Plant Growth Promoting Rhizobacteria Inoculation Research Articles

Transcriptomic analysis reveals the importance of JA-Ile turnover in the response of Arabidopsis plants to plant growth promoting rhizobacteria and salinity

The use of plant growth promoting rhizobacteria (PGPR) is a proven management strategy to improve plant growth. The aim was to reveal the genomic and proteomic basis of the plant tolerance to saline soil conditions. Combination of whole transcriptome analysis and proteomic profiling helped further the understanding of the complexity of salt tolerance responses. Arabidopsis plants were grown inoculated or not with Bacillus megaterium and irrigated or not with salt. Physiological, genomic and proteomic approaches were combined to reveal plant salt tolerance mechanisms. Microarray analyses revealed the up-regulation of the jasmonic acid metabolism (CYP94B3, lipooxigenase 4 and allene-oxide cyclase) under saline conditions. Knock-out mutants of the gene of interest CYP94B3, responsible of JA-Ile catabolism, were used to confirm the obtained results. Salinity resulted in leaf Na accumulation with decreased chlorophyll content, but PGPR inoculation helped to overcome the stress. Proteomic analysis showed enhanced monodehydroascorbate reductase (MDHAR) content together with ATP synthase. CYP94B3 knock-out plants confirmed the key role of JA-Ile turnover to overcome moderate saline stress. Subsequent experimentation showed that CYP94B3 was important for salt tolerance under moderate and severe salt stress. Inoculation with B. megaterium was a valuable tool to reveal the importance of JA-Ile turnover and to recover Arabidopsis plants from saline stress. A single gene can modify the response of plants to salt stress.

Effects of Plant Growth-Promoting Rhizobacteria and N Source on Plant Growth and N and P Uptake by Tomato Grown on Calcareous Soils

Introducing specific microorganisms into the soil ecological system is an important strategy for improving nutrient use efficiency. Two pot experiments were conducted in the greenhouse from December 3, 2012 to January 25, 2013 (Experiment 1) and March 11 to April 23, 2013 (Experiment 2) to evaluate the effect of nitrogen (N) source and inoculation with plant growth-promoting rhizobacteria (PGPR) on plant growth and N and phosphorus (P) uptake in tomato (Lycopersicon esculentum Mill.) grown on calcareous soils from South Florida, USA. Treatments included urea, controlled release urea (a controlled release fertilizer, CRF) each at low and high N rates and with or without inoculation of PGPR. A mixture of PGPR strains Bacillus amyloliquefaciens IN937a and Bacillus pumilus T4 was applied to the soil during growing periods of tomato. Treatments with PGPR inoculation increased plant height compared to treatments without PGPR in both experiments. Inoculation with PGPR increased shoot dry weight and shoot N uptake for the same N rate and N source. In both experiments, only at high N rate, CRF and urea treatments with PGPR had significantly (P < 0.05) greater shoot biomass than those without PGPR. Only at high N rate, CRF treatment with PGPR significantly increased shoot N uptake by 39.0% and 10.3% compared to that without PGPR in Experiments 1 and 2, respectively. Meanwhile, presence of PGPR in the soil increased shoot P uptake for all treatments in Experiment 1 and for most treatments in Experiment 2. In Experiment 1, only at low N rate, CRF treatment with PGPR significantly increased shoot P uptake compared with that without PGPR. In Experiment 2, a significant increase in shoot P uptake by inoculation of PGPR was only observed in CRF treatment at high N rate. Results from this study indicate that inoculation with PGPR may increase plant growth and N and P uptake by tomato grown on calcareous soils. However, the effect of PGPR varied and was influenced by many factors such as N source, N rate, and soil fertility. Further investigations are warranted to confirm the effect of PGPR under different soil conditions.

Increased root growth and nitrogen accumulation in common wheat following PGPR inoculation: Assessment of plant-microbe interactions by ESEM

Abstract The use of plant growth-promoting rhizobacteria (PGPR) meets the current need to reduce nitrogen input in order to attain greater sustainability in the production of crops, particularly cereals. This study investigated whether a commercial bio-fertiliser containing a consortium of PGPR and N-fixing bacteria (Azospirillum spp., Azoarcus spp. and Azorhizobium spp.) affects shoot and root growth, N accumulation and grain yield in common wheat (Triticum aestivum L.). Trials were conducted in a fertile, silty loam soil, firstly in rhizoboxes, by applying bacteria either as a seed-coating inoculum or by foliar + soil spraying, and then in the field by spraying the canopy at the tillering stage with decreasing levels of N fertilisation (160, 120 and 80 kg ha−1) in two consecutive years. Culm height, leaf chlorophyll content, nitrogen accumulation and yield were recorded above ground, while below ground Root Length Density (RLD) patterns were investigated by soil coring and image analysis at the flowering stage. Environmental Scanning Electron Microscope (ESEM) imaging revealed an excellent ability of bacteria to adhere to the surface of intact leaves and roots, and to colonise both leaf mesophyll and root vascular tissues in aseptic conditions. Bacteria increased the number of root tips and ramifications (+65% vs. non-inoculated controls) in sterilised rhizobox soil, regardless of the method of application, and the volumetric root length density in the open field with medium (+29%) and high (+11%) N supply, resulting in greater N accumulation (about +25 kg ha−1). Although the N dose had clear positive effects, no significant variations in grain yield (only + 1–3% vs. non-inoculated controls) or other agronomic parameters could be ascribed to bacteria inoculation. The conclusion drawn is that the use of a combination of PGPR and N-fixing bacteria offers an opportunity to improve root growth in wheat and increase plant resilience to environmental stresses, and helps to reduce N losses from agricultural ecosystems thereby offering partial fertiliser savings within crop rotations.

Silicon Application and Rhizobacterial Inoculation Regulate Mung Bean Response to Saline Water Irrigation

The present investigation examined the effects of silicon application and inoculation with the plant growth‐promoting rhizobacteria (PGPR) Enterobacter cloacae and Bacillus drentensis on alleviating ion toxicity, regulating osmolytes accumulation, and productivity of field‐grown mung bean under saline water irrigation. Inoculated seeds were grown in sandy loam soils (electrical conductivity: 2.78 dS m−1) and irrigated with water with three salinity levels (3.12, 5.46, and 7.81 dS m−1) using a drip irrigation system. Foliar application of silicon was conducted twice (3 wk after sowing and at flowering stage) at 1 or 2 kg ha−1. The silicon application and PGPR inoculation countered the adverse effects of salinity on mung bean by decreasing proline, total phenols, and lignin contents in leaves and increasing the total soluble sugars in comparison to the control. Silicon and rhizobacteria also reduced Na+ content and increased K+, Ca2+, and Si contents in shoots compared to the control under all salinity levels. Furthermore, the significant interaction between Si and PGPR increased the pod yield. Among the treatments, Si (2 kg ha−1) application and B. drentensis inoculation remained the most prominent for the studied traits at all salinity levels. Thus, the current study suggested that application of rhizobacteria along with Si mitigates the effects of salinity on mung bean by regulating osmolytes, reducing lignification, improving mineral uptake, and decreasing tissue Na+ content.

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

ABSTRACTStrains 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.

Symbiotic Parameters, Growth, Nutrient Uptake as Influenced by Biofertilizers under Conservation Agriculture Practices

A field experiment was conducted during rainy season 2014 and 2015 at Research Farm of the Punjab Agricultural University, Ludhiana, Punjab, India to study the effect of conservation agriculture practices (Happy Seeder sowing, Straw Chopper + Zero Tillage and Conventional tillage) and biofertilizers [Bradyrhizobium and plant growth promoting rhizobacteria (PGPR)] on growth, symbiotic parameters and productivity of soybean. The results showed that Happy Seeder sowing and Straw Chopper + Zero Tillage sowing improved the emergence count, leaf area index, photosynthetic active radiation interception, plant height of soybean and symbiotic traits (number and dry weight of nodules and leghaemoglobin content). Highest pods plant−1 and 100-seed weight were recorded in Happy Seeder sowing, followed by Straw Chopper + Zero Tillage sowing and conventional sowing. Based on the mean of two years, Happy Seeder sowing recorded 14.6 and 17.0% higher seed yield than Straw Chopper + Zero Tillage sowing and conventional sowing, respectively. Total nitrogen, phosphorus and potassium uptake were significantly higher in Happy Seeder sowing than the other planting methods. Single and dual inoculation of Bradyrhizobium and PGPR did not influence the growth parameters, symbiotic traits, yield attributes, biological, straw and seed yields, nutrient uptake and protein content of soybean significantly. Therefore, retention of crop residues with zero tillage (Happy Seeder sowing) proves to be the best planting method for sustainable productivity of soybean.

Economic analysis of application of phosphorus, single and dual inoculation of Rhizobium and plant growth promoting rhizobacteria in lentil (Lens culinaris Medikus)

This study investigates the economic returns of lentil (Lens culinaris Medikus) by the use of phosphorus and biofertilizers [Rhizobium and plant growth promoting rhizobacteria (PGPR)] in Indian Punjab. The field experiments were conducted during Rabi 2013-14 and 2014-15 with combinations of four levels of phosphorus (0, 20, 30 and 40 kg P2O5 ha-1) and two/four biofertilizer treatments [uninoculated control and Rhizobium (LLR 12) + PGPR (RB 2)] in 2013-14, and uninoculated control, Rhizobium, PGPR and Rhizobium + PGPR in 2014-15) by replicating thrice. The use of 40 kg P2O5 ha-1 provided the highest gross returns whereas net returns and B:C were highest at 30 kg P2O5 ha-1. The combination of Rhizobium + PGPR + 40 kg P2O5 ha-1 provided the highest gross returns (Rs. 45902) whereas Rhizobium + PGPR+ 20 kg P2O5 ha-1 provided the highest net returns (Rs 20620). Furthermore, the integrated use of Rhizobium + PGPR + 20 kg P2O5 ha-1 provided higher net returns (Rs 20620) and B:C (1.88) as compared to sole application of 40 kg P2O5 ha-1 (Rs 18792 and 1.72). Thus, there was a net saving of 20 kg P2O5 ha-1 with the use of Rhizobium + PGPR inoculation without sacrificing the economics returns.

Root plant growth promoting rhizobacteria inoculations increase ferric chelate reductase (FC-R) activity and Fe nutrition in pear under calcareous soil conditions

Iron deficiency occurring in calcareous soil is a problem in various plants. It is well known that some soil bacteria can release organic acids that can decrease the pH of the soil rhizosphere. However, there have been no attempts to study the effects of plant growth promoting rhizobacteria (PGPR), including organic acid releasing bacteria, on the organic acid contents of the leaf and FC-R activity in the roots and leaves under calcareous soil conditions. Therefore, pear plants were inoculated with 6 bacterial strains with the aim of acquiring iron under calcareous conditions. Uniform 1-year-old pear cv. Deveci sapling grafted on BA-29 and OHF-333 rootstocks were planted in plastic pots containing 10L of loamy soil at 29.6% CaCO3. All bacteria were applied to the roots as an inoculation before planting. The root and leaf Fe content, FC-R activity, leaf organic acids, and soil Fe content were compared in the Alcaligenes 637Ca, Agrobacterium A18, Staphylococcus MFDCa1, MFDCa2, Bacillus M3 and Pantoea FF1 strains. The study showed that the leaf organic acid content and the Fe content in the soil, root and leaf were significantly affected by the bacterial treatments in pear plants. It was determined that the total and active Fe in the leaf was higher in OHF-333 compared to BA-29 by 7% and 14%, respectively. Furthermore, the leaf FC-R activity of Deveci on OHF-333 was 8% higher than that on BA-29. In the Deveci/BA-29 plants, the 637Ca treatment had the highest root FC-R activity value (107nmolFe+2gr−1FW h−2). The highest leaf FC-R activity value was obtained from the MFDCa1, MFDCa2 and FF1 treatments (58.4, 56.3 and 55.7nmolFe+2gr−1FWh−2, respectively). The bacterial strains used in the present study have an important potential to be used as a biofertilizer to replace the use of iron fertilizers.

Alleviation of heavy metals toxicity by the application of plant growth promoting rhizobacteria and effects on wheat grown in saline sodic field

ABSTRACTThe aim of the study was to determine tolerance of plant growth promoting rhizobacteria (PGPR) in different concentrations of Cu, Cr, Co, Cd, Ni, Mn, and Pb and to evaluate the PGPR-modulated bioavailability of different heavy metals in the rhizosphere soil and wheat tissues, grown in saline sodic soil. Bacillus cereus and Pseudomonas moraviensis were isolated from Cenchrus ciliaris L. growing in the Khewra salt range. Seven-day-old cultures of PGPR were applied on wheat as single inoculum, co-inoculation and carrier-based biofertilizer (using maize straw and sugarcane husk as carrier). At 100 ppm of Cr and Cu, the survival rates of rhizobacteria were decreased by 40%. Single inoculation of PGPR decreased 50% of Co, Ni, Cr and Mn concentrations in the rhizosphere soil. Co-inoculation of PGPR and biofertilizer treatment further augmented the decreases by 15% in Co, Ni, Cr and Mn over single inoculation except Pb and Co where decreases were 40% and 77%, respectively. The maximum decrease in biological concentration factor (BCF) was observed for Cd, Co, Cr, and Mn. P. moraviensis inoculation decreases the biological accumulation coefficient (BAC) as well as translocation factor (TF) for Cd, Cr, Cu Mn, and Ni. The PGPR inoculation minimized the deleterious effects of heavy metals, and the addition of carriers further assisted the PGPR.

Root colonization by host-specific rhizobacteria alters indigenous root endophyte and rhizosphere soil bacterial communities and promotes the growth of mandarin orange

We assessed whether colonization of mandarin orange roots by host-specific plant growth promoting rhizobacteria (PGPR) can influence the composition of root endophyte, rhizosphere and non-rhizosphere bacterial communities and plant growth. The ability to colonize root by four host-specific PGPR, namely Enterobacter hormaechei RCE1, Enterobacter asburiae RCE2, Enterobacter ludwigii RCE5 and Klebsiella pneumoniae RCE7, was tested in axenically grown mandarin orange seedlings. The combined results of scanning and transmission electron microscopic images showed that PGPR inoculants successfully colonised on surface and within root tissues of axenic mandarin orange seedlings. The effect of root inoculation of 90 days old mandarin orange seedlings grown in natural soil with RCE1, RCE2, RCE5 and RCE7 either individually or as a consortium was evaluated in terms of plant biomass yield at 90, 180 and 360 days of post inoculation (dpi). Inoculation thus enhanced plant biomass yield over the uninoculated control. NMDS and hierarchical clustering of PCR DGGE fingerprints indicated that both the PGPR inoculation and plant age affected bacterial communities in root tissues and rhizosphere soils. Based on sequencing, the majority of the root endophyte and rhizosphere PCR-DGGE bands were from gammaproteobacteria, and the other were from Firmicutes and Deinococcus-Thermus. In conclusion, host-specific PGPR successfully colonized mandarin orange root tissue and rhizosphere soil and enhanced plant growth.

Amelioration of drought stress in wheat by combined application of PGPR, compost, and mineral fertilizer

ABSTRACTAmong various abiotic stresses, global drought reduces global growth and yield of wheat. Present research has been designed to ameliorate the adverse effects of drought stress on wheat by combined application of plant growth-promoting rhizobacteria (PGPR), compost, and mineral fertilizers. In this experiment, the role of fertilizer, compost, and PGPR inoculation to ameliorate drought stress was studied in two wheat varieties at vegetative stage. Water stress adversely affects morphology, physiology, and biochemistry of the wheat plant. Inoculated seed with compost and mineral fertilizer grown in drought condition showed 43% increase in relative water content (RWC) of 9.39% in Membrane Stability Index and 82.20% in chlorophyll as compared to control. Drought affected the accumulation of osmolytes, but PGPR in combination with compost and mineral fertilizer under drought stress triggered higher accumulation of soluble sugar and proline content, i.e., 28.96% and 73.91%, respectively. It is concluded from this research that PGPR in combination with compost and mineral fertilizer considerably reduces the effect of drought on wheat by enhancing the physiological (RWC, membrane stability, chlorophyll content, and water potential) and biochemical (proline and sugar) aspects of the plant.

Amelioration of drought tolerance in wheat by the interaction of plant growth-promoting rhizobacteria.

Drought stress adversely affects the growth and yield of wheat. The present study was planned to investigate the effect of inoculation of plant-growth promoting rhizobacteria (PGPR) strains IG 3 (Klebsiella sp.), IG 10 (Enterobacter ludwigii) and IG 15 (Flavobacterium sp.) in improving drought tolerance in wheat. These PGPR strains were screened for drought tolerance in nutrient broth supplemented with different concentrations (0-25%) of polyethylene glycol (PEG6000). Effect of PGPR inoculation on various physiological, biochemical parameters and gene expression of stress responsive genes were studied under drought stress. Root colonization at the surface and interiors of roots was demonstrated using scanning electron microscopy (SEM) and tetrazolium staining, respectively. Drought stress significantly affected various growth parameters, water status, membrane integrity, osmolyte accumulation and stress-responsive gene expressions, which were positively altered by PGPR-inoculation in wheat. Quantitative real-time (qRT)-PCR analysis revealed the up regulation of some stress-related genes (DREB2A and CAT1) in un-inoculated wheat plants exposed to drought stress. PGPR-inoculated plants showed attenuated transcript levels suggesting improved drought tolerance due to interaction of PGPRs. The PGPR strain IG 3 was found to be the best in terms of influencing biochemical and physiological status of the seedlings under drought stress. Our report demonstrates the role of PGPRs Enterobacter ludwigii and Flavobacterium sp. in plant growth promotion of wheat plants under drought stress. The study reports the potential of PGPR in alleviating drought stress in wheat which could be used as potent biofertilizers.

Integrated Use of Nitrogen Fertilization and Microbial Inoculation: Change in the Growth and Chemical Composition of White Cabbage

ABSTRACTField experiments were conducted to evaluate the impact of seed and seedling inoculation by plant growth promoting rhizobacteria (PGPR) on nitrogen use efficiency, growth, yield, and chemical composition of cabbage at varying levels of nitrogen (N). Data revealed that N alone or in combination with PGPR either as seed or seedling inoculation significantly improved growth, yield, and nutrients of cabbage. PGPR inoculations were more efficient than non-inoculated controls in terms of yield and yield parameters. This study showed that seed and seedling inoculation increased yield and yield parameters as well as chlorophyll reading value and stomatal conductance versus controls. Applications of mineral fertilizers with microbial applications save 25% of mineral fertilizers yet give 33% more yield versus full doses of mineral application without microbial applications. Both seed and seedling treatments increased the nitrogen use efficiency (NUE) rates by 45, 53, 58, 45, and 40%, and 49, 59, 68, 69, 60, and 55%, respectively.

Amino Acids Hydrolyzed from Animal Carcasses Are a Good Additive for the Production of Bio-organic Fertilizer

High-quality bio-organic fertilizers (BIOs) cannot be produced without the addition of some proteins. In this study, compound liquid amino acids (CLAA) from animal carcasses were utilized as additives into matured composts to create novel BIOs containing plant growth-promoting rhizobacteria (PGPR). The results showed that adding CLAA and inoculating bacteria meanwhile resulted in failed solid-state fermentation (SSF) due to the higher H+ contents. While after pre-compost for 4 days before PGPR inoculation, treatments of matured chicken or pig manure added with 0.2 ml g-1 of CLAA resulted in a maximum biomass of functional strains. Illumine-MiSeq sequencing and Real-Time PCR results showed that the CLAA addition decreased the bacterial abundance and richness, altered the bacterial community structure and changed the relative abundance of some microbial groups. This study offers a high value-added utilization of waste protein resources for producing economical, high-quality BIO.

Rhizobacteria enhance oil content and physiological status of Hyptis suaveolens under salinity stress

We investigate the effect of inoculation of plant growth promoting rhizobacteria (PGPR) i.e. Pseudomonas pseudoalcaligenes and Bacillus pumilus to evaluate their growth promoting ability in one of the medicinal important plant Hyptis suaveolens (L.) in terms of essential oil content and biomass under normal and salinity conditions. It reduced the negative effect of salinity and enhanced plant growth under salinity. The essential oil content and yield decreased due to inoculation of PGPR, but due to enhanced overall biomass, it showed a positive response. So for utilization of degraded wastelands, inoculation of such PGPR serves as a better and viable option for cultivation of medicinal, aromatic plants as well as other crops.

Selection of metal resistant plant growth promoting rhizobacteria for the growth and metal accumulation of energy maize in a mine soil — Effect of the inoculum size

Heavy metals, such as Cd and Zn, are spilled in soils by several anthropogenic sources, including mining activities. Their toxic effects can be minimized using plants especially when paired with plant growth promoting rhizobacteria (PGPR), under phytomanagement strategies. Several factors can contribute to the failure of rhizobacterial inoculation, such as bacteria selection and the inoculum size. In this work five metal resistant PGPR (Ralstonia eutropha 1C2, Chryseobacterium humi ECP37, Pseudomonas fluorescens S3X, Rhizobium radiobacter EC1B and Pseudomonas reactans EDP28) were investigated for their in vitro growth promoting traits and for their ability to induce growth of maize seedlings exposed to Zn and Cd. PGPR inoculum size (10 and 20mL) and inoculation effectiveness was assessed in energy maize sowed in a mine soil. The results showed that some bacteria only exhibited or enhanced PGP traits when exposed to metals. The bacterial strains ECP37 and EDP28 were the most efficient in improving seedling growth with increasing metal concentrations, followed by S3X. When inoculated in energy maize grown in mine soil, these same strains also outperformed the others by increasing shoot biomass and elongation, metal accumulation, and by decreasing it in roots. The most evident effect of doubling the inoculum size was the increase in Cd accumulation, which was of 17% and 31% in roots and shoots, respectively. Other effects included a slight reduction in shoots' biomass (13%) and a general decrease in P tissue content. The results obtained suggest that PGPR selection prior to inoculation in the target soils should be primarily based in seedling growth promotion under metal exposure. Additionally, the size of the inoculum applied in the soil rhizosphere appears to be important in remediation processes and should be taken into account when planning phytomanagement strategies, especially when the biomass of plants is an important demand.

Physiological and biochemical changes in tomato cultivar PT-3 with dual inoculation of mycorrhiza and PGPR against root-knot nematode

Arbuscular mycorrhizal fungi (AMF) and plant growth promoting rhizobacteria (PGPR) have potential to control soil-borne diseases including plant-parasitic nematodes. First, the effects of dual inoculation of mycorrhiza (Rhizophagus irregularis) and two stains of pseudomonads (Pseudomonas jessenii strain R62 and Pseudomonas synxantha strain R81) on tomato (Solanum lycopersicum cv. PT-3) growth were tested. Further, the physiological and biochemical changes caused by these beneficial organisms during infection by the root-knot nematode Meloidogyne incognita were studied. The experiment was conducted under glass house conditions and carried out up to one month after nematode inoculation. Plants treated with dual or individual inoculation of AMF and PGPR showed significantly enhanced plant growth and reduced nematode infection. In addition, they exhibited potent activity of phenolics (28 %) and defensive enzymes i.e. peroxidase (PO; 1.26 fold), polyphenyloxidase (PPO; 1.35 fold) and superoxide dismutase (SOD; 1.09 fold) while a significant reduction in malondialdehyde (MDA; 1.63 fold) and hydrogen peroxide (H2O2; 1.30 fold) content was recorded when compared to the nematode-infected plants. These findings indicate the feasibility of AMF and PGPR individually or in combinations as potential biocontrol agents for the management of root-knot nematodes.

Induced maize salt tolerance by rhizosphere inoculation of Bacillus amyloliquefaciens SQR9.

Salt stress reduces plant growth and is now becoming one of the most important factors restricting agricultural productivity. Inoculation of plant growth-promoting rhizobacteria (PGPR) has been shown to confer plant tolerance against abiotic stress, but the detailed mechanisms of how this occurs remain unclear. In this study, hydroponic experiments indicated that the PGPR strain Bacillus amyloliquefaciens SQR9 could help maize plants tolerate salt stress. After exposure to salt stress for 20 days, SQR9 significantly promoted the growth of maize seedlings and enhanced the chlorophyll content compared with the control. Additional analysis showed that the involved mechanisms could be the enhanced total soluble sugar content for decreasing cell destruction, improved peroxidase/catalase activity and glutathione content for scavenging reactive oxygen species, and reduced Na(+) levels in the plant to decrease Na(+) toxicity. These physiological appearances were further confirmed by the upregulation of RBCS, RBCL, H(+) -PPase, HKT1, NHX1, NHX2 and NHX3, as well as downregulation of NCED expression, as determined by quantitative reverse transcription-polymerase chain reaction. However, SQR9 counteracted the increase of abscisic acid in response to salt stress. In summary, these results show that SQR9 confers plant salt tolerance by protecting the plant cells and managing Na(+) homeostasis. Hence, it can be used in salt stress prone areas, thereby promoting agricultural production.

English

Growth promotion and disease control by rhizobacteria are complex interrelated processes that involve direct and indirect mechanisms. The mechanisms include synthesis of some metabolites (auxin, cytokinin and gibberellins), induction of 1-aminocyclopropane-1-carboxylate (ACC) deaminase, production of siderophore, antibiotics, hydrogen cyanide hydrogen cyanide (HCN) and volatile compounds. They also include mineral solubilization competition, and induction of systemic resistance. These bacteria are suitable as soil inoculants because they have the potential for rapid and aggressive colonization. This feature alone is characterised as a disease control mechanism, which prevents the invasion of detrimental soil microorganisms onto the root surface. Inoculant-based plant growth-promoting rhizobacteria (PGPR) is applied extensively on agricultural crops to improve plants’ growth and simultaneously reduce chemical inputs like fertilizer and pesticide which can cause environmental degradation. The structure of the rhizobacterial community is affected by several factors including plant genotype and is determined by the amount and composition of root exudates. In addition, soil type and fertility are the contributing factors that shape the community. This form of communication can affect plants’ growth, nutrient status and also susceptibility to stress and pathogens in the host plant. PGPR inoculants cause diverse beneficial interactions among plants, which leads to sustainable and environment-friendly agriculture. The application of rhizosphere soil of agricultural crops with desirable bacterial populations is considered promising in both laboratory and greenhouse experiment. Further, a clearer understanding of the way PGPRs promote plants’ growth can lead to expanded exploitation of these ‘biofertilizers’ in order to reduce the potential negative environmental effects associated with food and fiber production. Key words: Rhizobacteria, plant growth-promoting rhizobacteria (PGPR), root microbione, phytohormones, biocontrol, soil-borne phytopathgen, fluorescent pseudomonads.

Plant Growth-Promoting Rhizobacteria Inoculation to Enhance Vegetative Growth, Nitrogen Fixation and Nitrogen Remobilisation of Maize under Greenhouse Conditions.

Plant growth-promoting rhizobacteria (PGPR) may provide a biological alternative to fix atmospheric N2 and delay N remobilisation in maize plant to increase crop yield, based on an understanding that plant-N remobilisation is directly correlated to its plant senescence. Thus, four PGPR strains were selected from a series of bacterial strains isolated from maize roots at two locations in Malaysia. The PGPR strains were screened in vitro for their biochemical plant growth-promoting (PGP) abilities and plant growth promotion assays. These strains were identified as Klebsiella sp. Br1, Klebsiella pneumoniae Fr1, Bacillus pumilus S1r1 and Acinetobacter sp. S3r2 and a reference strain used was Bacillus subtilis UPMB10. All the PGPR strains were tested positive for N2 fixation, phosphate solubilisation and auxin production by in vitro tests. In a greenhouse experiment with reduced fertiliser-N input (a third of recommended fertiliser-N rate), the N2 fixation abilities of PGPR in association with maize were determined by 15N isotope dilution technique at two harvests, namely, prior to anthesis (D50) and ear harvest (D65). The results indicated that dry biomass of top, root and ear, total N content and bacterial colonisations in non-rhizosphere, rhizosphere and endosphere of maize roots were influenced by PGPR inoculation. In particular, the plants inoculated with B. pumilus S1r1 generally outperformed those with the other treatments. They produced the highest N2 fixing capacity of 30.5% (262 mg N2 fixed plant−1) and 25.5% (304 mg N2 fixed plant−1) of the total N requirement of maize top at D50 and D65, respectively. N remobilisation and plant senescence in maize were delayed by PGPR inoculation, which is an indicative of greater grain production. This is indicated by significant interactions between PGPR strains and time of harvests for parameters on N uptake and at. % 15Ne of tassel. The phenomenon is also supported by the lower N content in tassels of maize treated with PGPR, namely, B. pumilus S1r1, K. pneumoniae Fr1, B. subtilis UPMB10 and Acinetobacter sp. S3r2 at D65 harvest. This study provides evidence that PGPR inoculation, namely, B. pumilus S1r1 can biologically fix atmospheric N2 and provide an alternative technique, besides plant breeding, to delay N remobilisation in maize plant for higher ear yield (up to 30.9%) with reduced fertiliser-N input.