Trehalose can effectively protect the biomolecular structure, maintain the balance of cell metabolism, and improve the tolerance to various abiotic stresses in plants. However, the molecular mechanism underlying the improvement in salt tolerance by exogenous trehalose in watermelon (Citrullus lanatus) seedlings is still unclear. To understand these molecular mechanisms, in this study, watermelon seedlings under salt stress were treated with various concentrations of exogenous trehalose. An amount of 20 mM exogenous trehalose significantly improved the physiological status; increased the activities of enzymes such as POD, SOD, and CAT; and increased the K⁺/Na⁺ ratio in watermelon seedlings under salt stress. RNA-seq and metabolomic analysis were performed to identify the specifically expressed genes and metabolites after trehalose treatment. Watermelon seedlings were divided into salt stress (CK2), control (CK1) and trehalose treatment (T) groups as per the treatment. Overall, 421 shared differentially expressed genes (DEGs) were identified in the two comparison groups, namely CK2–CK1 and T–CK2. Functional annotation and enrichment analysis revealed that the DEGs were mainly involved in MAPK signaling pathway for plant hormone signal transduction and phenylpropanoid biosynthesis. Furthermore, 129 shared differential expressed metabolites (DEMs) were identified in the two comparison groups using liquid chromatography–mass spectrometry, which were mainly involved in the metabolic pathway and phenylpropanoid biosynthesis. The combined transcriptomic and metabolomic analyses revealed that genes involved in phenylpropanoid biosynthesis, plant hormone signal transduction, and carbohydrate biosynthesis pathways, especially bHLH family transcription factors, played an important role in improving salt tolerance of watermelon seedlings after exogenous trehalose treatment.

With the increase in watermelon cultivation area, there is an urgent need to explore enzymatic and genetic resources for the sustainable development of watermelon, especially under salt stress. Among the various compounds known, trehalose plays an important role in regulating abiotic stress tolerances in diverse organisms, including plants. Therefore, the present study comprehensively analyzed the trehalose-6-phosphate synthase (TPS) gene family in watermelon. The study analyzed the functional classification, evolutionary characteristics, and expression patterns of the watermelon TPS genes family. Seven ClTPSs were identified and classified into two distinct classes according to gene structure and phylogeny. Evolutionary analysis suggested the role of purifying selection in the evolution of the TPS family members. Further, cis-acting elements related to plant hormones and abiotic stress were identified in the promoter region of the TPS genes. The tissue-specific expression analysis showed that ClTPS genes were widely expressed in roots, stems, leaves, flowers, and fruits, while ClTPS3 was significantly induced under salt stress. The overexpression of ClTPS3 in Arabidopsis thaliana significantly improved salt tolerance. Finally, the STRING functional protein association networks suggested that the transcription factor ClMYB and ClbHLH regulate ClTPS3. Thus, the study indicates the critical role of ClTPS3 in watermelon response to salt stress.

Salt stress seriously reduced the yield and quality of watermelon and restricted the sustainable development of the watermelon industry. However, the molecular mechanism of watermelon in response to salt stress is still unclear. In this study, 150 mmol·L⁻¹ NaCl was used to deal with the seedlings of salt-tolerant and salt-sensitive watermelon varieties. Physiological characteristics showed that salt stress significantly reduced the biomass of watermelon seedlings and the accumulation of K⁺ in roots and leaves and significantly increased the content of Na⁺, Cl⁻, and malondialdehyde (MDA). Compared with the salt-sensitive variety, the salt-tolerant variety had higher K⁺ accumulation, lower Cl⁻, Cl⁻ accumulation, and MDA content in roots and leaves. Then, RNA-seq was performed on roots and leaves in normal culture and under 150 mmol·L⁻¹ NaCl treatment. A total of 21,069 genes were identified by RNA-seq analysis, of which 1412 were genes encoding transcription factors (TFs). In the comparison groups of roots and leaves, 122 and 123 shared differentially expressed genes (DEGs) were obtained, respectively. Gene ontology (GO) annotation and KEGG enrichment results showed that there were many identical GO terms and KEGG pathways in roots and leaves, especially the pathways that related to sugar or energy (ATP or NADP⁺/NADPH). In addition, some DEGs related to salt tolerance were identified, such as plant hormone indole-3-acetic acid (IAA) and gibberellin (GA) signal transduction pathway-related genes, K⁺/Na⁺/Ca²⁺-related genes, lignin biosynthesis-related genes, etc. At the same time, we also identified some TFs related to salt tolerance, such as AP2-EREBP, bZIP, bHLH, MYB, NAC, OFP, TCP, and WRKY and found that these TFs had high correlation coefficients with salt tolerance-related genes, indicating that they might have a potential regulatory relationship. Interestingly, one TCP TF (Cla97C09G174040) co-exists both in roots and leaves, and it is speculated that it may be regulated by miR319 to improve the salt tolerance of watermelon.

M. incognita is a major parasitic plant disease in watermelon production, causing serious economic losses. Although there are many studies on root-knot nematode, the resistance mechanism is still unclear. In this study, in order to fully understand the mechanism of watermelon resistance to root-knot nematode, the relatively strongly resistant ‘Hongzi watermelon’ variety and the susceptible ‘M16’ watermelon variety were used as materials, combined with RNA sequencing (RNA-seq), to analyze the expression abundance of resistant and susceptible varieties at 0, 2, 8 and 15 days post-infection (DPI) by M. incognita. The number of differentially expressed genes (DEGs) in the four comparison groups (A0_B0, A1_B1, A2_B2 and A3_B3) was 3645, 2306, 4449 and 2362, respectively, and there were 835 shared DEGs among them. GO annotation and KEGG pathway enrichment analysis showed that 835 DEGs were mainly involved in phenylpropane biosynthesis and carbon metabolism. Furthermore, lignin-biosynthesis-related genes (4CL (4-coumaric acid-CoA ligase), C3H (coumaric acid 3-hydroxylase), CSE (caffeoyl shikimate esterase), COMT (caffeic acid-O-methyltransferase), CCR (cinnamyl CoA reductase) and PRX (peroxidase)), defense-related proteins (UDP-glucoronosyl/UDP-glucosyl transferase, UGT84A13; salicylic acid binding protein, SABP2) and some transcription factors (TFs) were highlighted, which may be potential candidate genes for further analysis in the infection process of M. incognita. These results suggest that watermelon can achieve resistance to M. incognita by increasing the content of lignin and phenols in root or improving ROS level. These RNA-seq data provide new knowledge for future functional studies and will be helpful to further elucidate the molecular mechanism of resistance to M. incognita in watermelon.

The prolonged use of traditional polyethylene mulch (PM) films has resulted in significant environmental issues, such as soil residues and white pollution, which pose challenges to sustainable agriculture. The transition from PM to fully biodegradable mulch (BDM) films has emerged as a prominent trend in contemporary farming practices. This study investigates the effects of various colors of biodegradable mulches on watermelon production and quality, with a particular emphasis on BDM in comparison to conventional PM. Within the 0.2–15.3 µm wavelength range, transparent variants demonstrate high light transmission, while the silver–black treatment exhibits greater reflectivity. The silver–black surface effectively reduces evaporation, maintaining soil water content 5–8% higher than that of PM. However, its thermal profile reveals periodic temperature increases similar to those observed with PM. The results indicate that BDM silver–black enhances biomass, root N and P levels, and leaf NPK retention compared to PM. Notably, among the BDM treatments, silver–black yielded the highest average fruit weight and width (7.68 kg, 22.83 cm), although these differences were not statistically significant when compared to PM. Additionally, it produced the highest soluble solids content (13.2 °Brix) at a significance level of p < 0.05 relative to PM. This finding suggests an enhancement in the soil’s capacity to retain moisture and its nutrient availability, thereby fostering plant growth. All treatments proved profitable and economically viable; however, the total inputs and outputs associated with BDM silver–black and CK-PM transparent yielded a satisfactory profit, ranging from $1937 to $2503 per hectare. These results advocate for the utilization of sensor-embedded mulch films and the silver–black color to optimize water and nutrient utilization, thereby promoting sustainable watermelon cultivation.