**4. Citrus blast, black pit,** *Pseudomonas syringae* **pv.** *Syringae,* **and quorum sensing**

Citrus blast is an important bacterial disease that affects commercially important citrus fruits such as sweet orange (*C. sinensis*) and mandarin (*Citrus reticulata*). Black pit is a disease that affects sweet orange. Both diseases are caused by the bacterium *Pseusomonas syringae* pv. *syringae. P. syringae* pv. *syringae* becomes especially pathogenic for citrus fruits, when the prevailing environmental conditions are high humidity and temperatures around 18°C, which coupled with damage to shoots or fruits by wind, thorns, and hail [34]. Pathognomonic symptoms of disease are water-soaked lesions, which extend from the midrib to the minor ones that surround the base of the petiole. At the last phase of the disease, leaf desiccation and curling are observed. This trait is mainly present in the leaves that remain attached to the stem and finally fall. Necrotic area in twigs expands and finally dies after 4 weeks.

Within that vast number of bacteria that compound the plant microbiome (either rhizosphere or phyllosphere), Pseudomonas bacteria are the most versatile and metabolically varied. As mentioned above, due to its enormous genetic and metabolic plasticity, many species from Pseudomonas genus are successful colonizers of rhizosphere and phyllosphere inducing beneficial effects to the host plants. The most recognized effects are: plant growth promoters, biological control agents, and resistance auto-inducers [35]. Only one species of Pseudomonas is known to be pathogenic for a wide variety of plants; this species is *P. syringae*. P. *syringae* shows a high host plant specificity. Because of this specificity, these strains have been considered as pathovars within the *P. syringae* complex, depending on the type of plant in which the bacterium acts as a pathogen and produces the disease, actually around 50 pathovars are recognized to act as ethological agents in 180 different plant types [36]. *P. syringae* is a leaf-borne commensal bacterium in a wide variety of crop plants, fruit trees, vegetables, and ornamental plants. These bacteria are epiphytic colonizers, which could

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

reach the internal leaf tissue, once in the apoplast bacteria begins its multiplication producing disease symptoms. Therefore, the pathogenesis development is a multistep procedure such as: (i) entry to internal tissues plant to reaching the intercellular place the apoplast, (ii) to evade plants' resistance responses, and (iii) inducing disease and generating symptoms by particular invasive approaches and molecules [37].

#### **4.1 Quorum sensing in** *P. syringae* **pv***. syringae*

The quorum sensing system in *P. syringae* uses as auto-inducer a molecule from AHL signaling family: 3-oxo-hexanoyl-homoserine lactone (3-oxo-C6-HSL) molecule. Production of 3-oxo-C6-HSL is dependent on gene *ahlI* that encodes the synthase AhlI. The other component of the quorum sensing circuit is the gene *ahlR*, the signal regulator [38]. When the auto-inducer precursor is available, the synthase AhlI catalyzes the formation of 3-oxo-C6-HSL. Subsequently, the signal regulator AhlR forms a stable complex with 3-oxo-C6-HSL and promotes the transcription of *ahlI* via positive feedback increasing the concentration of 3-oxo-C6-HSL proportionally to cell population density. Additionally, this quorum sensing AhlI/R pathway is subject to effect of regulatory proteins as AefR, this protein actively participates in *ahlI* transcription. A novel regulator, GacA, displays a similar effect in the process of auto-induction. Together AefR and GacA seem to have participated in the activation of the AhlI-AhlR quorum sensing system via independent pathways [38]. The AHL quorum sensing system in *P. syringae* regulates the alginate production, the main component of EPS in *P. syringae. aefR, ahlI, and ahlR* deletion mutant strains display limited survival ability on dry leaves, which is due to the EPS helping epiphytic fitness and desiccation tolerance [39]. Motility is considered an indispensable epiphytic fitness trait, and swarming motility in *P. syringae* is coordinated by a bacterial social behavior [40]. In *P. syringae* swarming motility is regulated by AhlI-AhlR quorum sensing system and AefR. Deletion mutant strains of *aefR* and *ahlI*– *ahlR*– double mutant display a hypermotile phenotype compared with the wild-type strain [41].
