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Researchers have discovered seeds have an “internal thermometer” that tells them when to germinate.

Section of a seed of Arabidopsis thaliana, a model organism widely used in plant sciences

Germination

Germination is a crucial stage in the life of a plant as it transitions from a seed that is resistant to various environmental constraints (such as climatic conditions and absence of nutritive elements) to a seedling that is much more vulnerable. The survival of the young plant depends on the timing of this transition, making it essential that this stage be finely controlled. A Swiss team, led by scientists from the University of Geneva (UNIGE), has discovered the internal thermometer of seeds that can delay or even block germination if temperatures are too high for the future seedling.

Newly formed seeds are initially dormant and unable to germinate. After a period of time (which can range from a few days to several months, depending on the species), the seeds awaken and gain the ability to germinate during the most favorable time for seedling growth and new seed production. However, even a temperature fluctuation of just 1 to 2°C can delay the germination of a seed population, thus decreasing its chances of survival.

A key protein: phytochrome B

Luis Lopez-Molina’s group, based at the Department of Plant Sciences in the Faculty of Science at UNIGE, is researching the control of germination in Arabidopsis thaliana, a plant species belonging to the Brassicaceae family that is widely used as a model in research projects. To understand the mechanisms that allow seeds to trigger thermo-inhibition, scientists examined phenomena previously described in young plants, but at a more advanced stage of development.

Seedlings are also sensitive to changes in temperature, with even a slight increase promoting stem growth. This response is similar to the one observed when a plant is shaded and elongates to escape the shadow and expose itself to sunlight for photosynthesis. These changes are detected by a protein called phytochrome B, which is sensitive to both light and temperature and normally acts as a growth inhibitor. An increase in temperature of just 1 to 2°C can inactivate phytochrome B, making it less effective at preventing growth.

An internal thermometer

To investigate whether phytochrome B also contributes to thermo-inhibition during germination, the authors dissected the seeds to separate the two tissues inside: the embryo (which will eventually grow into a young plant) and the endosperm (the nourishing tissue that also controls germination in Arabidopsis seeds). The researchers found that, unlike embryos grown in contact with the endosperm, embryos deprived of their endosperm are unable to stop growing under high temperatures, which ultimately leads to their death.

Urszula Piskurewicz, researcher at the Department of Plant Sciences of the UNIGE Faculty of Science and first author of the study, explained,

‘‘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. 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.”

Optimizing crop germination

Thermal inhibition of germination is an example of how climatic variations affect certain cyclic phenomena in plant life, such as germination and flowering. According to the study’s last author, Luis Lopez-Molina, “This trait is expected to impact species distribution and plant agriculture, and this impact will be greater as temperatures increase worldwide.” A better understanding of how light and temperature trigger or delay seed germination could help optimize the growth of plants exposed to a wide range of climatic conditions.

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Section of a seed of Arabidopsis thaliana, a model organism widely used in plant sciences via UNIGE by Sylvain Loubery with usage type - News Release Media

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Section of a seed of Arabidopsis thaliana, a model organism widely used in plant sciences via UNIGE by Sylvain Loubery with usage type - News Release Media

 

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