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Considering the more frequent and longer drought events due to climate change, improving plant drought tolerance became a priority. The search for plant growth promoting rhizobacteria (PGPR) able to improve plant drought tolerance has been long addressed, but with inconsistent results. Here, we summarize the PGPR mechanisms that improve plant droug...
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The effects of climate change in the form of drought, heat stress, and irregular seasonal changes threaten global crop production. The ability of multi-omics data, such as transcripts and proteins, to reflect a plants response to such climatic factors can be capitalized in prediction models to maximize crop improvement. Implementing multi-omics cha...
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... It's interesting how root-associated microbes can help crops during drought by activating various molecular processes (Rosa et al., 2023). In a study involving rice, researchers found that when osmotolerant rhizobacteria were introduced, there was an increase in both root and shoot dry weight. ...
... In contrast, the nitrogen fixation and phosphate solubilization activities are crucial for plant growth, especially under nutrient-deficient conditions; thus, these functions may be preserved or regulated through other mechanisms to adapt to salt stress. The results presented in this paper also suggest that the ability of the strains to secrete IAA can serve as a preferred indicator for the preliminary screening of salt-tolerant PGPR, which would significantly improve the efficiency of selecting salt-tolerant PGPR [43]. ...
Alfalfa (Medicago sativa L.), a forage legume known for its moderate salt–alkali tolerance, offers notable economic and ecological benefits and aids in soil amelioration when cultivated in saline–alkaline soils. Nonetheless, the limited stress resistance of alfalfa could curtail its productivity. This study investigated the salt tolerance and growth-promoting characteristics (in vitro) of four strains of plant growth-promoting rhizobacteria (PGPR) that were pre-selected, as well as their effects on alfalfa at different growth stages (a pot experiment). The results showed that the selected strains belonged to the genera Priestia (HL3), Bacillus (HL6 and HG12), and Paenibacillus (HG24). All four strains exhibited the ability to solubilize phosphate and produce indole-3-acetic acid (IAA) and 1-aminocyclopropane-1-carboxylate (ACC) deaminase. Among them, except for strain HG24, the other strains could tolerate 9% NaCl stress. Treatment with 100 mM NaCl consistently decreased the IAA production levels of the selected strains, but inconsistent changes (either enhanced or reduced) in terms of phosphate solubilization, ACC deaminase, and exopolysaccharides (EPS) production were observed among the strains. During the various growth stages of alfalfa, PGPR exhibited different growth-promoting effects: at the seedling stage, they enhanced salt tolerance through the induction of physiological changes; at the flowering stage, they promoted growth through nutrient acquisition. The current findings suggest that strains HL3, HL6, and HG12 are effective microbial inoculants for alleviating salt stress in alfalfa plants in arid and semi-arid regions. This study not only reveals the potential of indigenous salt-tolerant PGPR in enhancing the salt tolerance of alfalfa but also provides new insights into the mechanisms of action of PGPR.
... Furthermore, bacterial biodiversity is decreased in the soil but increased in the root endosphere [92]. In rice plants, Si et al. [92] found that drought exerts a negligible effect on the alpha diversity of rhizosphere bacterial communities, but substantially enriches Actinobacteria and decreases Firmicutes [93]. Similarly, Santos-Medellin et al. [94] concluded that drought alters bacterial composition in the endosphere and rhizosphere compartments of rice, characterized by an enrichment of Actinobacteria and Chloroflexi but a depletion of Acidobacteria and Deltaproteobacteria. ...
... In conclusion, Actinobacteria enrichment within drought-stressed root microbiomes is strongly conserved among evolutionarily diverse plant species. Moreover, decreases in the phyla Proteobacteria and Verrucomicrobia, as well as increases in the ratio of Gram-positive to Gram-negative bacteria, are also frequently observed under drought conditions [93,97,98]. Drought significantly restructures the sorghum root microbiome composition and functionality during its early stage, which coincides with an increased abundance and activity of monodermal bacteria [99]. ...
Drought stress is an annual global phenomenon that has devastating effects on crop production, so numerous studies have been conducted to improve crop drought resistance. Plant-associated microbiota play a crucial role in crop health and growth; however, we have a limited understanding of the key processes involved in microbiome-induced crop adaptation to drought stress. In this review, we summarize the adverse effects of drought stress on crop growth in terms of germination, photosynthesis, nutrient uptake, biomass, and yield, with a focus on the response of soil microbial communities to drought stress and plant-microbe interactions under drought stress. Moreover, we review the morpho-physiological, biochemical, and molecular mechanisms underlying the mitigation effect of microbes on crop drought stress. Finally, we highlight future research directions, including the characterization of specific rhizosphere microbiome species with corresponding root exudates and the efficiency of rhizobacteria inoculants under drought conditions. Such research will advance our understanding of the complex interactions between crops and microbes and improve crop resistance to drought stress through the application of beneficial drought-adaptive microbes.
... The synthesis of phytohormones by Pseudomonas, Bacillus and Rhizobium isolated by PGPR is conducted to stimulate plant growth and overcome the stress of drought stress [114]. Under arid conditions, the introduction of beneficial microbial agents can generate extracellular polysaccharides (EPS), synthesize proline, and secrete phenolic compounds, and regulate plant growth and development to resist dehydration stress [115]. Salicylic acid (SA), primarily produced by microorganism-derived phenolic compounds, serves as a critical signaling molecule in arid conditions. ...
More food is needed to meet the demand of the global population, which is growing continuously. Chemical fertilizers have been used for a long time to increase crop yields, and may have negative effect on human health and the agricultural environment. In order to make ongoing agricultural development more sustainable, the use of chemical fertilizers will likely have to be reduced. Microbial fertilizer is a kind of nutrient-rich and environmentally friendly biological fertilizer made from plant growth-promoting bacteria (PGPR). Microbial fertilizers can regulate soil nutrient dynamics and promote soil nutrient cycling by improving soil microbial community changes. This process helps restore the soil ecosystem, which in turn promotes nutrient uptake, regulates crop growth, and enhances crop resistance to biotic and abiotic stresses. This paper reviews the classification of microbial fertilizers and their function in regulating crop growth, nitrogen fixation, phosphorus, potassium solubilization, and the production of phytohormones. We also summarize the role of PGPR in helping crops against biotic and abiotic stresses. Finally, we discuss the function and the mechanism of applying microbial fertilizers in soil remediation. This review helps us understand the research progress of microbial fertilizer and provides new perspectives regarding the future development of microbial agent in sustainable agriculture.
... Furthermore, the selection of microbial strains for the inoculation is generally performed under controlled laboratory conditions, which produce more reproducible results. Nevertheless, conditions at the rhizosphere are very different; thus, result outputs could not be as per choice (Rosa et al., 2023). ...
... Organic matter is a parameter of major importance to determine soil health and productivity [88,89]. Microorganisms are major participants in maintaining soil functioning [90][91][92], including the working of biogeochemical cycles of elements sustaining plant growth and interactions with plants [27,90,[92][93][94]. During extreme events (i.e., extreme temperature and desiccation), soil thermophiles can perform some of these functions during periods when other microorganisms are inhibited. ...
... Soil thermophiles' activity during hot and dry periods can represent an additional level of redundancy that enhances the role of microbial diversity by providing pathways leading to nutrient cycling and sustainability during extreme conditions [1,94,104,116,117]. Soil health requires microbial diversity so that changes can be compensated through different alternative pathways and soil functioning can continue providing similar services [6,8,118]. ...
... In order to confirm soil thermophiles' positive effects on plant growth, a series of studies have shown the potential capabilities of those soil thermophiles to enhance plant growth [62,85,93] and drought tolerance [62,94]. These reports showed that plants significantly benefit from soil thermophiles' activities, which contribute to compensating nutrient deficits by increasing decomposition of soil organic matter and releasing essential plant nutrients. ...
During this century, a number of reports have described the potential roles of thermophiles in the upper soil layers during high-temperature periods. This study evaluates the capabilities of these microorganisms and proposes some potential consequences and risks associated with the activity of soil thermophiles. They are active in organic matter mineralization, releasing inorganic nutrients (C, S, N, P) that otherwise remain trapped in the organic complexity of soil. To process complex organic compounds in soils, these thermophiles require extracellular enzymes to break down large polymers into simple compounds, which can be incorporated into the cells and processed. Soil thermophiles are able to adapt their extracellular enzyme activities to environmental conditions. These enzymes can present optimum activity under high temperatures and reduced water content. Consequently, these microorganisms have been shown to actively process and decompose substances (including pollutants) under extreme conditions (i.e., desiccation and heat) in soils. While nutrient cycling is a highly beneficial process to maintain soil service quality, progressive warming can lead to excessive activity of soil thermophiles and their extracellular enzymes. If this activity is too high, it may lead to reduction in soil organic matter, nutrient impoverishment and to an increased risk of aridity. This is a clear example of a potential effect of future predicted climate warming directly caused by soil microorganisms with major consequences for our understanding of ecosystem functioning, soil health and the risk of soil aridity.
Climate change has exacerbated the impact of abiotic stresses, mainly drought, on plant production. Plant selection, breeding, and genetic engineering to increase drought tolerance are costly and time-consuming. To mitigate drought stress, plants employ adaptive mechanisms and interact with beneficial microorganisms, such as plant growth-promoting rhizobacteria (PGPR). Inoculating plant roots with various PGPR species promotes drought tolerance through a network of cellular, physiological, and biochemical mechanisms, including enhanced root elongation, increased phytohormone production, and synthesis of volatile organic compounds. PGPR colonization represents an environmentally sustainable agricultural technique that enhances plant growth, development, and yield by facilitating improved tolerance to environmental challenges. The current review provides an overview of the impact of drought stress on plant growth and development, detailing how PGPR induce physiological, morphological, and molecular responses to mitigate drought stress.
Climate change will certainly intensify the prevalence of drought stress as a single most stressful environmental factor that put down the agri-industry, as it affects the water relations of a plant at the cellular and whole plant level, decreasing productivity and causing economic losses in agriculture. Furthermore, the frequency of drought occurrence increases every year eroding more and more agricultural land throughout the world, resulting in a 9–10% reduction in crop production which threatens world food security. Generally, drought stress can be addressed by breeding drought-resistant cultivars, adjusting crop calendars, strategic planning, conventional breeding and genetically engineered drought-resistant plants. However, the technical, economic and ecological limitations of these strategies have sparked interest in the exploration of alternative low-cost, natural and ecologically friendly approaches, i.e., the use of plant growth-promoting beneficial rhizobacterial biomes. Co-inoculation of plants with beneficial bacterial biomes that are adapted to moisture stress conditions may promote plant growth and protect the crops plants from the deleterious effects of extreme drought stress.
In the present chapter, we attempt to overview the current knowledge on how plant–rhizobacteria interactions help in alleviating drought stress naturally and their usage for sustainable crop production.
