Plants are often challenged with various environmental stresses simultaneously or consecutively. Cross-adaptation is the phenomenon that the adaptive changes caused by one stressor may make the organism more fit to resist the adverse effects of another type of stressor, which plays important roles in plant adapting to complex and changeable environments (Alcázar and Parker 2011; Hatmi et al. 2014; Xue and Yi 2018; Liu et al. 2021). In the present study, we report that SO2 pre-exposure resulted in an enhanced tolerance of Arabidopsis plants to drought stress. The enhanced drought tolerance is mainly attributed to the improving photosynthetic performance, the increasing biosynthesis of osmotic adjustment substances, and the enhancing antioxidant capability.
Photosynthesis of green plants is easily affected by drought stress (Chaves et al. 2009). Under water deficit conditions, the reduction of leaf RWC, the alteration of leaf gas exchange and the decrease of photosynthetic pigment contents could cause inhibition of photosynthesis (Degl’Innocenti et al. 2009; Misson et al. 2010; Hejnák et al. 2015). Here, SO2 pretreatment effectively decreased the Gs in drought-stressed plant leaves, which may be beneficial for the reduction of Tr and the improvement of RWC under drought stress (Fig. 2B and C). However, the decreases of Gs and Tr in SO2 + drought group did not directly lead to a lower Pn, which may be due to the enhanced iWUE in SO2 pre-exposed Arabidopsis plants (Fig. 2E). The higher leaf RWCs and the improvement of iWUE induced by SO2 pretreatment were of great relevance for drought tolerance of Arabidopsis plants under drought stress. Additionally, the increases of chlorophylls and carotenoid contents in SO2 pretreatment group (Fig. 2), could promote the light harvesting and photosynthetic efficiency of drought-stressed Arabidopsis plants, consistent with the higher Pn under drought stress. Taken together, the high photosynthetic performance was contributed to the enhanced drought tolerance of Arabidopsis plants.
Drought stress generally causes high levels of ROS in plant cells, which is considered to be the main cause for the harmful effects of drought stress on plants (Osakabe et al. 2014; Nakabayashi et al. 2014; Wei et al. 2019). Excessive accumulation of ROS would cause oxidative stress to the organisms and seriously affect plant growth and development (Tambussi et al. 2000; Cruz de Carvalho 2008; Noctor et al. 2014; Choudhury et al. 2017). In this study, SO2 pretreatment significantly decreased the contents of H2O2 and MDA in drought-stressed Arabidopsis plants, indicating that SO2 could alleviate oxidative stress and oxidative damage caused by drought stress. These positive effects were dependent on the SO2 pretreatment-triggered increases of antioxidant capability in Arabidopsis plants. The increased activities of SOD and POD, associated with high levels of antioxidant substances, could provide an enhanced ROS scavenging capacity to eliminate drought-induced ROS and alleviate oxidative damage.
It has been reported that ROS could act as signal molecules mediating the responses of plants to biotic and abiotic stresses, such as drought stress and high concentrations of SO2 (Cruz de Carvalho 2008; Yi et al. 2017; Mahmood et al. 2020). The increased ROS evoked by SO2 might trigger a series of responses including antioxidant defense, sulfur assimilation activation and proline synthesis contributing to drought tolerance of Arabidopsis plants, although the exact mechanisms need further study.
The sulfur assimilation and downstream metabolic pathways play important roles in plant response to drought stress (Chan et al. 2013; Stanislav et al. 2019). In the present study, SO2 pre-exposure promoted the sulfur assimilation in the leaves of drought-stressed Arabidopsis plants, as evidenced by the enhanced OASTL activity and the increased Cys content. The increased Cys content is beneficial for the biosynthesis of other thiol-containing compounds, including GSH and NPT (Fig. 4). High GSH level together with increased GPX and GR activities could enhance the ROS scavenging capacity of GSH-GSSG cycle in drought-treated Arabidopsis plants, and reduce drought-caused damage. Taken together, SO2 pretreatment enhanced the sulfur assimilation and further promoted the synthesis of GSH and NPT to complete the detoxification under drought stress.
The accumulation of osmoprotectants (such as soluble sugar, proline, and betaine) is a significant strategy for plants to maintain water potential and membrane stability to avoid drought-induced damage (Fang and Xiong 2015). In this study, drought stress resulted in a significant increase of the contents of proline and soluble sugar in Arabidopsis plants. Moreover, SO2 pretreatment induced a higher level of proline accumulation in drought-stressed Arabidopsis plants, consistent with the increased P5CS activity and decreased ProDH activity. In accordance with this result, the alteration of the transcription level of key genes involved in proline biosynthesis pathway (P5CS1, P5CS2 and P5CR) and degradation pathway (ProDH) showed the similar tendencies, indicating the involvement of gene expression and cellular metabolism in SO2 pretreatment-induced drought tolerance. In addition to its accepted role as an osmolyte, proline can act as a potent nonenzymatic antioxidant induced by ROS, contributing to overall antioxidant activity in plant response to limited water supply (Ben Rejeb et al. 2014). Therefore, proline accumulation induced by SO2 pretreatment might protect plant cells by osmotic adjustment, and also promote redox balance by scavenging ROS under drought stress, which contribute to the improvement of drought tolerance in Arabidopsis plants.
In conclusion, this study provides a systematic description of key events leading to enhanced drought tolerance in SO2-pretreated Arabidopsis plants (Fig. 7). SO2 pretreatment significantly reduced leaf water loss and maintained higher RWC, photosynthetic activity and proline levels, as well as higher P5CS activity under water deficit conditions. Importantly, SO2-induced antioxidant enzymes, together with high levels of nonenzymatic antioxidants increased the ROS scavenging capacity to recover drought-induced cellular redox imbalance. Taken together, SO2 exposure could enhance drought tolerance in Arabidopsis grown in water shortage conditions by increasing the photosynthesis performance, antioxidant defense, sulfur assimilation, and osmoprotectant accumulation. The present study reveals the molecular mechanism of cross-adaptation of plants under SO2 and drought stress, and provides a new perspective for future researches on stress-resistance mechanisms. However, more detailed analyses might be still needed to better understand the role of SO2 in plant adaptation to drought stress.