Impact of PGPR inoculation on growth and antioxidant status of wheat under saline conditions

Two plant growth‐promoting rhizobacterial (PGPR) strains, Bacillus subtilis SU47 and Arthrobacter sp. SU18, were found to tolerate 8% NaCl. Wheat co‐inoculated with these two PGPR strains, and grown under different salinity regimes (2–6 dS m^?1), showed an increase in dry biomass, total soluble sugars and proline content. Wheat sodium content was reduced under co‐inoculated conditions but not after single inoculation with either strain or in the control. The activity of antioxidant enzymes in wheat leaves decreased under salinity stress after PGPR co‐inoculation, suggesting these PGPR species could be used for amelioration of stress in wheat plants. Activity of three antioxidant enzymes in wheat grown with both PGPR strains was reduced, most notably that of catalase activity at a salinity of 6 dS m^?1, when compared with the control. The results indicate that co‐inoculation with B. subtilis and Arthrobacter sp. could alleviate the adverse effects of soil salinity on wheat growth.     

Study of mechanisms for plant growth promotion elicited by rhizobacteria in Arabidopsis thaliana

Plant growth-promoting rhizobacteria (PGPR) colonize plant roots and exert beneficial effects on plant health and development. We are investigating the mechanisms by which PGPR elicit plant growth promotion from the viewpoint of signal transduction pathways within plants. We report here our first study to determine if well-characterized PGPR strains, which previously demonstrated growth promotion of various other plants, also enhance plant growth in Arabidopsis thaliana. Eight different PGPR strains, including Bacillus subtilis GB03, B. amyloliquefaciens IN937a, B. pumilus SE-34, B. pumilus T4, B. pasteurii C9, Paenibacillus polymyxa E681, Pseudomonas fluorescens 89B-61, and Serratia marcescens 90-166, were evaluated for elicitation of growth promotion of wild-type and mutant Arabidopsis in vitro and in vivo. In vitro testing on MS medium indicated that all eight PGPR strains increased foliar fresh weight of Arabidopsis at distances of 2, 4, and 6 cm from the site of bacterial inoculation. Among the eight strains, IN937a and GB03 inhibited growth of Arabidopsis plants when the bacteria were inoculated 2 cm from the plants, while they significantly increased plant growth when inoculated 6 cm from the plants, suggesting that a bacterial metabolite that diffused into the agar accounted for growth promotion with this strain. In vivo, eight PGPR strains promoted foliar fresh weight under greenhouse conditions 4 weeks after sowing. To define signal transduction pathways associated with growth promotion elicited by PGPR, various plant-hormone mutants of Arabidopsis were evaluated in vitro and in vivo. Elicitation of growth promotion by PGPR strains in vitro involved signaling of brassinosteroid, IAA, salicylic acid, and gibberellins. In vivo testing indicated that ethylene signaling was involved in growth promotion. Results suggest that elicitation of growth promotion by PGPR in Arabidopsis is associated with several different signal transduction pathways and that such signaling may be different for plants grown in vitro vs. in vivo.     

Effect of salt‐tolerant plant growth‐promoting rhizobacteria on wheat plants and soil health in a saline environment

Salt‐tolerant plant growth‐promoting rhizobacteria (ST‐PGPR) significantly influence the growth and yield of wheat crops in saline soil. Wheat growth improved in pots with inoculation of all nine ST‐PGPR (ECe = 4.3 dS·m^?1; greenhouse experiment), while maximum growth and dry biomass was observed in isolate SU18 Arthrobacter sp.; simultaneously, all ST‐PGPR improved soil health in treated pot soil over controls. In the field experiment, maximum wheat root dry weight and shoot biomass was observed after inoculation with SU44 B. aquimaris, and SU8 B. aquimaris, respectively, after 60 and 90 days. Isolate SU8 B. aquimaris, induced significantly higher proline and total soluble sugar accumulation in wheat, while isolate SU44 B. aquimaris, resulted in higher accumulation of reducing sugars after 60 days. Percentage nitrogen (N), potassium (K) and phosphorus (P) in leaves of wheat increased significantly after inoculation with ST‐PGPR, as compared to un‐inoculated plants. Isolate SU47 B. subtilis showed maximum reduction of sodium (Na) content in wheat leaves of about 23% at both 60 and 90 days after sowing, and produced the best yield of around 17.8% more than the control.     

Effect of applying an arsenic-resistant and plant growth-promoting rhizobacterium to enhance soil arsenic phytoremediation by Populus deltoides LH05-17

Aims: Bioremediation of highly arsenic (As)‐contaminated soil is difficult because As is very toxic for plants and micro‐organisms. The aim of this study was to investigate soil arsenic removal effects using poplar in combination with the inoculation of a plant growth–promoting rhizobacterium (PGPR). Methods and Results: A rhizobacterium D14 was isolated and identified within Agrobacterium radiobacter. This strain was highly resistant to arsenic and produced indole acetic acid and siderophore. Greenhouse pot bioremediation experiments were performed for 5 months using poplar (Populus deltoides LH05‐17) grown on As‐amended soils, inoculated with strain D14. The results showed that P. deltoides was an efficient arsenic accumulator; however, high As concentrations (150 and 300 mg kg^?1) inhibited its growth. With the bacterial inoculation, in the 300 mg kg^?1 As‐amended soils, 54% As in the soil was removed, which was higher than the uninoculated treatments (43%), and As concentrations in roots, stems and leaves were significantly increased by 229, 113 and 291%, respectively. In addition, the As translocation ratio [(stems + leaves)/roots = 0·8] was significantly higher than the uninoculated treatments (0·5). About 45% As was translocated from roots to the above‐ground tissues. The plant height and dry weight of roots, stems and leaves were all enhanced; the contents of chlorophyll and soluble sugar, and the activities of superoxide dismutase and catalase were all increased; and the content of a toxic compound malondialdehyde was decreased. Conclusions: The results indicated that the inoculation of strain D14 could contribute to the increase in the As tolerance of P. deltoides, promotion of the growth, increase in the uptake efficiency and enhancement of As translocation. Significance and Impact of the Study: The use of P. deltoides in combination with the inoculation of strain D14 provides a potential application for efficient soil arsenic bioremediation.     

Seed inoculation with beneficial rhizobacteria affects European corn borer (Lepidoptera: Pyralidae) oviposition on maize plants

Larvae of Ostrinia nubilalis (Hübner) cause significant damage to maize ears and reduce market value of fresh sweet corn. Females rely on volatile cues to locate and oviposit preferentially on maize plants. In addition, oviposition behavior of females is influenced by soil management practices as they usually lay more eggs on maize plants grown on conventional soil than on organic soils that harbor rich microbial diversity. Since some plant growth‐promoting rhizobacteria (PGPR) are known to mediate plant health via suppression of soil pathogens and enhanced uptake of nutrients; we hypothesized that inoculation of maize seeds with PGPR will alter emission of maize volatile and reduce the attractiveness of plants to ovipositing O. nubilalis. Plants treated with the single PGPR strain Bacillus pumilus INR‐7, two PGPR mixtures (Blend‐8 or Blend‐9) or untreated plants were presented to O. nubilalis females in oviposition choice bioassays. Headspace volatile organic compounds (VOCs) from the plants were analyzed by gas chromatography–mass spectrometry (GC–MS). Ostrinia nubilalis laid significantly fewer eggs on PGPR‐treated plants compared to untreated plants. In two‐choice oviposition experiments, significantly higher numbers of eggs were laid on untreated plants compared to PGPR‐treated plants. PGPR‐treated plants emitted fewer VOCs than untreated plants which, in part, explains the relatively fewer eggs on PGPR‐treated plants. These results indicate that selected PGPR treatments can alter maize plant volatiles with important ramifications for plant‐insect interactions. The implication of this finding is discussed in the context of integrated management of soil health to improve crop resistance to biotic stressors.     

Effects of exogenously applied plant growth regulators in combination with PGPR on the physiology and root growth of chickpea (Cicer arietinum ) and their role in drought tolerance

Both the plant growth promoting rhizobacteria (PGPR) and plant growth regulators (PGR) exert beneficial effects on plant growth even under stress, but combined effect of both of them has not been evaluated yet. Present investigation was aimed to determine the responses of 2 chickpea varieties (differing in drought tolerance) to 3 PGPR viz. Bacillus subtilis, Bacillus thuringiensis and Bacillus megaterium and PGR (SA and Putrescine) on physiology of chickpea grown in sandy soil. The PGR, Salicylic acid (SA) and Putrescine (Put) were sprayed on the seedling 20 days after germination. Results revealed, synergistic effects of PGPR and PGR on chlorophyll, protein and sugar contents. Addition of PGR to PGPR inoculated plants assisted the plant in osmoregulation and amelioration of oxidative stresses and in induction of new proteins. Combined application of PGR and PGPR decreased lipid peroxidation more effectively but increased the leaf area. It is inferred that PGPR and PGR work synergistically to promote growth of plants under moisture and nutrient deficit condition of sandy soil. Since, SA induces Systemic Acquired Resistance (SAR) in plants hence the addition of SA along with PGPR may render the plant more productive and better tolerant to diseases/pathogen attack.     

Influence of arbuscular-mycorrhizal fungi, Rhizobium meliloti strains and PGPR inoculation on the growth of Medicago arborea used as model legume for re-vegetation and biological reactivation in a semi-arid mediterranean area

Medicago arborea can be used for re-vegetationpurposes under semiarid conditions. These woody legumes have the ability toforman association with arbuscular mycorrhizal (AM) fungi and rhizobial bacteria,which can be maximised by microorganisms producing certain stimulatingmetabolites acting as plant growth promoting rhizobacteria (PGPR). The effectsof single and combined inoculations using microorganisms with different andinteractive metabolic capacities, namely three Glomusspecies, two Rhizobium meliloti strains (a wild type, WTand its genetically modified derivative GM) and a plant growth promotingrhizobacterium, (PGPR), were evaluated. All three inoculated AM fungi affectedMedicago growth in different ways. Differences weremaintained when soil was co-inoculated with each of the rhizobial strains (WTorGM) and the PGPR. Mycorrhizal fungi were effective in all cases, but the PGPRonly affected plant growth specific microbial situations. PGPR increased growthof G. mosseae-colonised plants associated withRhizobium WT strain by 36% and those infected byG. deserticola when associated with the rhizobial GMstrainby 40%. The most efficient microbial treatments involved mycorrhizalinoculation, which was an indication of the AM dependency of this plantspecies.Moreover, PGPR inoculation was only effective when associated with specificmycorrhizal endophytes (G. mosseae plus WT andG.deserticola plus GM rhizobial strain). The reduced root/shoot (R/S)ratio resulting from PGPR inoculation, was an indication of more effective rootfunction in treated plants. AM colonisation and nodule formation wereunaffectedby the type of AM fungus or bacteria (rhizobial strain and/or PGPR). AM fromnatural soil were less infective and effective than those from the collection.The results supported the existence of selective microbial interactionsaffecting plant performance. The indigenous AM fungi appeared to be ineffectiveand M. arborea behaved as though it was highly dependentonAM colonisation, which implied that it must have a mycorrhizal association toreach maximum growth in the stressed conditions tested. Optimum growth ofmycorrhizal M. arborea plants was associated with specificmicrobial groups, accounting for a 355% increase in growth overnodulatedcontrol plants. The beneficial effect of PGPR in increasing the growth of awoody legume, such as M. arborea under stress, was onlyobserved with co-inoculation of specific AM endophytes. As a result of theinteraction, only shoot biomass was enhanced, but not as a consequence ofenhancing of the colonising abilities of the endophytes. The growthstimulation,occurring as a consequence of selected microbial groups, may be critical anddecisive for the successful establishment of plants under Mediterraneanclimaticand soil conditions.     

Does biopolymers composition in seeds contribute to the flax resistance against the Fusarium infection?

Over the last decades, the cultivation of fibrous flax declined heavily. There are number of reasons for that fact; one of them is flax susceptibility to the pathogen infection. Damages caused mainly by fungi from genus Fusarium lead to the significant losses when cultivating flax, which in turn discourage farmers to grow flax. Therefore, to launch the new products from flax with attractive properties there is a need to obtain new flax varieties with increased resistance to pathogens. In order to obtain the better quality of flax fiber, we previously generated flax with reduced pectin or lignin level (cell wall polymers). The modifications altered also plants' resistance to the Fusarium infection. Undoubtedly, the plant defense system is complex, however, in this article we aimed to investigate the composition of modified flax seeds and to correlate it with the observed changes in the flax resistance to the pathogen attack. In particular, we evaluated the content and composition of carbohydrates (cell wall polymers: pectin, cellulose, hemicelluloses and mucilage), and phenylpropanoid compounds (lignin, lignans, phenolics). From the obtained results we concluded that the observed changes in the vulnerability to pathogens putatively correlate with the antioxidant potential of phenylpropanoids accumulated in seeds, seco‐isolariciresinol and coumaric acid diglycosides in particular, and with pectin level as a carbon source for pathogens. Surprisingly, relatively less important for the resistance was the physical barrier, including lignin and cellulose amount and cellulose structure. Certainly, the hypothesis should be verified on a larger number of genotypes. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 30:992–1004, 2014     

Cytokinin-producing, plant growth-promoting rhizobacteria that confer resistance to drought stress in Platycladus orientalis container seedlings

One of the proposed mechanisms through which plant growth-promoting rhizobacteria (PGPR) enhance plant growth is the production of plant growth regulators, especially cytokinin. However, little information is available regarding cytokinin-producing PGPR inoculation on growth and water stress consistence of forest container seedlings under drought condition. This study determined the effects of Bacillus subtilis on hormone concentration, drought resistance, and plant growth under water-stressed conditions. Although no significant difference was observed under well-watered conditions, leaves of inoculated Platycladus orientalis (oriental thuja) seedlings under drought stress had higher relative water content and leaf water potential compared with those of noninoculated ones. Regardless of water supply levels, the root exudates, namely sugars, amino acids and organic acids, significantly increased because of B. subtilis inoculation. Water stress reduced shoot cytokinins by 39.14 %. However, inoculation decreased this deficit to only 10.22 %. The elevated levels of cytokinins in P. orientalis shoot were associated with higher concentration of abscisic acid (ABA). Stomatal conductance was significantly increased by B. subtilis inoculation in well-watered seedlings. However, the promoting effect of cytokinins on stomatal conductance was hampered, possibly by the combined action of elevated cytokinins and ABA. B. subtilis inoculation increased the shoot dry weight of well-watered and drought seedlings by 34.85 and 19.23 %, as well as the root by 15.445 and 13.99 %, respectively. Consequently, the root/shoot ratio significantly decreased, indicative of the greater benefits of PGPR on shoot growth than root. Thus, inoculation of cytokinin-producing PGPR in container seedlings can alleviate the drought stress and interfere with the suppression of shoot growth, showing a real potential to perform as a drought stress inhibitor in arid environments.     

Effects of inoculation with plant growth promoting rhizobacteria (PGPRs) andSinorhizobium fredii on biological nitrogen fixation, nodulation and growth ofGlycine max cv. Osumi

We investigated the effects of three plant growth promoting rhizobacteria (PGPR), on Biological Nitrogen Fixation (BNF), nodulation and growth promotion by soybean (Glycine max) var. Osumi plants. The strains, Aur 6, Aur 9 and Cell 4, belong toPsedomonas fluorescens, Chryseobacterium balustinum andSerratia fonticola, respectively. Inoculation modes for the PGPRs andSinorhizobium fredii (carried out through irrigation), were examined. In the first mode, PGPRs andS. fredii were co-inoculated. In the second mode, we first inoculatedS. fredii and after the PGPRs, which were added 5 or 10 days later (each inoculation being an independent treatment). In the third mode, the PGPRs were inoculated first, and theS. fredii was inoculated 5 days later. We also included treatments inoculated with only the PGPRs (one PGPR per treatment) and only withS. fredii. Plants were maintained in a greenhouse under controlled environmental conditions, and were sampled 3 months after sowing. The results obtained showed the effects of the inoculation sequence. The most significant effects on growth parameters (stem plus leaf weight and fresh root weight) were found when inoculations with PGPR andS. fredii were at different times or when we inoculated only with PGPR and the plants were watered with nitrogen. Co-inoculation had no positive effects on any parameter, probably due to competition between the PGPR andS. fredii. Our results indicate that the inoculation modes with PGPR and rhizobia play a very important role in the effects produced. Thus, although plant growth promoting rhizobacteria may interact synergistically with root-nodulating rhizobia, plant growth promoting rhizobacteria selected for one crop should be assessed for potentially hazardous effects on other crops before being used as inoculants.     

Effect of inoculation method and plant growth medium on endophytic colonization of sorghum by the entomopathogenic fungus <Emphasis Type="Italic">Beauveria bassiana</Emphasis>

A study was conducted to determine the effect of inoculation method and plant growth medium on colonization of sorghum by an endophytic Beauveria bassiana. Colonization of leaves, stems, and roots by B. bassiana was assessed 20-days after application of the fungus. Although B. bassiana established as an endophyte in sorghum leaves, stems, and roots regardless of inoculation method (leaf, seed, or soil inoculation), plant growth medium (sterile soil, non-sterile soil, or vermiculite) apparently influenced colonization rates. Seed inoculation with conidia caused no stem or leaf colonization by the fungus in non-sterile soil but did result in substantial endophytic colonization in vermiculite and sterile soil. Leaf inoculation did not result in root colonization, regardless of plant growth medium. Endophytic colonization was greater in leaves and stems than roots. Endophytic colonization by B. bassiana had no adverse effects on the growth of sorghum plants. Leaf inoculation with a conidial suspension proved to be the best method to introduce B. bassiana into sorghum leaves for plants growing in either sterile or non-sterile soil. Further research should focus on the virulence of endophytic B. bassiana against sorghum stem borers.     

Elicitation of peroxidase activity and lignin biosynthesis in pepper suspension cells by Phytophthora capsici

Cell suspension cultures of three varieties of Capsicum annuum L., each with a different degree of sensitivity to the fungus Phytophthora capsici, responded to elicitation by both lyophilized mycelium and fungus filtrate with a hypersensitive reaction. They showed the synthesis or accumulation of PR-proteins with peroxidase (EC 1.11.1.7) activity and the accumulation of lignin-like polymer (as measured by derivatization with thioglycolic acid). The cultivation medium was optimised for both plant and fungus growth in order to avoid any stress during their combination. The resistant pepper variety, Smith-5, showed a more intense response to the elicitor preparations than the sensitive varieties, Americano and Yolo Wonder. This was particularly evident when the cell suspensions were elicited with the filtrate. After elicitation, the cell walls thickened through the accumulation of lignin, as can be observed by staining microscope preparations with methylene blue. Elicitation also reduced the level of total peroxidase activity in the susceptible varieties, while such activity increased in resistant varieties, and was accompanied by de novo expression of acidic peroxidase isoenzymes in the extracellular and cell wall fractions. Of note was the PR protein of pI 5.7 showing peroxidase activity, which was induced by both elicitor types in the elicited cell suspensions of the resistant variety alone, making it a marker of resistance. The increases in the activity of these peroxidases in the resistant variety are in concordance with the accumulation of lignin observed 24 h after inoculation by both elicitors from the fungus. The possible role of these isoenzymes in lignin biosynthesis, used to reinforce the cell walls against fungal penetration of the cells, is discussed. These results are in accordance with those previously observed in plant stem sections.     

Effect of plant growth promoting rhizobacteria on bacterial canker of tomato

Use of plant growth promoting rhizobacteria in managing bacterial canker disease of tomato was studied in the present work. Tomato seeds were treated with PGPR strains viz., Bacillus pumilus INR7, Bacillus pumilus SE34, Bacillus pumilus T4, Bacillus subtilis GBO3, Bacillus amyloliquefaciens IN937a and Brevibacillus brevis IPC11 were subjected for seed germination and seedling vigor. Among the PGPR strains tested, only three strains (IN937a, GBO3 and IPC11) which showed enhancement in the seed quality parameters like seed germination and seedling vigor, were further subjected for estimation of one of the defence-related enzymes, Phenylalanine Ammonia Lyase (PAL) with total phenol contents. The same three strains were recorded for maximum disease protection under greenhouse conditions. The level of PAL and total phenol contents increased significantly upon the PGPR treatment. The rate of reduction in the bacterial canker disease incidence was directly proportional to the amount of increased level of PAL and total phenol content. The possible uses of these PGPR strains in effective management of bacterial canker of tomato were discussed in the present work.     

Differential Effects of Pseudomonas mendocina and Glomus intraradices on Lettuce Plants Physiological Response and Aquaporin PIP2 Gene Expression Under Elevated Atmospheric CO2 and Drought

Arbuscular mycorrhizal (AM) symbiosis and plant-growth-promoting rhizobacterium (PGPR) can alleviate the effects of water stress in plants, but it is unknown whether these benefits can be maintained at elevated CO₂. Therefore, we carried out a study where seedlings of Lactuca sativa were inoculated with the AM fungus (AMF) Glomus intraradices N.C. Schenk & G.S. Sm. or the PGPR Pseudomonas mendocina Palleroni and subjected to two levels of watering and two levels of atmospheric CO₂ to ascertain their effects on plant physiological parameters and gene expression of one PIP aquaporin in roots. The inoculation with PGPR produced the greatest growth in lettuce plants under all assayed treatments as well as the highest foliar potassium concentration and leaf relative water content under elevated [CO₂] and drought. However, under such conditions, the PIP2 gene expression remained almost unchanged. G. intraradices increased significantly the AMF colonization, foliar phosphorus concentration and leaf relative water content in plants grown under drought and elevated [CO₂]. Under drought and elevated [CO₂], the plants inoculated with G. intraradices showed enhanced expression of the PIP2 gene as compared to P. mendocina or control plants. Our results suggest that both microbial inoculation treatments could help to alleviate drought at elevated [CO₂]. However, the PIP2 gene expression was increased only by the AMF but not by the PGPR under these conditions.     

Phytochemical Variations and Enhanced Efficiency of Antioxidant and Antimicrobial Ingredients in Salvia officinalis as Inoculated with Different Rhizobacteria

Plants produce a variety of secondary metabolites to improve their performance upon exposure to pathogens, pests, herbivores, or environmental stresses. Secondary metabolism in plants is, therefore, highly regulated by presence of biotic or abiotic elicitors in the environment. The present research was undertaken to characterize plant growth‐promoting attributes of four plant growth‐promoting rhizobacteria (PGPR) including two Pseudomonas fluorescens (Pf Ap1, Pf Ap18) and two P. putida (Pp Ap9, Pp Ap14) strains, and to determine their role (individually or in consortium) on growth of Salvia officialis, and biosynthesis of secondary metabolites such as essential oils (EOs), total phenolics, and flavonoids. The antioxidant and antibacterial properties of the extracts and EOs obtained from the inoculated plants were also investigated. The PGPR inoculum was applied to soil, cuttings, and foliage. Results indicated that different PGPR strains varied in their efficiency for production of auxin, siderophore, 1‐aminocyclopropane‐1‐carboxylate deaminase, and phosphate solubilization. All individually inoculated plants had significantly higher shoot and root biomass, leaf P content, EOs yield, total phenolics, and flavonoids content compared to uninoculated control plants. The major constituents of EOs, cis‐thujene, camphor, and 1,8‐cineol, increased following inoculation with reference PGPRs. Although the extract from all inoculated plants had improved antioxidant activity, it was remarkable for the Pf Ap18 strain, which had the lowest IC₅₀ value across treatments. Antibacterial assay of various EOs and their major constituents against pathogenic bacteria showed that the highest activity was observed against Staphylococcus aureus using EOs of Pp Ap14 source. Based on our findings, we suggest that individual inoculation with effective PGPR strains can substantially improve plant growth and secondary metabolism in S. officinalis plants.