Thymol is a famous plant-derived compound that has been widely used in pharmacy due to its antioxidant and antimicrobial properties. However, the modulation of intrinsic plant physiology by thymol remains unclear. It is a significant challenge to confer plant tolerance to Cd (cadmium) stress. In the present study physiological, histochemical, and biochemical methods were applied to investigate thymol-induced Cd tolerance in tobacco (Nicotiana tabacum) seedlings. Thymol was able to alleviate Cd-induced growth inhibition of tobacco seedlings in both dose- and time-dependent manners. Both histochemical detection and in-tube assays suggested that thymol treatment blocked Cd-induced over-generation of reactive oxygen species (ROS), lipid peroxidation, and loss of membrane integrity in both leaves and roots. Thymol decreased Cd-induced cell death that was indicated in vivo by propidium iodide (PI) and trypan blue, respectively. Thymol stimulated glutathione (GSH) biosynthesis by upregulating the expression of γ-glutamylcysteine synthetase 1 (GSH1) in Cd-treated seedlings, which may contribute to the alleviation of Cd-induced oxidative injury. In situ fluorescent detection of intracellular Cd²⁺ revealed that thymol significantly decreased free Cd²⁺ in roots, which could be explained by the thymol-stimulated GSH biosynthesis and upregulation of the expression of phyochelatin synthase 1 (PCS1). Taken together, these results suggested that thymol has great potential to trigger plant resistant responses to combat heavy metal toxicity, which may help our understanding of the mechanism for thymol-modulated cell metabolic pathways in response to environmental stimuli.

Phytophthora capsici is one of the devastating pathogens, causing foliar blight, root rot, and fruit rot in peppers. Cinnamaldehyde (CA) is a natural compound coming from Cinnamomum cassia. The medicinal properties of CA have been widely identified. Limited knowledge is known about the application of CA in agriculture. In this study, CA significantly inhibited P. capsici, which further suppressed Phytophthora blights in both pepper seedlings and pepper fruits. Treatment with CA resulted in collapsed and fragmented hyphae, accompanying the increase in MDA (malondialdehyde) content and the decrease in intercellular glycerol content in hyphae. CA also inhibited the growth of wild type yeast. The yeast mutant ΔYvc1 with a deletion of Yvc1 (a Ca²⁺ transporter) showed decreased sensitivity to CA. The transformation of PcNCX1, a sodium/calcium exchanger from P. capsici, into ΔYvc1 restored its sensitivity to CA. The transformant carrying PcNCX1 also showed restored Ca²⁺ efflux upon CA treatment. RNA-seq analysis showed that CA treatments resulted in the down-regulation of a set of genes encoding for calcium-related proteins. Collectively, our study demonstrates that the antifungal activity of CA against P. capsici may be associated with PcNCX1-mediated Ca²⁺ efflux. Our results provide crucial insights into the antimicrobial action of CA.