**3. Molecular mechanisms in the bacteria-plant interactions under abiotic stress**

In spite of the abundant literature available on the improvement of plant growth and crop productivity using bacteria under abiotic stress conditions, the molecular mechanisms involved in these interactions remain elusive so far. This area has been studied by some authors to try to understand what changes bacteria elicit on plant gene expression finally leading to enhance the plant resilience to environmental stress. There are not many studies about genes involved in the plant-bacteria interactions, particularly in legumes, but in this section, some examples of these studies are exposed.

Plants have several mechanisms to adapt in the presence of any stress, whose genes are involved in the regulation of transporters, phytochelatins, glutathione reductase, phytohormones, oxidative stress reduction, phenolic compound, osmolytes, and low molecular weight organic acids, among others, and the bacteria role either expressing or repressing these genes has been investigated using real-time quantitative PCR, RNA Seq, and metabolomic and/or transcriptomic analyses. *OsGRAM* family genes are some of the most important in the plant growth and development under stress conditions, and it was demonstrated in rice under an array of stress situations, including salt. The plants inoculated with *B. amyloliquefaciens* SN13 showed overexpression of these genes [95].

Regarding heavy metal stress, one of the main genes expressed or repressed in plants is genes of transporters [96]. In the case of cadmium, *Tatm20* gene codifies a transmembrane transport that is expressed in the presence of Cd in wheat. When wheat plants were inoculated with *Azospirillum brasilense*, this gene was expressed much less than in the absence of the inoculation suggesting that *A. brasilense* helps plants to decrease Cd uptake and accumulation [97]. In this study, we also studied the expression of *TasSOS1* gene involving in keeping the plant cell homeostasis in the presence of high salt concentrations. In contrast to *Tatm20* gene, *TasSOS1* is overexpressed in inoculated plants under salinity stress. This overexpression maintains the homeostasis in plants and makes plant more tolerant to salt excess [97]. In *M. sativa* plants grown under heavy metal conditions, the genes involved in the phytochelatins synthesis were overexpressed in inoculated plants helping plant to detoxify and to grow in the presence of this stress. Moreover, the expression of glutathione reductase was improved by bacteria, so inoculated plants keep the redox status under heavy metal conditions [41]. Other examples are *Bacillus altitudinis*

*Legume Crops - Prospects, Production and Uses*

*2.2.3 Induced systemic resistance*

infection was prejudiced [88].

*2.2.4* Quorum sensing

*Bacillus altitudinis* BRHS/S-73

*B. subtilis* DSM1088*, P. fluorescens* ATCC13525 and *Glommus* spp.

producing these types of degrading enzymes.

attack by insects, or to a chemical treatment [42].

chitinase and protects *A. hypogea* against *Fusarium oxysporum* f. sp. *cubense* under salinity conditions [84]. **Table 5** shows some examples of bacteria capable of

There is an important feature of biocontrol that helps plant growth based on two defensive response mechanisms against various external agents known as mechanisms of induced systemic resistance (ISR) and mechanism of acquired systemic resistance (ASR) [56]. ISR is a physiological state of defensive capacity that plants present in response to an environmental stimulus [43] in which nonpathogenic microorganisms, including various PGPB, reduce the negative effects of pathogens of plants by activating a resistance mechanism without the need to target a specific pathogen and can develop this resistance in response to infection by pathogens, to

To stimulate this defensive response mechanism, the ISR uses phytohormones as jasmonic acid (JA) and ethylene (ET) that act as signaling molecules [64], although it has been shown that some organelles (such as flagella) and bacterial molecules (such as lipopolysaccharides of the outer membrane or antibiotics produced by bacteria) can also act as inducing agents activating ISR and generating a rapid accumulation of pathogenesis-related enzymes such as chitinase, β-1,3-glucanase, peroxidase, or liases, among others [85]. It is important to note that the ISR prepares plant to fight the pathogen with an improved defense [56]. A clear example of the ISR elicitor is the effect of *Bradyrhizobium japonicum* in soybean crops, where systemic redox changes are induced in plants [86]. The induction of ISR by *Bacillus* sp. CHEP5 and *Bradyrhizobium* sp. SEMIA 6144 in peanut plants against *S. rolfsii* also has been demonstrated [87]. However, this microbial induction could be limited by abiotic stress like the ISR induction of *B. amyloliquefaciens* S499 in different crops under heat and drought conditions where the response against *Botrytis cinerea*

*Quorum sensing* (QS) is a mechanism of genetic regulation in response to cell density mediated by small self-inducing molecules, which are usually secreted out of bacterial cells and act as chemical signals produced by an increase in the cell population. These molecules cause an alteration in bacterial metabolism by activating different sets of genes [89], so that similar bacteria that live in communities and are close to each other will begin to act in a coordinated way. The level of the autoinductors increases proportionally to the population of bacterial cells until it

**Microorganism Host plant Enzyme Phytopathogen References**

peroxidase and polyphenol oxidase *cubense*

*Thanatephorus cucumeris*

*Sclerotium rolfsii* [91]

[84]

[90]

*B. licheniformis* A2 *A. hypogea* Chitinase *F. oxysporum* f. sp.

*V. radiata* Chitinase and protease

*P. vulgaris* Chitinase,

*Lytic enzymes produced by PGPR and their effect on biocontrol of legume phytopathogens.*

**122**

**Table 5.**

FD48 that modules the expression of some genes involved in the synthesis of auxins improving root elongation in rice [98] and *Pseudomonas aeruginosa* and *Burkholderia gladioli* that reduced the expression of *CAT*, *GR*, *GPOX*, *APOX*, and *GST* genes in the presence of Cd stress in *Lycopersicon esculentum* plants modulating their antioxidative response [98]. *P. aeruginosa* and *Bacillus gladioli* also elevated the expression of phenolic compound, osmolytes, and molecular weight organic acid (citric acid, malic acid, fumaric acid, and succinic acid) genes, decreasing the physiological damage of *Solanum lycopersicum* plants under Cd toxicity [98].

A global transcriptomic analysis was carried in inoculated *Medicago truncatula* plants grown in the presence of arsenic and inoculated with an arsenic-resistant rhizobial strain. The results showed the overexpression of some defense genes in the inoculated plants (sulfur metabolism, several enzymes of the phytochelatins synthesis pathway, proline, heat shock proteins, and several transcription factors). Besides, secondary metabolism, isoflavonoids and phenylpropanoids, were activated. In contrast, the genes of nodulation were downregulated, particularly those involved in the early stages of the interaction [99, 100].

Under drought environments, *P. putida* MTCC5279 modulates the stress in plants of *Cicer arietinum* by the overexpression of *ACO* and *ACS* (involved in the synthesis of ethylene); *PR1* (synthesis of salicylic acid); *MYC2* (synthesis of jasmonate); *SOD, CAT, APX*, and *GST* (genes that codify the antioxidative enzymes in plants); *DREB1A* (response element to dehydration); *LEA* and *DHN* (dehydrins); and *NAC1* (transcription factors expressed under abiotic stress) genes [101]. In plants of sorghum inoculated with rhizobacterial endophytes, proline (a crucial molecule of maintaining the cellular functions under drought) accumulation was higher than in the noninoculated plants because bacteria induce the overexpression of *sbP5CS2* (pyrroline-5-carboxylate synthase 2) and *sbP5CS1* (pyrroline-5-carboxylate synthase 1) genes under drought stress [102]. This fact also was demonstrated in inoculated chickpea plants, where proline was accumulated by plants under drought conditions besides histidine, citrulline, and threonine [103].

Finally, the molecular mechanisms for plant alleviation in salt stress by bacteria are also reported by some authors. A transcriptome of rice plants inoculated with *Bacillus amyloliquefaciens* SN13 showed that the bacterial inoculation alters gene expression under salt stress. For example, genes of phytohormones, flavonoids, or photosynthesis are found in higher number in inoculated plants [104]. Other example is the inoculation of wheat with *Dietzia natrolimnaea* that causes an overexpression of genes involved in the ABA signaling cascade and in the salt sensitive pathway among others [105].

Recently, some authors have studied the miRNAs induced by PGPR as a possibility to regulate the stress in plants [106, 107]. miRNAs are RNA molecules of 20–24 nucleotides that do not codify proteins, and they get bound to mRNA or any transcriptional factor, regulating the expression of the target gene. However, the only study about miRNAs was performed in chickpea plants under drought stress, where plants were inoculated with *Pseudomonas putida* RA, and this inoculation improves plants adaptation to drought conditions through the regulations of miRNA expression [108].

#### **4. Conclusions and future perspectives**

One of the main problems of the rapid increase in the world's population lies in the challenge of having the necessary food for global supply, but the climate change and the pollution decrease the number of the agricultural soil, so a possible solution would be to encourage more widespread use of PGPB. The evidence that PGPR

**125**

**Author details**

benefit of all.

**Acknowledgements**

2020/00000092 project.

**Conflict of interest**

Salvadora Navarro-Torre1

Ignacio D. Rodríguez-Llorente1

Universidad de Sevilla, Sevilla, Spain

Bioresources in Dry lands (BVBZA), Gabès, Tunisia

The authors declare no conflict of interest.

\*Address all correspondence to: snavarro1@us.es

provided the original work is properly cited.

*Helping Legumes under Stress Situations: Inoculation with Beneficial Microorganisms*

promote the plant growth under stress environments is becoming increasingly true, being even more focused on the study of individual mechanisms than their combined mechanisms, which is why new paths are being opened toward the use of mixed inoculants that act jointly acquiring faster and improving results. There are also investigations in the area of genetics and molecular biology, where studies are being carried out based on the discovery of specific genes capable of motivating greater plant development as well as in the field of nanotechnology where nanoencapsulation and microencapsulation offer an alternative to produce effective formulations against pest control. However, this area needs more investigations and funding to solve the lack of development of new and better storage, shipping, formulation and application techniques of these PGPR, and the development of effective and consistent regulations regarding their use. In this way, the agricultural practice in degraded soils could become an effective and sustainable practice for the

This research was funded by Junta de Andalucia (Spain) under FEDER

\*, Khouloud Bessadok1,2, Noris J. Flores-Duarte1

, Miguel A. Caviedes1

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

1 Departamento de Microbiología y Parasitología, Facultad de Farmacia,

2 Faculty of Sciences of Gabès, Research Unit: Valuation of Biodiversity and

,

and Eloísa Pajuelo1

*DOI: http://dx.doi.org/10.5772/intechopen.91857*

*Helping Legumes under Stress Situations: Inoculation with Beneficial Microorganisms DOI: http://dx.doi.org/10.5772/intechopen.91857*

promote the plant growth under stress environments is becoming increasingly true, being even more focused on the study of individual mechanisms than their combined mechanisms, which is why new paths are being opened toward the use of mixed inoculants that act jointly acquiring faster and improving results. There are also investigations in the area of genetics and molecular biology, where studies are being carried out based on the discovery of specific genes capable of motivating greater plant development as well as in the field of nanotechnology where nanoencapsulation and microencapsulation offer an alternative to produce effective formulations against pest control. However, this area needs more investigations and funding to solve the lack of development of new and better storage, shipping, formulation and application techniques of these PGPR, and the development of effective and consistent regulations regarding their use. In this way, the agricultural practice in degraded soils could become an effective and sustainable practice for the benefit of all.
