Exogenous N-hexanoyl-l-homoserine lactone mitigates acid rain stress effects through modulation of structural and functional changes in Triticum aestivum leaf

Acid rain (AR) is a global environmental problem that causes a number of morphological, physiological and molecular changes in plants. N-acyl homoserine lactones, as bacterial quorum sensing signal molecules, mediate a number of physiological processes in plants. In this study, we have demonstrated for the first time the stress-protective role of AHL foliar treatment under conditions of abiotic stress - acid rain. The effects of simulated acid rain (SAR), treatment of plants with N-hexanoyl-l-homoserine lactone (C6-HSL) solution, and a combination of these two factors on the microstructure of the leaf surface, photosynthetic pigments, secondary metabolites, and lipoxygenase activity in winter wheat plants were analyzed. It was established that on the fourth day following treatment of 14-day-old plants with C6-HSL, the thickness of the outer cell wall of the leaf epidermis, together with the cuticle layer, increased by 15 %. SAR treatment resulted in a significant thinning of the cell wall (33 %), while pretreatment with C6-HSL followed by SAR reduced cell wall thickness by only 9 %. Under SAR, the cuticular wax layer was destroyed and there were unequal wax lamellae on the epidermal surface, while in C6-HSL treated plants only partial cracking of the cuticular wax layer, slight destruction of the wax lamellae, and formation of wax crusts took place. C6-HSL induced a significant increase in total chlorophyll content under non-SAR conditions, and significantly mitigated the decrease of total chlorophyll content caused by SAR. Lipoxygenase activity, the content of total phenols, flavonoids, and proline increased after treatment of plants with C6-HSL solution alone and in combination with SAR. This is the first report on the possibility of using C6-HSL as an ecological biostimulator under conditions of AR.