Internal thermometer helps seeds time germination which could be key to optimise plant growth in a warming world

How does a seed know when it is the right time to germinate? New research from Switzerland shows, for some seeds, it may have something to do with a mechanism that can tell temperature.

As a seed becomes a seedling, the plant removes its hard exterior shell and becomes vulnerable to the environment. Factors such as climate and nutrition levels could mean life or death for the germinating plant.

So, timing is important if the plant is to survive.

Seeds don’t automatically become plants. They start off dormant. Depending on species, days or even months can pass before the seed “awakens” and acquires the ability to germinate. Even then, though, for seedlings to survive, they need to germinate during a favourable period.

Non-dormant seeds, too, can choose whether the time is right. A non-dormant seed that is suddenly exposed to high temperatures can block germination, for example.

This repression by temperature (thermo-inhibition) can be sensitive to changes of temperature of only one or two degrees Celsius.

A Swiss team, led by University of Geneva researchers, has discovered an internal thermometer in seeds that can delay or block germination if temperatures are too high.

Looking at seeds of the small flowering plant, Arabidopsis thaliana, the team sought to understand the mechanism behind thermos-inhibition. They began by looking at data from young plants – further along in the plant’s development than germination.

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The team noted that temperature has an effect on seedlings. Slight temperature increases, for example, will promote stem growth. Such response to external stimuli is common in plants is referred to as a “tropism”.

The protein in plants sensitive to light and temperature is phytochrome B. It normally acts to halt plant growth. Increases in temperature sees the protein inactivated, making it less effective in stemming development.

University of Geneva scientists sought to understand if phytochrome B also plays a role in germination. They separated the embryo in the seed from the endosperm (the nourishing tissue that controls germination in Arabidopsis).

Section of a seed of Arabidopsis thaliana, a model organism widely used in plant sciences. Credit: UNIGE / Sylvain Loubéry.

Embryos separated from the endosperm were unable to stop germination under high temperatures, leading to the plant’s death.

“We found that thermo-inhibition in Arabidopsis is not autonomously controlled by the embryo but implemented by the endosperm, revealing a new essential function for this tissue,” explains Dr Urszula Piskurewicz. “In other words, in the absence of endosperm, the embryo within the seed would not perceive that the temperatures are too high and would begin its germination, which would be fatal.”

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The study represents a new example of climatic variation influence on plant life cycles. “This trait is expected to have an impact on species distribution and plant agriculture and this impact will be greater as temperatures increase worldwide,’’ says Dr Luis Lopez-Molina.

Knowledge of how light and temperature impact germination could help optimise plant growth in the context of climate change.

The research is published in Nature Communications.

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