4.2 Drought response differences of S. krylovii among different regions
Plant drought responses have been proven highly variable and significantly associated with higher environmental heterogeneity (Akman et al., 2021). In this study, the diagram based on gene expressions for all treatments also showed that individuals from eastern and middle regions under the same soil moisture treatment showed more similarity (Fig. 3), as shown by physiological traits in Table 2. In addition, under heavy drought treatment, the decline rates of the number of tillers in the western region were much smaller and the Pn ofindividuals from the western region were significantly greater than that of the other two regions (Table 2). Furthermore, under heavy drought treatment, the KEGG enriched pathways in the individuals from the western region were less than in the other two regions, which indicated that individuals from the western region were tolerant while individuals in the other two regions were sensitive to heavy drought treatment (Guo et al., 2023). All these findings indicated that the greater drought tolerance of individuals from the western region, which could explain the eastern shifts of the distribution areas ofS. krylovii to some extent (Liu, 2004).
Based on KEGG on DEGs between heavy drought treatment vs control, the present study showed that there were different response strategies between individuals from the western region and those from the other two regions. For DEGs of RNA-seq data, both up-regulated and down-regulated genes are important for species responses to stress conditions. However, sorting down-regulated genes into tolerance and response strategies is difficult due to the fact that many cellular processes are attenuated under stress conditions (such as heavy drought stress) to reduce growth and initiate dormancy (Bushman et al., 2021). Therefore, most studies have paid more attention to the significantly enriched pathways of DEGs .
First, metabolism of other amino acids and energy metabolism were significantly enriched and up-regulated in individuals from the eastern and middle regions but not the western region (Fig. 5a & b; Table S3 & S4). Many studies have shown that drought stress leads to changes in amino acid content and energy metabolism in plants (Heinemann et al., 2021; Ünlüsoy et al., 2023). Amino acid accumulation has a twofold role, their availability for protein biosynthesis and accelerated recovery after stress and osmoprotectant activity (Ashrafi, 2018). Wan et al. (2021) have found that differentially expressed metabolites related to the acid metabolism pathways may be key factors affecting drought resistance differences in two cherry (Prunus pseudocerasus L.) rootstocks. Plants can accumulate soluble proteins to maintain cell expansion and cell membrane stability, thereby protecting macromolecules from damage under drought stress (Fang & Xiong, 2015). Zhu et al. (2017) have found that individuals of Sorghum sudanense were significantly enriched in photosynthesis (energy metabolism) by KEGG on DEGs between drought treatment vs control. Plant photosynthesis can directly affect plant tissue material production and is extremely sensitive to environmental changes, thus, drought stress has a great impact on plant photosynthesis (Basu et al., 2016).
Second, carbohydrate metabolism and biosynthesis of other secondary metabolites were significantly enriched and up-regulated in individuals from the western region (Fig. 5c; Table S5). In response to drought stress, an increase in the content of flavonoids can help eliminate excess reactive oxygen species in the plant and help the plant to better adapt to the conditions of drought stress (Nakabayashi et al., 2014). Many studies have shown that drought stress usually induces the accumulation of flavonoids in plants (Gao et al., 2021; Yu et al., 2022). Glycolysis is an important metabolic pathway in carbohydrate metabolism, and changes in glycolysis and gluconeogenesis are considered an essential feature of plant adaptation to abiotic stresses (Broeckling et al., 2005). Shi et al. (2020) have conducted transcriptomic analysis of okra (Abelmoschus esculentus ) under drought stress and shown that most of the DEGs were mainly enriched in carbon metabolism, secondary metabolite synthesis, and glycolysis / glycolysis, etc. The increase in soluble sugar content plays an important role as a signaling molecule for plant growth and development under environmental stress, also is involved in the process of cellular carbon and energy metabolism (Rolland et al., 2006; Shahbazy et al., 2020). Using peach (Salvadora persica ) as the experimental material, Rangani et al. (2020) have found that galactose is up-regulated to provide roles such as osmoregulation, energy for antioxidant defense, carbon skeleton for secondary metabolite synthesis, and stress signaling under drought stress.