**Abstract**

In the upcoming years, legume crops will be subjected to multiple, diverse, and overlapping environmental stressors (raise in global temperatures and CO2, drought, salinity, and soil pollution). These factors will menace legume productivity and food quality and security. In this context, tolerant plant growth promoting rhizobacteria (PGPR) are useful biotechnological tools to assist legume establishment and growth. In this chapter, tolerant PGPR able to promote legume growth will be revised. Besides, in the era of -omics, the mechanisms underlying this interaction are being deciphered, particularly transcriptomic, proteomic, and metabolomic changes modulated by PGPR, as well as the molecular dialog legume-rhizobacteria.

**Keywords:** PGPR, legumes, abiotic stress, mechanisms, bacteria

## **1. Introduction**

Plants are colonized by several microorganisms, mainly bacteria, and the number of them sometimes is higher than the number of plant cells [1]. These microorganisms can live inside (endophytes) and outside (epiphytes) the plant tissues, and they do not cause diseases in the host plant [2]; rather, these microorganisms contribute to improve the health and the productivity of the plants [3].

Soils also have many microorganisms with potential to improve plant growth, and overall the rhizosphere. The rhizosphere is an area of interaction between microorganisms and plant roots, and it is inhabited by bacteria, fungi, protozoa, actinomycetes, and algae, with bacteria and mycorrhizal fungi being the main populations [4]. The size of the microbial population in the rhizosphere of plants is influenced by root exudates. The chemicals found in the soil along with the exudates from the plants cause changes in the pH and in the redox potentials that will be determined by the microbial community around the roots [5].

Among the bacteria that colonize the rhizosphere, those promoting plant growth, also known as PGPB (Plant Growth Promoting Bacteria), rhizobacteria or PGPR (Plant Growth Promoting Rhizobacteria), and nitrogen-fixing rhizobia, are the most remarkable because they provide beneficial effects in the development

of the plants being able to do it by direct or indirect mechanisms. Regarding legumes, they also interact with rhizobacteria, and the main interaction is the symbiosis between legumes and nitrogen-fixing rhizobia [4]. Rhizobia are bacteria that belong to the family Rhizobiaceae, and the most important genera are *Bradyrhizobium*, *Ensifer*, *Mesorhizobium*, *Rhizobium*, and *Sinorhizobium* [6]. During this interaction, rhizobia invade the root and group into a specialized organ called nodule. Inside the nodules, rhizobia become bacteroids, which transform N2 to NH4 + , molecule that can be assimilated by plants [7]. Inoculation of legumes with rhizobacteria produces a bioaugmentation of the microbial population in soils, thus contributing to plants growing bigger and faster [8–10], and this can solve the problem of the rapid growth of the world population that causes a great pressure in the area of land destined for food [11]. Moreover, the arable lands are decreasing because of the climate change and some human activities.

Climate change is one of the most important problems in the planet nowadays. Because of that, temperature and drought are increasing, involving an increase in arid and semi-arid zones and generating a loss of arable soils [12–14]. Drought is an abiotic stress that causes the highest losses in agriculture, so it is a very important factor in crop productivity [15]. Drought and heat involve the appearance of saline soils [16], although some human activities, like the increase of irrigation with bad water quality, are also responsible [17]. Salinity affects around 800 million of hectares in the world, and it is considered a global problem [18] being another stress that limits plant growth, productivity, nitrogen fixation in legumes, and the seed germination [19, 20] due to the uptake excess of NaCl by the plants [21]. Furthermore, the salt excess decreases the organic matter in soils and modifies the microbial population in the rhizosphere [22], so salinity also affects the nodulation negatively [23]. Finally, an additional abiotic stress is heavy metals. The increase of heavy metal concentrations becomes a pollution problem, being humans the main responsible of it [24]. Heavy metals affect plants and soils as the rest of abiotic stress does, and in legumes, they dramatically reduce nodule number and nitrogen fixation [25, 26].

To try to recover these affected soils, phytoremediation is being used, and several studies confirm that it is a very efficient tool, particularly in combination with bacterial inoculation since PGPB improve the potential of plants to phytoremediate soils [27–31]. Legumes belong to the plants used in phytoremediation because this family is one of the most diverse among other plant families in the world, and some of them are able to grow in degraded soils and can be used as pioneer plants in order to repopulate these degraded areas [25, 32]. In fact, legumes are usually used in intercropping with other crops to decrease the amount of pesticides and improve the quality of soils making legumes one of the most promising components of the Climate Smart Agriculture concept [33]. As described above, the symbiosis of legumes with rhizobia improves the growth of legumes and allows them to grow better in the degraded soils, but all the named abiotic stresses interfere with this interaction. For that, authors look for rhizobia resistant to these stresses that able to grow and form nodules even in degraded soils [34–37]. Furthermore, several studies demonstrate that coinoculation of legumes with rhizobia and another PGPR increases nodulation, plant growth, and the potential to phytoremediate soils of plants in the presence of abiotic stresses [38–41]. This improvement in legumes occurs for the interaction between plants and bacteria through different direct and indirect mechanisms that help the plant to grow in the presence of stress.

In this chapter, the different bacterial mechanisms used to improve the plant growth in the presence of the most important abiotic stresses nowadays are been reviewed, in order to help legumes to grow under stress situations and recover the degraded soils using the interaction between legumes and bacteria. Furthermore,

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

the transcriptomic, proteomic, and metabolomic studies so far.

**2. Effects and mechanisms of plant growth promotion** 

the molecular mechanisms involved in these interactions are being described with

As mentioned above, some bacteria are capable of promoting plant growth (PGPB and PGPR) through direct (biofertilization) and indirect (biocontrol) mechanisms. The direct mechanisms are based on the direct promotion of plant development, among which are nitrogen fixation, phosphate solubilization, production of phytohormones (auxins, cytokinins, and gibberellins), the enzymatic activity of the aminocyclopropane carboxylic acid (ACC) deaminase, and iron complexation by bacterial siderophores. On the other hand, indirect methods are responsible for inhibiting pathogenic organisms for plants. Among these methods are the synthesis of antibiotics, enzymes that degrade the cell wall, or the induced systemic resistance (ISR) process [42]. The mechanisms carried out by PGPB and PGPR will depend on the host plant and will be influenced in turn by biotic (such as plant defense mechanisms and genotype) and abiotic (such as weather conditions

However, bacteria are not the only microorganisms that are able to promote plant growth. Mycorrhizal fungi also can carry out this function, there being a symbiotic association between them and most terrestrial plants [44] favoring micronutrient absorption, resistance to diseases caused by pathogens, or reduction

Nitrogen is one of the essential elements for life that is present in biochemical structures as important as nucleotides and proteins, but atmospheric nitrogen is mostly nonassimilable for plants since about 78% is in a gaseous state, so it must become ammonia, thanks to the nitrogenase enzyme to be assimilable. This reaction is carried out by rhizobia under symbiosis with legumes, thanks to which the rhizobia obtain carbon provided by the legumes from photosynthesis, and they

Atmospheric nitrogen fixation occurs mainly in leguminous plants where rhizobia/plant interactions are highly specific [42]. However, certain free-living bacteria (such as *Frankia* spp. or Actinobacteria) are also able to fix atmospheric nitrogen to a much wider range of plants than rhizobia [47]. For example, coinoculation of *Bradyrhizobium* sp. UFLA 03-84 with *Actinomadura* sp. 183-EL, *Bacillus* sp. IPACC11, or *Streptomyces* sp. 212 in cowpea plants improves the nitrogen fixation even in the presence of salinity conditions [48]. Another example of the improvement of nitrogen fixation in the presence of a different abiotic stress is reported by Saia et al. [49] in which plants of *Trifolium alexandrinum* were inoculated with arbuscular mycorrhizal (AM) fungi leading to an improvement of nitrogen fixation

Phosphorus is also an essential element for life that is involved in such impor-

tant processes as energy transfer, respiration, or photosynthesis [50]. After

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

**by microorganisms**

and soil composition) factors [43].

provide the plant with nitrogen [46].

and the plant growth in water stress.

*2.1.2 Solubilization of phosphate*

**2.1 Direct mechanisms**

*2.1.1 Fixation of nitrogen*

of plant stress caused by environmental factors [45].

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

the molecular mechanisms involved in these interactions are being described with the transcriptomic, proteomic, and metabolomic studies so far.
