*2.2.4* Quorum sensing

*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


**123**

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

exceeds a defined threshold level (*quorum*) where it binds to bacterial cell receptors and triggers a signal transduction cascade leading to changes in the expression of

There are numerous bacterial signaling molecules among which the acyl-homoserine lactone (AHL) produced by Gram-negative bacteria consisting of a common lactone homo resin residue and an acyl chain that can vary between 4 and 18 carbon atoms. There are more than 100 species of proteobacteria that produce AHL, and three types of enzymes are known (LuxI, LuxM, and HdtS) capable of synthesizing AHL *in vivo.* Degradation of AHL can be carried out by various enzymes as is the case of the AHL lactonase that breaks the ester linkage of the lactose ring of the AHL to form N-acyl homoserine reversibly [92]. Most of these signal molecules are of the bacterial origin: *Bacillus* spp., *Klebsiella pneumoniae* KTCTC2241, or *Rhizobium* sp. NGR23 [93]. The detection of this bacterial communication system is interrupted by PGPB, thus preventing pathogen infection through immune

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

bacterial genes by the action of a group of cells [42].

responses and preventing plant growth inhibition [94].

*ciens* SN13 showed overexpression of these genes [95].

**abiotic stress**

are exposed.

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

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

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. amyloliquefa-*

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*

### **Table 5.**

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

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

exceeds a defined threshold level (*quorum*) where it binds to bacterial cell receptors and triggers a signal transduction cascade leading to changes in the expression of bacterial genes by the action of a group of cells [42].

There are numerous bacterial signaling molecules among which the acyl-homoserine lactone (AHL) produced by Gram-negative bacteria consisting of a common lactone homo resin residue and an acyl chain that can vary between 4 and 18 carbon atoms. There are more than 100 species of proteobacteria that produce AHL, and three types of enzymes are known (LuxI, LuxM, and HdtS) capable of synthesizing AHL *in vivo.* Degradation of AHL can be carried out by various enzymes as is the case of the AHL lactonase that breaks the ester linkage of the lactose ring of the AHL to form N-acyl homoserine reversibly [92]. Most of these signal molecules are of the bacterial origin: *Bacillus* spp., *Klebsiella pneumoniae* KTCTC2241, or *Rhizobium* sp. NGR23 [93]. The detection of this bacterial communication system is interrupted by PGPB, thus preventing pathogen infection through immune responses and preventing plant growth inhibition [94].
