**5. Microbes and drought stress mitigation**

#### **5.1 Plant growth-promoting rhizospheric bacteria**

The plant growth-promoting rhizobacteria (PGPR) have the potential to mitigate drought stress and alleviate the negative effects of climate change on plant growth and development in a sustainable way [73]. These microbes trigger the onset of biochemical changes, which enable the plant to set a response to alleviate drought stress [74]. The underlying mechanisms include optimization of exopolysaccharides and phytohormone production, antioxidant defense system, and cyclic metabolic pathways, involved in the deposition of sugars, polyamines, amino acids, and heat-shock protein synthesis [75]. Several studies have reported the positive effects of PGPR on plant growth and development under drought stress (**Table 2**). Particularly in wheat, the inoculation of PGPR mitigated drought stress. In one study, Li et al. [76] reported growth improvement in wheat plants upon inoculation with the Actinomycetes, *Streptomyces pactum* Act12. The bacterial inoculation significantly increased the overexpression of several genes including P5CS, EXPA6, SnRK2, and EXPA2. Overall, the root length, shoot length, and fresh biomass were significantly increased. Enhanced levels of sugars and antioxidant enzymes were detected in the exposed seedlings under stress condition. Inoculation of wheat plants with *Pseudomonas libanensis* EU-LWNA-33 increased the root length and biomass under drought stress [77]. The biochemical analysis revealed an increased production of osmolytes, that is, proline and glycine betaine. At the cellular level, proline and glycine betaine production regulates osmotic homeostasis, as well as the phosphorus solubilization and uptake. Phosphorus availability is a crucial factor in the overall growth and development of plants. In this study, the inoculated strains showed solubilization of phosphorus. In previous studies, Jochum et al. [79] reported drought stress tolerance in wheat and maize plants when inoculated with *Bacillus* sp. 12D6 and *Enterobacter* sp. 16i. The inoculation improved root length, surface area, and plant productivity. The study further revealed that *Bacillus* sp. 12D6 was comparatively more effective in countering drought stress. This enhanced drought stress mitigation was possible due to the production of phytohormones such as IAA and salicylic acid. In another study, Raheem et al. [80] isolated and investigated the impact of PGPR, namely *Bacillus*, *Moraxella, Enterobacter*, and *Pseudomonas,* on wheat plants under drought stress. Biochemical analysis of the inoculated stressed plants revealed production of increased levels of auxin that obviously helped plants to avoid the negative impact of drought stress. It was further concluded that the enhanced auxin production triggered by the *Bacillus* species improved the field capacity by 10% and crop yield by 34%. The drought stress mitigation effects of the plant growth-promoting bacteria (PGPB), *Azospirillum* were investigated in wheat plants [78]. The stressed plants showed drought tolerance, which was attributed to the microbial-mediated production of phytohormones, solutes, ACC deaminase, exopolysaccharides, chlorophyll synthesis, and increased mineral solubilization.

Drought stress imposes osmotic and oxidative stresses, which negatively affect the crops' growth and productivity. Microbial inoculation has been the most preferred strategy to reduce stress-associated losses in crop plants. In this connection, Kour et al. [81] investigated the effects of bacterial inoculation on foxtail millet crop subjected to drought stress. Inoculation of plants with *Acinetobacter calcoaceticus* EU-LRNA-72 and *Penicillium* sp. EU-FTF-6 showed drought stress tolerance. The drought stress mitigation was mainly due to the accumulation of osmolytes


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

#### **Table 2.**

*Rhizospheric plant-growth-promoting bacteria and drought stress mitigation in plants.*

(proline and glycine betaine) and increased levels of chlorophyll a and b contents. In this study, the increased proline and glycine betaine levels improved osmotic adjustment and membrane integrity, while the increased chlorophyll content resulted in plant growth and development.

In one study, Chiappero et al. [82] investigated the positive impact of PGPR inoculation on peppermint subjected to drought stress. Two rhizospheric bacteria, *Pseudomonas fluorescens* WCS417r and *Bacillus amyloliquefaciens* GB03, were used in the inoculation and drought experiment. The results revealed a significant improvement in drought stress tolerance, which was attributed mainly to the upregulation of the antioxidant defense system and phenolic components.

The effects of bacterial inoculation on drought stress were further tested in soybean plants [83]. The plants were inoculated with *Bacillus thuringiensis, Bacillus cereus*, and *B. subtilis* strains. The inoculation of plants with these strains resulted into an improved efficiency of the photosystem II (PS-II) and maintained the overall photosynthetic rates of the plants, transpiration rate, and stomatal conductance, which in turn improved the overall growth of inoculated plants compared with that of control plants. In addition, the genomic analysis revealed that the overexpression of *Gmdreb1a* might partly be responsible for drought stress mitigation.

The PGPBs have proven their potential as ecofriendly biofertilizers that can alleviate the negative effects of drought stress on plants. In a previous study, the PGPR strains, *Pseudomonas putida* and *B. amyloliquefaciens,* were isolated from alkaline

soils and then were used in the inoculation of chickpea plants under drought stress in the greenhouse and *in vitro* experiments [84]. The inoculation of plants with the strains in combination showed increased chlorophyll content, osmolyte production, and improved photosynthesis and biomass compared with plants inoculated with a single strain. In conclusion, the PGPR enhances the overall growth and development, as well as the biotic and abiotic stress tolerance of plants through a wide range of mechanisms.

#### **5.2 Endophytic bacteria and fungi**

Endophytes bacteria and fungi reside in different organs and tissues of plants and establish symbiotic relationship. The endophytes get their prepared food, while plants are benefited in different ways such as access to limited nutrients in the soil and biotic and abiotic stress tolerance. Endophytes have been specifically focused due to their crucial role in abiotic stress tolerance of plants [85]. It was previously reported that endophytes assist their host plants to increase their biomass under stress conditions [86]. However, different plant species showed variable levels of endophytic-mediated biomass accumulation under stress condition. For example, eudicots and C4 plants exhibited increased biomass accumulation compared with C3 and monocots [87].

Endophytic microbes play a very important role in reducing the damaging effects of abiotic stresses on plants. Several studies have demonstrated the drought stress mitigation in plants with endophytic bacterial inoculation (**Table 3**). Previously, Singh et al. [88] investigated the inoculation effects of endophytic bacterial strains, *Trichoderma* T42 and *Pseudomonas* on the growth and metabolic alterations in rice plants subjected to drought stress. The inoculated plants showed significantly improved metabolic activity such as induction of the antioxidant enzymes, and increases in the total polyphenolic content, which in turn, conferred oxidative stress tolerance. In another study, rice seedlings were treated with *Gluconacetobacter diazotrophicus* strain Pal5, and the drought stress tolerance was tested in the inoculated and uninoculated plants [89]. The plants were subjected to various drought stress levels for 15 days. The inoculated plants showed increased levels of proline and glycine betaine, which conferred the plants drought stress tolerance. Molecular analysis revealed relative expression of several genes such as cat, gor, sod, BADH, and P5CR. In conclusion, inoculation with *G. diazotrophicus* mitigated the effects of drought stress on rice plants.

The effects of bacterial inoculation on the growth of *Glycine max* were evaluated under drought stress [90]. Inoculation with bacterial strains, LHL10 and LHL06, produced positive improvement in plant growth under stress condition. The inoculated plants showed increased roots, shoot length, leaf area, and dry biomass. The underlying mechanisms included an increase in *HSP90* expression levels: lipid peroxidation, increased calcium levels, and phosphate solubilization. In a study, Kour et al. [91] investigated the effects of *Streptomyces laurentii* EU-LWT3–69 and *Penicillium* sp. EU-DSF-10 on sorghum plants subjected to drought stress. Bioavailability of phosphorus to plants is reduced under drought stress. However, the plant-associated microbes contain the active form of phosphate that is provided to plants to counter the stress effects. Both the strains used in the study might solubilize phosphate and ensure its availability to the plants. The biochemical alterations mediated by bacterial inoculation included an increase in the proline and glycine betaine levels and chlorophyll content, while a decrease in the lipid peroxidation. Overall, the study suggested that bacterial inoculation enabled the plants to grow better under the drought stress.

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

Microbial inoculation confers drought stress tolerance to the plants partly by an induction in the growth hormone levels and acquisition to soil mineral content. This was demonstrated in a study conducted by Kang et al. [100], in which the alfalfa plants were inoculated with two *Enterobacter ludwigii* strains, namely AFFR02 and Mj1212. The inoculated plants were assessed under drought stress for hormones and mineral concentrations. The results showed that the inoculated plants were more drought-tolerant than the uninoculated plants. Growth attributes such as fresh and dry biomass, root/shoot elongation, and stalk diameter were significantly higher in the inoculated plants than in the uninoculated plants. It was also observed that the treated plants accumulated higher levels of flavonoids, minerals, and ABA than those of untreated plants. In a previous study, Silambarasan et al. [92] used *Rhodotorula mucilaginosa* strain CAM4 in the inoculation of *Lactuca sativa* subjected to drought stress. The inoculated plants showed drought tolerance at various developmental stages. The treated plants showed a clear increase in the growth, dry biomass, root proliferation, and stem elongation as compared with the untreated plants. The inoculation caused a significant increase in the content of chlorophyll, carotenoids, and proline, while a decrease in the malondialdehyde (MDA) levels, indicating lipid peroxidation.

Fungal endophytic species have been extensively studied for their positive effects on plant growth, stress tolerance, and disease resistance. They exert their positive effects through production of growth hormones, siderophores, secondary metabolites, and phosphate solubilization. Several studies have demonstrated the drought stress-mitigating effects of fungal inoculation on plants (**Table 3**). Fungal endophytes, specifically isolated from desert plants, have shown promising results when used for stress mitigation in crop plants. Desert plants are usually exposed to high magnitudes of drought conditions and thus may harbor fungal endophytes that may confer drought and salt stress tolerance under arid environments. Jain et al. [101] used halotolerant fungal endophytic strains, namely *Neocamarosporium chichastianum*, *Neocamarosporium goegapense*, and *Periconia macrospinosa* in the inoculation of tomato and cucumber seedlings. The treated plants showed stress tolerance, which was evident from the increased plant growth, chlorophyll content, proline, and antioxidant enzyme levels.

Osmotic adjustment is one of the key physiological mechanisms, which have been observed in the drought-tolerant plants. Endophytic fungal strains confer drought tolerance in plants by maintaining osmotic balance and water uptake efficiency. In one study, Dastogeer et al. [94] investigated the role of the fungal endophyte *Neotyphodium coenophialum* in the drought tolerance of *Lolium arundinaceum*. The results revealed that the treated plants had high drought tolerance than the untreated plants. The underlying mechanism of this enhanced tolerance was dependent on the osmotic balance and improved water uptake efficiency, which in turn enhanced the gene expression and photosynthesis rate. In addition, drought tolerance was achieved in *Nicotiana benthamiana*, when inoculated with fungal endophytes isolated from a *Nicotiana* plant. Overall, the fungal endophytes contribute to the drought tolerance trait mainly by increasing the water-use efficiency, nutrient uptake and maintaining the ion homeostasis to induce stress tolerance in the associated plants.

#### **5.3 Mycorrhizae**

Mycorrhizae are fungal species that establish a symbiotic relationship with higher plants and play a significant role in plant growth, nutrient acquisition, soil fertility,


#### **Table 3.**

*Microbial mitigation of drought stress in plants.*

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

and biotic and abiotic stress tolerance. They have interspecific functionality and are generally host-specific [102]. The endophytic microbes induce stress tolerance in the associated plants mainly through producing phytohormones and induction of the synthesis of secondary metabolites. On the contrary, arbuscular mycorrhizae confer stress tolerance to host plants by maintaining a steady flow of water and nutrient absorption from the soil [103].

The role of arbuscular mycorrhizae in drought stress tolerance of plants has been documented in several previous studies (**Table 3**). In one study, the impact of inoculation of arbuscular mycorrhizal strains, namely *Septoglomus constrictum*, *Glomus* sp., and *Glomus aggregatum* was studied in soybean, which is highly sensitive to abiotic stress [95]. The plants were subjected to drought stress after inoculation, and the biochemical, physiological, and molecular attributes were investigated. The treated plants showed increased levels of soluble sugars, proline, and glycine betaine and reduced MDA levels. The increased osmolyte levels in the treated plants conferred increased protection against the drought stress, while the lowered MDA content reduced the osmotic stress. The induction of phenolic compounds is one of the key mechanisms through which plants generate a response not only to infectious diseases but also to drought stress. In one study, Cheng et al. [97] used *Funneliformis mosseae* in the inoculation of trifoliate orange. The inoculated plants exhibited a marked increase in several growth attributes such as stem elongation, leaf number, leaf area, and root architecture. Biochemical analysis revealed induction in the contents of coumarin, terpene, and phenolic contents in the root exudates of the treated plants as compared with those of untreated plants. The drought stress tolerance in the inoculated plants was attributed to the induction of phenolic components, as they reduce oxidative stress in plants. In a similar study, a mycorrhizal fungal strain, *Glomus mosseae* was used to inoculate bread and durum wheat cultivars, and the plants were then exposed to drought stress [98]. The drought stress tolerance mechanism was evaluated through measurement of growth parameters and proteomics analysis. The inoculated plants showed increased dry weight, and the two genotypes responded differently to the fungal inoculation in terms of stress tolerance. A significant upregulation in the osmolytes concentrations was observed. Moreover, the inoculated plants accumulated lower ethylene levels an indication of stress tolerance.

Oxidative stress triggers the production of reactive oxygen species (ROS), which further causes irreversible damages to the macromolecules and key enzymes. Microbes have an ameliorating impact on oxidative stress in plants. Drought stress imposes oxidative stress on plants with associated growth and yield reduction. Plants respond to ROS generation by triggering the induction of ROS scavengers, which protect the cellular machinery. In one study, Zou et al. [104] used *Gigaspora margarita* and *Glomus intraradices* strains in the inoculation of host plants subjected to drought stress. Molecular analysis revealed upregulation of the expression of *GintSOD*, *GmarCuZnSOD*, *GintPDX1*, and *GintMT1* in the inoculated plants. Moreover, it was concluded that the drought stress tolerance mechanism also involved reduction in the cytoplasmic protein levels and regulation of redox status through synthesis of pyridoxamine. The drought-associated secondary stresses negatively impact both quality and quantity of crop plants. However, these negative effects can be efficiently mitigated through inoculation of various AMF strains. In one study, Al-Arjani et al. [99] isolated three AMF strains, namely *Glomus mosseae*, *Glomus etunicatum*, and *Glomus intraradices* from the rhizosphere of *Acacia gerrardii*. These strains were used to inoculate the *Ephedra foliata* Boiss plants,

subjected to drought stress. Compared with the untreated plants, the treated plants showed a significant increase in the chlorophyll and carotenoid contents. In addition, the treated plants showed increased levels of sucrose-phosphate synthase and osmolyte levels, which might be responsible for the enhanced drought stress tolerance. In another study, Moradtalab et al. [96] inoculated strawberry seedlings with AMF and silicon to evaluate their combined effects against drought stress. It was observed that the AMF and silicone inoculation caused a marked increase in the water uptake, mineral content, and overall biomass. The antioxidant defense system was also triggered, which reduced the drought-associated damages and conferred stress tolerance.
