**4. Microbial mechanisms governing drought stress tolerance**

Microbes have the potential to promote plant growth directly and indirectly through several mechanisms. The indirect activation of plant growth involves a series of events by which microbes prevent the inhibition of plant growth and development induced by pathogens [32]. During direct activation, microbes biosynthesize bacterial compounds that promote the uptake of nutrients from the soil and stimulate plant growth and development [33]. Microbes trigger local or systemic stress mitigation response mechanisms that enable plants to survive and overcome the negative effects of abiotic stress conditions. The mechanisms governing microbial-mediated stress tolerance may include such as drought stress and help plants sustain growth and development through the production, mobilization of nutrients, and induction of the levels of hormones and organic phytostimulants [34]. Below are the fundamental mechanisms governing drought stress tolerance in plants.

#### **4.1 Microbial production of aminocyclopropane-1-carboxylate deaminase**

Aminocyclopropane-1-carboxylic acid (ACC) is a precursor of ethylene, and its production increases in plants during stress conditions. Plants enhance their ethylene production under drought stress, which inhibits plant growth by affecting root enlargement and seed germination. The production of higher ACC levels in plants is a strategy to combat severe drought stress [35]. A group of beneficial microbes have the potential to produce ACC deaminase that regulates plant growth and development by sequestering the plant-produced ACC, responsible for ethylene production in plants. A large number of microorganisms have been reported to produce ACC deaminase that in turn reduces ACC, thereby lowering the increased ethylene levels in plants under stress conditions [36]. These microbes play a vital role in plants' adaptation to stress conditions. In particular, drought stress tolerance has been achieved in several plants through the production of ACC deaminase. Some of the prominent examples of the microbial production of ACC deaminase and its mitigation effects on drought stress in several plants have been summarized (**Table 1**). The ACC deaminase production by *Bacillus subtilis* Rhizo SF 48 strain conferred maximum seed and plant growth promotion in tomato plants under drought stress [37]. The underlying biochemical mechanisms for this improved drought stress tolerance included induction in the proline, SOD, and APX activities, whereas reduction in the MDA and H2O2 contents. The maize plant-associated rhizospheric microbial species; that is, *Pseudomonas aeruginosa*, *Enterobacter cloacae*, *Achromobacter xylosoxidans,* and *Leclercia adecarboxylata* were reported to produce ACC deaminase that resulted into enhanced drought stress tolerance. The plants showed improved grain yield plant−1, photosynthetic rate, and stomatal conductance, enhanced chlorophyll a, total chlorophyll, and carotenoid contents under drought stress [38, 42]. Chandra et al. [40] reported ACC deaminase production in the wheat-associated microbes, that is, *Variovorax paradoxus* RAA3,


#### **Table 1.**

*Microbial ACC deaminase production confers drought stress tolerance in plants.*

*Pseudomonas* spp. *Achromobacter* spp. and *Ochrobactrum anthropi* DPC9 that improved plant growth and foliar nutrient concentrations in the wheat plants subjected to drought stress under glasshouse conditions. The plants also showed positive changes in the antioxidant properties under drought stress. Similarly, the rhizospheric *Serratia* 

#### *Microbial Mitigation of Drought Stress in Plants: Adaptations to Climate Change DOI: http://dx.doi.org/10.5772/intechopen.109669*

*marcescens* and *Pseudomonas* sp. conferred drought stress tolerance to wheat plants, which showed improved harvest index, water status, reactive oxygen species scavenging, osmolyte accumulation, chlorophyll and carotenoid content [40]. Moreover, the rhizospheric *Ochrobactrum pseudogrignonense* RJ12, *Pseudomonas* sp. RJ15, and *B. subtilis*RJ46 exhibited increased seed germination percentage, root length, shoot length, and dry weight of treated plants, *Vigna mungo* L. and *Pisum sativum* L under drought stress. The ACC deaminase production triggered the induction of ROS scavenging enzymes and cellular osmolytes, and higher leaf chlorophyll content. The microbial production of ACC deaminase showed growth improvement and drought stress tolerance in other plants such as *Capsicum anum*, *Vitis vinifera,* and *Cyamopsis tetragonoloba* [39, 41, 43].

#### **4.2 Microbial production of phytohormone**

Phytohormones such as indole acetic acid (IAA), gibberellins, ABA, ethylene, and cytokinin are organic chemical messengers that coordinate cellular events in plants and, therefore, play a crucial role in plant development and drought stress tolerance [44]. These phytohormones are generally produced by plants; however, they also produced by some plant-associated microorganisms. Phytohormones such as auxins indirectly regulate drought stress through modification of root growth and root hairs in a manner that enable plants to absorb maximum water and nutrients from the soil. In plants, auxins are produced through the tryptophan-dependent pathways [46]. Downward in the tryptophan pathway, indole-3-acetamide is produced that is converted into indole-3-acetaldoxime and tryptamine that further give indole-3-pyruvic acid, the final product [47]. The role of IAA in drought stress mitigation through inducing the drought signaling pathways has been reported in several studies [48]. Auxin production has the potential to induce the elongation of stems and coleoptiles of plants under stress conditions; thus, its production in the microbe-treated plants may trigger such modifications [49]. In a similar passion, plant-associated microbes also induce the production of plant gibberellins. These are diterpeniods and are responsible for the hyperactive elongation of stems under stress conditions. Gibberellins are in association with carotenes and isoprene, bioactive compounds [50]. Carotenes protect the plant cells from harmful photodynamic reactions through triggering the photosynthesis, whereas isoprene regulates the turgor pressure that provides stability to the cell membranes [51]. In plants, the cytokinin biosynthesis increases in association with auxin and regulation of developmental responses under abiotic stress conditions. Cytokinin mediates in the phosphorylation of sugars leading to cellular accumulation in cells and also helps to prevent the reverse diffusion of sugars [52]. This cytokinin-mediated competitive phosphorylation enables plant cells to adapt to stress and provides protection against the damaging effects of oxidative stress [53]. Abscisic acid is generally known as a universal stress hormone because of its very important in plant adaptation to stress condition. In plants, the stomatal closure and inhibitor of stomatal opening are the underling mechanisms of the ABA-mediated stress tolerance [54]. The ABA production also confers desiccation tolerance through regulation of gene expression [55].

The ethylene production in plants plays a significant role in regulation of plant growth and senescence. In interaction with other hormones, ethylene serves as a messenger hormone that regulates developmental processes, ranging from seed germination to the plant vegetative and reproductive stages [56].

The plant-associated microbes have the potential to produce phytohormones, which in turn help plants to adapt to the stress condition. Phytohormones mitigate stress through triggering a complex signaling network of genes and production of metabolites, which protect the inner cellular machinery and function as a response to environmental stress condition. The underlying mechanisms may include hormone regulation, and production of osmolytes and antioxidant enzymes.

#### **4.3 Osmotic adjustment**

Plants experience a sudden osmotic shock upon exposure to abiotic stresses. However, the plant-associated microbial communities and their interactions assist in osmotic adjustments. In response to drought stress, the plant-microbe interactions are governed by a series of biochemical and molecular changes leading to secretion of metabolites such as glycine, proline, organic acids, sugars, betaine, trehalose, calcium, chloride, and potassium ions. Proline is one of the most important osmolytes that is accumulated in plants and provides maintenance and protection to vital cellular organs as an adaptation to osmotic stress. Proline production has been reported in several rhizospheric bacteria [57]. Trehalose, a non-reducing sugar, is produced in plants under stress condition. Trehalose contains two glucose molecules that store energy for utilization under stress conditions. Trehalose biosynthesis in microbes is accelerated through the TPS/trehalose-6-phosphate phosphatases (TPS/TPP) pathway. Trehalose stabilizes turgor pressure and maintains osmotic adjustment in plant cells [58]. Production of organic acids has been one of the key mechanisms that microbes utilize to benefit the associated plants. Microbial inoculation of plants triggers the secretion of organic acids such as oxalic acid, malic acid, citric acid, and minerals, for example, chlorine, potassium, and sodium. These are very important for metabolic reactions, maintenance of osmoregulation, and nutrient availability in plant cells [59].

#### **4.4 Microbial production of exopolysaccharides for drought stress mitigation**

Exopolysaccharides are long-chain polymers of repeating sugar units (e.g., glucose, galactose, and rhannose) [60]. Exopolysaccharides play a crucial role by forming hydrophilic biofilms, which provide protection against aridness during osmotic stress. Exopolysaccharides enhance the water-retaining potential and regulate the distribution of biological carbon sources in the soil. Microbes protect the roots from dehydration and maintain the moisture content by forming sheaths of exopolysaccharides [61]. Microbes release exopolysaccharides in the soil as slime ingredients comprising van der Waals linkages, anion adsorption interactions, and cation hydrogen bridges, which improve the biological properties of the soil [62]. The vital microbe-plant interaction regulates the production of biofilms, which facilitate microbial attachment to the plant roots, and imparts a strong root adhering capability. In conclusion, the microbial production of exopolysaccharide is one of the important strategies that protect plants against the damaging effects of abiotic stresses including drought stress [63].

#### **4.5 Effects of microbial volatile organic compounds against osmotic stress**

The plant growth-promoting microbes produce volatile compounds, which increase plant growth and development, iron uptake, photosynthesis, and overall

### *Microbial Mitigation of Drought Stress in Plants: Adaptations to Climate Change DOI: http://dx.doi.org/10.5772/intechopen.109669*

crop productivity. Organic acid production helps plants against disease-causing pathogens. The stress-induced organic compounds such as 2-pentylfuran, 3-hydroxy-2-butanone, and 2,3-butanediol play an important role in plant growth and development [64]. These compounds regulate stomatal closure and impart systemic stress resistance, and thus ensure plant growth and development under abiotic stress conditions [65]. These volatile compounds help boost plant growth by acting as insect repellents owing to their strong odor. The microbial production of these compounds triggers stress tolerance in the associated plants through inducing the biosynthesis of ROS scavengers and gene expression.
