Melatonin-Soaked Seeds Help Tomato Seedlings Fight Cadmium and Drought

A simple soak in melatonin, the same hormone that governs sleep in humans, may hold the key to protecting one of the world’s most valuable vegetable crops from two of its most punishing environmental threats. New research published in Plant Biosystems shows that treating tomato seeds with melatonin before sowing dramatically improves the ability of young seedlings to withstand both cadmium contamination and drought-induced water stress. The findings, from a team at the Xinjiang Academy of Agricultural Sciences in Urumqi, China, offer a low-cost, chemically simple intervention that could help farmers establish crops successfully in degraded, contaminated, or water-limited soils, conditions that are becoming increasingly common across major production regions.

The study, led by Tayeb Muhammad and colleagues including Tao Yang, Juan Wang, Baike Wang, and corresponding author Qinghui Yu, focused on the earliest and most vulnerable phase of a crop plant’s life: germination and seedling establishment. This window is decisive for final yield, because seedlings that fail to establish vigor in their first days rarely recover. The researchers compared three groups of tomato seeds: untreated controls, seeds soaked in water alone (hydro-primed), and seeds primed in a melatonin solution. The primed and control seeds were then exposed to cadmium, a toxic heavy metal that accumulates in agricultural soils worldwide, and to polyethylene glycol, a laboratory-standard agent that mimics the osmotic stress plants experience during drought by restricting water availability.

Without protection, both stressors inflicted severe damage. Cadmium and osmotic stress significantly inhibited seed germination, stunted seedling growth, and reduced biomass accumulation. The mechanism behind this damage is well understood in plant physiology: both stresses trigger the overproduction of reactive oxygen species, including hydrogen peroxide, which attack cellular membranes and macromolecules. The team measured this oxidative assault directly, recording elevated hydrogen peroxide levels and increased lipid peroxidation, the destructive degradation of membrane fats that is tracked through malondialdehyde, a classic biomarker of oxidative injury. They also observed increased electrolyte leakage, a sign that cell membranes had lost their integrity and were allowing ions to escape from the cells.

One of the most scientifically interesting aspects of the work is the discovery that the two stressors behave in strikingly different ways within the plant. Cadmium stress primarily targeted the root system, which is unsurprising given that roots are the first point of contact for the metal in the soil solution and the main site of cadmium uptake. Osmotic stress imposed by polyethylene glycol, by contrast, had its strongest impact on shoot growth and above-ground physiological performance, reflecting the plant’s struggle to take up water and maintain turgor in its leaves. This differential targeting pattern matters for breeders and agronomists, because it suggests that no single trait or treatment can fully shield a seedling; a successful protective strategy must strengthen both root and shoot systems simultaneously, which is precisely what melatonin priming achieved.

Melatonin, chemically known as N-acetyl-5-methoxytryptamine, has emerged over the past decade as a master regulator of stress responses in plants, functioning alongside its better-known role as a circadian signal. In this study, melatonin priming significantly improved germination efficiency and seedling vigor under both stress conditions. Primed seedlings displayed enhanced root and shoot elongation, accumulated greater biomass, and maintained higher levels of chlorophyll, the photosynthetic pigment whose degradation is an early casualty of stress. The treatment also boosted proline accumulation, a compatible osmolyte that plants deploy to protect proteins and membranes while adjusting their internal water balance, an adaptation especially valuable under drought.

The protective effect extended deep into the plant’s biochemical defenses. Melatonin-primed seedlings showed markedly lower levels of electrolyte leakage, malondialdehyde, and hydrogen peroxide, indicating that the hormone had blunted the oxidative cascade at its source. The team traced this resilience to a strengthened enzymatic antioxidant system. Activities of four key antioxidant enzymes rose in primed seedlings: superoxide dismutase, which converts superoxide radicals into hydrogen peroxide; peroxidase and catalase, which break hydrogen peroxide down into water and oxygen; and ascorbate peroxidase, a central component of the ascorbate-glutathione cycle that detoxifies peroxide within chloroplasts and other compartments. Crucially, this enzymatic upregulation was mirrored at the molecular level, with increased transcript abundance of the genes encoding these enzymes, showing that melatonin primes the seedlings’ stress-response machinery at the level of gene expression rather than merely acting as a chemical antioxidant itself.

The researchers went further, profiling stress-responsive genes that coordinate the plant’s broader adaptive response. Primed seedlings showed upregulation of SlGST, a glutathione S-transferase involved in detoxifying reactive compounds and sequestering heavy metals; SlDREB, a transcription factor that switches on suites of genes during drought and other abiotic stresses; SlNHX1, a sodium/proton antiporter that contributes to ion homeostasis and vacuolar sequestration of toxic ions; and SlDHN, a dehydrin gene whose products stabilize proteins and membranes during dehydration. Together, these molecular signatures indicate that melatonin priming activates both the ROS-scavenging frontline and the downstream regulatory and protective networks that give seedlings lasting tolerance rather than a fleeting chemical shield.

The practical implications of the work are considerable. Cadmium contamination of agricultural soils is a global problem driven by industrial emissions, phosphate fertilizers, and wastewater irrigation, and drought is intensifying across many of the same regions. Tomato, one of the most widely grown and economically important horticultural crops, is particularly sensitive during establishment. Seed priming is attractive as a technology because it is inexpensive, requires no genetic modification, applies no chemical load to soil, and can be integrated into existing seed conditioning pipelines before sowing. Compared with hydro-priming, melatonin priming conferred a clear and consistent advantage, suggesting that the hormone acts as a genuine elicitor of stress memory rather than simply accelerating germination. The study builds on a growing body of evidence from wheat, maize, rice, rapeseed, and other crops showing that melatonin seed treatments can improve germination under salinity, drought, and heavy metal stress, and extends this evidence to the combined challenge of cadmium and osmotic stress in tomato specifically.

The authors, whose work was supported by the Research Fund for Young Tianchi Talent program of the Xinjiang Uyghur Autonomous Region, published their findings in Volume 160 of Plant Biosystems, article number 245, on 20 August 2026. As climate change expands the footprint of drought and legacy contamination continues to constrain arable land, strategies that harden crops at the very start of their lives are likely to attract growing attention. This study demonstrates that a molecule best known for regulating sleep can, at the scale of a seed, set a young tomato plant on a fundamentally more resilient developmental trajectory, strengthening antioxidant defenses, reprogramming stress-responsive gene expression, and preserving the growth and physiological functions on which a season’s harvest ultimately depends. Future work will need to confirm these benefits under field conditions, but the message from the laboratory is clear: what happens to a seed before it meets the soil may determine how well the plant survives what the soil has in store.

Subject of Research: Melatonin seed priming as a strategy to alleviate cadmium and drought stress in tomato seedlings

Article Title: Seed priming with melatonin alleviates cadmium and drought stress by regulating growth, physiological functions and defense mechanisms in tomato

Article References: Muhammad, T., Yang, T., Wang, J., Wang, B., & Yu, Q. (2026). Seed priming with melatonin alleviates cadmium and drought stress by regulating growth, physiological functions and defense mechanisms in tomato. Plant Biosystems, 160(5), Article 245. https://doi.org/10.1007/s44473-026-00261-y

Image Credits: AI Generated

DOI: 10.1007/s44473-026-00261-y

Keywords: melatonin, seed priming, tomato, cadmium stress, drought stress, oxidative stress, antioxidant enzymes, reactive oxygen species, germination, seedling vigor, stress-responsive genes, plant stress physiology