Mechanisms and Applications of Bacterial Inoculants in Plant Drought Stress Tolerance

Abstract

Agricultural systems are highly affected by climatic factors such as temperature, rain, humidity, wind, and solar radiation, so the climate and its changes are major risk factors for agricultural activities. A small portion of the agricultural areas of Brazil is irrigated, while the vast majority directly depends on the natural variations of the rains. The increase in temperatures due to climate change will lead to increased water consumption by farmers and a reduction in water availability, putting production capacity at risk. Drought is a limiting environmental factor for plant growth and one of the natural phenomena that most affects agricultural productivity. The response of plants to water stress is complex and involves coordination between gene expression and its integration with hormones. Studies suggest that bacteria have mechanisms to mitigate the effects of water stress and promote more significant growth in these plant species. The underlined mechanism involves root-to-shoot phenotypic changes in growth rate, architecture, hydraulic conductivity, water conservation, plant cell protection, and damage restoration through integrating phytohormones modulation, stress-induced enzymatic apparatus, and metabolites. Thus, this review aims to demonstrate how plant growth-promoting bacteria could mitigate negative responses in plants exposed to water stress and provide examples of technological conversion applied to agroecosystems.

Keywords: PGPB; abiotic stress; bioinoculant; endophytic bacteria; sustainable agriculture; water-use efficiency.

Figure 1

Schematic illustration of the plant growth promotion mechanisms by PGPB. The direct mechanism includes biological nitrogen fixation (BNF) by the activity of the nitrogenase enzyme complex; solubilization of inorganic phosphate in the soil; production of siderophores, increasing the availability of iron, and the production of hormones such as auxins, gibberellins, and cytokinin that modulate the hormonal balance of the plant host. Indirect mechanisms are related to the occupation of niches by PGPB and the production of substances with repelling functions, preventing colonization by phytopathogens and nematodes.

Figure 2

Microorganism–plant interaction. Plant roots and bacterial cells synthesize metabolites as substrates and signaling molecules. Microorganisms used as biofertilizers promote plant growth through biological nitrogen fixation, nutrient solubilization (phosphate and iron), and the production of hormones and other compounds. A dashed line indicates a positive relationship between plants and bacteria. Abbreviations: ACC, 1-aminocyclopropane-1-carboxylate; ACC deaminase, 1-aminocyclopropane-1 carboxylate deaminase; BNF, biological nitrogen fixation. Organisms and cells are not to scale.

Figure 3

Schematic representation of bacteria with ACC deaminase activity. Abbreviations: AIA, auxin; SAM, S-adenosylmethionine; ACC, 1-aminocyclopropane-1-carboxylate; ACC deaminase, 1-aminocyclopropane-1-carboxylate deaminase; BNF, biological nitrogen fixation. Organisms and cells are not to scale.