Plant Growth-promoting Activities Research Articles

Enhancing plant growth promoting rhizobacterial activities through consortium exposure: A review.

Plant Growth Promoting Rhizobacteria (PGPR) has gained immense importance in the last decade due to its in-depth study and the role of the rhizosphere as an ecological unit in the biosphere. A putative PGPR is considered PGPR only when it may have a positive impact on the plant after inoculation. From the various pieces of literature, it has been found that these bacteria improve the growth of plants and their products through their plant growth-promoting activities. A microbial consortium has a positive effect on plant growth-promoting (PGP) activities evident by the literature. In the natural ecosystem, rhizobacteria interact synergistically and antagonistically with each other in the form of a consortium, but in a natural consortium, there are various oscillating environmental conditions that affect the potential mechanism of the consortium. For the sustainable development of our ecological environment, it is our utmost necessity to maintain the stability of the rhizobacterial consortium in fluctuating environmental conditions. In the last decade, various studies have been conducted to design synthetic rhizobacterial consortium that helps to integrate cross-feeding over microbial strains and reveal their social interactions. In this review, the authors have emphasized covering all the studies on designing synthetic rhizobacterial consortiums, their strategies, mechanism, and their application in the field of environmental ecology and biotechnology.

Plant-beneficial Bacillus, Pseudomonas, and Staphylococcus spp. from Kumaon Himalayas and their drought tolerance response

Plant growth-promoting rhizobacteria (PGPR) have been shown to augment plant responses against drought and other abiotic stresses. In the present study, we isolated 27 bacteria from the rhizosphere of various plants cultivated in the Kumaon Himalayas., and to measure their abiotic stress tolerance, these 27 isolates were subjected to variations in pH, temperature, and drought. All 27 isolates were also screened for various plant growth-promoting traits. Among these, the four isolates RR1, ASC1, AFS3, and NG4 demonstrated various plant growth promotion activities including the synthesis of indole-3-acetic acid (IAA), siderophores, ammonia, and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase production, and concomitantly high tolerance to abiotic stresses. Moreover, 16S rRNA sequencing of these four isolates validated their identities as Bacillus, Pseudomonas, and Staphylococcus sp. Finally, to assess the in-vivo drought tolerance potential of these four isolates, a pot-trial experiment was undertaken in wheat cultivar VL-892. The results demonstrated that inoculating wheat plants with these four PGPR isolates greatly improved plant growth under drought circumstances by increasing root and shoot length and both fresh and dry weight of root and shoot. This study endeavors to discover the biochemical and molecular diversity of cultivable PGPR in six remotely located districts of Uttarakhand. In conclusion, the drought-tolerant PGPR strains described in this study are plant-beneficial and can effectively mobilize nutrients under drought conditions. Consequently, they could be used as bioinoculants to alleviate drought stress in wheat plants, in a sustainable manner. To the best of our knowledge, this is the first report of exploring the diversity and characterization of PGPR from the Kumaon Himalayas and their drought evaluation.

Replacement of water yam (Dioscorea alata L.) indigenous root endophytes and rhizosphere bacterial communities via inoculation with a synthetic bacterial community of dominant nitrogen-fixing bacteria.

Biofertilizers containing high-density plant growth-promoting bacteria are gaining interest as a sustainable solution to environmental problems caused by eutrophication. However, owing to the limitations of current investigative techniques, the selected microorganisms are not always preferred by the host plant, preventing recruitment into the native microbiota or failing to induce plant growth-promoting effects. To address this, five nitrogen-fixing bacteria previously isolated from water yam (Dioscorea alata L.) plants and showing dominant abundance of 1% or more in the water yam microbiota were selected for analysis of their plant growth-promoting activities when used as a synthetic bacterial inoculant. Water yam cv. A-19 plants were inoculated twice at 10 and 12 weeks after planting under greenhouse conditions. Bacterial communities in root, rhizosphere, and bulk soil samples were characterized using high-throughput 16S rRNA amplicon sequencing. Compared with non-inoculated plants, all bacterial communities were significantly altered by inoculation, mainly at the genus level. The inoculation effects were apparently found in the root communities at 16 weeks after planting, with all inoculated genera showing dominance (in the top 35 genera) compared with the control samples. However, no significant differences in any of the growth parameters or nitrogen contents were observed between treatments. At 20 weeks after planting, the dominance of Stenotrophomonas in the inoculated roots decreased, indicating a decline in the inoculation effects. Interestingly, only the Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium clade was dominant (>1% relative abundance) across all samples, suggesting that bacteria related to this clade are essential core bacteria for water yam growth. This is the first report on addition of a synthetic nitrogen-fixing bacterial community in water yam plants showing that native bacterial communities can be replaced by a synthetic bacterial community, with declining in the effects of Stenotrophomonas on the modified communities several weeks after inoculation.

Evaluation of Soil Streptomyces spp. for the Biological Control of Fusarium Wilt Disease and Growth Promotion in Tomato and Banana

Fusarium oxysporum f. sp. lycopersici (Fol) and Fusarium oxysporum f. sp. cubense (Foc), are the causal agent of Fusarium wilt disease of tomato and banana, respectively, and cause significant yield losses worldwide. A cost-effective measure, such as biological control agents, was used as an alternative method to control these pathogens. Therefore, in this study, six isolates of the Streptomyces-like colony were isolated from soils and their antagonistic activity against phytopathogenic fungi and plant growth-promoting (PGP) activity were assessed. The results showed that these isolates could inhibit the mycelial growth of Fol and Foc. Among them, isolate STRM304 showed the highest percentage of mycelial growth reduction and broad-spectrum antagonistic activity against all tested fungi. In the pot experiment study, the culture filtrate of isolates STRM103 and STRM104 significantly decreased disease severity and symptoms in Fol inoculated plants. Similarly, the culture filtrate of the STRM304 isolate significantly reduced the severity of the disease and symptoms of the disease in Foc inoculated plants. The PGP activity test presents PGP activities, such as indole acetic acid production, phosphate solubilization, starch hydrolysis, lignin hydrolysis, and cellulase activity. Interestingly, the application of the culture filtrate from all isolates increased the percentage of tomato seed germination and stimulated the growth of tomato plants and banana seedlings, increasing the elongation of the shoot and the root and shoot and root weight compared to the control treatment. Therefore, the isolate STRM103 and STRM104, and STRM304 could be used as biocontrol and PGP agents for tomato and banana, respectively, in sustainable agriculture.

Screening for indole-3-acetic acid synthesis and 1-aminocyclopropane-carboxylate deaminase activity in soil yeasts from Chile uncovers Solicoccozyma aeria as an effective plant growth promoter

Abstract Background and aims Plant growth-promoting microorganisms (PGPMs) stimulate plant growth by a series of mechanisms, including atmospheric nitrogen fixation, phosphorus solubilization and the synthesis of plant hormones. Within the group of PGPMs, several species of bacteria and fungi have been extensively studied. However, little information is available with regard to soil yeasts. The present study was conducted to identify yeast strains isolated from different soils in Chile that present plant growth promotion activity. Methods Twenty-three strains were evaluated either for their capacity to synthesize indole-3-acetic acid or show aminocyclopropane-1-carboxylate deaminase activity. The effect on tomato seedling growth was evaluated in vitro, and two strains were selected for in vivo evaluation of plant growth and root ethylene synthesis. Results All the strains analyzed presented IAA synthesis within the range between 0.8 and 3.3 μg IAA mL−1. Solicoccozyma aeria (YCPUC75 and YCPUC79 strains) was the only yeast with ACC deaminase activity. In vitro inoculation of tomato seeds with eight of the evaluated strains resulted in an increase in the root volume and the number of lateral roots. In the second experiment, a 40% reduction in root ethylene synthesis was achieved by adding S. aeria (YCPUC79) to the root zone, which resulted in a 26% increase in plant growth. Conclusions Solicoccozyma aeria YCPUC79 is an effective plant growth promoter stimulating root growth and reducing root ethylene synthesis.

Halophytic Plant Halostachys belangeriana (Moq.) Botsch as a Source of Plant Growth-Promoting Endophytic Bacteria

Halostachys belangeriana (Moq.) Botsch also known as Halostachys caspica C. A. Mey belongs to the Chenopodiaceae family and is distributed in deserts of Asian countries. The plant grows in severe salinity and drought conditions and its survival and growth can be associated with the activity of endophytic bacteria. The objective of our research was to isolate and screen endophytic bacteria from Halostachys belangeriana for plant growth promotion and reveal their plant-beneficial traits. Methods. Halostachys belangeriana (Moq.) Botsch plants were collected from the saline soil of the Kyzylkum desert in Uzbekistan in spring. The endophytic bacteria were isolated from the tissues of plants by cutting the outer sterilized shoots and roots and putting them into the water to let bacteria come from the tissues into the water. The suspension was transferred onto Tryptic Soy Agar to let bacteria grow and form separate colonies. The colonies different in shape and color were used to get pure cultures of bacteria. The bacteria were screened using plant growth-promoting activity in Petri plates by inoculating wheat seeds with the suspension of isolated bacteria. The best plant growth promoters were identified by analyzing their 16S rRNA gene and comparing it with sequences registered in GenBank of NCBI. The strains were tested for wheat growth promotion in a pot experiment and then examined for their plant-benefi cial traits: N2-fixation, phosphates solubilization, production of indole-3-acetic acid (IAA), 1-aminocyclopropane-1-carboxylate deaminase (ACC-deaminase), and siderophores. Results. A total of 25 isolates of endophytic bacteria were obtained from the tissues of Halostachys belangeriana (Moq.) Botsch. Due to the high efficiency of isolates SSU-4, SSU-7, SSU-16, SSU-18, and SSU-21 in the stimulation of wheat shoot and root growth, they were chosen for identification and (OK559720), Bacillus endophyticus SSU-7 (OK559721), Bacillus subtilis SSU-16 (OK559722), Isoptericola halotolerans SSU-18 (OK559723) and Pseudomonas kilonensis SSU-21 (OK559724), respectively. The single inoculation of seeds with tested strains increased the root and shoot length and plant fresh weight. The coinoculation of seeds with a mixture of five strains resulted in an even more increase in plant growth parameters. It was revealed that the tested strains had at least two plant-beneficial properties. The strains B. pumilus SSU-4 and P. kilonensis SSU-21 had the ability for nitrogen fixation. All strains produced IAA; however, the most active IAA producer was P. kilonensis SSU-21. Three of five strains had phosphates solubilization ability and produced ACC-deaminase and siderophores. The strains B. pumilus SSU-4 and P. kilonensis SSU-21 possessed four of five tested plant-beneficial properties. The strains B. endophyticus SSU-7 and I. halotolerans SSU-18 had three of five tested plant-beneficial traits, and B. subtilis SSU-16 could just produce IAA and ACC-deaminase. Conclusions. This is the first report about the isolation of plant growth-promoting endophytic bacteria from the desert halophytic plant Halostachys belangeriana (Moq.) Botsch. The most efficient plant growth-promoting strains were: B. pumilus SSU-4, B. endophyticus SSU-7, B. subtilis SSU-16, I. halotolerans SSU-18, and P. kilonensis SSU-21. After field experiments, these strains can be suggested for use as bioinoculants improving plants growth.

Biochemical and Molecular Evaluation of Rhizobium spp. and its Growth Promotion Studies with Lentil (Lens culinaris Medik. L.)

Growth promoting Rhizobium spp are frequently used as biofertilizers for agricultural cropping system. Furthermore, Isolation, screening and biochemical characterization of Rhizobium for a specific plant is necessary to examine ability of isolated bacteria to affect the growth and development of host plant in various ways. The current study was aimed to isolate plant specific rhizobacterial strains which are compatible with lentil (Lens culinaris Medik.L.) plant. 20 bacterial isolates have been isolated from root nodules of lentil from various agro ecological area and their biochemical characterization was performed by different plant growth promotion activities. The result showed that, among 20 isolates, four isolates have vigorous plant growth promoting activities. Four bacterial strains were able to solubilise phosphorous along with hormone production. Moreover, among four bacterial strains, two strongly produced HCN and siderophore in vitro. Subsequently, all selected bacterial isolates were inoculated in lentil seeds of variety HUL57 to study germination percentage and vigour index of the crop. Out of four isolates 26N isolate performed best growth promotion activities on lentil seedlings. Finally, on the basis of performance of bacteria on plant, four isolates were characterized using molecular approach of species identification such as 16S rRNA sequencing.

Identification and plant growth-promoting activity of endophytic bacteria from sugarcane roots and genome analysis of Gluconacetobacter strain PS25

Identification and plant growth-promoting activity of endophytic bacteria from sugarcane roots and genome analysis of Gluconacetobacter strain PS25

Culturable bacteria diversity in stem liquid and resina from Populus euphratica and screening of plant growth-promoting bacteria

BackgroundPopulus euphratica Olivier is a kind of tree capable of growing in extremely arid desert and semi-desert environments. In this study, a culture-dependent method was used to analyze the bacterial diversity of stem liquid of P. euphratica and resina of P. euphratica, and to further evaluate plant growth promoting (PGP) activity.ResultsA total of 434 bacteria were isolated from stem fluid and resina of P. euphratica in Ebinur Lake Wetland Nature Reserve and Mulei Primitive forest. The results of taxonomic composition analysis shows that Gammaproteobacteria, Firmicutes, and Actinobacteria_c are the three dominant groups in all the communities, and the representative genera are Bacillus, Nesterenkonia and Halomonas. The diversity analysis shows that the culturable bacterial community diversity of P. euphratica in Ebinur Lake Wetland Nature Reserve is higher than that in Mulei Primitive forest, and the bacterial community diversity of P. euphratica stem fluid is higher than that of resina. According to PGP activity evaluation, 158 functional bacteria with plant growth promoting potential were screened. Among them, 61 strains havephosphorus solubilizing abilities, 80 strains have potassium solubilizing abilities, 32 strains have nitrogen fixation abilities, and 151 strains have iron ammonia salt utilization abilities. The germination rate, plant height, and dry weight of the maize seedlings treated with strains BB33-1, TC10 and RC6 are significantly higher than those of the control group.ConclusionIn this study, a large number of culturable bacteria were isolated from P. euphratica, which provides new functional bacteria sources for promoting plant growth.

Molecular identification and characterization of phytobeneficial osmotolerant endophytic bacteria inhabiting root nodules of the Saharan tree Vachellia tortilis subsp. raddiana.

Nodular endophytes of drought-tolerant legumes are understudied. For this reason, we have isolated and studied non-symbiotic endophytic bacteria from nodules of Vachellia tortilis subsp. raddiana, a leguminous tree adapted to the harsh arid climate of Southern Morocco. Rep-PCR analysis followed by 16S rDNA sequencing revealed two main genera, Pseudomonas and Bacillus. Isolates responded variably to salt and water stresses, and mostly produced exopolysaccharides. Differences concerned also plant growth-promoting activities: phosphate, potassium, and zinc solubilization; biological nitrogen fixation; auxin, siderophore, ammonia, and HCN production; and ACC deaminase activity. Some strains exhibited antagonistic activities against phytopathogenic fungi (Fusarium oxysporum and Botrytis cinerea) and showed at least two enzymatic activities (cellulase, protease, chitinase). Four selected strains inoculated to vachellia plants under controlled conditions have shown significant positive impacts on plant growth parameters. These strains are promising bio-inoculants for vachellia plants to be used in reforestation programs in arid areas increasingly threatened by desertification.

Exploration of the rhizosphere microbiome of native plant Ceanothus velutinus - an excellent resource of plant growth-promoting bacteria.

Continuous demand for an increase in food production due to climate change and a steady rise in world population requires stress-resilient, sustainable agriculture. Overuse of chemical fertilizers and monoculture farming to achieve this goal deteriorated soil health and negatively affected its microbiome. The rhizosphere microbiome of a plant plays a significant role in its growth and development and promotes the plant's overall health through nutrient uptake/availability, stress tolerance, and biocontrol activity. The Intermountain West (IW) region of the US is rich in native plants recommended for low water use landscaping because of their drought tolerance. The rhizosphere microbiome of these native plants is an excellent resource for plant growth-promoting rhizobacteria (PGPR) to use these microbes as biofertilizers and biostimulants to enhance food production, mitigate environmental stresses and an alternative for chemical fertilizer, and improve soil health. Here, we isolated, purified, identified, and characterized 64 bacterial isolates from a native plant, Ceanothus velutinus, commonly known as snowbrush ceanothus, from the natural habitat and the greenhouse-grown native soil-treated snowbrush ceanothus plants. We also conducted a microbial diversity analysis of the rhizosphere of greenhouse-grown native soil-treated and untreated plants (control). Twenty-seven of the 64 isolates were from the rhizosphere of the native region, and 36 were from the greenhouse-grown native soil-treated plants. These isolates were also tested for plant growth-promoting (PGP) traits such as their ability to produce catalase, siderophore, and indole acetic acid, fix atmospheric nitrogen and solubilize phosphate. Thirteen bacterial isolates tested positive for all five plant growth-promoting abilities and belonged to the genera Pantoea, Pseudomonas, Bacillus, and Ancylobacter. Besides, there are isolates belonging to the genus Streptomyces, Bacillus, Peribacillus, Variovorax, Xenophilus, Brevundimonas, and Priestia, which exhibit at least one of the plant growth-promoting activities. This initial screen provided a list of potential PGPR to test for plant health improvement on model and crop plants. Most of the bacterial isolates in this study have a great potential to become biofertilizers and bio-stimulants.

Application of data integration for rice bacterial strain selection by combining their osmotic stress response and plant growth-promoting traits.

Agricultural application of plant-beneficial bacteria to improve crop yield and alleviate the stress caused by environmental conditions, pests, and pathogens is gaining popularity. However, before using these bacterial strains in plant experiments, their environmental stress responses and plant health improvement potential should be examined. In this study, we explored the applicability of three unsupervised machine learning-based data integration methods, including principal component analysis (PCA) of concatenated data, multiple co-inertia analysis (MCIA), and multiple kernel learning (MKL), to select osmotic stress-tolerant plant growth-promoting (PGP) bacterial strains isolated from the rice phyllosphere. The studied datasets consisted of direct and indirect PGP activity measurements and osmotic stress responses of eight bacterial strains previously isolated from the phyllosphere of drought-tolerant rice cultivar. The production of phytohormones, such as indole-acetic acid (IAA), gibberellic acid (GA), abscisic acid (ABA), and cytokinin, were used as direct PGP traits, whereas the production of hydrogen cyanide and siderophore and antagonistic activity against the foliar pathogens Pyricularia oryzae and Helminthosporium oryzae were evaluated as measures of indirect PGP activity. The strains were subjected to a range of osmotic stress levels by adding PEG 6000 (0, 11, 21, and 32.6%) to their growth medium. The results of the osmotic stress response experiments showed that all bacterial strains accumulated endogenous proline and glycine betaine (GB) and exhibited an increase in growth, when osmotic stress levels were increased to a specific degree, while the production of IAA and GA considerably decreased. The three applied data integration methods did not provide a similar grouping of the strains. Especially deviant was the ordination of microbial strains based on the PCA of concatenated data. However, all three data integration methods indicated that the strains Bacillus altitudinis PB46 and B. megaterium PB50 shared high similarity in PGP traits and osmotic stress response. Overall, our results indicate that data integration methods complement the single-table data analysis approach and improve the selection process for PGP microbial strains.

C–H arylation enables synthesis of imidazole-4-carboxamide (ICA) based fairy chemicals with plant growth-promoting activity

Imidazole-4-carboxamide (ICA), which is one of the groups of “fairy chemicals” (FCs) that cause the fairy ring phenomena, has plant growth inhibitory activity. FCs have the potential as candidates for a new family of plant hormones as they have been found endogenously in all plant species tested and show growth-regulating activities against the plants. While basic research on FCs is progressing, they are also expected to be applied not only to agrochemicals but also as pharmaceuticals. Derivatization of one of the FCs, 2-azahypoxanthine, and the structure–activity relationship (SAR) studies have clarified its activity as a plant growth promoter. Yet, ICA has not been derivatized at all and SAR regarding its activity remains unknown. In this study, we synthesized the derivatives of ICA by direct C–H arylation of ICA precursors and evaluated its activity in rice. The 12 total compounds including the arylated ICAs and their precursors were evaluated for root and shoot elongation in rice, resulting in the discovery that a number of compounds unexpectedly have an elongation activity in the root and shoot.

ACC Deaminase Produced by PGPR Mitigates the Adverse Effect of Osmotic and Salinity Stresses in Pisum sativum through Modulating the Antioxidants Activities.

Salinity-induced ethylene production and reactive oxygen species (ROS) inhibit agricultural productivity. The plant synthesizes ethylene directly from aminocyclopropane-1-carboxylic acid (ACC). By using ACC as a nitrogen source, bacteria with ACC deaminase (ACCD) inhibit the overproduction of ethylene, thereby maintaining the ROS. The present study investigated the ACCD activity of previously identified rhizobacterial strains in Dworkin and Foster (DF) minimal salt media supplemented with 5 mM ACC (as N-source). Bacterial isolates GKP KS2_7 (Pseudomonas aeruginosa) and MBD 133 (Bacillus subtilis) could degrade ACC into α-ketobutyrate, exhibiting ACCD activity producing more than ~257 nmol of α-ketobutyrate mg protein−1 h−1, and were evaluated for other plant growth-promoting (PGP) traits including indole acetic acid production (>63 µg/mL), phosphate solubilization (>86 µg mL−1), siderophore (>20%) ammonia and exopolysaccharide production. Furthermore, Fourier Transform Infrared analysis also demonstrated α-ketobutyrate liberation from ACC deamination in DF minimal salt media, thereby confirming the ACCD activity. These isolates also showed enhanced tolerance to salinity stress of 3% w/v NaCl in vitro, in addition to facilitating multifarious PGP activities. Seed bacterization by these ACCD-producing bacterial isolates (GKP KS2_7 and MBD 133) revealed a significant decline in stress-stimulated ethylene levels and its associated growth inhibition during seedling germination. They also mitigated the negative effects of salt stress and increased the root-shoot length, fresh and dry weight of root and shoot, root-shoot biomass, total sugar, protein, reducing sugar, chlorophyll content, and antioxidants enzymes in Pisum sativum. As a result, these strains (GKP KS2_7 and MBD 133) might be applied as biofertilizers to counteract the negative effects of soil salinity.

Diversity, mechanisms and beneficial features of phosphate-solubilizing Streptomyces in sustainable agriculture: A review.

Currently, the use of phosphate (P) biofertilizers among many bioformulations has attracted a large amount of interest for sustainable agriculture. By acting as growth promoters, members of the Streptomyces genus can positively interact with plants. Several studies have shown the great potential of this bacterial group in supplementing P in a soluble, plant-available form by several mechanisms. Furthermore, some P-solubilizing Streptomyces (PSS) species are known as plant growth-promoting rhizobacteria that are able to promote plant growth through other means, such as increasing the availability of soil nutrients and producing a wide range of antibiotics, phytohormones, bioactive compounds, and secondary metabolites other than antimicrobial compounds. Therefore, the use of PSS with multiple plant growth-promoting activities as an alternative strategy appears to limit the negative impacts of chemical fertilizers in agricultural practices on environmental and human health, and the potential effects of these PSS on enhancing plant fitness and crop yields have been explored. However, compared with studies on the use of other gram-positive bacteria, studies on the use of Streptomyces as P solubilizers are still lacking, and their results are unclear. Although PSS have been reported as potential bioinoculants in both greenhouse and field experiments, no PSS-based biofertilizers have been commercialized to date. In this regard, this review provides an overview mainly of the P solubilization activity of Streptomyces species, including their use as P biofertilizers in competitive agronomic practices and the mechanisms through which they release P by solubilization/mineralization, for both increasing P use efficiency in the soil and plant growth. This review further highlights and discusses the beneficial association of PSS with plants in detail with the latest developments and research to expand the knowledge concerning the use of PSS as P biofertilizers for field applications by exploiting their numerous advantages in improving crop production to meet global food demands.

Exploring the Plant Growth-Promotion of Four Streptomyces Strains from Rhizosphere Soil to Enhance Cucumber Growth and Yield

The genus Streptomyces is the most abundant and essential microbes in the soil microbial community. Streptomyces are familiar and have great potential to produce a large variety of bioactive compounds. This genus considers an efficient biofertilizer based on its plant growth-promoting activities. Based on their ability to produce a wide varieties of bioactive molecules, the present study aimed to explore the potential plant growth promotion of four Streptomyces strains and their role in enhancing cucumber growth and yield under greenhouse conditions. Streptomyces sp. strain HM2, Streptomyces thinghirensis strain HM3, Streptomyces sp. strain HM8, and Streptomyces tricolor strain HM10 were chosen for the current study. Plant growth-promoting (PGP) features, i.e., indole acetic acid (IAA) production, siderophore excretion, and solubilizing phosphate, were evaluated in vitro. All four strains produced IAA, siderophore, and immobilized inorganic phosphate. Following 4 days of incubation at 30 °C, strains HM2, HM3, HM8, and HM10 produced copious amounts of IAA (18, 22, 62, and 146 µg/mL, respectively) and siderophores (42.59, 40.01, 16.84, 64.14% SU, respectively). At the same time, P solubilization efficacy scored 64.3%, 84.4%, 57.2%, and 81.6% with the same frequency. During in planta evaluation, selected Streptomyces strains combined with rock phosphate were assessed as biofertilizers on the growth and yield of cucumber plants. Under all treatments, positive and significant differences in studied traits were manifested except dry stem matter (SDM), net assimilation rate (NAR), relative growth rate (RGR), and fruit firmness (FF). Treatment T4 (rock phosphate + strain HM3) followed by T5 (rock phosphate + strain HM8) revealed the best results for plant height (PH), number of leaves per plant (NLPP), root length (RL), number of fruits per plant (NFPP), fruit length (FL), fruit diameter (FD), fruit fresh weight per plant (FFWPP), soil P (SP) after 21 DAT, and soil P at the end of the experiment. Notably, T6 (rock phosphate + strain HM10) caused a considerable increase in leaf area (LA). Plant growth-promoting bacteria enhance plant growth and yield through phosphorus solubilizing, improve nutrient availability, produce phytohormones, and support plant growth under abiotic stress. These features are important for sustainable agriculture and reducing environmental pollution with chemical fertilizers and pesticides.

Whole-genome analysis revealed the growth-promoting mechanism of endophytic bacterial strain Q2H1 in potato plants.

Endophytes are non-pathogenic inhabitants of healthy plant tissues and have been found to promote plant growth and health. The endophytic bacterial strain Q2H1 was isolated from the roots of the potato and was identified to exhibit growth-promoting effects in potato plants. Whole-genome sequencing was performed to reveal the mechanism underlying its growth-promoting effect. The obtained sequencing data of approximately 5.65 MB encompassed 5,533 coding sequences. Of note, nine secondary metabolite gene clusters, including siderophore gene clusters, closely associated with plant growth promotion (PGP) were predicted by antiSMASH software. Comparative genomic analysis revealed that Q2H1 belongs to the genus Peribacillus. By gene function annotation, those genes related to plant growth-promoting activities, including indole-3-acetic acid (IAA) synthesis in tryptophan metabolism, siderophore biosynthetic activity, phosphate solubilization, nitrogen fixation, and related genes, were summarized. IAA (14.4 μg/ml) was presumptively produced by Q2H1 using the Salkowski colorimetric method. A total of five genes, namely, phoU, pstB, pstA1, pstC, and pstS, were annotated for phosphate solubilization, which is associated with the ability of the Q2H1 strain to solubilize phosphate under in vitro conditions. It is revealed that genes in the Q2H1 genome associated with nitrogen fixation belonged to three groups, namely, nitrogen fixation (nifU, sufU, salA, and nifS), nitrogen metabolism (nirA, nrtB, and nasA), and glutamate synthesis (glnA, gltB, gltD, and gudB), supported by evidence that Q2H1 grew on medium without nitrogen. We have also identified a siderophore gene cluster located on the chromosome of Q2H1, including seven genes (viz., rbsR, rhbf, rhbE, rhbD, rhbC, rhbA, ddc, and an unknown gene). In the in vitro assay, a prominent brown circle around the colony was produced on the chrome azurol S medium at 48 and 72 h post-inoculation, indicating that the siderophore gene cluster in Q2H1 harbored the ability to produce siderophores. In summary, these findings implied that identifying strain-specific genes for their metabolic pathways in bacterial endophytes may reveal a variety of significant functions of plant growth-promoting mechanisms.

Observations on the interaction between plant growth-promoting bacteria and the root-knot nematode Meloidogyne javanica.

Pseudomonas fluorescens, strains L124, L228, L321, and the positive control strain F113 used in this study, produce compounds associated with plant growth promotion, biocontrol, antimicrobial and antiviral activity, and adaptation to stresses. These bacterial strains were tested in vitro and in vivo in tomato plants, to determine their potential role in Meloidogyne javanica suppression. In laboratory experiments, only 2% of M. javanica eggs hatched when exposed to the metabolites of each bacterial strain. Additionally, 100% M. javanica J2 mortality was recorded when nematodes were exposed to the metabolites of F113 and L228. In greenhouse experiments, M. javanica infected tomato plants, which were also inoculated with the bacterial strains F113 and L124, displayed the highest biomass (height, number of leaves, fresh and dry weight) of all bacterial treatments tested. Results from the development and induced systemic resistance experiments indicated that the bacterial strains F113 and L321 had the most effective biocontrol capacity over nematode infection, delayed nematode development (J3/J4, adults and galls), and reduced nematode fecundity. In addition, these results indicated that the bacterial strain L124 is an effective plant growth promoter of tomato plants. Furthermore, it was determined that the bacterial strain L321 was capable of M. javanica biocontrol. P. fluorescens F113 was effective at both increasing tomato plant biomass and M. javanica biocontrol. In an agricultural context, applying successional drenches with these beneficial plant growth promoting rhizobacteria would ensure bacteria viability in the rhizosphere of the plants, encourage positive plant bacterial interactions and increase biocontrol against M. javanica.

Nematicidal, Acaricidal and Plant Growth-Promoting Activity of Enterobacter Endophytic Strains and Identification of Genes Associated with These Biological Activities in the Genomes.

In the present study, the nematicidal and acaricidal activity of three Enterobacter endophytic strains isolated from Mimosa pudica nodules was evaluated. The percentages of mortality of Enterobacter NOD4 against Panagrellus redivivus was 81.2%, and against Nacobbus aberrans 70.1%, Enterobacter NOD8 72.4% and 62.5%, and Enterobacter NOD10 64.8% and 58.7%, respectively. While against the Tyrophagus putrescentiae mite, the mortality percentages were 68.2% due to Enterobacter NOD4, 64.3% due to Enterobacter NOD8 and 77.8% due to Enterobacter NOD10. On the other hand, the ability of the three Enterobacter strains to produce indole acetic acid and phosphate solubilization, characteristics related to plant growth-promoting bacteria, was detected. Bioinformatic analysis of the genomes showed the presence of genes related to IAA production, phosphate solubilization, and nitrogen fixation. Phylogenetic analyzes of the recA gene, phylogenomics, and average nucleotide identity (ANI) allowed us to identify the strain Enterobacter NOD8 related to E. mori and Enterobacter NOD10 as E. asburiae, while Enterobacter NOD4 was identified as a possible new species of this species. The plant growth-promoting, acaricidal and nematicidal activity of the three Enterobacter strains makes them a potential agent to include in biocontrol alternatives and as growth-promoting bacteria in crops of agricultural interest.

Isolation and Characterization of Endophytic Kosakonia radicincitans to Stimulate Wheat Growth in Saline Soil

Among abiotic stresses, salinity is a significant limiting factor affecting agricultural productivity, survival, and production, resulting in significant economic losses. The goal of this study was to isolate and characterize the endophytic Kosakonia radicincitans bacteria and their plant growth-promoting (PGP) activities to determine the effect of its inoculation on wheat plants under salinity stress conditions. One endophytic bacterial isolate was obtained from root nodules of faba bean (Vicia faba) plants grown in the salt-affected clay soil of Egypt. The isolate was identified and characterized using 16S rRNA gene sequencing. This isolate Was phylogenetically related to Kosakonia radicincitans strain DSM 16656 with accession number OM980222.1. The isolate was evaluated for its ability to promote plant growth in vitro. Results revealed that the bacterial isolate of Kosakonia radicincitans produced indole-3-acetic acid (40.44 μgml-1) and exopolysaccharides (14.2 g/L), fixed nitrogen and could solubilize phosphate and potassium (273 mg/L and 42.8 μg/ml) respectively. In a field trial, this bacterial isolate reduced proline, and showed the best results for chlorophyll, grain yield, and 100-grain weight of wheat plants grown in the salt-affected soil as compared to the control plants. Wheat plants may benefit from the use of Kosakonia radicincitans as biofertilizers in sustainable farming techniques. Additional research in this area can support our findings.