Plant responses to bacterial quorum sensing molecules

The newest advances in research on interactions between plants and their associated bacteria have shown that inter-kingdom communication uses bacterial compounds like the homoserine lactones. Coevolution of plants and bacteria has resulted in diverse microbial relations ranging from loose surface biofilms, close endophytes, symbioses, and pathogenic intearctions. Both, symbiotic and pathogenic bacteria rely on intercellular communication to coordinate the collective behavior towards the host plant. Many Gram-negative bacteria use N-acyl-homoserine lactones (HSLs) in this communication. Bacteria synthesize and secret HSLs into the medium; through specialized receptors neighboring cells can perceive them. In this way HSLs influence gene transcription and hence the behavior of bacterial populations. This phenomenon is called quorum sensing. Plants too, have evolved means to perceive HSLs and respond to them with changes in gene expression or modifications in development. The consequences of this response may be improved root growth, altered plant architecture, induction of root hair formation, and even the formation of adventitious roots. Recently it became clear, that plants respond differently towards HSLs and structural differences in HSLs greatly influence the type of responses. Furthermore, some evidence that HSLs induce plant defense mechanisms were presented. Several plant-associated bacteria, known to induce systemic resistance lose this ability if blocked in HSL synthesis. Moreover, treatment with the oxo-C14-HSL induces systemic resistance to biotrophic fungi and bacteria, as shown in several plants. This increased resistance is associated with a stronger activation of mitogen-activated protein kinases, followed by a higher expression of the defense-related transcription factors. The molecular basis of HSL’s perception in plant cells is still largely unknown. Nevertheless, two necessary steps in HSLs perception were identified; MPK6 and the GCR1 G-protein are required for HSL signaling in Arabidopsis. Plants and plant-growth promoting rhizobacteria also produce quorum-quenching molecules and enzymes, which may alter QS in the plant pathogens. In addition, some plants have high activities of lactonases, while others lack them and thus the HSLs can be transported from the root to the shoot. Interestingly, the perception of HSL seems to have opposite effects on plant and animal immunity. While AHLs abolish some of the immune responses in animals, they potentiate the defense mechanisms of plants indicating that plants evolved mechanisms to benefit from HSLs presence, probably a consequence of their sessile lifestyle.
This research topic aims to present research on different levels of the plant-bacteria interaction, involving the chemical base of ligand – receptor interaction, signaling cascades, transcriptome and proteome changes in plants to changes in pathogenicity of bacterial populations associated with plants upon modifications of HSL content. We anticipate that in upcoming years research in this field will concentrate on the differences between animal and plant perception mechanisms, as well as on the molecular bases of the growth promotion and induced resistance provoked by different HSLs in plants. Very important will be of course the question how this knowledge will affect biotechnology approaches, which could possibly use HSLs as modulators of QS or biostimulants.