Integrated cytological, physiological, and transcriptomic analyses elucidate the genes and regulatory network underlying sulfur stress responses in melon seedlings

Abstract

Sulfur (S) is a vital macronutrient, which plays a crucial role in the plant growth and development of defense, but its excessive usage causes severe phytotoxicity. The melon plant is very sensitive to sulfur stress because of routine-wise applications of sulfur-based fungicide for controlling the powdery mildew. In this experiment, we performed an integrative cytological, physiological, and transcriptomic study to elucidate the physiological and molecular mechanisms regulating sulfur tolerance in comparative melon lines (‘M531’ as sulfur-tolerant and ‘M446’ as sulfur-sensitive).The relative ultrastructural observations exposed that the ‘M531’ line had tightly organized palisade tissue, intact organelles, and stable chloroplast morphology under sulfur stress, but the ‘M446’ line depicted excessive membranal peroxidation, organelle rupture, and cell destruction. Physiological evaluations also displayed that the ‘M531’ line effectively modulated the metabolism of reactive oxygen species (ROS) by improved activities of superoxide dismutase (SOD) and peroxidase (POD), restricting oxidative injury and lipid peroxidation. Transcriptomic profiling across various time intervals (0, 24, 48, and 72 h) exposed a total of 556 differentially expressed genes (DEGs) linked with redox modulation, cell wall transformation, and secondary metabolism. Pathway enrichment analysis determined the phenylpropanoid activation and biosynthesis of flavonoid in the ‘M531’ line, triggering accumulation of flavonoid and lignin that enclosed the integrity of the cell wall and enhanced scavenging of ROS. The upregulated transcription of the DGAT and other glycerolipid metabolic genes continued membranal homeostasis under the extended stress phase. Further, the pathway suppression in the ‘M446’ line disturbed metabolic symmetry and intensified cell destruction. A total of ten significant DEGs involved in sulfur metabolism, glycerolipid pathways, and phenylpropanoid biosynthesis were verified through qRT-PCR, confirming the functional transcriptomics regulation through a wide-range systematic framework for sulfur tolerance, linking coordinated cytological resilience, and antioxidative defense. The current study provides fundamental insights into adaptation of sulfur-induced stress and categorizes robust genetic targets for developing sulfur-tolerant cultivars of melon through advanced breeding and sustainable cultivation.