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Phytobeneficial microbes, particularly endophytes, such as fungi and bacteria, are concomitant partners of plants throughout its developmental stages, including seed germination, root and stem growth, and fruiting. Endophytic microbes have been identified in plants that grow in a wide array of habitats; however, seed-borne endophytic microbes have not been fully explored yet. Seed-borne endophytes are of great interest because of their vertical transmission; their potential to produce various phytohormones, enzymes, antimicrobial compounds, and other secondary metabolites; and improve plant biomass and yield under biotic and abiotic stresses. This review addresses the current knowledge on endophytes, their ability to produce metabolites, and their influence on plant growth and stress mitigation.

Melatonin is an indolamine bioactive molecule that regulates a wide range of physiological processes during plant growth and enhances abiotic stress tolerance. Here we examined the putative role of exogenous melatonin application (foliar or root zone) in improving drought stress tolerance in soybean seedlings. Pre-treatment of soybean seedlings with melatonin (50 and 100 µM) was found to significantly mitigate the negative effects of drought stress on plant growth-related parameters and chlorophyll content. The beneficial impacts against drought were more pronounced by melatonin application in the rhizosphere than in foliar treatments. The melatonin-induced enhanced tolerance could be attributed to improved photosynthetic activity, reduction of abscisic acid and drought-induced oxidative damage by lowering the accumulation of reactive oxygen species and malondialdehyde. Interestingly, the contents of jasmonic acid and salicylic acid were significantly higher following melatonin treatment in the root zone than in foliar treatment compared with the control. The activity of major antioxidant enzymes such as superoxide dismutase, catalase, polyphenol oxidase, peroxidase and ascorbate peroxidase was stimulated by melatonin application. In addition, melatonin counteracted the drought-induced increase in proline and sugar content. These findings revealed that modifying the endogenous plant hormone content and antioxidant enzymes by melatonin application improved drought tolerance in soybean seedlings. Our findings provide evidence for the stronger physiological role of melatonin in the root zone than in leaves, which may be useful in the large-scale field level application during drought. Melatonin is an indolamine bioactive molecule that regulates a wide range of physiological processes during plant growth and enhances abiotic stress tolerance. Here we examined the putative role of exogenous melatonin application (foliar or root zone) in improving drought stress tolerance in soybean seedlings. Pre-treatment of soybean seedlings with melatonin (50 and 100 µM) was found to significantly mitigate the negative effects of drought stress on plant growth-related parameters and chlorophyll content. The beneficial impacts against drought were more pronounced by melatonin application in the rhizosphere than in foliar treatments. The melatonin-induced enhanced tolerance could be attributed to improved photosynthetic activity, reduction of abscisic acid and drought-induced oxidative damage by lowering the accumulation of reactive oxygen species and malondialdehyde. Interestingly, the contents of jasmonic acid and salicylic acid were significantly higher following melatonin treatment in the root zone than following foliar application as compared to the control. The activity of major antioxidant enzymes such as superoxide dismutase, catalase, polyphenol oxidase, peroxidase and ascorbate peroxidase was largely stimulated by melatonin application. In addition, melatonin counteracted the drought-induced increase in proline and sugar content. These findings revealed that modifying the endogenous plant hormone content and antioxidant enzymes by melatonin improved drought tolerance in soybean seedlings. Our findings provide evidence for the stronger physiological role of melatonin in the root zone rather than foliar zone, which may be useful in the large-scale field level application during drought.

Background The utilization of plant growth-promoting microbes is an environment friendly strategy to counteract stressful condition and encourage plants tolerance. In this regards, the current study was designed to isolate ACC deaminase and indole-3-acetic acid (IAA) producing halotolerant bacteria to promote tomato ( Solanum lycopersicum L.) growth and tolerance against salinity stress. Results The selected bacterial isolate MO1 was identified as Leclercia adecarboxylata and IAA quantification results revealed that MO1 produced significant amount of IAA (9.815 ± 0.6293 μg mL − 1 ). The MO1 showed the presence of ACC (1-Aminocyclopropane-1-Carboxylate) deaminase responsible acdS gene and tolerance against salinity stress. A plant microbe interaction experiment using tomato ( Solanum lycopersicum L.) with glycine betaine (GB) as a positive control was carried out to investigate the positive role MO1 in improving plant growth and stress tolerance. The results indicated that MO1 inoculation and GB application significantly increased growth attributes under normal as well as saline condition (120 mM NaCl). The MO1 inoculation and GB treatment approach conferred good protection against salinity stress by significantly improving glucose by 17.57 and 18.76%, sucrose by 34.2 and 12.49%, fructose by 19.9 and 10.9%, citric acid by 47.48 and 34.57%, malic acid by 52.19 and 28.38%, serine by 43.78 and 69.42%, glycine by 14.48 and 22.76%, methionine by 100 and 124.99%, threonine by 70 and 63.08%, and proline by 36.92 and 48.38%, respectively, while under normal conditions MO1 inoculation and GB treatment also enhanced glucose by 19.83 and 13.19%, sucrose by 23.43 and 15.75%, fructose by 15.79 and 8.18%, citric acid by 43.26 and 33.14%, malic acid by 36.18 and 14.48%, serine by 46.5 and 48.55%, glycine by 19.85 and 29.77%, methionine by 22.22 and 38.89%, threonine by 21.95 and 17.07%, and proline by 29.61 and 34.68% compared to levels in non-treated plants, respectively. In addition, the endogenous abscisic acid (ABA) level was noticeably lower in MO1-inoculated (30.28 and 30.04%) and GB-treated plants (45 and 35.35%) compared to their corresponding control plants under normal condition as well as salinity stress, respectively. Conclusion The current findings suggest that the IAA- and ACC-deaminase-producing abilities MO1 can improve plants tolerance to salinity stress.

Global warming is impacting the growth and development of economically important but sensitive crops, such as soybean (Glycine max L.). Using pleiotropic signaling molecules, melatonin can relieve the negative effects of high temperature by enhancing plant growth and development as well as modulating the defense system against abiotic stresses. However, less is known about how melatonin regulates the phytohormones and polyamines during heat stress. Our results showed that high temperature significantly increased ROS and decreased photosynthesis efficiency in soybean plants. Conversely, pretreatment with melatonin increased plant growth and photosynthetic pigments (chl a and chl b) and reduced oxidative stress via scavenging hydrogen peroxide and superoxide and reducing the MDA and electrolyte leakage contents. The inherent stress defense responses were further strengthened by the enhanced activities of antioxidants and upregulation of the expression of ascorbate–glutathione cycle genes. Melatonin mitigates heat stress by increasing several biochemicals (phenolics, flavonoids, and proline), as well as the endogenous melatonin and polyamines (spermine, spermidine, and putrescine). Furthermore, the positive effects of melatonin treatment also correlated with a reduced abscisic acid content, down-regulation of the gmNCED3, and up-regulation of catabolic genes (CYP707A1 and CYP707A2) during heat stress. Contrarily, an increase in salicylic acid and up-regulated expression of the defense-related gene PAL2 were revealed. In addition, melatonin induced the expression of heat shock protein 90 (gmHsp90) and heat shock transcription factor (gmHsfA2), suggesting promotion of ROS detoxification via the hydrogen peroxide-mediated signaling pathway. In conclusion, exogenous melatonin improves the thermotolerance of soybean plants and enhances plant growth and development by activating antioxidant defense mechanisms, interacting with plant hormones, and reprogramming the biochemical metabolism.

Plant growth promoting rhizobacteria (PGPR) are diverse, naturally occurring bacteria that establish a close association with plant roots and promote the growth and immunity of plants. Established mechanisms involved in PGPR-mediated plant growth promotion include regulation of phytohormones, improved nutrient availability, and antagonistic effects on plant pathogens. In this study, we isolated a bacterium from the rhizospheric soil of a soybean field in Chungcheong buk-do, South Korea. Using 16S rRNA sequencing, the bacterium was identified as Bacillus aryabhattai strain SRB02. Here we show that this strain significantly promotes the growth of soybean. Gas chromatography-mass spectrometry analysis showed that SRB02 produced significant amounts of abscisic acid, indole acetic acid, cytokinin and different gibberellic acids in culture. SRB02-treated soybean plants showed significantly better heat stress tolerance than did untreated plants. These plants also produced consistent levels of ABA under heat stress and exhibited ABA-mediated stomatal closure. High levels of IAA, JA, GA12, GA4, and GA7, were recorded in SRB02-treated plants. These plants produced longer roots and shoots than those of control plants. B. aryabhattai SRB02 was found to be highly tolerant to oxidative stress induced by H2O2 and MV potentiated by high catalase (CAT) and superoxide dismutase (SOD) activities. SRB02 also tolerated high nitrosative stress induced by the nitric oxide donors GSNO and CysNO. Because of these attributes, B. aryabhattai SRB02 may prove to be a valuable resource for incorporation in biofertilizers and other soil amendments that seek to improve crop productivity.

Heavy metal accumulation in crop grains due to hazardous metal contamination is considered a great concern. However, phytobeneficial fungi are reported to have important abilities for the biosafety of crops grown in contaminated soil. Therefore, the current study was undertaken to explore the mutualistic association of plant growth-promoting endophytic fungi in reducing heavy metal concentration in the seeds of soybean plants subsequently grown in contaminated soil, without comprising seed quality and biochemical profile. The results revealed that endophytic Paecilomyces formosus LHL10 and Penicillium funiculosum LHL06 synergistically produced higher amounts of GAs and IAA in a co-cultured medium. Moreover, the co-inoculation of LHL06 and LHL10 to soybean plants grown under multi-metal toxic conditions significantly mitigated the adverse effects of heavy metal toxicity and increased the seed production (number of pods per plants, number of seeds per pod, and 100 seed weight) of soybean plants grown under control and multi-metal toxic conditions. Moreover, the levels of carbohydrates (glucose, sucrose, and fructose), minerals (iron, calcium, magnesium, and potassium), amino acids (serine, glutamic acids, glycine, methionine, lysine, arginine, and proline), and antioxidants (superoxide dismutase, catalase, and peroxidase) were significantly enhanced in sole and co-inoculated plants under control and stress conditions. Whereas organic acids (citric acid, tartaric acid, malic acid, and succinic acid), lipid peroxidation (MDA) products, multi-metal accumulation (nickel, cadmium, copper, lead, chromium, and aluminum), and stress-responsive endogenous abscisic acid levels were significantly decreased in seeds of soybean plants grown under control and multi-metal toxic conditions upon LHL06 and LHL10 sole and co-inoculation. The current results suggested the positive biochemical regulation in seeds for improving the nutritional status and making it safe for human consumption.

Soybean (Glycine max L.) is a good source of natural antioxidants and commonly consumed as fermented products such as cheonggukjang, miso, tempeh, and sufu in Asian countries. The aim of the current study was to examine the influence of novel endophytic bacterial strain, Bacillus amyloliquefaciens RWL-1 as a starter for soybean fermentation. During fermentation, the cooked soybeans were inoculated with different concentrations (1%, 3%, and 5%) of B. amyloliquefaciens RWL-1. The changes in 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging activities, total phenolic contents, isoflavones (Daidzin, Genistin, Glycitin, Daidzein, Glycitein, and Genistein), amino acids (aspartic acid, threonine, serine, glutamic acid, glycine, alanine, cysteine, valine, methionine, isoleucine, leucine, tyrosine, phenylalanine, lysine, histidine, arginine, and proline) composition, and minerals (calcium, copper, iron, potassium, magnesium, manganese, sodium, nickel, lead, arsenic, and zinc) were investigated. The level of antioxidants, total phenolic contents, isoflavones, and total amino acids were higher in fermented soybean inoculated with 1% B. amyloliquefaciens RWL-1 after 60 h of fermentation as compared to control, 3% and 5% B. amyloliquefaciens RWL-1. Additionally, fermented soybean inoculated with 5% B. amyloliquefaciens RWL-1 showed the highest values for mineral contents. Changes in antioxidant activities and bioactive compounds depended on the concentration of the strain used for fermentation. From these results, we conclude that fermented soybean has strong antioxidant activity, probably due to its increased total phenolic contents and aglycone isoflavone that resulted from fermentation. Such natural antioxidants could be used in drug and food industries and can be considered to alleviate oxidative stress.

This study investigated the regulatory role of exogenous salicylic acid (SA) in rice and its effects on toxic reactive oxygen and nitrogen species during short-term salinity stress. SA application (0.5 and 1.0 mM) during salinity-induced stress (100 mM NaCl) resulted in significantly longer shoot length and higher chlorophyll and biomass accumulation than with salinity stress alone. NaCl-induced reactive oxygen species production led to increased levels of lipid peroxidation in rice plants, which were significantly reduced following SA application. A similar finding was observed for superoxide dismutase; however, catalase (CAT) and ascorbate peroxidase (APX) were significantly reduced in rice plants treated with SA and NaCl alone and in combination. The relative mRNA expression of OsCATA and OsAPX1 was lower in rice plants during SA stress. Regarding nitrogenous species, S-nitrosothiol (SNO) was significantly reduced initially (one day after treatment [DAT]) but then increased in plants subjected to single or combined stress conditions. Genes related to SNO biosynthesis, S-nitrosoglutathione reductase (GSNOR1), NO synthase-like activity (NOA), and nitrite reductase (NIR) were also assessed. The mRNA expression of GSNOR1 was increased relative to that of the control, whereas OsNOA was expressed at higher levels in plants treated with SA and NaCl alone relative to the control. The mRNA expression of OsNR was decreased in plants subjected to single or combination treatment, except at 2 DAT, compared to the control. In conclusion, the current findings suggest that SA can regulate the generation of NaCl-induced oxygen and nitrogen reactive species in rice plants.

The compatible microbial consortia containing fungal and bacterial symbionts acting synergistically are applied to improve plant growth and eco-physiological responses in extreme crop growth conditions. However, the interactive effects of phytohormones-producing endophytic fungal and bacterial symbionts plant growth and stress tolerance under heavy metal stress have been least known. In the current study, the phytohormones-producing endophytic LHL10 and sp. LK11 revealed potent growth and tolerance during their initial screening against combined Al and Zn (2.5 mM each) stress. This was followed with their co-inoculation in the Al- and Zn-stressed L. plants, showing significantly higher plant growth attributes (shoot/root length, fresh/dry weight, and chlorophyll content) than the plants solely inoculated with LHL10 or LK11 and the non-inoculated (control) plants under metal stresses. Interestingly, under metal stress, the consortia exhibited lower metal uptake and inhibited metal transport in roots. Metal-induced oxidative stresses were modulated in co-inoculated plants through reduced hydrogen peroxide, lipid peroxidation, and antioxidant enzymes (catalase and superoxide dismutase) in comparison to the non-inoculated plants. In addition, endophytic co-inoculation enhanced plant macronutrient uptake (P, K, S, and N) and modulated soil enzymatic activities under stress conditions. It significantly downregulated the expression of heavy metal ATPase genes , and and upregulated the expression of an ariadne-like ubiquitin ligase gene under heavy metals stress. Furthermore, the endogenous phytohormonal contents of co-inoculated plants revealed significantly enhanced gibberellins and reduced abscisic acid and jasmonic acid contents, suggesting that this endophytic interaction mitigated the adverse effect of metal stresses in host plants. In conclusion, the co-inoculation of the endophytic fungus LHL10 and bacteria LK11 actively contributed to the tripartite mutualistic symbiosis in under heavy metal stresses; this could be used an excellent strategy for sustainable agriculture in the heavy metal-contaminated fields.

, a tetraploid wild relative of cultivated rice (family Poaceae), possesses a BBCC genome and contains genes that confer resistance to bacterial blight (BB) and white-backed (WBPH) and brown (BPH) plant hoppers. Based on the importance of this wild species, this study aimed to understand the phylogenetic relationships of with other species through an in-depth analysis of the composition and diversity of the chloroplast (cp) genome. The analysis revealed a cp genome size of 135,094 bp with a typical quadripartite structure and consisting of a pair of inverted repeats separated by small and large single copies, 139 representative genes, and 419 randomly distributed microsatellites. The genomic organization, gene order, GC content and codon usage are similar to those of typical angiosperm cp genomes. Approximately 30 forward, 28 tandem and 20 palindromic repeats were detected in the . cp genome. Comparison of the complete cp genome with another eleven species showed a high degree of sequence similarity and relatively high divergence of intergenic spacers. Phylogenetic analyses were conducted based on the complete genome sequence, 65 shared genes and gene showed same topologies and O. forms a single clade with parental . Thus, the complete . cp genome provides interesting insights and valuable information that can be used to identify related species and reconstruct its phylogeny.