AHL-priming functions via oxylipin and salicylic acid

Abstract

Collaborative action between the host plant and associated bacteria is crucial for the establishment of an efficient interaction. In bacteria, the synchronized behavior of a population is often achieved by a density-dependent communication called quorum sensing. This behavior is based on signaling molecules, which influence bacterial gene expression. N-acyl homoserine lactones (AHLs) are such molecules in many Gram-negative bacteria. Moreover, some AHLs are responsible for the beneficial effect of bacteria on plants, for example the long chain N-3-oxo-tetradecanoyl-L-homoserine lactone (oxo-C14-HSL) can prime Arabidopsis and barley plants for an enhanced defense. This AHL-induced resistance phenomenon, named AHL-priming, was observed in several independent laboratories during the last two decades. Very recently, the mechanism of priming with oxo-C14-HSL was shown to depend on an oxylipin and salicylic acid (SA). SA is a key element in plant defense, it accumulates during different plant resistance responses and is the base of systemic acquired resistance. In addition, SA itself can prime plants for an enhanced resistance against pathogen attack. On the other side, oxylipins, including jasmonic acid (JA) and related metabolites, are lipid-derived signaling compounds. Especially the oxidized fatty acid derivative cis-OPDA, which is the precursor of JA, is a newly described player in plant defense. Unlike the antagonistic effect of SA and JA in plant-microbe interactions, the recently described pathway functions through a synergistic effect of oxylipins and SA, and is independent of the JA signaling cascade. Interestingly, the oxo-C14-HSL-induced oxylipin/SA signaling pathway induces stomata defense responses and cell wall strengthening thus prevents pathogen invasion. In this review, we summarize the findings on AHL-priming and the related signaling cascade. In addition, we discuss the potential of AHL-induced resistance in new strategies of plant protection.

Keywords: AHL; SA; oxylipins; priming; quorum sensing (QS).

FIGURE 1

Function of SA and oxylipins in AHL-induced priming. Signaling steps of AHL-induced mechanisms leading to the reinforcement of resistance against several pathogens. The perception mechanism(s) of AHL in plant tissues is not known, indicated by “?”. Nonetheless, in local tissues of Arabidopsis plants, the oxo-C14-HSL-priming is manifested through the prolonged and stronger activation of MAPKs and the enhanced expression of WRKY transcription factors, followed by transcriptional reprogramming of genes related to Ca²⁺-signaling, defense, G-proteins, cell wall, and flavonoid metabolism. Furthermore, AHLs induced a higher accumulation of ROS, phenolic compounds, and callose in the cell walls. Even though long chain AHLs are not translocated to distal tissues, elevated production of the phytohormones oxylipin (cis-OPDA) and SA was observed in distal tissues, indicating that a systemic signaling is involved in this phenomenon. The proteins NPR1 and LOX2, as well as the TGA2/5/6 transcription factors were required. Like in the local tissue, the long chain AHL oxo-C14-HSL induced callose depositions, accumulation of phenolic compounds, and enhanced stomatal closure.