. As an initial step, we review the distribution and significance of such bacterial phyla in soil, their ecophysiological roles, and their hidden plant growth promoting potential. The ability to select and deploy plant probiotic bacteria from the difficult-to-culture fraction of the bacterial community might open new avenues for improving crop health.

Effects of plant probiotic bacteria and herbivore-induced plant volatiles on life table parameters of Tetranychus urticae (Acari: Tetranychidae) on kidney bean’s attached leaves

ABSTRACT The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is an agricultural important polyphagous pest of many crops that causes significant yield loss. In this study, the effect of four herbivore-induced plant volatiles (HIPVs) (methyl jasmonate, methyl salicylate, indole, and 3-pentanol) with a concentration of 100 mmol/L and two plant probiotic bacteria (PBB) (Bacillus pumilus INR7 and Bacillus velezensis H3), were investigated on life table parameters of T. urticae under laboratory conditions (25 ± 1°C, 65 ± 5% RH, and 16 L: 8D). The T. urticae cohorts were reared on attached leaves of kidney bean plants (Phaseolus vulgaris L.) treated with HIPVs and PBBs, separately. The life history data were analysed using age-stage, two-sex life table theory. The intrinsic rate of increase (r) on methyl salicylate (0.1667 ± 0.0087 day-1) and B. velezensis treatment (0.1640 ± 0.0099 day-1) was significantly lower than the control group (0.1934 ± 0.0100 day-1). The finite rate of increase (λ) ranged from 1.178 ± 0.0116 day-1 on the B. velezensis to 1.2134 ± 0.0121 day-1 on the control with a significant difference (P < 0.05). The gross reproduction rate (GRR) on the indole treatment was significantly lower than the other treatments (28.14 ± 3.66 offspring). Based on the results, methyl salicylate, B. velezensis and indole could be considered resistance inducing materials against T. urticae.

N,N-dimethyl-hexadecylamine modulates Arabidopsis root growth through modifying the balance between stem cell niche and jasmonic acid-dependent gene expression

N,N-dimethyl-hexadecylamine (DMHDA) is released as part of volatile blends emitted by plant probiotic bacteria and affects root architecture, defense and nutrition of plants. Here, we investigated the changes in gene expression of transcription factors responsible of maintenance of the root stem cell niche and jasmonic acid signaling in Arabidopsis seedlings in response to this volatile. Concentrations of DMHDA that repress primary root growth were found to alter cell size and division augmenting cell tissue layers in the meristem and causing root widening. DMHDA triggered the division of quiescent center cells, which correlated with repression of SHORT ROOT (SHR), SCARECROW (SCR), and PLETHORA 1 (PLT1) proteins and induction of WUSCHEL-RELATED HOMEOBOX 5 (WOX5) transcription factor. Interestingly, an activation of the expression of the jasmonic acid-related reporter genes JAZ1/TIFY10A-GFP and JAZ10pro::JAZ10-GFP suggests that the halted growth of the primary root inversely correlated with expression patterns underlying the defense reaction, which may be of adaptive importance to protect roots against biotic stress. Our data help to unravel the gene expression signatures upon sensing of a highly active bacterial volatile in Arabidopsis seedlings.

Survivability and controlled release of alginate-microencapsulated Pseudomonas fluorescens VUPF506 and their effects on biocontrol of Rhizoctonia solani on potato

Preserving the efficacy of plant probiotic bacteria in soil is a major challenge to the biological control of plant diseases. The microencapsulation technique is an important step in preserving the viability and activity of probiotics in adverse environmental conditions. The main objective of this study was to choose an appropriate coating for probiotic encapsulation. For this purpose, the survivability and controlled release of Pseudomonas fluorescens VUPF506 encapsulated with alginate (Alg) combined with whey protein concentrate (WPC), carboxymethyl cellulose (CMC), and peanut butter (PB) were evaluated. Moreover, the encapsulated cells were evaluated to control for Rhizoctonia solani in potato plants under in vivo conditions. The results showed that all tested wall material maintained more than 80% of the bacterial cells. The Alg-WPC microcapsules provided a better controlled release over two months. Interestingly, the greenhouse experiment also revealed that the treatment of potato plants with Alg-WPC microcapsules was the most effective treatment, suppressing 90% of the pathogen. The results showed that Alg-WPC is the most promising combination to improve the survivability of P. fluorescens VUPF506. Moreover, it can be used as a fertilizer due to its content of valuable amino acids.