Seed germination is controlled by an internal thermometer

Germination represents the first big step in a plant’s development. It involves a transition from the embryo state, inside the seed, to a fragile, small seedling that is exposed to the environment. A dormant embryo inside a seed is protected from environmental conditions by a sturdy coat, whereas the seedling is far more vulnerable. The timing of this change is critical if the tender new seedling is to survive, but how exactly is this controlled?

Newly formed seeds are unable to germinate but, after a while (a few days or months), they acquire the ability to begin this transition to seedling, as long as the environmental conditions are favorable. However, even after they have ‘awakened,’ seeds can still choose not to germinate. For example, a non-dormant seed that is suddenly subjected to excessively high temperatures (>28°C) can block its own germination to prevent the potentially fatal consequences; this allows for very fine control of the timing of germination.

This mechanism of repression by temperature (thermo-inhibition) enables a seed population to delay germination, even if there is only a small increase in temperature (1 to 2°C) that threatens the seedlings’ chances of survival. However, it is not known how a seed detects this change in environmental temperature or how it prevents germination.

In order to understand the detection mechanism that allows seeds to trigger thermo-inhibition, a group of researchers, led by Luis Lopez-Molina, professor at the Department of Plant Sciences of the Faculty of Science of the University of Geneva (UNIGE), has investigated the phenomenon in Arabidopsis thaliana. This is a species of plant belonging to the Brassicaceae family (cabbage, kale, broccoli) and used as a model in many research projects. They have published their findings in the journal Nature Communications.

The researchers first considered the mechanisms whereby young seedlings manage to respond to temperature changes, to help them pinpoint how seeds may accomplish this feat. Indeed, in seedlings a slight increase in temperature results in more rapid stem growth. This adaptation is similar to the one observed when a plant finds itself growing in the shadow of another: it lengthens to escape the shadow in order to expose itself to the sunlight which is more favorable for photosynthesis. 

In seedlings, these variations are detected by a protein called phytochrome B that is sensitive to both light and temperature. This protein normally acts as a brake on plant growth in seedlings. An increase of 1 to 2°C tends to inactivate phytochrome B, making it less effective at preventing growth. 

However, the role of phytochrome B in thermo-inhibition of seed germination was not known. The experts began by dissecting some Arabidopsis thaliana seeds to separate the two tissues inside the seed: the embryo (which will grow into the young plant) and the endosperm (which provides nutrition for the seed during dormancy and germination). They found that separated embryos were unable to prevent themselves from growing when temperatures were too high, whereas intact embryos showed thermo-inhibition. This indicated to the researchers that some factor within the endosperm was controlling the seeds’ ability to delay germination when it was too warm. 

“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 Urszula Piskurewicz, researcher at the Department of Plant Sciences of the UNIGE Faculty of Science, and first author of the study. ‘”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.”

Further investigation of genetic factors, as well as experimentation with mutant plants, indicated that phytochrome B in the endosperm of a seed is indeed involved with this ability to delay germination. High temperatures were found to lower phytochrome B signaling in the endosperm, leading to changes in the production and release of abscisic acid into the embryo. This had the result of preventing growth of the embryo and germination of the seed. 

Thermal inhibition of germination is a new example of the ways in which climatic variations can influence certain cyclic phenomena in plant life, such as growth and flowering. It is important to understand the mechanisms involved in this phenomenon, particularly as Earth’s climate warms and the germination of crop seeds may be affected.

“This trait is expected to have an impact on species distribution and plant agriculture and this impact will be greater as temperatures increase worldwide,” reports Luis Lopez-Molina, the study’s last author. A better understanding of how light and temperature trigger or delay seed germination could indeed help optimize the growth of plants exposed to a wide range of climatic conditions.”

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By Alison Bosman, Earth.com Staff Writer

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