Plant seeds go dormant to avoid harsh conditions

Plants are extremely adaptable organisms that have evolved remarkable strategies to survive in various environments. One such strategy is seed dormancy, which prevents seeds from germinating even under optimal conditions, reducing the risk to seedlings during adverse periods.

In Mediterranean climates like Cyprus, summers can be extremely hot and dry, posing a significant threat to young seedlings. 

A subspecies of Aethionema arabicum in Cyprus has evolved a unique adaptation to avoid germination during these harsh months. In a recent study, experts investigated the molecular basis of this adaptation.

Long-term suppression of germination 

Previously, researchers discovered that direct light exposure inhibits the germination of Aethionema arabicum seeds. 

The new study explored whether light exposure could also induce long-term suppression of germination, known as secondary dormancy. The investigation revealed that the length of daily light exposure was crucial in establishing secondary dormancy. 

“This mechanism makes a lot of sense: day length is a much more stable condition than temperature or humidity, which may vary significantly from one day to the next,” said lead author Zsuzsanna Mérai, a postdoctoral fellow at the Gregor Mendel Institute. 

Protecting the seedlings

As days lengthen in summer, seeds exposed to longer light periods avoid germinating, thereby preventing seedlings from facing lethal conditions.

The researchers demonstrated that exposing seeds to 16 hours of light per day resulted in long-term suppression of germination, even when the seeds were later placed in darkness. 

“Even if we put the seeds back to dark conditions, they will not germinate because they will remember that the conditions were not good for sprouting,” Mérai explained.

Triggers of seed dormancy

While secondary dormancy has been observed before, its molecular triggers were not well understood. By screening a library of mutated seeds unable to establish secondary dormancy, the researchers identified RGL2 as the primary regulator. 

“Seeds with mutations in the RGL2 gene are still inhibited by light, but only short term. Lacking RGL2, secondary dormancy induction fails,” Mérai said. 

Moreover, the analyses revealed that RGL2 regulates over 3,300 genes associated with secondary dormancy, affecting both pro-dormancy and dormancy-repressing genes.

Molecular basis of secondary dormancy 

Previous research has shown that seed dormancy results from the balance between abscisic acid (ABA), a dormancy-promoting hormone, and gibberellin (GA), a dormancy-repressing hormone. 

The study showed that RGL2 suppresses GA production and enhances ABA synthesis in seeds exposed to light. 

However, the scientists discovered that only the reduction of GA levels, not the increase of ABA levels, was essential for inducing secondary dormancy. 

Adapting germination patterns

These findings reveal how some plants adapt their germination patterns to avoid harsh summer conditions, differing from the traditional view that light induces germination. 

“Some plants seem to have tweaked germination to adapt to specific climates,” Mérai said. 

The study also showed that summer heat and drought trigger the end of dormancy, preparing seeds for favorable conditions. 

Ongoing research

The researchers plan to investigate if similar mechanisms exist in other plants, which could have significant implications for crop resilience in warming climates. 

“Our findings could be applied to make new crops that are better adapted to warmer climates and the continuously rising temperatures,” Mérai concluded. 

Which plants use seed dormancy?

Seed dormancy is a survival strategy that is used across a wide range of plant species, each adapted to their unique ecological niches.

Trees

Trees such as oak, pine, and maple exhibit seed dormancy to ensure germination at the optimal time for growth. 

Grasses

Several grass species, including wild grasses and some cultivated varieties like wheat and barley, also use seed dormancy to avoid germination under unfavorable conditions. 

Legumes 

Plants in the legume family, such as peas, beans, and clovers, often have hard seed coats that contribute to dormancy. 

Wildflowers

Many wildflower species, like lupines and poppies, exhibit seed dormancy to ensure they germinate when conditions are ideal for survival. 

Shrubs 

Various shrubs, including those in the genus Rosa (roses) and Rubus (blackberries and raspberries), also use seed dormancy to synchronize germination with favorable environmental conditions. 

The study is published in the journal Current Biology.

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