QUORUM SENSING IN PLANT PATHOGENIC BACTERIA

Bacteria communicate with one another by using chemical signal molecules. As in higher organism , the information supplied by these molecule is critical for synchronizing the activities of large group of cells. Quorum sensing is a process of cell-cell communication in bacteria. In bacteria, chemical communication involves producing, releasing, detecting and responding to small hormone like molecules termed as autoinducer (Christopher and Bassler, 2005). These regulate a diverse array of physiological activities such as bioluminescence, sporulation, competence, biofilm formation and virulence factor secretion (Rutherford and Bassler, 2012).

There are three kinds of quorum sencing molecules such as Acyl-homoserine lactones (AHLs), Autoinducer peptides (AIPs) and Autoinducer-2 (AI-2). AHLs regulate quorum sensing in Gram-negative bacteria and synthesized by an autoinducer synthase LuxI and recognized by a autoinducer receptor. AIPs regulate gene expression in Gram positive bacteria such as Bacillus subtilis, Staphylococcus aureuas etc. AI-2 Present in both Gram (+) and Gram (-) bacteria (Christopher and Bassler, 2005).

QS molecular signaling network of Gram-negative bacteria produces the autoinducers and diffused freely out of the cell. The autoinducers produced will bind to their cognate receptor to form an autoinducer-receptor complex which will then binds to the target promoter that lead to QS gene regulation and Gram-positive bacteria produces the precursor peptide autoinducers are modified and transported out of the cell by ATP binding cassette exporter complex. Then sensor kinase protein will be activated and phosphorylate the response regulator protein, which will then binds to the target promoter that will lead to QS gene regulation (Koh et al., 2013).

Guo et al. (2012) Diffusible signal factor (DSF) family signal-mediated quorum sensing (QS). This QS pathway of Xanthomonas spp. consists of three major QS components: RpfF, RpfC, and RpfG. The rpfF gene encodes a putative enoyl-CoA hydratase that catalyzes the synthesis of the signal molecule. RpfC and RpfG serve as a two-component system for the perception and transduction of the extracellular DSF family signals. This suggesting that the RpfC-RpfG two-component system is a major and conserved signal perception and transduction system for DSF family signal-mediated QS in X. citri subsp. citri.

Studied molecular factors involved in the survival or growth of X. albilineans on sugarcane leaves. Six mutants of strain XaFL07-1 affected in surface polysaccharide production completely lost their capacity to survive on the sugarcane leaf surface. These mutants produced more biofilm in vitro and accumulated more cellular poly-β hydroxybutyrate than the wild-type strain. The attachment of bacteria on sugarcane leaves varied among mutants of the rpf gene cluster involved in bacterial quorum sensing. Therefore, quorum sensing may affect polysaccharide production, or both polysaccharides and quorum sensing may be involved in the survival or growth of X. albilineans on sugarcane leaves (Mensi et al., 2016).

Many important plant pathogenic bacteria use quorum sencing to regulate a diverse array of physiological activities such as bioluminescence, sporulation, competence, biofilm formation and virulence factor secretion, strategies designed to interfere with these signaling system will likely have broad applicability for biological control of diseases. This natural strategy provides the blue-print for genetic engineering and to improve crop protection by identifying and targeting critical pathogen factors.

Selected References: GUO, Y., ZHANG, Y., LI, J. AND WANG, N., 2012, Diffusible Signal Factor-Mediated Quorum sensing plays a central role in coordinating gene expression of Xanthomonas citri subsp. Citri. The American Phytopathological Society. 25:165-179. MENSI, I., DAUGROIS, J., PIERETTI, I., GARGANI, D., FLEITES, L. A., NOELL ,J., BONNOT, F., GABRIEL, D. W. AND ROTT, P., 2016, Surface polysaccharides and quorum sensing are involved in the attachment and survival of Xanthomonas albilineans on sugarcane leaves. Mol. Pl. pathol., 17 (2): 236- 246.