Azospirillum brasilense and Azospirillum lipoferum hydrolyze conjugates of GA20 and metabolize the resultant aglycones to GA1 in seedlings of rice dwarf mutants

Efforts towards overcoming drought stress in crops: Revisiting the mechanisms employed by plant growth-promoting bacteria

Globally, agriculture is under a lot of pressure due to rising population and corresponding increases in food demand. However, several variables, including improper mechanization, limited arable land, and the presence of several biotic and abiotic pressures, continually impact agricultural productivity. Drought is a notable destructive abiotic stress and may be the most serious challenge confronting sustainable agriculture, resulting in a significant crop output deficiency. Numerous morphological and physiological changes occur in plants as a result of drought stress. Hence, there is a need to create mitigation techniques since these changes might permanently harm the plant. Current methods used to reduce the effects of drought stress include the use of film farming, super-absorbent hydrogels, nanoparticles, biochar, and drought-resistant plant cultivars. However, most of these activities are money and labor-intensive, which offer limited plant improvement. The use of plant-growth-promoting bacteria (PGPB) has proven to be a preferred method that offers several indirect and direct advantages in drought mitigation. PGPB are critical biological elements which have favorable impacts on plants’ biochemical and physiological features, leading to improved sugar production, relative water content, leaf number, ascorbic acid levels, and photosynthetic pigment quantities. This present review revisited the impacts of PGPB in ameliorating the detrimental effects of drought stress on plants, explored the mechanism of action employed, as well as the major challenges encountered in their application for plant growth and development.

Bacillus cereus Improves Performance of Brazilian Green Dwarf Coconut Palms Seedlings With Reduced Chemical Fertilization

Coconut production in the Amazon requires the knowledge and development of sustainable technologies to alleviate the detrimental effects of inorganic chemical fertilizers and intensive farming practices. In this study, we investigated the effects of plant growth-promoting rhizobacteria (PGPR) isolated from coconut seedlings on nutrient use efficiency (NUE) and physiological mechanisms related to biomass accumulation of seedlings grown with reduced inorganic fertilizer levels. Of the 96 PGPR isolates tested on rice plants, the isolate Bacillus cereus (UFRABC40) was selected, as it resulted in the most significant gain in growth variables. In a commercial coconut tree nursery, we subjected seedlings to two treatments, both with seven replications: control 100% NPK chemical fertilizer (CF) and B. cereus + 50% NPK CF. The results indicated that the inoculation increased phytohormone levels [190% indole acetic acid (IAA), 31% gibberellic acid GA3, and 17% gibberellic acid GA4] and leaf gas exchange [48% by assimilation of CO2 (A), 35% stomatal conductance to water vapor (gs), 33% transpiration, and 57% instantaneous carboxylation efficiency] in leaves. Furthermore, growth parameters (shoot, root, and total dry weight, height, and diameter) and macro- and micronutrient levels (95% N, 44% P, 92% K, 103 Ca, 46% Fe, 84% B) were improved. Our results show the potential ability of strain Bacillus cereus UFRABC40 to promote the growth performance of coconut seedlings under decreased application of inorganic fertilizers. The application of microbial-based products in coconut seedling production systems improves plants' physiological performance and the efficiency of nutrient use.

Plant Growth Promoting and Stress Mitigating Abilities of Soil Born Microorganisms

Abiotic stresses affect the plant growth in different ways and at different developmental stages that reduce the crop yields. The increasing world population continually demands more crop yields; therefore it is important to use low-cost technologies against abiotic stresses to increase crop productivity. Soil microorganisms survive in the soil associated with plants in extreme condition. It was demonstrated that these beneficial microorganisms promote plant growth and development under various stresses. The soil microbes interact with the plant through rhizospheric or endophytic association and promote the plant growth through different processes such as nutrients mobilization, disease suppression, and hormone secretions. The microorganisms colonized in the rhizospheric region and imparted the abiotic stress tolerance by producing 1-aminocyclopropane-1- carboxylate (ACC) deaminase, antioxidant, and volatile compounds, inducing the accumulation of osmolytes, production of exopolysaccharide, upregulation or downregulation of stress genes, phytohormones and change the root morphology. A large number of these rhizosphere microorganisms are now patented. In the present review, an attempt was made to throw light on the mechanism of micro-organism that operates during abiotic stresses and promotes plant survival and productivity.

Climate Change and Salinity Effects on Crops and Chemical Communication Between Plants and Plant Growth-Promoting Microorganisms Under Stress

During the last two decades the world has experienced an abrupt change in climate. Both natural and artificial factors are climate change drivers, although the effect of natural factors are lesser than the anthropogenic drivers. These factors have changed the pattern of precipitation resulting in a rise in sea levels, changes in evapotranspiration, occurrence of flood overwintering of pathogens, increased resistance of pests and parasites, and reduced productivity of plants. Although excess CO2 promotes growth of C3 plants, high temperatures reduce the yield of important agricultural crops due to high evapotranspiration. These two factors have an impact on soil salinization and agriculture production, leading to the issue of water and food security. Farmers have adopted different strategies to cope with agriculture production in saline and saline sodic soil. Recently the inoculation of halotolerant plant growth promoting rhizobacteria (PGPR) in saline fields is an environmentally friendly and sustainable approach to overcome salinity and promote crop growth and yield in saline and saline sodic soil. These halotolerant bacteria synthesize certain metabolites which help crops in adopting a saline condition and promote their growth without any negative effects. There is a complex interkingdom signaling between host and microbes for mutual interaction, which is also influenced by environmental factors. For mutual survival, nature induces a strong positive relationship between host and microbes in the rhizosphere. Commercialization of such PGPR in the form of biofertilizers, biostimulants, and biopower are needed to build climate resilience in agriculture. The production of phytohormones, particularly auxins, have been demonstrated by PGPR, even the pathogenic bacteria and fungi which also modulate the endogenous level of auxins in plants, subsequently enhancing plant resistance to various stresses. The present review focuses on plant-microbe communication and elaborates on their role in plant tolerance under changing climatic conditions.

Alternative Strategies for Multi-Stress Tolerance and Yield Improvement in Millets

Millets are important cereal crops cultivated in arid and semiarid regions of the world, particularly Africa and southeast Asia. Climate change has triggered multiple abiotic stresses in plants that are the main causes of crop loss worldwide, reducing average yield for most crops by more than 50%. Although millets are tolerant to most abiotic stresses including drought and high temperatures, further improvement is needed to make them more resilient to unprecedented effects of climate change and associated environmental stresses. Incorporation of stress tolerance traits in millets will improve their productivity in marginal environments and will help in overcoming future food shortage due to climate change. Recently, approaches such as application of plant growth-promoting rhizobacteria (PGPRs) have been used to improve growth and development, as well as stress tolerance of crops. Moreover, with the advance of next-generation sequencing technology, genome editing, using the clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) system are increasingly used to develop stress tolerant varieties in different crops. In this paper, the innate ability of millets to tolerate abiotic stresses and alternative approaches to boost stress resistance were thoroughly reviewed. Moreover, several stress-resistant genes were identified in related monocots such as rice (Oryza sativa), wheat (Triticum aestivum), and maize (Zea mays), and other related species for which orthologs in millets could be manipulated by CRISPR/Cas9 and related genome-editing techniques to improve stress resilience and productivity. These cutting-edge alternative strategies are expected to bring this group of orphan crops at the forefront of scientific research for their potential contribution to global food security.

Lipopeptide production by Bacillus atrophaeus strain B44 and its biocontrol efficacy against cotton rhizoctoniosis

OBJECTIVES

An assay was conducted to show the comparisons the effects of nine metal ions on antagonistic metabolites (lipopeptides, siderophores and gibberellins) by Bacillus atrophaeus strain B44 using well-diffusion assays, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis, chrome azurol S plus mannitol salt agar (CAS-MSA) tests, and reversed-phase high-performance liquid chromatography (RP-HPLC) analysis. This assay is also designed to demonstrate the biocontrol efficacy of B44 against cotton rhizoctoniosis using pot culture tests.

RESULTS

Both the lipopeptide yield and the antimicrobial activity of B44 increase with the MnSO₄, MgSO₄, CaCO₃, and CuSO₄ treatments and either have no effect or decreased lipopeptide yield and antimicrobial activity with the FeSO₄, K₂HPO₄, KCl, KH₂PO₄ and ZnSO₄ treatments. The medium containing MgSO₄ has no significant effect on either the lipopeptide yield or antimicrobial activity. MALDI-TOF-MS analysis shows a broad range of m/z peaks, indicating that strain B44 produces a complex mixture of iturin, surfactin, and fengycin lipopeptides. Gibberellin production by strain B44 varies greatly depending on the culture medium, and the siderophore production is not significantly affected by the culture medium. Pot tests show that lipopeptide production affects the disease control efficacy of strain B44.

CONCLUSION

The biocontrol efficacy of B. atrophaeus strain B44 is related to the lipopeptide yield. Moreover, B. atrophaeus strain B44 significantly increases the size of cotton seedlings, which is related to the GA₃ concentration.

Rhizosphere Bacterium Rhodococcus sp. P1Y Metabolizes Abscisic Acid to Form Dehydrovomifoliol

The phytohormone abscisic acid (ABA) plays an important role in plant growth and in response to abiotic stress factors. At the same time, its accumulation in soil can negatively affect seed germination, inhibit root growth and increase plant sensitivity to pathogens. ABA is an inert compound resistant to spontaneous hydrolysis and its biological transformation is scarcely understood. Recently, the strain Rhodococcus sp. P1Y was described as a rhizosphere bacterium assimilating ABA as a sole carbon source in batch culture and affecting ABA concentrations in plant roots. In this work, the intermediate product of ABA decomposition by this bacterium was isolated and purified by preparative HPLC techniques. Proof that this compound belongs to ABA derivatives was carried out by measuring the molar radioactivity of the conversion products of this phytohormone labeled with tritium. The chemical structure of this compound was determined by instrumental techniques including high-resolution mass spectrometry, NMR spectrometry, FTIR and UV spectroscopies. As a result, the metabolite was identified as (4RS)-4-hydroxy-3,5,5-trimethyl-4-[(E)-3-oxobut-1-enyl]cyclohex-2-en-1-one (dehydrovomifoliol). Based on the data obtained, it was concluded that the pathway of bacterial degradation and assimilation of ABA begins with a gradual shortening of the acyl part of the molecule.

PGPR Mediated Alterations in Root Traits: Way Toward Sustainable Crop Production

The above ground growth of the plant is highly dependent on the belowground root system. Rhizosphere is the zone of continuous interplay between plant roots and soil microbial communities. Plants, through root exudates, attract rhizosphere microorganisms to colonize the root surface and internal tissues. Many of these microorganisms known as plant growth promoting rhizobacteria (PGPR) improve plant growth through several direct and indirect mechanisms including biological nitrogen fixation, nutrient solubilization, and disease-control. Many PGPR, by producing phytohormones, volatile organic compounds, and secondary metabolites play important role in influencing the root architecture and growth, resulting in increased surface area for nutrient exchange and other rhizosphere effects. PGPR also improve resource use efficiency of the root system by improving the root system functioning at physiological levels. PGPR mediated root trait alterations can contribute to agroecosystem through improving crop stand, resource use efficiency, stress tolerance, soil structure etc. Thus, PGPR capable of modulating root traits can play important role in agricultural sustainability and root traits can be used as a primary criterion for the selection of potential PGPR strains. Available PGPR studies emphasize root morphological and physiological traits to assess the effect of PGPR. However, these traits can be influenced by various external factors and may give varying results. Therefore, it is important to understand the pathways and genes involved in plant root traits and the microbial signals/metabolites that can intercept and/or intersect these pathways for modulating root traits. The use of advanced tools and technologies can help to decipher the mechanisms involved in PGPR mediated determinants affecting the root traits. Further identification of PGPR based determinants/signaling molecules capable of regulating root trait genes and pathways can open up new avenues in PGPR research. The present review updates recent knowledge on the PGPR influence on root architecture and root functional traits and its benefits to the agro-ecosystem. Efforts have been made to understand the bacterial signals/determinants that can play regulatory role in the expression of root traits and their prospects in sustainable agriculture. The review will be helpful in providing future directions to the researchers working on PGPR and root system functioning.

What Did We Learn From Plant Growth-Promoting Rhizobacteria (PGPR)-Grass Associations Studies Through Proteomic and Metabolomic Approaches?

Plant growth stimulation by microorganisms that interact in a mutually beneficial manner remains poorly understood. Understanding the nature of plant-bacteria interactions may open new routes for plant productivity enhancement, especially cereal crops consumed by humans. Proteomic and metabolomic analyses are particularly useful for elucidating these mechanisms. A complete depiction of these mechanisms will prompt researchers to develop more efficient plant-bacteria associations. The success of microorganisms as biofertilizers may replace the current massive use of chemical fertilizers, mitigating many environmental and economic issues. In this review, we discuss the recent advances and current state of the art in proteomics and metabolomics studies involving grass-bacteria associations. We also discuss essential subjects involved in the bacterial plant-growth promotion, such, nitrogen fixation, plant stress, defense responses, and siderophore production.

Modeling vegetative vigour in grapevine: unraveling underlying mechanisms

Mechanistic modeling constitutes a powerful tool to unravel complex biological phenomena. This study describes the construction of a mechanistic, dynamic model for grapevine plant growth and canopy biomass (vigor). To parametrize and validate the model, the progeny from a cross of Ramsey (Vitis champinii) × Riparia Gloire (V. riparia) was evaluated. Plants with different vigor were grown in a greenhouse during the summer of 2014 and 2015. One set of plants was grafted with Cabernet Sauvignon. Shoot growth rate (b), leaf area (LA), dry biomass, whole plant and root specific hydraulic conductance (k_H and L_pr), stomatal conductance (g_s), and water potential (Ψ) were measured. Partitioning indices and specific leaf area (SLA) were calculated. The model includes an empirical fit of a purported seasonal pattern of bioactive GAs based on published seasonal evolutionary levels and reference values. The model provided a good fit of the experimental data, with R = 0.85. Simulation of single trait variations defined the individual effect of each variable on vigor determination. The model predicts, with acceptable accuracy, the vigor of a young plant through the measurement of L_pr and SLA. The model also permits further understanding of the functional traits that govern vigor, and, ultimately, could be considered useful for growers, breeders and those studying climate change.

Significance of Plant Growth Promoting Rhizobacteria in Grain Legumes: Growth Promotion and Crop Production

Grain legumes are an important component of sustainable agri-food systems. They establish symbiotic association with rhizobia and arbuscular mycorrhizal fungi, thus reducing the use of chemical fertilizers. Several other free-living microbial communities (PGPR—plant growth promoting rhizobacteria) residing in the soil-root interface are also known to influence biogeochemical cycles and improve legume productivity. The growth and function of these microorganisms are affected by root exudate molecules secreted in the rhizosphere region. PGPRs produce the chemicals which stimulate growth and functions of leguminous crops at different growth stages. They promote plant growth by nitrogen fixation, solubilization as well as mineralization of phosphorus, and production of phytohormone(s). The co-inoculation of PGPRs along with rhizobia has shown to enhance nodulation and symbiotic interaction. The recent molecular tools are helpful to understand and predict the establishment and function of PGPRs and plant response. In this review, we provide an overview of various growth promoting mechanisms of PGPR inoculations in the production of leguminous crops.

Plant growth-promoting endophytic bacteria augment growth and salinity tolerance in rice plants

Salt stress negatively affects growth and development of plants. However, it is hypothesized that plant growth-promoting endophytic bacteria can greatly alleviate the adverse effects of salinity and can promote growth and development of plants. In the present research, we aimed to isolate endophytic bacteria from halotolerant plants and evaluate their capacity for promoting crop plant growth. The bacterial endophytes were isolated from selected plants inhabiting sand dunes at Pohang beach, screened for plant growth-promoting traits and applied to rice seedlings under salt stress (NaCl; 150 mm). Out of 59 endophytic bacterial isolates, only six isolates, i.e. Curtobacterium oceanosedimentum SAK1, Curtobacterium luteum SAK2, Enterobacter ludwigii SAK5, Bacillus cereus SA1, Micrococcus yunnanensis SA2, Enterobacter tabaci SA3, resulted in a significant increase in the growth of Waito-C rice. The cultural filtrates of bacterial endophytes were tested for phytohormones, including indole-3-acetic acid, gibberellins and organic acids. Inoculation of the selected strains considerably reduced the amount of endogenous ABA in rice plants under NaCl stress, however, they increased GSH and sugar content. Similarly, these strains augmented the expression of flavin monooxygenase (OsYUCCA1) and auxin efflux carrier (OsPIN1) genes under salt stress. In conclusion, the pragmatic application of the above selected bacterial strains alleviated the adverse effects of NaCl stress and enhanced rice growth attributes by producing various phytohormones.

Roles of microbes in supporting sustainable rice production using the system of rice intensification

The system of rice intensification (SRI) is an agroecological approach to rice cultivation that seeks to create optimal conditions for healthy plant growth by minimizing inter-plant competition, transplanting widely spaced young single seedlings, and optimizing favorable soil conditions with organic amendments, increased soil aeration by weeding, and controlled water management. These practices improve rice plant growth with yields up to three times more than with conventional cultivation methods, and increase crop resilience under biotic and abiotic stresses. This review discusses the roles of beneficial microbes in improving rice plant growth, yield, and resilience when SRI practices are used, and how these modifications in plant, soil, water, and nutrient management affect the populations and diversity of soil microorganisms. Mechanisms whereby symbiotic microbes support rice plants' growth and performance are also discussed.

Klebsiella variicola and Klebsiella quasipneumoniae with capacity to adapt to clinical and plant settings

OBJECTIVE

To compare the genetic determinants involved in plant colonization or virulence in the reported genomes of K. variicola, K. quasipneumoniae and K. pneumoniae.

MATERIALS AND METHODS

In silico comparisons and Jaccard analysis of genomic data were used. Fimbrial genes were detected by PCR. Biological assays were performed with plant and clinical isolates.

RESULTS

Plant colonization genes such as cellulases, catalases and hemagglutinins were mainly present in K. variicola genomes. Chromosomal β-lactamases were characteristic of this species and had been previously misclassified. K. variicola and K. pneumoniae isolates produced plant hormones.

CONCLUSIONS

A mosaic distribution of different virulence- and plant-associated genes was found in K. variicola and in K. quasipneumoniae genomes. Some plant colonizing genes were found mainly in K. variicola genomes. The term plantanosis is proposed for plant-borne human infections.

Inoculation with Azospirillum sp. and Herbaspirillum sp. Bacteria Increases the Tolerance of Maize to Drought Stress

Stress drought is an important abiotic factor that leads to immense losses in crop yields around the world. Strategies are urgently needed to help plants adapt to drought in order to mitigate crop losses. Here we investigated the bioprotective effects of inoculating corn grown under drought conditions with two types of plant growth-promoting rhizobacteria (PGPR), A. brasilense, strain SP-7, and H. seropedicae, strain Z-152. Plants inoculated with the bacteria were grown in a greenhouse with perlite as a substrate. Two hydric conditions were tested: normal well-watered conditions and drought conditions. Compared to control non-inoculated plants, those that were inoculated with PGPR bacteria showed a higher tolerance to the negative effects of water stress in drought conditions, with higher biomass production; higher carbon, nitrogen, and chlorophyll levels; and lower levels of abscisic acid and ethylene, which are plant hormones that affect the stress response. The oxidative stress levels of these plants were similar to those of non-inoculated plants grown in well-watered conditions, showing fewer injuries to the cell membrane. We also noted higher relative water content in the vegetal tissue and better osmoregulation in drought conditions in inoculated plants, as reflected by significantly lower proline content. Finally, we observed lower gene expression of ZmVP14 in the inoculated plants; notably, ZmVP14 is involved in the biosynthesis of abscisic acid. Taken together, these results demonstrate that these bacteria could be used to help plants cope with the negative effects of drought stress conditions.

PROMOCIÓN DEL CRECIMIENTO DE Baccharis macrantha (ASTERACEAE) CON BACTERIAS SOLUBILIZADORAS DE FOSFATOS ASOCIADAS A SU RIZOSFERA

<p>El objetivo de esta investigación fue aislar y caracterizar bacterias solubilizadoras de fosfatos (BSF) asociadas a la rizosfera de <em>Baccharis macrantha </em>y <em>Viburnum triphyllum,</em> y evaluar su capacidad para solubilizar fosfatos en condiciones <em>in vitro</em>. Además se determinó el efecto de la inoculaciónde las cepas de BSF más eficientes sobre el crecimiento de <em>B. macrantha</em>. Las muestras de suelo rizosférico de <em>B. macrantha </em>y <em>V. triphyllum </em>fueron colectadas en los meses de mayo-período de lluvia y septiembre-período seco del 2012. Para la cuantificación de bacterias heterótrofas cultivables y BSF se empleó el método de recuento en placa en los medios Agar Tripticasa de Soya y Pikovskaya (PVK) respectivamente. La capacidad de solubilización de fosfatos de las cepas aisladas se estimó a partir del diámetro de los halos formados alrededor de las colonias en el medio de cultivo PVK después de 7 días de incubación a 28 °C. Los ensayos de inoculación en <em>B. macrantha </em>se realizaron con las BSF más eficientes<em>. </em>La inoculación de las BSF <em>B. firmus y P. fluorescens</em> de forma individual y como inoculante combinado mostro un efecto benéfico, incrementando significativamente el porcentaje de germinación de semillas, la altura de la plántula, la longitud de la raíz y el peso seco de <em>B. macrantha</em>. La inoculación de BSF podría ser considerada una estrategia para mejorar el crecimiento y establecimiento de <em>B. macrantha</em> en pastizales abandonados.</p><p><strong>Growth Promotion of <em>Baccharis macrantha </em>(Asteraceae) by Phosphate Solubilizing Rhizosphere Bacteria</strong>     </p><p>The objectives of this research was to isolate and characterize phosphate solubilizing bacteria (BSF) associated to the rhizosphere of <em>Baccharis macrantha</em> and <em>Viburnum triphyllum</em>, and to assess their ability to solubilize phosphate under conditions in vitro. Furthermore to determine the effect of inoculation of the strains BSF more efficient on the growth of <em>B. macrantha</em>. Rhizosphere soil samples of <em>B. macrantha</em> and <em>V. triphyllum </em>were collected in the months of May-rainy season and September-period dry the 2012. Trypticase Soya Agar and Pikovskaya (PVK) were used for quantification of culturable heterotrophic bacteria and BSF, respectively. The phosphate solubilizing capacity of the isolated strains was estimated from the diameter of the halo around the colonies formed in the culture medium PVK after 7 days incubation at 28 °C. Inoculation assays were performed with more efficient BSF in <em>B. macrantha. </em>Inoculation of BSF <em>Bacillus firmus</em> and <em>Pseudomona fluorescens </em>individually and as inoculant combined showed a beneficial effect, significantly increasing the percentage of seed germination, seedling height, root length and dry weight of <em>B . macrantha</em>. Inoculation the BSF could be considered a strategy to improve the growth and development of <em>B. macrantha</em> in abandoned pastures</p>

Azospirillum brasilense ameliorates the response of Arabidopsis thaliana to drought mainly via enhancement of ABA levels

Production of phytohormones is one of the main mechanisms to explain the beneficial effects of plant growth-promoting rhizobacteria (PGPR) such as Azospirillum sp. The PGPRs induce plant growth and development, and reduce stress susceptibility. However, little is known regarding the stress-related phytohormone abscisic acid (ABA) produced by bacteria. We investigated the effects of Azospirillum brasilense Sp 245 strain on Arabidopsis thaliana Col-0 and aba2-1 mutant plants, evaluating the morphophysiological and biochemical responses when watered and in drought. We used an in vitro-grown system to study changes in the root volume and architecture after inoculation with Azospirillum in Arabidopsis wild-type Col-0 and on the mutant aba2-1, during early growth. To examine Arabidopsis development and reproductive success as affected by the bacteria, ABA and drought, a pot experiment using Arabidopsis Col-0 plants was also carried out. Azospirillum brasilense augmented plant biomass, altered root architecture by increasing lateral roots number, stimulated photosynthetic and photoprotective pigments and retarded water loss in correlation with incremented ABA levels. As well, inoculation improved plants seed yield, plants survival, proline levels and relative leaf water content; it also decreased stomatal conductance, malondialdehyde and relative soil water content in plants submitted to drought. Arabidopsis inoculation with A. brasilense improved plants performance, especially in drought.

Gibberellin oxidase activities in Bradyrhizobium japonicum bacteroids

Bradyrhizobium japonicum bacteroids isolated from root nodules of soybean (Glycine max.) plants converted the gibberellin (GA) precursor [(14)C1]GA12 into several products identified by combined gas chromatography-mass spectrometry as [(14)C1]GA24, [(14)C1]GA9, [(14)C1]GA15, GA9 17-nor-16-one and unidentified products. The oxidation of GA12, catalyzed by the GA 20-oxidase, was present in symbiotic bacteroids from plants around flowering, but not in bacteroids from plants at either an early vegetative stage or at late growth stages. Expression of cps and ks genes, involved in ent-kaurene biosynthesis, was also demonstrated in bacteroids from soybean plants around flowering. Earlier precursors of the GA pathway, ent-[(14)C1]kaurenoic acid or [(14)C4]GA12-aldehyde, were efficiently utilized by B. japonicum bacteroids to give labelled GA9 plus intermediates partially oxidized at C-20, as well as GA9 17-nor-16-one and an unidentified product. No 3β or 13-hydroxylated [(14)C]GAs were detected in any of the incubations. Moreover the C19-GAs [(14)C1]GA4 or [(14)C1]GA20 were recovered unconverted upon incubation with the bacteroids which supports the absence of GA 3β-hydroxylase activity in B. japonicum. The bacterial 20-oxidase utilized the 13-hydroxylated substrates [(14)C1]GA53, [(14)C1]GA44 or [(14)C1]GA19, although with less efficiency than [(14)C1]GA12 to give [(14)C1]GA20 as final product, while the 3β-hydroxylated substrate [(14)C1]GA14 was converted to [(14)C1]GA4 to a very small extent. Endogenous GA9 and GA24 were identified by GC-MS in methanolic nodule extracts. These results suggest that B. japonicum bacteroids would synthesize GA9 under the symbiotic conditions present in soybean root nodules.

Azospirillum improves lettuce growth and transplant under saline conditions

BACKGROUND

Recent studies have shown that as a plant-growth-promoting rhizobacteria (PGPR), Azospirillum inoculation could contribute to the mitigation of the negative effects caused by salt on lettuce growth. Moreover, the use of PGPR to alleviate the effects of transplant in vegetables has also been recognized. However, the scarce data available on the use of Azospirillum to improve lettuce growth before and after transplant under saline conditions prompted us to focus our research on this topic.

RESULTS

Early germination and seedling settlement of seeds exposed to 0 and 40 mol m(-3) NaCl were clearly improved by Azospirillum inoculation. At 0 mol m(-3) NaCl, plant establishment, leaf mass and root mass parameters before transplant were significantly higher in inoculated plants than in non-inoculated controls. At harvest, leaf fresh weight, ascorbic acid content and plant survival to transplant were also significantly higher in Azospirillum-inoculated plants grown at 0 mol m(-3) NaCl. In addition to these effects, leaf dry weight, area and chlorophyll content were also increased by Azospirillum inoculation when plants were grown at 40 mol m(-3) NaCl.

CONCLUSION

Azospirillum-inoculated lettuce seeds yield a higher number of transplanted plants with superior quality than non-inoculated controls grown at 0 or at 40 mol m(-3) NaCl.

Plant-rhizobacteria interactions alleviate abiotic stress conditions

Root-colonizing non-pathogenic bacteria can increase plant resistance to biotic and abiotic stress factors. Bacterial inoculates have been applied as biofertilizers and can increase the effectiveness of phytoremediation. Inoculating plants with non-pathogenic bacteria can provide 'bioprotection' against biotic stresses, and some root-colonizing bacteria increase tolerance against abiotic stresses such as drought, salinity and metal toxicity. Systematic identification of bacterial strains providing cross-protection against multiple stressors would be highly valuable for agricultural production in changing environmental conditions. For bacterial cross-protection to be an effective tool, a better understanding of the underlying morphological, physiological and molecular mechanisms of bacterially mediated stress tolerance, and the phenomenon of cross-protection is critical. Beneficial bacteria-mediated plant gene expression studies under non-stress conditions or during pathogenic rhizobacteria-plant interactions are plentiful, but only few molecular studies on beneficial interactions under abiotic stress situations have been reported. Thus, here we attempt an overview of current knowledge on physiological impacts and modes of action of bacterial mitigation of abiotic stress symptoms in plants. Where available, molecular data will be provided to support physiological or morphological observations. We indicate further research avenues to enable better use of cross-protection capacities of root-colonizing non-pathogenic bacteria in agricultural production systems affected by a changing climate.

Diversity, regulation, and evolution of the gibberellin biosynthetic pathway in fungi compared to plants and bacteria

Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. GAs were first isolated as metabolites of a fungal rice pathogen, Gibberella fujikuroi, since renamed Fusarium fujikuroi. Although higher plants and the fungus produce structurally identical GAs, significant differences in their GA pathways, enzymes involved and gene regulation became apparent with the identification of GA biosynthetic genes in Arabidopsis thaliana and F. fujikuroi. Recent identifications of GA biosynthetic gene clusters in two other fungi, Phaeosphaeria spp. and Sphaceloma manihoticola, and the high conservation of GA cluster organization in these distantly related fungal species indicate that fungi evolved GA and other diterpene biosynthetic pathways independently from plants. Furthermore, the occurrence of GAs and recent identification of the first GA biosynthetic genes in the bacterium Bradyrhizobium japonicum make it possible to study evolution of GA pathways in general. In this review, we summarize our current understanding of the GA biosynthesis pathway, specifically the genes and enzymes involved as well as gene regulation and localization in the genomes of different fungi and compare it with that in higher and lower plants and bacteria.

Aerobic nitric oxide production by Azospirillum brasilense Sp245 and its influence on root architecture in tomato

The major feature of the plant-growth-promoting bacteria Azospirillum brasilense is its ability to modify plant root architecture. In plants, nitric oxide (NO) mediates indole-3-acetic acid (IAA)-signaling pathways leading to both lateral (LR) and adventitious (AR) root formation. Here, we analyzed aerobic NO production by A. brasilense Sp245 wild type (wt) and its mutants Faj009 (IAA-attenuated) and Faj164 (periplasmic nitrate reductase negative), and its correlation with tomato root-growth-promoting effects. The wt and Faj009 strains produced 120 nmol NO per gram of bacteria in aerated nitrate-containing medium. In contrast, Faj164 produced 5.6 nmol NO per gram of bacteria, indicating that aerobic denitrification could be considered an important source of NO. Inoculation of tomato (Solanum lycopersicum Mill.) seedlings with both wt and Faj009 induced LR and AR development. In contrast, Faj164 mutant was not able to promote LR or AR when seedlings grew in nitrate. When NO was removed with the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), both LR and AR formation were inhibited, providing evidence that NO mediated Azospirillum-induced root branching. These results show that aerobic NO synthesis in A. brasilense could be achieved by different pathways and give evidence for an NO-dependent promoting activity on tomato root branching regardless of bacterial capacity for IAA synthesis.

Phytohormone production by three strains of Bradyrhizobium japonicum and possible physiological and technological implications

The aim of this work was to evaluate phytohormone biosynthesis, siderophores production, and phosphate solubilization in three strains (E109, USDA110, and SEMIA5080) of Bradyrhizobium japonicum, most commonly used for inoculation of soybean and nonlegumes in USA, Canada, and South America. Siderophore production and phosphate solubilization were evaluated in selective culture conditions, which had negative results. Indole-3-acetic acid (IAA), gibberellic acid (GA(3)), and abscisic acid (ABA) production were analyzed by gas chromatography-mass spectrometry (GC-MS). Ethylene and zeatin biosynthesis were determined by GS-flame ionization detection and high-performance liquid chromatography (HPLC-UV), respectively. IAA, zeatin, and GA(3) were found in all three strains; however, their levels were significantly higher (p < 0.01) in SEMIA5080 (3.8 microg ml(-1)), USDA110 (2.5 microg ml(-1)), and E109 (0.87 microg ml(-1)), respectively. ABA biosynthesis was detected only in USDA110 (0.019 microg ml(-1)). Ethylene was found in all three strains, with highest production rate (18.1 ng ml(-1) h(-1)) in E109 cultured in yeast extract mannitol medium plus L-methionine. This is the first report of IAA, GA(3), zeatin, ethylene, and ABA production by B. japonicum in pure cultures, using quantitative physicochemical methodology. The three strains have differential capability to produce the five major phytohormones and this fact may have an important technological implication for inoculant formulation.

Nitric oxide is involved in the Azospirillum brasilense-induced lateral root formation in tomato

Azospirillum spp. is a well known plant-growth-promoting rhizobacterium. Azospirillum-inoculated plants have shown to display enhanced lateral root and root hair development. These promoting effects have been attributed mainly to the production of hormone-like substances. Nitric oxide (NO) has recently been described to act as a signal molecule in the hormonal cascade leading to root formation. However, data on the possible role of NO in free-living diazotrophs associated to plant roots, is unavailable. In this work, NO production by Azospirillum brasilense Sp245 was detected by electron paramagnetic resonance (6.4 nmol. g-1 of bacteria) and confirmed by the NO-specific fluorescent probe 4,5-diaminofluorescein diacetate (DAF-2 DA). The observed green fluorescence was significantly diminished by the addition of the specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO). Azospirillum-inoculated and noninoculated tomato (Lycopersicon esculentum L.) roots were incubated with DAF-2 DA and examined by epifluorescence microscopy. Azospirillum-inoculated roots displayed higher fluorescence intensity which was located mainly at the vascular tissues and subepidermal cells of roots. The Azospirillum-mediated induction of lateral root formation (LRF) appears to be NO-dependent since it was completely blocked by treatment with cPTIO, whereas the addition of the NO donor sodium nitroprusside partially reverted the inhibitory effect of cPTIO. Overall, the results strongly support the participation of NO in the Azospirillum-promoted LRF in tomato seedlings.

Azospirillum-plant relationships: physiological, molecular, agricultural, and environmental advances (1997-2003)

This review presents a critical and comprehensive documentation and analysis of the developments in agricultural, environmental, molecular, and physiological studies related to Azospirillum cells, and to Azospirillum interactions with plants, based solely on information published between 1997 and 2003. It was designed as an update of previous reviews (Bashan and Levanony 1990; Bashan and Holguin 1997a), with a similar scope of interest. Apart from an update and critical analysis of the current knowledge, this review focuses on the central issues of Azospirillum research today, such as, (i) physiological and molecular studies as a general model for rhizosphere bacteria; (ii) co-inoculation with other microorganisms; (iii) hormonal studies and re-consideration of the nitrogen contribution by the bacteria under specific environmental conditions; (iv) proposed Azospirillum as a non-specific plant-growth-promoting bacterium; (v) re-introduction of the "Additive Hypothesis," which suggests involvement of multiple mechanisms employed by the bacteria to affect plant growth; (vi) comment on the less researched areas, such as inoculant and pesticide research; and (vii) proposes possible avenues for the exploitation of this bacterium in environmental areas other than agriculture.Key words: Azospirillum, plant–bacteria interaction, plant-growth-promoting bacteria, PGPB, PGPR, rhizosphere bacteria.