Deciphering the antagonistic effect of Streptomyces spp. and host-plant resistance induction against charcoal rot of sorghum.

MAIN CONCLUSION: The findings of this study suggest that the selected five strains of Streptomyces spp. could be used for biological control of charcoal rot disease in sorghum. Two strains each of Streptomyces albus (CAI-17 and KAI-27) and Streptomyces griseus (KAI-26 and MMA-32) and one strain of Streptomyces cavourensis (SAI-13) previously reported to have plant growth-promotion activity in chickpea, rice and sorghum were evaluated for their antagonistic potential against Macrophomina phaseolina, which causes charcoal rot in sorghum. The antagonistic potential of these strains against M. phaseolina was assessed through dual culture assay, metabolite production assay, blotter paper assay in greenhouse and field disease screens. In both dual culture and metabolite production assays, the selected strains significantly inhibited the growth of M. phaseolina (63-74%). In the blotter paper assay, all the five strains of Streptomyces spp. inhibited the pathogen (80-90%). When these five strains were tested for their antagonistic potential under the greenhouse (two times) and field (two seasons) conditions by toothpick method of inoculation, significant differences were observed for charcoal rot severity. Principal component analysis capturing 91.3% phenotypic variations, revealed that the shoot samples treated with both Streptomyces and the pathogen exhibited significantly enhanced antioxidant parameters including superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, glutathione reductase, phenylalanine ammonia-lyase, polyphenol oxidase, and total phenolic contents when compared to shoot samples treated with only M. phaseolina. Scanning electron microscope analysis revealed that the phloem and xylem tissues of the Streptomyces treated stem samples were intact compared to that of pathogen inoculated plants. This study indicated that the selected strains of Streptomyces spp. have the potential for biological control of charcoal rot disease in sorghum.

Streptomyces consortia-mediated plant defense against Fusarium wilt and plant growth-promotion in chickpea.

Three strains of Streptomyces griseus (CAI-24, CAI-121 and CAI-127) and one strain each of Streptomyces africanus (KAI-32) and Streptomyces coelicolor (KAI-90) were reported by us as biocontrol agents against Fusarium wilt, caused by Fusarium oxysporum f. sp. ciceri (FOC), and as plant growth-promoters (PGP) in chickpea. In the present study, the combined effect of these Streptomyces strains as a consortium were assessed for their biocontrol potential against Fusarium wilt and PGP in chickpea. Based on their compatibility, biocontrol ability and PGP performance, two consortia were assembled, consortium-1 having all the five strains of Streptomyces sp. and consortium-2 having the two promising strains (CAI-127 and KAI-32). Under greenhouse conditions, consortium-1 and consortium-2 were found to reduce the Fusarium wilt disease incidence by 55% and 74%, while under field conditions, these were by 86% and 96% in year-1 and by 54% and 69% in year-2, respectively, when compared to the positive control (only FOC treated). Shoot samples treated with consortia + FOC contained significantly enhanced antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, glutathione reductase and phenylalanine ammonia-lyase, when compared to the positive control (only FOC treated) or the negative control samples (neither FOC nor consortia treated). When the consortia were evaluated for their PGP traits under field conditions in two chickpea cultivars, JG11 and ICCV2, and in two consecutive years, nodule number was found to enhance up to 25%, nodule weight up to 49%, leaf area up to 37%, leaf weight up to 43%, root weight up to 23%, shoot weight up to 35%, seed weight up to 30%, seed number up to 29%, total dry matter up to 22% and grain yield up to 22% over the un-inoculated control plants. This study had demonstrated that the selected consortium of Streptomyces spp. has a greater potential for biological control of Fusarium wilt disease and PGP in chickpea.

Deciphering the tri-dimensional effect of endophytic Streptomyces sp. on chickpea for plant growth promotion, helper effect with Mesorhizobium ciceri and host-plant resistance induction against Botrytis cinerea.

A total of 219 endophytic actinobacteria, isolated from roots, stems and leaves of chickpea, were characterized for antagonistic potential against Botrytis cinerea, causal organism of Botrytis grey mold (BGM) disease, in chickpea. Among them, three most potential endophytes, AUR2, AUR4 and ARR4 were further characterized for their plant growth-promoting (PGP) and nodulating potentials and host-plant resistance against B. cinerea, in chickpea. The sequences of 16 S rDNA gene of the three endophytes were matched with Streptomyces but different species. In planta, the isolate AUR4 alone was able to significantly enhance PGP traits including seed numbers (11.8 vs. 9.8/Plant), seed weight (8 vs. 6.8 g/Plant), pod numbers (13.6 vs. 11.5/Plant), pod weight (9.3 vs. 7.5 g/Plant) and biomass (10.9 vs. 8 g/Plant) over the un-inoculated control in chickpea genotype JG11. Interestingly, consortium of the selected endophytes, AUR2, AUR4 and ARR4 were found less effective than single inoculation. Co-inoculation of the selected endophytes with Mesorhizobium ciceri significantly enhanced nodulation and nitrogenase activity in five chickpea genotypes including ICCV2, ICCV10, ICC4958, Annigeri and JG11 over the un-inoculated control. The selected endophytes showed antagonistic potential in planta by significant reduction of disease incidence (28─52%) in both single inoculation and consortium treatments over the un-inoculated control across the genotypes ICC4954 (susceptible), ICCV05530 (moderately resistant) and JG11 (unknown resistance). Further, antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, glutathione reductase, phenylalanine ammonia-lyase and polyphenol oxidase and phenolics were found induced in the leaves of chickpea inoculated with selected endophytes over un-inoculated control. Principal component analysis revealed that, the antioxidant enzymes and phenolics were found in the magnitude of ICC4954 < JG11 < ICCV05530 which correlates with their resistance level. The selected endophytes enhanced the plant growth and also host plant resistance against BGM in chickpea.

Evaluation of Streptomyces spp. for their plant-growth-promotion traits in rice.

Five strains of Streptomyces (CAI-17, CAI-68, CAI-78, KAI-26, and KAI-27) were previously reported to have potential for charcoal rot control and plant growth promotion (PGP) in sorghum. In this study, those 5 Streptomyces strains were characterized for their enzymatic activities and evaluated for their PGP capabilities on rice. All the Streptomyces strains were able to produce lipase and ß-1,3-glucanase; grew in NaCl (up to 8%), at pH 5-13, and at temperatures 20-40 °C; and were resistant to ampicillin, sensitive to nalidixic acid, and highly sensitive to chloramphenicol, kanamycin, streptomycin, and tetracycline. They were highly tolerant to the fungicide bavistin but were highly sensitive to benlate, benomyl, and radonil. When evaluated on rice in the field, Streptomyces significantly enhanced tiller and panicle numbers, stover and grain yields, dry matter, root length, volume and dry weight, compared with the control. In the rhizosphere at harvest, microbial biomass carbon and nitrogen, dehydrogenase activity, total nitrogen, available phosphorus, and % organic carbon were also found significantly higher in Streptomyces-treated plots than in the control plots. This study further confirms that the selected Streptomyces have PGP activities.

Streptomyces consortia-mediated plant growth-promotion and yield performance in chickpea.

Fourteen Streptomyces strains reported earlier as plant growth promoters (PGP) in chickpea were characterized for production of ammonia and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase and solubilization of silica and zinc. The results showed that nine (CAI-17, CAI-78, KAI-26, CAI-21, CAI-26, MMA-32, CAI-140, CAI-155 and KAI-180) and six (CAI-17, CAI-21, CAI-26, CAI-13, CAI-93 and KAI-180) strains were found to produce ammonia and ACC deaminase, respectively, while one (KAI-180) and eight (CAI-17, CAI-21, CAI-26, MMA-32, CAI-13, CAI-85, CAI-93 and KAI-180) strains solubilized silica and zinc, respectively. The selected 14 Streptomyces strains were categorized into three consortia groups, consortium-1 (CAI-17, CAI-68, CAI-78, KAI-26 and KAI-27), consortium-2 (CAI-21, CAI-26 and MMA-32) and consortium-3 (CAI-13, CAI-85, CAI-93, CAI-140, CAI-155 and KAI-180), based on their compatibility, and evaluated for their PGP traits in chickpea. The experiment was conducted under field conditions with two chickpea varieties over two years. The consortia-treated plots enhanced nodule number up to 23%, nodule weight up to 36%, root weight up to 27% and shoot weight up to 26% at 30 days after sowing and pod weight up to 35%, pod number up to 34% and grain yield up to 24% at harvest over the un-inoculated control plots. The harvested grains of consortia treatments were found to enhance crude protein up to 14%, crude fibre up to 17% and crude fat up to 16% over the grains from un-inoculated control. The rhizosphere soils of the consortia-treated plots enhanced total nitrogen up to 21%, organic carbon up to 8% and available phosphorous up to 16% over the un-inoculated control plots. This investigation demonstrated the potential use of the selected consortium of Streptomyces strains in the farmers' fields to improve the chickpea yields and soil fertility.

Effects of Plastic Film Mulching on Soil Enzyme Activities and Stoichiometry in Dryland Agroecosystems.

Soil extracellular enzymes are pivotal for microbial nutrient cycling in the ecosystem. In order to study the effects of different nitrogen application rates under plastic film mulching on soil extracellular enzyme activities and stoichiometry, five nitrogen application levels (i.e., 0, 90, 150, 225 and 300 kg·hm-2) were set based on two treatments: plastic film mulching (PM) and no film mulching (LD). We measured the soil extracellular enzyme activities (EEAs) and stoichiometry (EES) of four enzymes (i.e., ß-1,4-glucosidase (ßG), leucine aminopeptidase (LAP), ß-1,4-N-acetylaminoglucosidase (NAG) and alkaline phosphatase (AP)) involved in the C, N and P cycles of soil microorganisms in surface soil at five maize growth stages (seedling stage, jointing stage, trumpet stage, grout stage and harvest stage). The results showed that there were significant differences in soil EEA at different maize growth stages. The soil nutrient content and soil EEA were significantly improved under PM, and the stoichiometric ratio of extracellular enzymes (EC:N:P) was closer to 1:1:1, which indicated that PM was beneficial to the balance of soil nutrients and the activity of microorganisms. At each stage, with the increase in nitrogen application levels, the soil EEA showed a trend of increasing first and then decreasing (or remained unchanged), and both LD and PM treatments reached their highest activity at the 225 kg·hm-2 nitrogen application rate. When the nitrogen application level was less than 225 kg·hm-2, the soil enzyme activity was mainly limited by the N nutrient, and when the nitrogen application level reached 300 kg·hm-2, it was mainly limited by the P nutrient. RDA and correlation analysis showed that the soil C:P, C:N, N:P and pH had significant effects on soil ßG, NAG + LAP and AP activities as well as EC:N, EC:P and EN:P.

Response of the Fate of In-Season Fertilizer Nitrogen to Plastic Mulching in Rainfed Maize Croplands of the Loess Plateau.

As plastic mulching is widely used for maize production on Loess Plateau, study of the fate of fertilizer nitrogen (N) in rain-fed croplands is of great significance. Field experiments were conducted during 2015-2016 at a typical dry-land farm on the Loess Plateau, China. The stable isotope tracer technique was applied to analyze the effects of plastic mulching on the maize crop yield, N content in the grain, and mechanism of N uptake and utilization in maize plants with plastic mulch (PM) and without plastic mulch (CK) on the Loess Plateau. Maize yield, aboveground dry matter, grain N concentration, and N uptake in aboveground biomass for PM significantly increased, in addition to fertilizer nitrogen recovery and nitrogen production efficiency. Compared to CK, PM improved the total N uptake from the soil in the aboveground biomass by 16.39 and 27.75 kg ha-1 and fertilizer nitrogen recovery by 10.89 and 22.02 kg ha-1, respectively. Furthermore, PM increased in-season fertilizer N retention in the soil by 11.9-24.8 kg ha-1, and the uncountable fertilizer N decreased by approximately 33.8 kg ha-1 on average. In conclusion, PM simultaneously improved the maize yield and N utilization, which provides a scientific basis for nitrogen management in maize croplands.

Changes in Ammonia-Oxidizing Archaea and Bacterial Communities and Soil Nitrogen Dynamics in Response to Long-Term Nitrogen Fertilization.

Ammonia oxidizing archaea (AOA) and bacteria (AOB) mediate a crucial step in nitrogen (N) metabolism. The effect of N fertilizer rates on AOA and AOB communities is less studied in the wheat-fallow system from semi-arid areas. Based on a 17-year wheat field experiment, we explored the effect of five N fertilizer rates (0, 52.5, 105, 157.5, and 210 kg ha-1 yr-1) on the AOA and AOB community composition. This study showed that the grain yield of wheat reached the maximum at 105 kg N ha-1 (49% higher than control), and no further significant increase was observed at higher N rates. With the increase of N, AOA abundance decreased in a regular trend from 4.88 × 107 to 1.05 × 107 copies g-1 dry soil, while AOB abundance increased from 3.63 × 107 up to a maximum of 8.24 × 107 copies g-1 dry soil with the N105 treatment (105 kg N ha-1 yr-1). Application rates of N fertilizer had a more significant impact on the AOB diversity than on AOA diversity, and the highest AOB diversity was found under the N105 treatment in this weak alkaline soil. The predominant phyla of AOA and AOB were Thaumarchaeota and Proteobacteria, respectively, and higher N treatment (N210) resulted in a significant decrease in the relative abundance of genus Nitrosospira. In addition, AOA and AOB communities were significantly associated with grain yield of wheat, soil potential nitrification activity (PNA), and some soil physicochemical parameters such as pH, NH4-N, and NO3-N. Among them, soil moisture was the most influential edaphic factor for structuring the AOA community and NH4-N for the AOB community. Overall, 105 kg N ha-1 yr-1 was optimum for the AOB community and wheat yield in the semi-arid area.

Characterization of rhizobia isolated from leguminous plants and their impact on the growth of ICCV 2 variety of chickpea (Cicer arietinum L.).

Six rhizobia-like-bacterial strains in total, secluded from the root and stem nodules of various leguminous plants were characterized for growth promoting ability on ICCV 2 variety of chickpea. Bacterial strains showed production of IAA, NH3, siderophore, HCN, ACC deaminase, hydrolytic enzyme production such as chitinase, amylase, protease, lipase, ß-1, 3-glucanase and solubilization of nutrients such as phosphate, zinc and potassium. However the performance of PGP traits characterized in-vitro varied among the six bacterial strains. The sequences of 16S rRNA gene of bacterial strains IHSR, IHRG, IHAA, IHGN-3, IHCP-1 and IHCP-2 showed maximum identity with Rhizobium sp., Rhizobium tropici, Rhizobium multihospitium, Mesorhizobium sp., Burkholderia cepacia and Rhizobium pusense. In plate culture conditions the bacterial strains changed the colour of media (NFB) from green to blue and showed amplification of nifH gene by PCR, and also enhanced nodule formation in chickpea under greenhouse conditions, which explains their nitrogen fixing ability. Scanning electron microscopy studies of chickpea roots showed colonization by all the six bacterial strains in solo and by consortium (IHRG + IHGN-3). Under greenhouse conditions, chickpea plants inoculated with different strains showed improvement in plant height, number of branches, total chlorophyll, nodule number, nodule weight, shoot weight, root weight, root volume and root surface area at 30 and 45 days after sowing (DAS) over the uninoculated control plants. It was also observed at the crop maturity stage all the bacterial strains inoculated separately enhanced pod number, seed number and total NPK compared to uninoculated control plants. This study suggests that bacteria associated with root and stem nodules can be a promising resource to enhance nodulation, PGP and crop yields in chickpea.

Substitution of Chemical Fertilizer with Organic Fertilizer Affects Soil Total Nitrogen and Its Fractions in Northern China.

The impact of chemical to organic fertilizer substitution on soil labile organic and stabilized N pools under intensive farming systems is unclear. Therefore, we analyzed the distribution of soil total N (STN), particulate organic N (PON), microbial biomass N (MBN), dissolved organic N (DON), and mineral N (NO3- and NH4+) levels down to 100 cm profile under wheat-maize rotation system in northern China. The experiment was established with four 270 kg ha-1 N equivalent fertilizer treatments: Organic manure (OM); Organic manure with nitrogen fertilizer (OM + NF); Nitrogen fertilizer (NF); and Control (CK). Results found that the OM and OM + NF treatments had significantly higher STN, PON, MBN, DON, and NO3- contents in 0-20 cm topsoil depths. Conversely, the NF treatment resulted in the highest (p < 0.01) DON and NO3- depositions in 40-100 cm subsoil depths. The NH4+ contents in selected profile depths were significantly highest (p < 0.01) under OM treatment. The correlations between STN and its fractions were positively significant at 0-10 and 10-20 cm topsoil depths. Our results suggest that partial substitution of chemical fertilizer with organic manure could be a sustainable option for soil N management of intensive farming systems.

Exploiting Biological Nitrogen Fixation: A Route Towards a Sustainable Agriculture.

For all living organisms, nitrogen is an essential element, while being the most limiting in ecosystems and for crop production. Despite the significant contribution of synthetic fertilizers, nitrogen requirements for food production increase from year to year, while the overuse of agrochemicals compromise soil health and agricultural sustainability. One alternative to overcome this problem is biological nitrogen fixation (BNF). Indeed, more than 60% of the fixed N on Earth results from BNF. Therefore, optimizing BNF in agriculture is more and more urgent to help meet the demand of the food production needs for the growing world population. This optimization will require a good knowledge of the diversity of nitrogen-fixing microorganisms, the mechanisms of fixation, and the selection and formulation of efficient N-fixing microorganisms as biofertilizers. Good understanding of BNF process may allow the transfer of this ability to other non-fixing microorganisms or to non-leguminous plants with high added value. This minireview covers a brief history on BNF, cycle and mechanisms of nitrogen fixation, biofertilizers market value, and use of biofertilizers in agriculture. The minireview focuses particularly on some of the most effective microbial products marketed to date, their efficiency, and success-limiting in agriculture. It also highlights opportunities and difficulties of transferring nitrogen fixation capacity in cereals.

Soil Bacterial Diversity and Potential Functions Are Regulated by Long-Term Conservation Tillage and Straw Mulching.

Soil physiochemical properties are regulated by cropping practices, but little is known about how tillage influences soil microbial community diversity and functions. Here, we assessed soil bacterial community assembly and functional profiles in relation to tillage. Soils, collected in 2018 from a 17-year field experiment in northwestern China, were analyzed using high-throughput sequencing and the PICRUSt approach. The taxonomic diversity of bacterial communities was dominated primarily by the phyla Proteobacteria (32-56%), Bacteroidetes (12-33%), and Actinobacteria (17-27%). Alpha diversity (Chao1, Shannon, Simpson, and operational taxonomic unit (OTU) richness) was highest under no-tillage with crop residue removed (NT). Crop residue retention on the soil surface (NTS) or incorporated into soil (TS) promoted the abundance of Proteobacteria by 16 to 74% as compared to conventional tillage (T). Tillage practices mainly affected the pathways of soil metabolism, genetic information processing, and environmental information processing. Soil organic C and NH4-N were the principal contributors to the diversity and composition of soil microbiota, whereas soil pH, total nitrogen, total P, and moisture had little effect. Our results suggest that long-term conservation practices with no-tillage and crop residue retention shape soil bacterial community composition through modifying soil physicochemical properties and promoting the metabolic function of soil microbiomes.

Identification and Characterization of a Streptomyces albus Strain and Its Secondary Metabolite Organophosphate against Charcoal Rot of Sorghum.

Streptomycesalbus strain CAI-21 has been previously reported to have plant growth-promotion abilities in chickpea, pigeonpea, rice, and sorghum. The strain CAI-21 and its secondary metabolite were evaluated for their biocontrol potential against charcoal rot disease in sorghum caused by Macrophomina phaseolina. Results exhibited that CAI-21 significantly inhibited the growth of the pathogen, M. phaseolina, in dual-culture (15 mm; zone of inhibition), metabolite production (74% inhibition), and blotter paper (90% inhibition) assays. When CAI-21 was tested for its biocontrol potential under greenhouse and field conditions following inoculation of M. phaseolina by toothpick method, it significantly reduced the number of internodes infected (75% and 45% less, respectively) and length of infection (75% and 51% less, respectively) over the positive control (only M. phaseolina inoculated) plants. Under greenhouse conditions, scanning electron microscopic analysis showed that the phloem and xylem tissues of the CAI-21-treated shoot samples were intact compared to those of the diseased stem samples. The culture filtrate of the CAI-21 was purified by various chromatographic techniques, and the active compound was identified as "organophosphate" by NMR and MS. The efficacy of organophosphate was found to inhibit the growth of M. phaseolina in the poisoned food technique. This study indicates that S.albus CAI-21 and its active metabolite organophosphate have the potential to control charcoal rot in sorghum.

Molecular Basis of Root Nodule Symbiosis between Bradyrhizobium and 'Crack-Entry' Legume Groundnut (Arachis hypogaea L.).

Nitrogen is one of the essential plant nutrients and a major factor limiting crop productivity. To meet the requirements of sustainable agriculture, there is a need to maximize biological nitrogen fixation in different crop species. Legumes are able to establish root nodule symbiosis (RNS) with nitrogen-fixing soil bacteria which are collectively called rhizobia. This mutualistic association is highly specific, and each rhizobia species/strain interacts with only a specific group of legumes, and vice versa. Nodulation involves multiple phases of interactions ranging from initial bacterial attachment and infection establishment to late nodule development, characterized by a complex molecular signalling between plants and rhizobia. Characteristically, legumes like groundnut display a bacterial invasion strategy popularly known as "crack-entry'' mechanism, which is reported approximately in 25% of all legumes. This article accommodates critical discussions on the bacterial infection mode, dynamics of nodulation, components of symbiotic signalling pathway, and also the effects of abiotic stresses and phytohormone homeostasis related to the root nodule symbiosis of groundnut and Bradyrhizobium. These parameters can help to understand how groundnut RNS is programmed to recognize and establish symbiotic relationships with rhizobia, adjusting gene expression in response to various regulations. This review further attempts to emphasize the current understanding of advancements regarding RNS research in the groundnut and speculates on prospective improvement possibilities in addition to ways for expanding it to other crops towards achieving sustainable agriculture and overcoming environmental challenges.

Detection, isolation and characterization of a root-exuded compound, methyl 3-(4-hydroxyphenyl) propionate, responsible for biological nitrification inhibition by sorghum (Sorghum bicolor).

Nitrification results in poor nitrogen (N) recovery and negative environmental impacts in most agricultural systems. Some plant species release secondary metabolites from their roots that inhibit nitrification, a phenomenon known as biological nitrification inhibition (BNI). Here, we attempt to characterize BNI in sorghum (Sorghum bicolor). In solution culture, the effect of N nutrition and plant age was studied on BNI activity from roots. A bioluminescence assay using recombinant Nitrosomonas europaea was employed to determine the inhibitory effect of root exudates. One major active constituent was isolated by activity-guided HPLC fractionations. The structure was analysed using NMR and mass spectrometry. Properties and the 70% inhibitory concentration (IC(70)) of this compound were determined by in vitro assay. Sorghum had significant BNI capacity, releasing 20 allylthiourea units (ATU) g(-1) root DW d(-1). Release of BNI compounds increased with growth stage and concentration of supply. NH4+ -grown plants released several-fold higher BNI compounds than NO3- -grown plants. The active constituent was identified as methyl 3-(4-hydroxyphenyl) propionate. BNI compound release from roots is a physiologically active process, stimulated by the presence of NH4+. Methyl 3-(4-hydroxyphenyl) propionate is the first compound purified from the root exudates of any species; this is an important step towards better understanding BNI in sorghum.

Nitrification Inhibitors from the root tissues of Brachiaria humidicola, a tropical grass.

Nitrification inhibitory activity was found in root tissue extracts of Brachiaria humidicola, a tropical pasture grass. Two active inhibitory compounds were isolated by activity-guided fractionation, using recombinant Nitrosomonas europaea containing luxAB genes derived from the bioluminescent marine gram-negative bacterium Vibrio harveyi. The compounds were identified as methyl-p-coumarate and methyl ferulate, respectively. Their nitrification inhibitory properties were confirmed in chemically synthesized preparations of each. The IC50 values of chemically synthesized preparations were 19.5 and 4.4 microM, respectively. The ethyl, propyl, and butyl esters of p-coumaric and ferulic acids inhibited nitrification, whereas the free acid forms did not show inhibitory activity.

Plant growth-promoting actinobacteria: a new strategy for enhancing sustainable production and protection of grain legumes.

Grain legumes are a cost-effective alternative for the animal protein in improving the diets of the poor in South-East Asia and Africa. Legumes, through symbiotic nitrogen fixation, meet a major part of their own N demand and partially benefit the following crops of the system by enriching soil. In realization of this sustainability advantage and to promote pulse production, United Nations had declared 2016 as the "International Year of pulses". Grain legumes are frequently subjected to both abiotic and biotic stresses resulting in severe yield losses. Global yields of legumes have been stagnant for the past five decades in spite of adopting various conventional and molecular breeding approaches. Furthermore, the increasing costs and negative effects of pesticides and fertilizers for crop production necessitate the use of biological options of crop production and protection. The use of plant growth-promoting (PGP) bacteria for improving soil and plant health has become one of the attractive strategies for developing sustainable agricultural systems due to their eco-friendliness, low production cost and minimizing consumption of non-renewable resources. This review emphasizes on how the PGP actinobacteria and their metabolites can be used effectively in enhancing the yield and controlling the pests and pathogens of grain legumes.

Plant growth-promoting actinobacteria on chickpea seed mineral density: an upcoming complementary tool for sustainable biofortification strategy.

The present study was evaluated to test the potential of plant growth-promoting actinobacteria in increasing seed mineral density of chickpea under field conditions. Among the 19 isolates of actinobacteria tested, significant (p < 0.05) increase of minerals over the uninoculated control treatments was noticed on all the isolates for Fe (10-38 %), 17 for Zn (13-30 %), 16 for Ca (14-26 %), 9 for Cu (11-54 %) and 10 for Mn (18-35 %) and Mg (14-21 %). The increase might be due to the production of siderophore-producing capacity of the tested actinobacteria, which was confirmed in our previous studies by q-RT PCR on siderophore genes expressing up to 1.4- to 25-fold increased relative transcription levels. The chickpea seeds were subjected to processing to increase the mineral availability during consumption. The processed seeds were found to meet the recommended daily intake of FDA by 24-28 % for Fe, 25-28 % for Zn, 28-35 % for Cu, 12-14 % for Ca, 160-167 % for Mn and 34-37 % for Mg. It is suggested that the microbial inoculum can serve as a complementary sustainable tool for the existing biofortification strategies and substantially reduce the chemical fertilizer inputs.

Plant growth-promotion and biofortification of chickpea and pigeonpea through inoculation of biocontrol potential bacteria, isolated from organic soils.

Seven strains of bacteria [Pseudomonas plecoglossicida SRI-156, Brevibacterium antiquum SRI-158, Bacillus altitudinis SRI-178, Enterobacter ludwigii SRI-211, E. ludwigii SRI-229, Acinetobacter tandoii SRI-305 and Pseudomonas monteilii SRI-360; demonstrated previously for control of charcoal rot disease in sorghum and plant growth-promotion (PGP) in rice] were evaluated for their PGP and biofortification traits in chickpea and pigeonpea under field conditions. When treated on seed, the seven selected bacteria significantly enhanced the shoot height and root length of both chickpea and pigeonpea over the un-inoculated control. Under field conditions, in both chickpea and pigeonpea, the plots inoculated with test bacteria enhanced the nodule number, nodule weight, root and shoot weights, pod number, pod weight, leaf weight, leaf area and grain yield over the un-inoculated control plots. Among the seven bacteria, SRI-229 was found to significantly and consistently enhance all the studied PGP and yield traits including nodule number (24 and 36%), nodule weight (11 and 44%), shoot weight (22 and 20%), root weight (23 and 16%) and grain yield (19 and 26%) for both chickpea and pigeonpea, respectively. When the harvested grains were evaluated for their mineral contents, iron (up to 18 and 12%), zinc (up to 23 and 5%), copper (up to 19 and 8%), manganese (up to 2 and 39%) and calcium (up to 22 and 11%) contents in chickpea and pigeonpea, respectively, were found enhanced in test bacteria inoculated plots over the un-inoculated control plots. This study further confirms that the selected bacterial isolates not only have the potential for PGP in cereals and legumes but also have the potential for biofortification of mineral nutrients.

Insecticidal activity of a novel fatty acid amide derivative from Streptomyces species against Helicoverpa armigera.

Helicoverpa armigera, an important pest causes serious damage to grain legumes. The main objective of this study was to isolate and identify the metabolite against H. armigera from a previously characterised Streptomyces sp. CAI-155. The culture filtrate of CAI-155 was extracted using Diaion HP-20 and the active fractions were fractionated on Silica and C18 column chromatography. The C18 active fraction was further fractionated on Silica gel 60 F254 thin layer chromatography (TLC). The most active fraction (Rf 0.64) purified from TLC led to the identification of a novel metabolite N-(1-(2,2-dimethyl-5-undecyl-1,3-dioxolan-4-yl)-2-hydroxyethyl)stearamide by spectral studies. The purified metabolite showed 70-78% mortality in 2nd instar H. armigera by diet impregnation assay, detached leaf assay and greenhouse assay. The LD50 and LD90 values of the purified metabolite were 627 and 2276 ppm, respectively. Hence, this novel metabolite can be exploited for pest management in future.