Thymol has drawn great attention from scientific researchers because of its antioxidant properties [17]. In this study, we found that thymol enhanced plant tolerance by increasing NO and GSH content, and modulating Na+/K+ homeostasis in tobacco seedlings.

Increased ROS formation was observed in plants in response to both osmotic and ionic stresses associated with soil salinity, further resulting in oxidative stress and cell damage [18]. ROS (H2O2 and superoxide anion), can directly damage most cellular macromolecules and cause irreversible damage [8]. In contrast, ROS have also been proposed to act as signaling mediators of plant salinity tolerance. Plants scavenge excess ROS through the antioxidant system to maintain cell homeostasis and reduce the harm caused by oxidative stress.

In the present study, we found that the growth of tobacco seedlings was strongly affected by salt stress. Salt stress significantly inhibited the root growth of tobacco seedlings, which was attenuated by thymol treatment (Fig. 1). Under salt stress, H2O2 and ({mathrm{O}}_2^{bullet -}) both in leaves and roots were decreased by thymol, suggesting that thymol effectively prevented the over-accumulation of ROS in tobacco leaves and roots (Fig. 2). Furthermore, thymol led to a decrease in MDA content and the lighter staining of Schiff’s reagent in tobacco seedlings under salt stress, suggesting that lipid peroxidation and oxidative injury caused by salt stress were ameliorated by thymol (Fig. 3). ROS directly affects normal cellular functioning and leads to cell death [19]. Therefore, our experiments proposed that the decrease in ROS accumulation caused by thymol resulted in the mitigation of lipid peroxidation and cell death in salt-treated tobacco seedlings.

Plants scavenge excess ROS through the antioxidant system to maintain cell homeostasis and reduce the harm caused by oxidative stress. Antioxidant systems include enzymes such as SOD, POD, and CAT and non-enzymatic antioxidants, such as glutathione, ascorbic acid, and carotenoids, which are important mechanisms for plants to resist salt stress [20]. Under salt stress, the activities of SOD, POD, and CAT were increased to scavenge the over-accumulation of ROS. Furthermore, it was significantly increased when treated with thymol, suggesting that thymol is an active antioxidant system that modulates ROS homeostasis (Fig. 2).

NO, an essential messenger that exists widely in plants, regulates multiple plant growth and development processes. NO also modulates plant responses to various abiotic stresses, including salt, drought, and heavy metals. NO helps eliminate ROS through two pathways. First, NO can regulate ROS levels [21,22,23] by regulating ({mathrm{O}}_2^{bullet -}) and H2O2 production [21]. Second, nitroso glutathione (GSNO), generated via the reversible reaction of NO and GSH, is an endogenous regulator of NO and GSH homeostasis [24]. GSH reacts directly with ROS species [25], and participates in the ASA-GSH cycle to scavenge ROS [26, 27]. In this study, we found that thymol enhanced endogenous NO and GSH content in tobacco seedlings under salt stress. This may explain the decreased ROS content in the salt-treated seedlings in the presence of thymol. However, further studies are needed to determine whether thymol modulates GSNO to maintain the homeostasis of NO, GSH, and ROS upon salt stress.

Ionic stress, caused by excess Na+ accumulation, is an important component of salt stress. To relieve Na+ toxicity, some strategies have been developed to decrease Na+ content in the cytosol of plant cells, including suppression of Na+ uptake and enhancement of Na+ compartmentalization [28]. The plasmalemma Na+/H+ antiporter, SOS1, transports Na+ out of the cell; high-affinity K+ transporter, HKT, transports Na+ from stems to xylem; sodium-hydrogen exchanger, NHX, transports Na+ into the vacuole [29, 30]. In this study, Na+ influx was observed in roots treated with salt, which was repressed by the addition of thymol due to the activation of NtSOS1 by thymol. Thymol also induced the expression of NtHKT1, which is responsible for the tissue distribution of Na+, which may also contribute to the decrease in total Na+ content in roots. K+ in tobacco seedlings was not significantly affected. Thus, thymol seems to maintain Na+ / K+ balance, likely through the regulation of Na+ transportation. In addition, NtNHX1 was induced by thymol, which may help root cells compartmentalize Na+ inside the vacuole to avoid Na+ toxicity in the cytosol. However, the subcellular distribution of Na+ in thymol-treated tobacco seedlings requires further investigation (Fig. 6).

Fig. 6
figure6

Schematic model for thymol-induced salt tolerance in tobacco seedlings. Thymol deploys two strategies to increase the salt tolerance of tobacco seedlings. First, thymol promotes the scavenging of ROS by promoting the decomposition of GSNO and increasing the content of NO and GSH. Second, thymol activates NtNHX1 to stimulate vacuole retention of Na+, NtSOS1 to promote Na+ rejection, and NtHKT1 to promote the transfer of Na+ to xylem and phloem, thus helping to reduce the level of Na+ in leaves and roots. (Created with BioRender.com)

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