Bacterial Quorum Sensing Regulation by Strigolactones

Introduction:

Various bacteria communicate with each other and other cells through a process called quorum sensing (QS), by producing, releasing and sensing small signal molecules called autoinducers (AIs). The signal response in QS is dependent on the cell density and species composition found in the surrounding area. When the bacteria reach a certain cell density their AIs reach a threshold concentration, which leads to coordinated expression of bioluminescence, biofilm formation, virulence and public goods production.

Plant derived compounds such as coumarins, indoles and karrikins have been shown to cause inhibition of bacterial QS. Karrikins are growth regulators found in smoke of burnt plants and they were found to stimulate germination. Another class of plant-derived molecules called strigolactones (SLs) shares a structural and functional similarity to karrikins. SLs are phytohormones that stimulate germination in parasitic plant seeds (striga, orobanche sp.), hyphal branching in arbuscular mycorrhizal fungi and they inhibit shoot branching. There are over 20 natural occurring SLs found in various plants, and analogues with a simpler structure, GR24 and Nijmegen, were evaluated as QS modulators.

Results:

Both compounds, GR24 and Nijmegen, showed inhibitory effects on V. cholerae MM920 (CqsS system reporter strain) with IC₅₀ values 19 µM and 9 µM respectively. Furthermore, GR24 and Nijmegen were tested on V. cholerae MM825, a CqsS/LuxPQ deletion strain, and showed QS inihibition with an IC₅₀ of 78 µM and 21 µM respectively. Nijmegen also showed a minor inhibition effect on A. tumefaciens with an IC₅₀ of 160 µM.

Conclusions:

The inhibitory effects of several plant hormone analogs on V. cholera QS reporter strains suggest that there may be unknown interactions between the signaling systems of these two representative species from different kingdoms. We are examining the specificity, selectivity and mechanism of action behind this phenomenon in order to understand its relevance to plant-bacterial interactions.