Xanthomonas spp. are yellow-pigmented bacteria, several species of which are phytopathogens associated with economically important crops worldwide. Bacteria from this genus are responsible for diseases on approximatively 400 plant species (124 monocots and 268 dicots), including rice, wheat, citrus, tomato, pepper, banana, and bean. There is a high degree of specificity between the host plant and the Xanthomonas species and pathovars [81]. Infection is divided into two stages: the epiphytic stage and the endophytic stage. Briefly, the pathogen penetrates into a new host via plant natural openings and wounds, colonizes the host, and invades either the vascular system or the intercellular spaces of the mesophyll parenchyma tissue. Virulence factors are mainly type III secretion systems (T3SS) and the effectors secreted by these systems. In addition, other pathogenicity factors have been involved, directly or indirectly, in the virulence, such as PCWDEs, type IV secreted effectors, adhesins, and lipopolysaccharides [82]. It has been clearly demonstrated that Xcc recruits DSF to synchronize virulence gene expression. Transcriptome analyses of mutant strains modified in the production of DSF led to the identification of 165 genes whose expression is significantly varied. Among these genes are those involved in extracellular enzyme and extracellular polysaccharide production, flagella synthesis, iron uptake, aerobic respiration, resistance to toxins, and oxidative stress [83]. Phenotypes associated with these genes are complemented by the addition of DSF signals [83,95]. Additionally, DSF is involved in biofilm formation [92]. DSF has also been implicated in the intricate crosstalk between Xanthomonas spp. and their host plants. DSF facilitates Xanthomonas spp. entry into host plants [84]. DSF elicits innate immunity and the secretion of xanthan, the main exopolysaccharide that suppresses the DSF-induced innate immunity in Xcc [85]. Taken together, these indicate that plants have evolved to recognize a widely conserved bacterial communication system [85].

Finally, studies of the DSF system in Xoo, the causative agent of rice bacterial blight disease, and in Xanthomonas axonopodis pv. glycines (Xag), which is responsible for the bacterial pustule of soybean, demonstrated that the mechanisms for DSF biosynthesis and regulation are conserved and promote the regulation of virulence factor productions or factors associated with virulence in both pathovars [86,87]. In Xag, the regulated extracellular enzymes are mainly carboxyl–methyl cellulases, proteases, endo-β-1,4-mannanases, and pectate lyases [87].

Xyllella fastidiosa, which is divided into four subspecies, belongs to the Xanthomonadaceae genus [88]. Xyllella fastidiosa is a vascular wilt pathogen that has a wide host range of over 350 plant species, including almond leaves, lemons, and olive trees. X. fastidiosa is always found in the xylem tissue of its plant host or in the foregut of its xylem-feeding hemipteran insect vector. In contrast to other Xanthomonads, X. fastidiosa does not carry T3SS, and the symptoms associated with this pathogen depend on the capacity of the bacteria to colonize and block xylem vessels, leading to drought stress. Hence, attachment, motility, and biofilm formation are important for infection [7,88]. A study of rpfF and rpfC mutants in X. fastidiosa showed that switching between the plant host and insect vectors is regulated by QS (Table 3). DSF-mediated QS also controls virulence factors, such as adhesins, migration in xylem vessels, and the colonization of insect vectors [88,89]. At a high cell density, DSF-mediated QS promotes cell stickiness, thereby encouraging attachment to the xylem wall [88]. In addition, outer membrane vesicle (OMV) production is repressed via DSF by an unknown mechanism [96]. OMVs block bacterial interactions with the xylem vessel walls. Taken together, this interaction indicates that DSF-mediated QS favors X. fastidiosa attachment to the vessels. This phenomenon has been proposed to help bacterial acquisition by insects [89].

In summary, X. fastidiosa coordinates its behavior according to its infection stages (insect vectors vs. plant host) and population size using a DSF-mediated QS similar to that present in Xanthomonas spp. [89].

As mentioned before, the R. solanacearum species complex carries two QS systems—one depending on AHL signals, which does not seem to be involved in the regulation of virulence, and a second one whose signal molecules are close to the DSF family. This system regulates the virulence of R. solanacearum species [97]. The R. solanacearum species complex is divided into two clades, depending on their QS signal types, with one being the (R)-methyl 3-hydroxymyristate (or 3-OH-MAME) and the second one being (R)-methyl 3-hydroxypalmitate (or 3-OH-PAME) [48,98]. PhcB methyltransferases synthesize both QS signals from the cognate fatty acids, but the specific production of signals depends on the strains [49]. The QS signal is recognized by the two-component regulatory system PhcS–PhcR. In the absence of a signal, PhcR inhibits the PhcA regulator, which activates the production of exopolysaccharides and PCWDEs. In the presence of the signal, PhcR is phosphorylated by PhcS, which inhibits it, and thereby increases the number of functional PhcA and the production of virulence factors [97]. In this way, early and late virulence factors are coordinately controlled by cell density, allowing the proper expression of genes encoding virulence factors, which is important for the success of the infectious process.

In Xcc, DSF biosynthesis is autoregulated by a posttranslational mechanism [95]. In addition to its interaction with RpfG, RpfC can also bind to RpfF using its C-terminal REC domain and negatively regulate DSF biosynthesis [76,77] (Figure 4). At low cell density, unphosphorylated RpfC conformation facilitates the formation of the RpfC–RpfF complex blocking the enzymatic activity of RpfF and the production of DSF signals. At a higher cell density, RpfF is released and produces DSF signals, which allows the induction of QS regulation [95]. This mechanism demonstrated in Xcc seems also to be present in X. fastidiosa.

In Xcc and Xoo, DSF signals accumulate in the early stationary phase, and their levels decline rapidly afterwards, suggesting the existence of a DSF signal turnover system [99]. Studies of RpfB in both Xcc and X. fastidiosa have shown that RpfB is involved in DSF processing, as DSF-like fatty acid profiles whose production depends on RpfF are affected in rpfB mutants [78]. In addition, rpfB mutants boost DSF production during growth, while the overproduction of RpfB abolishes the DSF signal [94]. A reduction in insect colonization and transmission was observed, but not a reduction in plant colonization. A biochemical analysis performed in vitro suggested fatty acyl-CoA ligase activity for RpfB, but, surprisingly, its effects on the DSF and BDSF signals was limited, indicating that RpfB plays a more important role in pathogenesis by counteracting RpfF thioesterase activity [100]. Discrepancies in RpfB enzymatic activities measured in vitro and in vivo suggest the involvement of an additional factor.

The expression of rpfB is negatively regulated by RpfC, RpfG, and Clp, which directly bind to the rpfB promoter region when it is complexed with di-GMP-cyclic [101] (Figure 4). At a low cell density, the di-GMP-cyclic-Clp complex represses rpfB expression, whereas at a high cell density, di-GMP-cyclic is degraded by RpfG, and rpfB is expressed. Finally, the RpfB-dependent signal turnover system was also detected in several Xanthomonas spp. including Xoo, but discrepancies were observed in bacterial virulence-associated traits [94].