The escalating problems of salt and drought stress in agriculture pose a significant danger to crop production and food security. Plants have developed certain adaptation mechanisms to cope with challenges in order to ensure their survival. These mechanisms include the ability to detect stress signals and rapidly modify their biological responses. The investigation into the mechanism of gene stress resistance has garnered significant interest. Kenaf, a crop known for its high stress resistance, can serve as a valuable tool for understanding the underlying mechanisms of its stress response through the analysis of its gene function. This study provide evidence that HcMDH1 gene is strongly associated to kenaf signal transduction in response to salt and drought and plays an important regulatory role. Additionally, under salt and drought stress, transgenic Arabidopsis HcMDH1 can greatly increase root length, seed germination rate, and survival rate (Fig. 5–7).
This study shows that the salt and drought stress significantly increased the expression of HcMDH1 (Fig. 1C). The phylogenetic analysis demonstrated that HcMDH1 exhibited the closest evolutionary relationship to other MDHs within the Malvaceae family (Fig. 1B). This suggests that HcMDH1 in kenaf has been highly conserved throughout the species' evolution. These malate dehydrogenases may have distinct biological roles due to variations in subcellular localization [8, 11, 32]. The subcellular localization experiment indicated that HcMDH1 is mainly located in chloroplasts (Fig. 1A), leading us to hypothesize that it is primarily involved in photosynthesis. Furthermore, heterologous expression of HcMDH1 gene into a yeast system improves the ability of yeast strains to withstand high salt and drought conditions, providing evidence of its capacity to boost stress resistance.
Plants have developed several strategies and advanced mechanisms to cope with drought stress, which include both physiological and morphological alterations [33, 34]. Exposure of plants to abiotic stress leads to the accumulation of ROS within their bodies. The antioxidant enzyme system, which is comprised of SOD, CAT, and POD, is enhanced to a certain extent in order to eliminate toxic compounds from the plant, mitigate oxidative stress caused by ROS, and preserve normal development [35–37]. SOD, POD, and CAT are important indicators reflecting cell oxidative damage [38]. In addition, proline helps plant tissues to retain water and prevent dehydration by regulating cellular osmosis under stress [39]. Thus, the mechanism of plant drought resistance is intimately linked to the accumulation of soluble compounds like proline [40]. Proline and MDA can show the extent of damage caused by stress [41]. Chlorophyll, a key component of chloroplasts, captures and transmits sunlight during photosynthesis [42]. Yao et al. (2011) found that the overexpression of MdcyMDH gene is involved in salt and cold stress resistance, as it enhances the activity of SOD and CAT in transgenic apple callus and tomato, reduces the production of ROS, and improves the resistance of transgenic plants to cold and salt stress [12]. The mMDH1 mutant of soybean showed the phenotype of yellow leaf flower, and the photosynthetic efficiency decreased significantly [13]. In our investigation, allogeneic overexpression of HcMDH1 significantly increased Arabidopsis thaliana seed germination, seedling length, and survival rates under salt and drought stress (Fig. 5–7). The histochemical staining and physiological parameter analysis revealed a reduced accumulation of H2O2 and O2− in the leaves of HcMDH1-OE Arabidopsis plants subjected to salt and drought stress. This was accompanied by a significant enhancement in the activity of SOD, POD, and CAT enzymes, as well as a noticeable increase in chlorophyll, proline, and soluble sugar levels (Figs. 7 and 8). These results were similar to the overexpression of ZmNADP-MDH in Arabidopsis, which also resulted in improved resistance physiology, including a decrease in ROS and an increase in soluble substance and chlorophyll content [15]. Conversely, the HcMDH1-silenced lines were more sensitive to salt and drought stress than CK, due to more ROS accumulation in the HcMDH1-silenced kenaf (Fig. 9). Therefore, the HcMDH1 gene closely relates to antioxidant enzyme activity and osmotic regulation in kenaf under salt and drought stress.
Glucose, being the primary carbon source in cells, supplies energy to many cellular components and has a vital function in intracellular regulation. Gluconeogenesis and glycolysis are the main pathways involved in the metabolism of glucose. One of the essential enzymes in the processes of glycolysis and gluconogenesis is glyceraldehyde-3-phosphate dehydrogenase (GAPDH). It facilitates the process of oxidative phosphorylation of glyceraldehyde-3-phosphate, resulting in the production of 1, 3-diphosphoglyceric acid. This enzyme is essential for sustaining vital biological functions. Research has demonstrated that GAPDH is present in a wide range of tissues and cells. It has a role in plant immune response and stress response, and is tightly linked to several cellular activities including DNA repair, apoptosis, and cell homeostasis [43]. Fructose-1, 6-bisphosphate aldolase (FBA) and fructose-1, 6-biphosphatase (FBPase) are metabolic enzymes that play crucial roles in glycolysis, gluconeogenesis, and the regulation of photosynthetic rate [44, 45]. Photorespiration interacts with numerous metabolic processes in plants and is thus an integral aspect of plant metabolism [46]. The process of photorespiration is facilitated by 3-phosphoglycerate (3-PGA) and D-glycerate 3-kinase (GLYK) [47, 48]. GLYK catalyzes the phosphorylation of glycerate to produce 3-PGA, which is the final step in the photorespiration pathway [49]. The genes responsible for lesion simulating disease (LSD) encode a group of zinc finger proteins that have a role in programmed cell death (PCD) and serve as a crucial regulatory factor for plants in their response to biotic and abiotic stress [50]. The study found that HcMDH1-silencing kenaf plants had lower transcript levels of these stress-related genes relative to the WT control under salt and drought stress (Fig. 10). These findings suggested that HcMDH1 might engage in interactions with these stress related genes to jointly regulate kenaf resistance to stress. Nevertheless, further investigation is needed to determine the precise role of HcMDH1 in regulating these genes and the specific pathways involved in kenaf's response to abiotic stress.
The promoter region of a gene often comprises several cis-elements that have significant functions in responding to various stress stimuli [51]. The cis-elements have a direct impact on the regulation of stress responsive gene expression [51]. Transcriptional initiation events are determined by the molecular switches that are created by the interactions between transcription factors and cis-acting elements [52]. Hence, it is crucial to identify cis-acting elements in the promoter region in order to understand the function of transcription factors in stress response. The study conducted bioinformatics analysis on the promoter region of HcMDH1 gene. The promoter region contained various types of cis-acting elements, including those associated with photoresponse, hormonal response, development, and abiotic stress response (Fig. 2, Table 1). As a result, it is hypothesized that HcMDH1 can respond to salt and drought stress in kenaf because the cis-acting elements in its promoter play an important regulatory role. It is imperative to examine these crucial cis-acting elements in future trials, carry out relevant investigations on the specific transcription factors that govern these cis-acting elements under hormone or abiotic stress, and elucidate the molecular mechanism by which HcMDH1 responds to abiotic stress.