The worldwide increases in both environmental damage and human population pressure have the unfortunate consequence that global food production may soon become insufficient to feed all of the world's people. It is therefore essential that agricultural productivity be significantly increased within the next few decades. To this end, agricultural practice is moving toward a more sustainable and environmentally friendly approach. This includes both the increasing use of transgenic plants and plant growth-promoting bacteria as a part of mainstream agricultural practice. Here, a number of the mechanisms utilized by plant growth-promoting bacteria are discussed and considered. It is envisioned that in the not too distant future, plant growth-promoting bacteria (PGPB) will begin to replace the use of chemicals in agriculture, horticulture, silviculture, and environmental cleanup strategies. While there may not be one simple strategy that can effectively promote the growth of all plants under all conditions, some of the strategies that are discussed already show great promise.

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Plant Growth-Promoting Bacteria: Mechanisms and Applications

Enhancement of drought stress tolerance in crops by plant growth promoting rhizobacteria.

During the last 35 years of studies of Azospirillum–plant interaction, over 20 proposals were suggested for the mechanism of action by which Azospirillum spp., the most intensively studied plant growth-promoting bacteria, enhances plant growth. The proposals include a single phytohormone activity, multiple phytohormones, nitrogen fixation, assortments of small-sized molecules and enzymes, enhanced membrane activity, proliferation of the root system, enhanced water and mineral uptake, mobilization of minerals, mitigation of environmental stressors of plants, and direct and indirect biological control of numerous phytopathogens. By volume, the largest number of published information involves hormonal activities, nitrogen fixation, and root proliferation. After analyzing the accumulated knowledge, it was concluded that this versatile genus possesses a large array of potential mechanisms by which it can effect plant growth. Consequently, this review proposes the “Multiple Mechanisms Theory,” based on the assumption that there is no single mechanism involved in promotion of plant growth by Azospirillum, but a combination of a few or many mechanisms in each case of inoculation. These may vary according to the plant species, the Azospirillum strain, and environmental conditions when the interaction occurred. The effect can be cumulative, an “additive hypothesis” (proposed before), where the effects of small mechanisms operating at the same time or consecutively create a larger final effect on plant. Additionally, the observed effect on plant growth can be the result of a tandem or a cascade of mechanisms in which one mechanism stimulates another, yielding enhanced plant growth, such as the plausible relations among phytohormones, nitric oxide, membrane activities, and proliferation of roots. Finally, the growth promotion can also be a combination of unrelated mechanisms that operate under environmental or agricultural conditions needed by the crop at particular locations, such as mitigating stress (salt, drought, toxic compounds, adverse environment), and the need for biological control of or reducing pathogenic microflora.

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How the Plant Growth-Promoting Bacterium Azospirillum Promotes Plant Growth—A Critical Assessment

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1574-6941.2010.00900.x

Climate change effects on beneficial plant–microorganism interactions

In mammals, cadmium is widely considered as a non-genotoxic carcinogen acting through a methylation-dependent epigenetic mechanism. Here, the effects of Cd treatment on the DNA methylation patten are examined together with its effect on chromatin reconfiguration in Posidonia oceanica. DNA methylation level and pattern were analysed in actively growing organs, under short- (6 h) and long- (2 d or 4 d) term and low (10 mM) and high (50 mM) doses of Cd, through a Methylation-Sensitive Amplification Polymorphism technique and an immunocytological approach, respectively. The expression of one member of the CHROMOMETHYLASE (CMT) family, a DNA methyltransferase, was also assessed by qRT-PCR. Nuclear chromatin ultrastructure was investigated by transmission electron microscopy. Cd treatment induced a DNA hypermethylation, as well as an up-regulation of CMT, indicating that de novo methylation did indeed occur. Moreover, a high dose of Cd led to a progressive heterochromatinization of interphase nuclei and apoptotic figures were also observed after long-term treatment. The data demonstrate that Cd perturbs the DNA methylation status through the involvement of a specific methyltransferase. Such changes are linked to nuclear chromatin reconfiguration likely to establish a new balance of expressed/repressed chromatin. Overall, the data show an epigenetic basis to the mechanism underlying Cd toxicity in plants.

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In Posidonia oceanica cadmium induces changes in DNA methylation and chromatin patterning

Bacteria of the genus Azospirillum increase the grain yield of several grass crops. In this work the effect of inoculating maize plants with genetically engineered Azospirillum brasilense for trehalose biosynthesis was determined. Transformed bacteria with a plasmid harboring a trehalose biosynthesis gene-fusion from Saccharomyces cerevisiae were able to grow up to 0.5M NaCl and to accumulate trehalose, whereas wild-type A. brasilense did not tolerate osmotic stress or accumulate significant levels of the disaccharide. Moreover, 85% of maize plants inoculated with transformed A. brasilense survived drought stress, in contrast with only 55% of plants inoculated with the wild-type strain. A 73% increase in biomass of maize plants inoculated with transformed A. brasilense compared with inoculation with the wild-type strain was found. In addition, there was a significant increase of leaf and root length in maize plants inoculated with transformed A. brasilense. Therefore, inoculation of maize plants with A. brasilense containing higher levels of trehalose confers drought tolerance and a significant increase in leaf and root biomass. This work opens the possibility that A. brasilense modified with a chimeric trehalose biosynthetic gene from yeast could increase the biomass, grain yield and stress tolerance in other relevant crops.

Julieta Rodríguez-Salazar

Papers

Trehalose accumulation in Azospirillum brasilense improves drought tolerance and biomass in maize plants

Evaluation of a bacterial group 1 LEA protein as an enzyme protectant from stress-induced inactivation