**3. Progress in understanding interkingdom QS**

As noted in the discussion above, some plant pathogenic bacteria encode LuxR homologs that are capable of 'eavesdropping' by utilizing AHL mimicking low molecular weight compounds that are produced by plants. In place of the LuxI, the LuxR homologs in plant pathogenic bacteria are in most oftenly in close proximity to the *pip* gene [119]. The *pip* harbors an inverted repeat unit similar to *luxI* and is directly involved in pathogenicity, hence its biological role merits further investigation. Over the past decade, researchers have attempted to investigate these LuxR proteins especially on deciphering their role in QS signaling. The binding motifs of these LuxR homologs is unique and distinct from the conventional LuxR homolog, they lack one or two of the several conserved regions required for AHL binding [5, 8, 119]. The AHL binding domain of these proteins are substituted by methionine and tryptophan in the conserved region allowing specificity for binding to plant derived molecules [119]. The orthologs of these LuxR proteins are also encoded on the genomes of AHL producing bacteria including *Pseudomonas syringae* [8]. Consequently, questions arise, do these LuxR homologs bind to the AHL mimicking compounds and function in a similar way in the AHL producing and non AHL producing bacteria? In addition, the AHL mimicking molecules produced by plants still need to be characterized.

## **4. Conclusions**

A variety of bacterial species are increasingly becoming resistant to the antimicrobial agents that are currently in use [120]. Resistance to streptomycin in plant pathogenic bacteria was reported within a decade of its use in controlling plant infections and diseases [121]. Research efforts are now focusing on alternative bacterial control strategies. The discovery of the involvement of QS in the regulation of bacterial virulence has led to escalated research efforts towards discovering possible biological control measures that target QS systems. The main advantage of control measures that target QS systems, though not yet scientifically proven, is that they are less prone to selective pressure [122].

For an effective application of QS inhibition as a biological antimicrobial measure, a better understanding of the genes influenced by QS is crucial. Latest technology including research tools such as RNA-Seq has made it possible for whole transcriptome investigations to be conducted. In addition, targeted mutation and characterization of mutants has helped in unveiling the biological significance of specific genes in bacteria, the complexity of bacterial transcriptomes and thus regulation of gene expression. Nonetheless, as additional experimental and analytical tools become available, the critical role of bacterial QS to plant pathogenesis will undoubtedly become much clearer.

The literature cited in this chapter reflects on QS and its role in influencing pathogenicity and pathogenicity-associated traits in Gram-negative plant pathogenic bacteria. The different QS systems, the extent of those QS regulons that have been elucidated as well as the different signaling molecules employed by plant pathogenic bacteria have been explored. This chapter highlights interesting similarities and differences of QS systems and the diversity of QS signal molecules utilized by plant pathogenic bacteria. Understanding QS regulation in plant pathogenic bacteria could provide useful tools for control and management of bacterial plant diseases.
