**5. Quorum sensing silencing in bacteria: a valuable tool for phytopathogens control**

Quorum sensing is a cell-to-cell communication system that depends on population density. This communication system favors the adaptation to new ecological niches, promotes the exploitation of new metabolic resources, and affords competitive advantages to the bacteria that use it. All the aforementioned is directly related to the regulation of virulence, bacterial resistance, and biofilm formation among other phenotypes observed in bacterial population. Quorum sensing disruption is an alternative to reduce the pathogenicity in bacteria. There are two main approaches in order to silence the quorum sensing system in phytopathogenic bacteria: (i) signal degradation/modification termed quorum quenching, (ii) signal overproduction termed pathogen confusion [42]. Quorum quenching is a mechanism approved by several bacteria groups in order to disrupt the QS signaling of contenders, offering to these bacterial cells an additional benefit within a specific niche. It is rational that microorganisms can develop mechanisms to neutralize the QS systems of competing organisms in order to increase their competitive strength in an ecosystem. In a previous study, we have isolated and identified bacterial from citrus phylloplane that display the ability to modify the structure of DSF signal molecule

the cis-11-methyl-2-dodecenoic acid in *Xanthomonas citri* subsp. *citri* the etiological agent of bacterial citrus canker. The bacteria *Bacillus vallismortis*, *Pseudomonas oryzihabitans*, *Pseudomonas aeruginosa*, *Raoultella planticola*, *Kosakonia cowanii*, and *Citrobacter freundii* were characterized by molecular technics and display the ability to reduce DSF/rpf communication pathway [43]. We show that these quorum quenching bacteria use a DSF molecule as a substrate for the UDP-sugar transferase enzyme. These bacteria added to DSF molecule one unit of sugar from UDP sugar pools. Thus, the recognition for the RpfC sensor of this modified DSF molecule was impaired. Subsequently, a substantial reduction in the canker lesions was obtained in *Citrus sinesis.*

A previous study used the pathogen confusion approach expressing rpfF from *X. fastidiosa* in sweet orange (*C. sinensis* L. Osb.) using *agrobacterium tumefaciens* in order to reduce citrus canker disease severity. Ectopic expression of xfDSF molecule in *C. sinensis* reduces its susceptibility to *Xcc.* Transgenic plants display a reduction in the number of citrus lesions presumably for the effect on its motility and attachment, also genes involved in the flagella function, pili formation, and T3SS were downregulated in Xcc when they were infiltrated into the leaves of transgenic plants [44].

The bacteria *P. syringae* strain B728a displays the ability to degrade enzymatically different types of AHL. The HacA and HacB are acylases that cleavage the amide bonds of AHL. These enzymes do not have any affect over 3OC6-HSL endogenous accumulation. The heterologous expression of the secreted HacA acylase produced in *P. syringae* strain B728a could become a potential tool in biological control agents, because it might enable the quorum sensing disruption in phytopathogenic bacteria [45].

## **6. Conclusions**

Bacterial coordinated behaviors such as virulence factors production, motility, biofilm formation, and antibiotic resistance are regulated by cell–cell communication system often called quorum sensing. For all aforementioned, quorum sensing silencing has arisen as a good-looking approach to reduce the disease spread and severity. The major bacteria that affect citrus cultivar are endowed with several quorum sensing pathways. The quorum quenching and pathogen confusion implementation approaches could afford new and environmental-friendly strategies for control of this bacterial disease.

## **Acknowledgements**

The authors thank Professor Jesus A Ferro from the Technology Department, Faculdade de Ciencias Agrarias e Veterinarias, Universidade estadual Paulista, UNESP, Jaboticabal, SP, Brasil.

## **Conflict of interest**

The authors declare no conflict of interest.

*Quorum Quenching Bacteria: An Approach for Phytopathogens Control in Citrus Cultivars DOI: http://dx.doi.org/10.5772/intechopen.107902*
