The Response of Plants toward N-Acyl Homoserine Lactones of Quorum-Sensing-Active Bacteria in the Rhizosphere

Gram-negative bacteria communicate within a population on species or even genus level using small autoinducer signal molecules of the N-acyl homoserine lactone (HSL) type, which occur in several structural variations. Plants colonized by these bacteria are exposed to these signaling molecules and are able to recognize and even distinguish between different quorum-sensing QS-active molecules. Plants responses to those molecules are manifold, ranging from modifications of the defense system to modulation of the immune response or changes in the hormonal status. The type of response is dependent on the structural features of the HSL molecule and the plant species. In some plants, the HSL molecules with side chain length up to C10 are taken up, while in other plants, lactonases efficiently hydrolyse the molecules. It appears that short side chain (C4–C10) and long side chain (C12 and C14) HSLs induce different types of responses by activating different perception mechanisms and response cascades. Plants and microbes evolved together and thus complex trans-kingdom assemblages were formed. Apparently, in addition to the microbe-associated molecular pattern (MAMP) recognition, multiple communication possibilities have been formed, many of them only poorly understood at the present. All these signals can be integrated within a “superorganism, ” which is characterized by its healthy state and well-balanced “immunity homoeostasis.”