Quorum Sensing: Communication Complexity for Resilience of Plant-Microbe Interaction

Abstract

Microorganisms employ a precised communication pattern among themselves in order to coordinate between various processes during their growth. Both unicellular and multicellular microbes are found to show cell density-driven gene expression. This phenomenon of density-dependent cell regulation used for survival, prevalence and colonization of specific host is generally termed as quorum sensing (QS). Microorganisms respond to this stimulus once the signalling molecule reaches its threshold concentration. Since they are found to be able to regulate their own production, they are termed as autoinducers (quorum sensing molecules). These molecules function by sensing their own population with respect to their density and distribution pattern in the prevailing environment. Hence, microorganisms use such environmental sensing mechanisms to get adapted as well as for their survival in the existing conditions in their habitat, thereby maintaining healthy cell population. The autoinducers occur widespread in several microorganisms and differ from each other in their molecular structures. Acyl homoserine lactones (AHL), Autoinducer (AI), i.e., AI-2, AI-3 and quinolones are the common QS signalling molecules in Gram-negative bacteria, while cyclic peptides, AI-2 and butyrolactones are observed as signalling molecules in Gram-positive bacteria. In the case of actinomycetes, small diffusible molecules called autoregulators, A-factor and 2-iso-octanoyl-(3R)-hydroxymethyl-γ-butyrolactone act as QS signalling molecules. Understanding the connection between genomes, gene expression and the molecules in complex environment is considered to be a tough task. Increasing interest towards studying the underlying mechanisms has led to the development of various model systems. Among them, plant-microbe symbiotic system is considered to be the best one to study the inter-kingdom molecular cross-talk. During the process of evolution, plants started to respond to the external stimuli in different and more specific ways. One such way includes production of AHL-like molecules to regulate the QS of plant-associated microorganisms. In view of this, the present chapter will be focused on quorum sensing molecules and their role in plant-microbe interaction.