Melatonin Mediates Methylglyoxal Homeostasis and Autophagy During Seed Germination Under Polyethylene Glycol-Induced Drought Stress in Upland Cotton.

Methylglyoxal (MGO), a toxic byproduct of glycolysis, acts as a signaling molecule at low levels, but its overaccumulation during drought stress disrupts redox balance and accelerates cell death in plants. Contrarily, melatonin maintains redox balance, particularly during stress. The redox status and MGO levels might differ in drought-sensitive and drought-tolerant varieties, so shall the melatonin's effect. This present study evaluated the effect of melatonin priming on MGO detoxification and autophagy during seed germination under polyethylene glycol (PEG)-induced drought stress in drought-sensitive (L-799) and drought-tolerant (Suraj) varieties of upland cotton. Melatonin priming increased endogenous melatonin content, reduced MGO accumulation and advanced glycation end-products (AGEs), and downregulated the expression of MGO biosynthesis genes in L-799 under stress. The expression and activities of glyoxalases and nonglyoxalases were upregulated, showing melatonin's effectiveness in MGO detoxification. Additionally, melatonin priming upregulated TPI1, PGK5, and PK1 expressions and downregulated HK3 expression, allowing better conversion of glucose to pyruvate, leading to reduced MGO in L-799. The downregulated expression of necrosis-related genes with reduced cell death in L-799 shows the potential of melatonin priming in maintaining cell viability under stress. Furthermore, the upregulated expression of SnRK1.1 and SnRK2.6 genes and the KIN10 protein levels confirmed improved autophagy in melatonin-primed L-799 under stress, as evidenced by enhanced autophagy markers (ATGs, MDC-stained bodies, lipidated-ATG8). Despite lowered ABA, melatonin-mediated MGO homeostasis likely activated MAPK6, thus inducing autophagy independent of ABA in stressed plants. Conversely, Suraj seedlings showed a limited response to melatonin priming under stress possibly owing to its inherent stress tolerance and higher endogenous melatonin. Overall, this study illustrates melatonin's role in regulating MGO homeostasis and autophagy under drought stress in cotton.