Response of Azospirillum brasilense Cd to sodium chloride stress

Molecular Mechanisms Determining the Role of Bacteria from the Genus Azospirillum in Plant Adaptation to Damaging Environmental Factors

Agricultural plants are continuously exposed to environmental stressors, which can lead to a significant reduction in yield and even the death of plants. One of the ways to mitigate stress impacts is the inoculation of plant growth-promoting rhizobacteria (PGPR), including bacteria from the genus Azospirillum, into the rhizosphere of plants. Different representatives of this genus have different sensitivities or resistances to osmotic stress, pesticides, heavy metals, hydrocarbons, and perchlorate and also have the ability to mitigate the consequences of such stresses for plants. Bacteria from the genus Azospirillum contribute to the bioremediation of polluted soils and induce systemic resistance and have a positive effect on plants under stress by synthesizing siderophores and polysaccharides and modulating the levels of phytohormones, osmolytes, and volatile organic compounds in plants, as well as altering the efficiency of photosynthesis and the antioxidant defense system. In this review, we focus on molecular genetic features that provide bacterial resistance to various stress factors as well as on Azospirillum-related pathways for increasing plant resistance to unfavorable anthropogenic and natural factors.

Microorganisms associated to tomato seedlings growing in saline culture act as osmoprotectant

Less than 0.5% of total water in the world is available for human consumption and agriculture. The major part of the world’s water is saline and salinity in soils interferes in germination of seeds and the posterior development of the plant. In order to increase the osmotolerance of tomato, seedlings were associated with Azospirillum brasilense Cd, Azospirillum brasilense Cd transformed bacteria with a plasmid harboring a trehalose biosynthesis gene-fusion or Chlorella vulgaris. Two plant culture media: Hydroponic and Murashige and Skoog were tested. In the first set of studies seedlings were associated to single free cells meanwhile in a second set single and combined free cells were studied. A positive interaction between transformed Azospirillum and Chlorella vulagris and tomato plants was observed. Seedlings showed a salt concentration tolerance, as sodium chloride, up to 200 mM. According to our results, the association of plants with A. brasilense Cd-BIF and C. vulgaris is a viable approach to increase their salt tolerance and biomass, as consequence the possible use of sea water to irrigate horticultural plants.

Responses of Azospirillum brasilense to nitrogen deficiency and to wheat lectin: a diffuse reflectance infrared fourier transform (DRIFT) spectroscopic study

For the rhizobacterium Azospirillum brasilense, the optimal nutritional range of C:N ratios corresponds to the presence of malate (ca. 3 to 5 g l(-1) of its sodium salt) and ammonium (ca. 0.5 to 3 g l(-1) of NH4Cl) as preferred carbon and nitrogen sources, respectively. This microaerophilic aerotactic bacterium is known to have a narrow optimal oxygen concentration range of ca. 3 to 5 microM, which is 1.2% to 2% of oxygen solubility in air-saturated water under normal conditions. In this work, the effects of stress conditions (bound-nitrogen deficiency related to a high C:N ratio in the medium; excess of oxygen) on aerobically grown A. brasilense Sp245, a native wheat-associated endophyte, were investigated in the absence and presence of wheat germ agglutinin (WGA, plant stress protein and a molecular host-plant signal for the bacterium) using FTIR spectroscopy of whole cells in the diffuse reflectance mode (DRIFT). The nutritional stress resulted in the appearance of prominent spectroscopic signs of poly-3-hydroxybutyrate (PHB) accumulation in the bacterial cells; in addition, splitting of the amide I band related to bacterial cellular proteins indicated some stress-induced alterations in their secondary structure components. Similar structural changes were observed in the presence of nanomolar WGA both in stressed A. brasilense cells and under normal nutritional conditions. Comparative analysis of the data obtained and the relevant literature data indicated that the stress conditions applied (which resulted in the accumulation of PHB involved in stress tolerance) and/or the presence of nanomolar concentrations of WGA induced synthesis of bacterial cell-surface (glyco)proteins rich in beta-structures, that could be represented by hemagglutinin and/or porin.

Glutamate Is Involved in Acid Stress Response in Bradyrhizobium sp. SEMIA 6144 (Arachis hypogaea L.) Microsymbiont

In the present study, the effect of acid stress on ammonium assimilation in Bradyrhizobium sp. SEMIA 6144 (Arachis hypogaea L.) microsymbiont was analyzed. The bacterial growth rate was decreased by 50%, and a significant increase in intracellular glutamate concentration was detected when the strain grew at acid pH (5.5). Assays of the enzymes involved in glutamate synthesis showed increased activities of glutamine synthetase (GS) and glutamate synthase (NADPH-GOGAT) under acid stress condition. This would support the contention that the GS/NADPH-GOGAT pathway contributes to the increase of glutamate synthesis as a compatible solute in response to acid stress.

Cultivation factors and population size control the uptake of nitrogen by the microalgae Chlorella vulgaris when interacting with the microalgae growth-promoting bacterium Azospirillum brasilense

Growth of and the capacity to take up nitrogen in the freshwater microalgae Chlorella vulgaris were studied while varying the concentrations of ammonium and nitrate, the pH and the source of carbon in a synthetic wastewater growth medium when co-immobilized in alginate beads with the microalgae growth-promoting bacterium Azospirillum brasilense. Analyses of 29 independent experiments showed that co-immobilization of the microalgae with A. brasilense could result in two independent phenomena directly affected by cultivation factors, such as nitrogen species, pH and presence of a carbon source. First, growth of the microalgal population increased without an increase in the capacity of the single cells to take up nitrogen, or second, the capacity of cells to take up nitrogen increased without an increase of the total microalgal population. These phenomena were dependent on the population density of the microalgae, which was in turn affected by cultivation factors. This supports the conclusion that the size of the microalgal population controls the uptake of nitrogen in C. vulgaris cells - the higher the population (regardless the experimental parameters), the less nitrogen each cell takes up.

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.