Induction of abiotic stress tolerance in plants by endophytic microbes

Endophytes are micro-organisms including bacteria and fungi that survive within healthy plant tissues and promote plant growth under stress. This review focuses on the potential of endophytic microbes that induce abiotic stress tolerance in plants. How endophytes promote plant growth under stressful conditions, like drought and heat, high salinity and poor nutrient availability will be discussed. The molecular mechanisms for increasing stress tolerance in plants by endophytes include induction of plant stress genes as well as biomolecules like reactive oxygen species scavengers. This review may help in the development of biotechnological applications of endophytic microbes in plant growth promotion and crop improvement under abiotic stress conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Increasing human populations demand more crop yield for food security while crop production is adversely affected by abiotic stresses like drought, salinity and high temperature. Development of stress tolerance in plants is a strategy to cope with the negative effects of adverse environmental conditions. Endophytes are well recognized for plant growth promotion and production of natural compounds. The property of endophytes to induce stress tolerance in plants can be applied to increase crop yields. With this review, we intend to promote application of endophytes in biotechnology and genetic engineering for the development of stress-tolerant plants.

The endophytic mycoflora of bark, leaf, and stem tissues of Azadirachta indica A. Juss (neem) from Varanasi (India)

A systematic study was made of the endophytes of Azadirachta indica A. Juss (the neem tree) growing in several of its natural habitats in India. A total of 233 isolates of endophytic fungi representing 18 fungal taxa were obtained from segments of bark, stem, and leaves of this tree. Hyphomycetes (62.2%) were the most prevalent followed by the Coelomycetes (27.4%) and Mycelia Sterilia (7.7%). As mathematically determined, the maximum species richness and frequency of colonization of endophytes appeared in leaf segments rather than stem and bark tissues from each location. Endophytic colonization frequency was also greater in leaves (45.5%) than bark (31.5%). The leaf samples from all locations were nearly constant in their endophytic composition, whereas bark samples showed maximum diversity at different locations. Inter-site comparisons for endophytic diversity, however, were not significantly different with Loc1 and Loc2 having a maximum of 66.67% Jc. The smallest similarity was between Loc2 and Loc3 of 54.17% Jc. The dominant endophytic fungi isolated were Phomopsis oblonga, Cladosporium cladosporioides, Pestalotiopsis sp., Trichoderma sp, and Aspergillus sp. Genera such as Periconia, Stenella, and Drechslera are reported here for the first time as endophytes from this host plant. This report illustrates the value of sampling different tissues of a given plant in several locations to obtain the greatest species diversity of endophytes. The rich and sizeable collection of endophytic fungi from this specific plant may represent a unique source of one or more of the interesting and useful bioactive compounds normally associated with A. indica such as the azadirachtins and related tetranortriterpenoids.

Endophyte roles in nutrient acquisition, root system architecture development and oxidative stress tolerance

Plants associate with communities of microbes (bacteria and fungi) that play critical roles in plant development, nutrient acquisition and oxidative stress tolerance. The major share of plant microbiota is endophytes which inhabit plant tissues and help them in various capacities. In this article, we have reviewed what is presently known with regard to how endophytic microbes interact with plants to modulate root development, branching, root hair formation and their implications in overall plant development. Endophytic microbes link the interactions of plants, rhizospheric microbes and soil to promote nutrient solubilization and further vectoring these nutrients to the plant roots making the soil-plant-microbe continuum. Further, plant roots internalize microbes and oxidatively extract nutrients from microbes in the rhizophagy cycle. The oxidative interactions between endophytes and plants result in the acquisition of nutrients by plants and are also instrumental in oxidative stress tolerance of plants. It is evident that plants actively cultivate microbes internally, on surfaces and in soils to acquire nutrients, modulate development and improve health. Understanding this continuum could be of greater significance in connecting endophytes with the hidden half of the plant that can also be harnessed in applied terms to enhance nutrient acquisition through the development of favourable root system architecture for sustainable production under stress conditions.

Bacillus siamensis CNE6- a multifaceted plant growth promoting endophyte of Cicer arietinum L. having broad spectrum antifungal activities and host colonizing potential

Exploration of endophytic bacteria with multiple plant growth promoting (PGP) attributes is considered as an eco-friendly and cost-effective alternative to agricultural chemicals for increasing crop productivity. In the present endeavor, healthy chickpea plants (Cicer arietinum L.) collected from district Birbhum, West Bengal, India were subjected for the isolation of endophytic bacteria having multifarious PGP properties. One potent endophytic Gram positive bacterial strain CNE6 was isolated from the nodule of chickpea and was identified as Bacillus siamensis based on 16S rDNA sequence homologies. The isolate showed a number of PGP properties like phosphate solubilization, IAA production, nitrogen fixation, hydroxamate type of siderophore production and ACC deaminase activities. The isolate CNE6 produced 33.27 ± 2.16 μg/mL of IAA in the presence of tryptophan. Production of IAA was also confirmed by HPLC analysis and it was found effective for inducing lateral root branching in chickpea. In addition, the isolate displayed significant antagonistic activity against a number of plant pathogenic fungi when tested by dual culture overlay and agar well diffusion assay. 50 % cell free supernatant of CNE6 was found effective for 60-80 % inhibition of radial growth of pathogenic fungi tested. Scanning electron microscopic observation revealed massive degradation of pathogenic fungal mycelia by the antifungal metabolites of CNE6. LC-MS analysis of bacterial lipopeptides suggested the production of antifungal antibiotics like surfactin, fengycin and iturin by the isolate. The presence of genes encoding antifungal lipopeptides was also confirmed by PCR amplification using specific primers. Green fluorescent protein (GFP) tagging of CNE6 using broad host range plasmid vector (pDSK-GFPuv) followed by colonization study indicated very good host colonization potential of the isolate and its probable movement through xylem vessels. Enhanced shoot and root length and chlorophyll content upon treatment with CNE6 as observed in in vivo pot experiments also supported the positive role of the endophytic isolate on overall development and growth of the chickpea plants. This is the first report of Bacillus siamensis as an endophyte of Cicer arietinum L. which can be successfully applied for improving the productivity of this crop plant.

Management of Black Root Disease-Causing Fungus Fusarium solani CRP1 by Endophytic Bacillus siamensis CNE6 through Its Metabolites and Activation of Plant Defense Genes

Black root rot disease of Cicer arietinum L. is accountable for substantial loss in chickpea production worldwide. Endophytic Bacillus siamensis CNE6 has previously shown multifaceted plant growth-promoting, broad-spectrum antifungal, and chickpea plant-colonizing potential. In the present study, the strain Bacillus siamensis CNE6 was used for controlling black root rot disease caused by Fusarium solani CRP1 in chickpea. CNE6 showed strong antagonistic potential against CRP1 both in vivo and in vitro. Scanning electron microscopic studies indicated cellular deformation of CRP1 due to production of β-glucanase, protease, and other secondary metabolites. A total of five compounds were detected from the cell-free supernatant (CFS) of the ethyl acetate (EA) fraction of CNE6 through gas chromatography-mass spectrometry analysis. A confocal microscopic study demonstrated strong inhibition of biofilm formation of the pathogen CRP1 by the EA fraction of CFS of CNE6. Molecular docking analysis revealed that one compound, (2E)-6-methoxy-2-[(4-methoxyphenyl)methylidene]-2,3-dihydro-1-benzofuran-3-one, may inhibit the activity of lanosterol 14-alpha demethylase, which is involved in ergosterol biosynthesis and beta-tubulin assembling. In vivo experiments also showed the efficacy of CNE6 for increasing chickpea growth as well as upregulation of four defense genes (CHI1, PAMP, PR2B, and TF1082) upon pathogenic challenge. Thus, our results strongly suggest a positive role for CNE6 as a prospective biocontrol agent for combating Fusarium solani in chickpea. IMPORTANCE The present work was undertaken to explore an effective biocontrol agent against the destructive black root rot disease of chickpea. We have used an efficient bacterial endophyte, CNE6, which can colonize in the chickpea root system, produce secondary metabolites and enzymes to degrade pathogenic cellular integrity, inhibit pathogenic establishment by rupturing biofilm formation, and induce host immunity upon treatment. Interaction of the bacterial metabolite was also observed with lanosterol 14-alpha demethylase, which is an important component in fungal membrane functioning. Being an endophyte, Bacillus siamensis CNE6 fulfills a suitable criterion as a biocontrol agent to control black root rot disease in chickpea and has huge prospects for use commercially.

Detection of vinblastine and related bioactive compounds from culture extracts of endophytic fungi of Catharanthus roseus

This study investigates the synthesis of vinblastine by endophytic fungi isolated from leaf of C. roseus. A total of 10 endophytic fungi were selected for secretion of vinca alkaloids based on the initial screening by biochemical tests and thin-layer chromatography (TLC). Out of these ten, only four fungal extracts showed positive results for presence of vinblastine at same retention time (10 min.) compared to reference compound on HPLC analysis. The detected concentration of vinblastine was maximum (17 µg/ml) in isolate no. CRL 22 followed by CRL 52, CRL 17 and CRL 28. To validate the presence of vinblastine, ultra-high-performance liquid chromatography coupled with high-resolution accurate mass spectrometry (HRMS) was employed. This analysis confirmed the presence of anhydrovinblastine, a precursor of vinblastine through the detection of molecular ions at m/z 793.4185 in extract of CRL 17. In addition to anhydrovinblastine, the intermediate compounds essential to the biosynthetic pathway of vinblastine were also detected in the extract of CRL 17. These host-origin compounds strongly suggest the presence of a biosynthetic pathway within the endophytic fungus. Based on morphological observation and sequence analysis of the ITS region of rDNA, endophytic fungi were identified as Alternaria alternata (CRL 17), Curvularia lunata (CRL 28), Aspergillus terrus (CRL 52), and Aspergillus clavatonanicus (CRL 22).

Bio-control potential of Cladosporium sp. (MCPL-461), against a noxious weed Parthenium hysterophorus L

The phenological survey of Parthenium hysterophorus L., in and around the campus of Banaras Hindu University (BHU) was done for about two years (2004-06). During Nov 2004, a few Parthenium plants were found diseased, and symptoms were restricted to the flowers, buds, and inflorescences. The disease causes sterility and reduces seed viability, which was observed with seed germination test from infected and healthy plants. The fungal pathogen was isolated and identified as Cladosporium sp. (MCPL-461). The severity of pathogen to the reproductive organs led to serious damages of the Parthenium plants. Thus in vitro and in vivo experiments were conducted to determine the bio-control potential of Cladosporium sp. (MCPL 461) against Parthenium weed. A combinatorial effort of Cladosporium sp. (MCPL 461) bio-control potential was evaluated with different culture media, incubation periods and spores strength. Spore suspension of 10(5) to 10(12) spores ml(-1) were used to spray on healthy Parthenium plants, and it was found that severe infection symptoms were appeared at 10(10) to 10(12) spores ml(-1) suspension. LD50 was found at 10(7) spores ml(-1). To enhance the myco-herbicide activity 3% sucrose was added to the spore suspension, which further resolute the bio-control efficacy of the isolates. Only 20-30% seeds of infected plants could germinate. However the safety of non-targeted and wild plants was also tested with Lantana camera, Chromolaena odorata and found that suspension up to 10(12) spores ml(-1) were not sufficient for disease outbreak in them.

Antibacterial and antioxidant potentials, detection of host origin compounds, and metabolic profiling of endophytic Bacillus spp. isolated from Rauvolfia serpentina (L.) Benth. ex Kurz

The research highlights the importance of exploring endophytic microbiomes of medicinal plants to uncover their potential for secondary metabolite production and their role in the biosynthesis of host-derived compounds. This study was aimed to isolate leaf endophytic bacteria of Rauvolfia serpentina, investigate their antibacterial, antioxidant potentials and detect host-origin compound reserpine using Reverse Phase High-Performance Liquid Chromatography (RPHPLC). Untargeted analysis via Ultra High-Performance Liquid Chromatography-High-Resolution Mass Spectrometry (UHPLC-HRMS/MS) was conducted for profiling main phytochemicals in the leaves and to explore potential bioactive compounds in bacterial extracts. Nine bacterial isolates were obtained from R. serpentina leaves. These isolates exhibited positive results in various biochemical tests including indole production, methyl red, Voges-Proskauer, citrate utilization, catalase and oxidase production, nitrate reduction, oxidative fermentation, and citrate reduction tests. Endophytic isolates RSLB3 and RSLB18 exhibited most potential antibacterial activity against tested human pathogenic bacteria and were identified as Bacillus sp. The extract of RSLB3 and RSLB18 also showed significant antioxidant activity compared to leaf extract. The total phenol content was similar in both these isolates while flavonoids content and DPPH scavenging activity was higher in isolate RSLB3. RPHPLC analysis confirmed the presence of reserpine in bacterial metabolites when compared to a standard reference. UHPLC-HRMS profiling unveiled a diverse range of host-derived compounds and reaction intermediates with known and unknown bioactive properties in leaf extract, RSLB3, and RSLB18. To our knowledge, this is the first study to achieve a comprehensive profiling.