Plant Growth Promoting Rhizobacteria

Plant growth promoting rhizobacteria (PGPR) are soil bacteria with some beneficial effects on soil properties, plant growth and the environment. In this article, some of the most important advancements in the field of PGPR and their related properties are presented. Such knowledge can be important for understanding regarding the use of PGPR for different uses such as biological fertilization and alleviation of different stresses on plant growth and the environment.     

Impact of single and co-inoculations with Rhizobial and PGPR isolates on chickpea (Cicer arietinum) in cereal-growing zone soil

Strains isolated from chickpea (Cicer arietinum L.) rhizospheric soil from selected sites in Algeria were screened for their plant-growth-promoting potential, for indole acetic acid production and P solubilization ability. Then, we selected native rhizobial strains with high nitrogen-fixing potential. On the basis of their efficiency under controlled conditions, two plant-growth-promoting rhizobacteria (PGPR) isolates and three nodulating bacteria were selected. Then, the effect of single PGPR isolates inoculation was compared to their combination with rhizobial inoculants on plant growth, on native cereal-growing soils under greenhouse conditions. No effects were observed on chickpea yield by using rhizobial inoculation alone, nor by PGPR-rhizobial co-inoculation on two soils presenting weak and no nodulation pattern in natural conditions. Only PGPR inoculation improved growth of plants on soil with no nodulation pattern. These findings emphasized inoculation on native soils at a little scale before large assays on field because no one could predict inocula behavior with native soil microflora.     

Phosphate-solubilizing rhizobacteria enhance the growth and yield but not phosphorus uptake of canola (Brassica napus L.)

The ability of phosphate-solubilizing rhizobacteria to enhance the growth and phosphorus uptake of canola (Brassica napus L., cv. Legend) was studied in potted soil experiments in the growth chamber. One hundred and eleven bacteria isolated from the rhizosphere of field-grown plants, and a collection of nine bacteria known to be effective plant growth-promoting rhizobacteria (PGPR), were screened for P-solubilization in vitro. All rhizobacteria were identified using whole-cell fatty acids methyl ester (FAME) profiles. The best P-solubilizing isolates were two Bacillus brevis strains, B. megaterium, B. polymyxa, B. sphaericus, B. thuringiensis, and Xanthomonas maltophilia (PGPR strain R85). The P-solubilizers were tested for their effects on growth and P-uptake of canola plants in a P-deficient soil amended with rock phosphate. Although some of the P-solubilizing rhizobacteria significantly increased plant height or pod yield, none increased P-uptake. The most effective inoculant was a B. thuringiensis isolate which significantly increased the number and weight of pods and seed yield without rock phosphate. Xanthomonas maltophilia increased plant height, whereas the other bacilli increased the number on weight of pods. These results demonstrate the potential use of these P-solubilizing rhizobacteria as inoculants for canola, but indicate that P-solubilization was not the main mechanism responsible for positive growth response. Received: 8 February 1996     

Nitrogen‐enriched compost application combined with plant growth‐promoting rhizobacteria (PGPR) improves seed quality and nutrient use efficiency of sunflower

Ecological benefits associated with plant growth‐promoting rhizobacteria (PGPR) inoculants offer a promising integrated nutrient management option to counteract plant nitrogen (N) deficiency. We performed field experiments to evaluate the effect of integrated N fertilizer regime involving chemical N fertilizer (CNF) and N‐enriched compost (NEC), either alone or combined with selected PGPR (Pseudomonas aeruginosa ) on sunflower seed quality, N use efficiency (NUE) and soil fertility during 2014–2015. We found that integrated N biofertilizer application resulted in significantly higher seed oil concentration, fatty acid composition, and harvest index in both cropping years. Greater effects on N yield efficiency (NYE), N use efficiency (NUE), N physiological efficiency (NPE), and photosynthetic N use efficiency (PNUE) were recorded in nitrogen‐enriched compost+PGPR inoculant (NECPI) treatment followed by chemical N fertilizer+PGPR inoculant (CNFPI) treatment. Statistically significant differences were observed in linoleic and linolenic acid, NYE, and NUE for treatment × year interaction, thus, suggesting that the integrated N biofertilizer approach facilitates the efficient N use by sunflower for improving yield and seed quality. Moreover, we also found considerable enhancement of soil N fertility after two consecutive cropping years of sunflower. The enhancement of seed quality, N use efficiencies, and soil N fertility through integrated N biofertilizer application emphasizes the importance of balanced crop N nutrition, ensuring sufficient N supply to sunflower with adequate N balance in soil for the next crop. Overall, combination of PGPR with NEC amendment may optimize N uptake efficiency and reduce N fertilizer losses, which is necessarily required for the sustainable sunflower production.     

Crop improvement and root rot suppression by seed bacterization in chickpea

Abstract

Field experiments were conducted at the Regional Research Station, CCS Haryana Agricultural University, Bawal, India, to evaluate the contribution of different bioinoculants in terms of nodule number, nodule biomass, root rot incidence and seed yield in chickpea. Nodule number and biomass were positively affected by the application of bioinoculants. Plant growth promoting rhizobacteria (PGPR) alone or in combination with bioinoculants reduced plant mortality and increased seed yield of the crop. Seed yield at 50% fertilizer dose (RF) plus all the three inoculants was at a maximum during all the three years of experimentation.     

根际促生菌及其在污染土壤植物修复中的应用

植物对重金属吸收、转运和积累以及植物生物学特征使其成为修复重金属污染土壤的重要手段之一。然而,由于植物对重金属的耐受性有限而限制其广泛实际应用,因而探讨植物修复技术强化措施就显得尤为重要。随着自然资源的开发和技术的发展,微生物调控使植物修复技术变得更为可行和更有价值。回顾近年来新兴的微生物调控技术,植物根际促生菌资源因其对环境无污染,可利用自身的抗性系统减缓重金属对植物的毒性,促进植物的生长和影响重金属的迁移等优势,在修复过程中发挥着重要作用。目前,国内外就植物根际促生菌的筛选、鉴定和应用价值等方面已经做了大量的相关研究。本文综述了根际促生菌-植物相互作用的机制及其促进植物修复重金属污染土壤的作用原理。     

Plant growth-promoting rhizobacteria-alleviators of abiotic stresses in soil: A review

With the continuous increase in human population,there is widespread usage of chemical fertilizers that are responsible for introducing abiotic stresses in agricultural crop lands.Abiotic stresses are major constraints for crop yield and global food security and therefore require an immediate response.The implementation of plant growth-promoting rhizobacteria(PGPR)into the agricultural production system can be a profitable alternative because of its efficiency in plant growth regulation and abiotic stress management.These bacteria have the potential to promote plant growth and to aid in the management of plant diseases and abiotic stresses in the soil through production of bacterial phytohormones and associated metabolites as well as through significant root morphological changes.These changes result in improved plant-water relations and nutritional status in plants and stimulate plants’defensive mechanisms to overcome unfavorable environmental conditions.Here,we describe the significance of plant-microbe interactions,highlighting the role of PGPR,bacterial phytohormones,and bacterial metabolites in relieving abiotic environmental stress in soil.Further research is necessary to gather in-depth knowledge on PGPR-associated mechanisms and plant-microbe interactions in order to pave a way for field-scale application of beneficial rhizobacteria,with the aim of building a healthy and sustainable agricultural system.Therefore,this review aims to emphasize the role of PGPR in growth promotion and management of abiotic soil stress with the goal of developing an eco-friendly and cost-effective strategy for future agricultural sustainability.     

Plant growth-promoting rhizobacteria: strategies to improve abiotic stresses under sustainable agriculture

The abiotic stresses like drought, heavy metal and salts directly or indirectly influence the global environmental pollution and decrease the agricultural productivity. The stress tolerant plant growth promoting rhizobacteria (PGPR) play an important role against the abiotic stresses in terms of enhancing the efficacy of soil, plant growth promotion (PGP). Stress tolerance PGPRs have certain specific PGP properties such as hormones synthesis, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, Indole-3-acetic acid (IAA) production, Abscisic acid (ABA) synthesis, enzymes production, nitrogen fixation, phosphorus (P) and Potassium (K) solubilization, as results which protect various crops during such stress conditions and consequently enhance crop sustainability. Efficient PGPRs isolated from various stress conditions have certainly, more useful against that particular stress. This article highlighted the isolation of various stresses tolerant PGPRs from varieties of crops under different stress conditions and their effect on varieties of crops to enhance their plant growth promotion.     

Encapsulation of plant growth promoting Rhizobacteria—prospects and potential in agricultural sector: a review

Plant growth promoting rhizobacteria (PGPR) are a group of bacteria that can enhance plant growth. In fact, PGPR are biologically unstable and the bacteria activity degrades over time due to environmental factors, survival rate in soils, the compatibility with the crop and the interaction ability with the indigenous microflora in soil. Therefore, the utilization of PGPR as plant growth promoter agent is a major challenge in the agricultural sectors because of their bioactivity degradation needs to be inhibited to maximize its function as a plant growth promoter. The application of delivery system based on encapsulation technology shows a promising technique to store and deliver PGPR. However, the task to find the appropriate PGPR encapsulation method is the most challenging for agricultural industry. In addition, the lack of knowledge on the action mechanism of encapsulated PGPR, physico-chemical properties and their survival in the environment are the many challenges need to be addressed. In the present review, the encapsulation technology of PGPR and its properties have been reviewed in detail. Moreover, the remaining technical challenges of encapsulation systems including insignificant stabilization of PGPR, instability of the environmental and difficulty of their preparation are also extensively discussed here.     

Screening of Multi-Trait Rhizobacteria for Improving the Growth,Enzyme Activities,and Nutrient Uptake of Tea (Camellia sinensis)

The use of plant growth-promoting rhizobacteria (PGPR) as agricultural inputs for increasing crop production needs the selection of efficient bacteria with plant growth-promoting (PGP) attributes. Therefore, the purpose of this study was to evaluate the effects of 20 multi-traits bacteria on tea growth, nutrient uptake, chlorophyll contents, and enzyme activities under field conditions for over 3 years. These isolates were screened in vitro for their PGP traits such as the production of indole acetic acid (IAA), nitrogenase activity, phosphorus (P) solubilization, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. Screening of rhizobacteria that show multiple PGP traits suggests that they stimulated overall plant growth, including shoot development and leaf yield, improving macro- and micro-nutrient uptake, chlorophyll contents, and activities of enzymes of tea plant. Use of strains with multiple PGP traits could be a more effective approach and have great potential for the environmentally-friendly tea production.     

PGPRs of plum (Prunus domestica) rhizosphere enhance plant growth and antagonise fungal activity in vitro

Plant growth promoting rhizobacteria (PGPR) promote the plant growth by various direct and indirect mechanisms. The present study was undertaken to isolate and characterise the PGPRs of plum (Prunus domestica) rhizosphere in Pakistan. A total of 95 rhizobacteria were isolated, out of which 40 strains were selected on the basis of morphological, biochemical and Gram staining characteristics. The selected isolates were screened for in vitro plant growth promoting potential and were subsequently evaluated for host plant growth promotion. The selected isolates demonstrated strong lytic enzymatic activities and were able to produce ammonia, siderophore, Hydrogen cyanide along with capability of phosphate solubilisation. Moreover, the results showed a significant growth suppression of pathogenic Fusarium oxysporum and Rhizoctonia solani in an in vitro assay. The plant microbe interaction study was carried out using 11 most efficient rhizobacterial strains inoculated to roots of plum plants. The inoculated PGPRs significantly augmented the leaves number per shoot, shoot diameter, shoot length and plant height. The inoculation also significantly increased the chlorophyll contents of leaves, concentration of micro and macro nutrients compared with control. The current study shows the importance of these selected PGPRs as bio-fertilizer to improve the health and productivity of plum species in Pakistan.     

Effects of mineral and biofertilizers on barley growth on compacted soil

Abstract

Biofertilizers are an alternative to mineral fertilizers for increasing soil productivity and plant growth in sustainable agriculture. The objective of this study was to evaluate possible effects of three mineral fertilizers and four plant growth promoting rhizobacteria (PGPR) strains as biofertilizer on soil properties and seedling growth of barley (Hordeum vulgare) at three different soil bulk densities, and in three harvest periods. The application treatments included the control (without bacteria inoculation and mineral fertilizers), mineral fertilizers (N, NP and P) and plant growth promoting rhizobacteria species (Bacillus licheniformis RC04, Paenibacillus polymyxa RC05, Pseudomonas putida RC06, and Bacillus OSU-142) in sterilized soil. The PGPR, fungi, seedling growth, soil pH, organic matter content, available P and mineral nitrogen were determined in soil compacted artificially to three bulk density levels (1.1, 1.25 and 1.40 Mg m^?3) at 15, 30, and 45 days of plant harvest. The results showed that all the inoculated bacteria contributed to the amount of mineral nitrogen. Seed inoculation significantly increased the count of bacteria and fungi. Data suggest that seed inoculation of barley with PGPR strains tested increased root weight by 9–12.2%, and shoot weight by 29.7–43.3% compared with control. The N, NP and P application, however, increased root weight up to 18.2, 25.0 and 7.4% and shoot weight by 31.6, 43.4 and 26.4%, respectively. Our data show that PGPR stimulate barley growth and could be used as an alternative to chemical fertilizer. Soil compaction hampers the beneficial plant growth promoting properties of PGPR and should be avoided.     

MOBILIZATION OF POTASSIUM FROM WASTE MICA BY PLANT GROWTH PROMOTING RHIZOBACTERIA AND ITS ASSIMILATION BY MAIZE (ZEA MAYS) AND WHEAT (TRITICUM AESTIVUM L.): A HYDROPONICS STUDY UNDER PHYTOTRON GROWTH CHAMBER

A hydroponics study was carried out to evaluate the effect of three plant growth promoting rhizobacteria (PGPR) namely, Bacillus mucilaginosus, Azotobacter chroococcum, and Rhizobium spp. on their ability to mobilize potassium from waste mica using maize and wheat as the test crops under a phytotron growth chamber. Results revealed that PGPR significantly improved the assimilation of potassium by both maize and wheat, where waste mica was the sole source of potassium. This was translated into higher biomass accumulation, potassium content and uptake by plants as well as chlorophyll and crude protein content in plant tissue. Among the rhizobacteria, Bacillus mucilaginosus resulted in significantly higher mobilization of potassium than Azotobacter chroococcum and Rhizobium inoculation. Overall, inoculation of maize and wheat plants with these bacteria could be used to mobilize potassium from waste mica, which in turn could be used as a source of potassium for plant growth.     

Evidence for enhanced N availability during switchgrass establishment and seeding year production following inoculation with rhizosphere endophytes

Improvement in sustainable production of switchgrass (SG, Panicum virgatum L), as a purpose-grown biomass feedstock crop, could be realized through investigation of plant–microbe interactions associated with plant growth promoting rhizobacteria (PGPR), capable of biological nitrogen fixation (BNF). The objective of this study is to increase establishment year production of SG biofuels by inoculation with a mixed PGPR inoculum. We isolated pure strains of N₂-fixing, and other PGPR, from SG rhizomes. The bacteria were identified as Paenibacillus polymyxa, an N₂-fixing bacterium, and other PGPR capable of solubilizing phosphate and/or producing auxins. Field trials utilizing these strains in a mixed PGPR inoculum showed that inoculated plants contained more N in tillers during anthesis but not at senescence, suggesting that more N could be cycled to belowground roots and rhizomes for winter storage. The amount of N removal in biomass and recovery of fertilizer N were also greater for inoculated than uninoculated plants. PGPR inoculation also resulted in positive N balances, suggesting improved access to N from non-fertilizer N sources, possibly through BNF and improved soil N uptake. Overall, inoculation of SG with PGPR enhanced N acquisition and could be an effective strategy to increase the establishment year production of this crop.     

Plant growth-promoting rhizobacteria alter rooting patterns and arbuscular mycorrhizal fungi colonization of field-grown spring wheat

The impact of plant growth-promoting rhizobacteria (PGPR) inoculants on the growth, yield and interactions of spring wheat with arbuscular mycorrhizal fungi (AMF) was assessed in field studies. The pseudomonad inoculants P. cepacia R55, R85, P. aeruginosa R80, P. fluorescens R92 and P. putida R104, which enhance growth and yield of winter wheat, were applied at a rate of ca. 10⁷–10⁸ cfu seed^-1 and plots established on pea stubble or summer fallow at two different sites in Saskatchewan. Plant shoot and root biomass, yield and AMF colonization were determined at four intervals. Plant growth responses were variable and dependent on the inoculant strain, harvest date and growth parameter evaluated. Significant increases or decreases were measured at different intervals but these were usually transient and final seed yield was not significantly affected. Harvest index was consistently increased by all pseudomonad inoculants; responses to strain R55 and R104 were significant. Root biomass to 60 cm depth was not significantly affected by inoculants except strain R104, which significantly reduced root dry weight. However, root distribution, root length and AMF colonization of roots within the soil profile to 60 cm were significantly altered by inoculants. Most of these responses were reductions in the assessed parameter and occurred at depths below 15 cm; however, strains R85 and R92 significantly increased root dry weight in the 0- to 15-cm zone. These results indicate that some PGPR inoculants may adversely affect mutualistic associations between plants and indigenous soil microorganisms, and suggest a possible reason as to why spring wheat growth was not consistently enhanced by these pseudomonad PGPR.     

Root colonization and growth promotion of wheat and maize by Pseudomonas aurantiaca SR1

Wheat (Triticum aestivum L.), rice (Oryza sativa) and maize (Zea mays L.) are the most important cereals worldwide. However, in the last few years, soil has been submitted to both use and handling pressures due to the increase in agricultural practices, which are leading to its degradation. The use of plant growth-promoting rhizobacteria (PGPR) as inoculants constitutes a biological alternative for sustainable production. Pseudomonas aurantiaca SR1 was formulated as an inoculant in order to evaluate its growth promotion effect in the field when applied on maize and wheat seeds at the sowing time. P and N fertilization treatments were also included in the assays. P. aurantiaca SR1 colonized the root system of both crops and it persisted at appropriate population densities. It also showed a significant plant growth-promoting effect that was reflected in the yield. Another relevant finding was that both crops, when inoculated with P. aurantiaca SR1, presented higher yields with fertilization doses lower than those conventionally applied. This indicated its potential use as a reasonable alternative for crop production, with a minimization of the ecological impact.     

含PGPR菌株LZ-8生物育苗基质的研制与促生效应研究

根际促生细菌(PGPR)与普通育苗基质联合形成生物育苗基质,能够有效促进PGPR菌株的根际定殖,从而增强菌株促生效果的发挥。本研究以辣椒和番茄两种经济作物为供试材料,采用拌土的方式向基质中添加PGPR菌株LZ-8发酵液形成生物育苗基质,研究了该生物基质对这两种作物苗期的促生效果及种苗移栽后大田产量的增加情况。结果表明:含PGPR菌株的生物育苗基质对辣椒和番茄苗期均具有显著的促生效果,其中苗期辣椒和番茄的株高、茎粗、叶面积、鲜重和干重分别比对照高出22%、15.9%、33.6%、21.84%、31.25%和26.8%、29.4%、62%、72.7%、83.3%;生物基质所育种苗移栽至大田后显著增加了辣椒和番茄的产量,分别比对照增产22%和11%。     

Influence of Plant Growth-Promoting Rhizobacteria on Corn Growth Under Different Fertility Sources

A greenhouse study was conducted to evaluate the effects of plant growth-promoting rhizobacteria (PGPR) on root establishment and biomass production of corn (Zea mays L.) using three fertility sources (poultry litter (PL), biosolids, and urea). Applying PL significantly improved root morphological parameters and increased plant biomass at the V4, V6, and VT growth stages when compared to the other fertility sources. At the V4 stage, PGPR stimulated root growth and enhanced aboveground biomass with urea and PL, while no differences were observed with biosolids. At the V6 stage, PL, biosolids, and urea with PGPR significantly increased some growth parameters (e.g., plant height, leaf area, and root morphology). However, at the VT stage, PGPR’s influence on plant growth was minimal regardless of fertility source. Applying the fertility sources at 135 kg N ha^?1 may have masked PGPR’s influence on corn growth as the plants reached their later vegetative growth stages.     

Enhancement in Nutrient Accumulation and Growth of Oil Palm Seedlings Caused by PGPR Under Field Nursery Conditions

Abstract

Plant growth promoting rhizobacteria (PGPR) (e.g., Azospirillum and Bacillus spp.) have been reported to enhance growth and fix N₂ with several nonleguminous crops. These rhizobacteria have the potential to be applied to oil palm seedlings and, consequently, reduce the cost of nitrogenous fertilizer. The rhizobacteria are also known as a bioenhancer for the ability to increase root growth and enhanced water and nutrient absorption by the host plants. An experiment was carried out in the field nursery station, Federal Land Development Authorities (FELDA), Bukit Mendi, Pahang, Malaysia, to observe the effects of PGPR inoculation on enhanced nutrient accumulation and plant growth (tops and roots) of oil palm seedlings under field nursery conditions. The inoculation process showed positive response in enhancing higher accumulation of nitrogen (N), phosphorus (P), and potassium (K) in the plant tissues, enhanced root dry weight and top growth (dry matter and leaf chlorophyll content) of the host plants under field nursery conditions.     

Siderophore-Producing Rhizobacteria as a Promising Tool for Empowering Plants to Cope with Iron Limitation in Saline Soils: A Review

Iron (Fe) bioavailability to plants is reduced in saline soils; however, the exact mechanisms underlying this effect are not yet completely understood. Siderophore-expressing rhizobacteria may represent a promising alternative to chemical fertilizers by simultaneously tackling salt-stress effects and Fe limitation in saline soils. In addition to draught, plants growing in arid soils face two other major challenges:high salinity and Fe deficiency. Salinity attenuates growth, affects plant physiology, and causes nutrient imbalance, which is, in fact, one of the major consequences of saline stress. Iron is a micronutrient essential for plant development, and it is required by several metalloenzymes involved in photosynthesis and respiration. Iron deficiency is associated with chlorosis and low crop productivity. The role of microbial siderophores in Fe supply to plants and the effect of plant growth-promoting rhizobacteria (PGPR) on the mitigation of saline stress in crop culture are well documented. However, the dual effect of siderophore-producing PGPR, both on salt stress and Fe limitation, is still poorly explored. This review provides a critical overview of the combined effects of Fe limitation and soil salinization as challenges to modern agriculture and intends to summarize some indirect evidence that argues in favour of siderophore-producing PGPR as biofertilization agents in salinized soils. Recent developments and future perspectives on the use of PGPR are discussed as clues to sustainable agricultural practices in the context of present and future climate change scenarios.