**3. Variegated chlorosis,** *Xylella fastidiosa and* **quorum sensing**

CVC (Citrus Variegated Chlorosis) is a disease that affects sweet orange (*Citrus sinensis*) and grapefruit (Citrus x paradise); no other citrus are susceptible to disease *Quorum Quenching Bacteria: An Approach for Phytopathogens Control in Citrus Cultivars DOI: http://dx.doi.org/10.5772/intechopen.107902*

[20]. CVC is caused by the Gram-negative bacterium *X. fastidiosa (Xf).* Transmission paths of CVC are: (i) *Propagative vegetative material*: CVC is a paradigm of accidental long-distance propagation of phytopathogen; this is mainly due to the movement made by agriculture workers of infected plant material [21]. (ii) *Insect vector borne:* vectors of *Xf* are xylem sap sucking insects belonging to the Hemiptera order including sharpshooter, leafhoppers, and spittlebugs [22].

CVC symptomatology in susceptible host sweet orange initiates as a small chlorosis, which extents irregularly on the upper surface of mature leaves, the affected leaves display a consequent brownish gum-like material on the lower surface. At this disease stage, the lesions are present only in one or two branches. At the later disease stage, bacteria became systemic spreading in the plant canopy, symptoms become apparent between 3 and 6 months [22].

The bacterium *X. fastidiosa* belongs to the gammaproteobacteria group, *Xanthomonadaceae* family. *Xf* is a xylem-limited bacterium, which is obligatory colonizer of plants and insect vectors [21]. There are three major monophyletic subspecies of *X. fastidiosa: Xf* subsp*. multiplex, Xf* subsp*. Fastidiosa,* and *Xf* subsp*. pauca*, endemic respectively in North, Central, and South America [23–25]. Nowadays, it is widely accepted that *Xf* has a commensal relationship with a huge number of plant species; however, just a few numbers of clades and specific bacterial genotypes are associated as phytopathogens. Contrasting with the relationship between *X. fastidiosa* and the plant, the association of *X. fastidiosa* with insect vector is independent of plant-pathogen mixtures. Vectorial transmission is the only natural mechanism spread. There exist two principal xylem-sap feeders insect vector groups: the sharpshooter leafhoppers (Cicadellidae subfamily Cicadellinae) and spittlebugs (Cercopoidea, families Aphrophoridae, Cercopidae, and Clastopteridae) [26, 27]. Actually it is extensively known that all insect vectors aforementioned display the ability to transmit all genotypes of *X. fastidiosa* without any specificity [28]. *Xyllela fastidiosa* colonize plants gradually, the initial stage is a vessel obstruction because of bacterial multiplication, reduction of sap flow in the xylem system is due to plant response (i.e., tylose) and bacterial dissemination between vessel via pit membrane [29].

#### **3.1 Quorum sensing in** *Xyllela fastidiosa*

As mention before, the virulence of *X. fastidiosa* is related with its capability to travel and to multiply within xylem vessels, and symptoms could basically be an unintended effect caused by effective colonization that restricts with xylem sap flow. *X. fastidiosa* as similar to related phyto-pathogenes Xanthomonas and Stenotrophomonas use small molecules from DSF family in order to coordinate its behavior in a celldensity-dependent fashion [30]. Cell-to-cell communication system in *X. fastidiosa* involving the production of DSF *rpfF* gene is responsible for the DSF production as same of Xanthomonas bacteria. Unlike, *Xanthomonas* bacteria, in which the disruption of DSF quorum sensing pathway reduces its virulence and pathogenicity in *X. fastidiosa,* mutants of *rpfF* and subsequently deficient in DSF production show an hypervirulent phenotype behavior in a susceptible host such as sweet orange and grapefruit [31]. In *X. fastidiosa,* the DSF molecule is 12 methyl tetradecanoic acid *xf*DSF (**Figure 1**). The whole mechanistic details in DSF pathway are not yet completely understood. Previous studies have proposed the existence of two different types of receptors in *X. fastidiosa:* the first one RpfC transmembrane has a high sequence similarity with the RpfC of *Xanthomonas campestris*, the only difference lies that in the *X. fastidiosa* is truncated at the N terminus, apparently its function is to

bind DSF and in that way to execute a negative feedback regulation in DSF production [32]. Another potential DSF sensor has an intracytoplasmic localization, and its function could be the perception of DSF accumulated within the cell. Once DSF bound to intracellular DSF receptor the autophosphorylation and phosphorelay to a response regulator as RPFG is triggered. It allows the expression of genes involved in attachment and biofilm formation [32].

Previous studies have shown that *rpfF* mutants of *X. fastidiosa* display a hypervirulent phenotype in a susceptible plant host, and this mutant strain was incapable to colonize and be spread by insect vectors. These observations arose the hypothesis that DSF signaling is used as a lifestyle dependent switch, because *rpfF* promotes the genes expression involved in attachment and biofilm formation in the xylem vessels. By contrast, *rpfF* in *X. fastidiosa* represses the expression of genes intricated in motility and hydrolytic enzyme production, which are responsible for the cell migration and pit membrane disruption. *X. fastidiosa* xylem vessels attached cells display reduced pathogenicity and a phenotype highly favorable to be acquired by the insect vector [33]. On the other hand, *rpfC* deletion mutants displayed an avirulent phenotype, because the great DSF production, these mutants were capable to successfully colonize the insect vector, whoever these bacterial cells display an impaired ability to be transferred to another susceptible plants. Finally, similar to Xanthomonas bacteria, DSF-dependent signaling regulates decyclic di-GMP in *X. fastidiosa* [32].
