Plants use melatonin as a secret weapon - and it's not for sleep

Plants stay active inside, even when nothing seems to move on the surface. Roots, stems, and seeds send signals to each other to guide growth and respond to changes in light, water, and soil.

Researchers found that melatonin plays an important part in these signals. Humans know melatonin as a sleep aid.

Plants use the same molecule for very different jobs. It helps plants grow, keep a daily rhythm, and handle stress from heat, cold, or poor soil.

Scientists also discovered a close link between plant melatonin and soil microbes. Some microbes make melatonin around plant roots. That extra support can help plants stay healthy.

Because of this, melatonin research now points toward new tools that may improve farming and protect crops more naturally.

How plants use melatonin

Human brains release melatonin before sleep. Many adults use supplements for regular rest. Plant tissues make melatonin in a different context.

“Melatonin has emerged as a pivotal molecule in agriculture due to its ability to promote plant growth and alleviate abiotic stresses,” said Abdul Latif Khan of the University of Houston.

“In plants, the internal clock can adjust the phase of various biological processes, such as gene expression, metabolic regulation and protein stability, to coincide with daily and/or seasonal cycles,” noted study lead author Imad Aijaz.

Plant cells turn tryptophan into serotonin and then into melatonin, with several enzymes guiding each step. Melatonin levels rise in many parts of a plant during early growth and seed development.

Research shows that plant cells can make melatonin in different places, including chloroplasts, the cytoplasm, and the endoplasmic reticulum.

These flexible pathways help plants cope with heat, cold, drought, and harmful substances.

Microbes support growth

“Melatonin-producing microbes can enrich soils, enhancing melatonin availability, uptake, and transport within plants to improve stress tolerance and growth,” said Khan.

Many microbes, such as Bacillus, Pseudomonas, Lactobacillus, and yeast, can build high melatonin levels when salt, cold, or metal stress appears.

Some microbes release even more melatonin during fermentation as energy use changes.

This extra melatonin from microbes often improves plant health, leading to stronger roots, richer pigments, and better stress tolerance in many crops.

How plants use melatonin cycles

Plant life follows a clear melatonin cycle. Night brings a rise in melatonin, while sunlight brings a drop.

An internal clock uses melatonin to guide actions such as opening and closing pores on leaves, managing energy, and starting flowers.

Signals move through special chemical routes and receptors such as PMTR1. As melatonin moves through plant tissues, growth patterns change.

Research shows wide differences across many plant groups, seasons, and organs, with strong links to seed sprouting, photosynthesis, and seed production.

Melatonin supports growth hormones

Melatonin works alongside many plant hormones and helps plants adjust to changing conditions.

Cytokinin (CK) is a plant hormone that keeps leaves green, healthy, and photosynthetically active.

Under stressful conditions – too much light or dry soil – leaves normally age and fade faster. Melatonin boosts the effect of cytokinin, making the “stay green and active” signal stronger.

Jasmonic acid (JA) is a hormone plants use to respond to stress, damage, or attack. When JA levels rise, plants shift into defense mode.

Under salt stress, plants often over-activate jasmonic acid, which can slow growth because the plant is busy defending itself. Melatonin tones down JA’s alarm response during salt exposure.

As a result, the plant doesn’t waste energy on unnecessary defenses and can keep growing steadily.

Melatonin and soil bacteria

“So far, most work has focused on species that are important for agriculture or health, but we know very little about how melatonin works in wild plants or those with cultural value,” noted Khan.

Melatonin in soil supports useful bacteria such as Proteobacteria and Actinobacteria, while harmful fungi drop in number.

Soil processes that supply nitrogen and phosphorus also improve under melatonin, and helpful mycorrhizal fungi gain extra strength and support plant roots more effectively.

Scientists now design microbes that can produce melatonin on their own, with some strains able to sense plant signals and release melatonin only when needed.

Early studies show stronger roots, better nutrient uptake, and higher survival under heat or salty conditions.

The study is published in the journal iScience.

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