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In a new study, scientists at Umeå University have found a way to break open the protective walls of pollen grains—one of the hardest biomaterials in the world—without damaging the inside cell and its components.

This achievement opens the possibility of isolating and studying mitochondria—parts of the cell essential for energy production. To their surprise, several proteins that are key for maintaining the energy production of the mitochondria were nowhere to be found.

Pollen grains: Nature's resilient capsules

"Flowering plants are dependent on pollen to reproduce, and the pollen grains are very special in many ways," says Olivier Keech, Associate Professor at the Department of Plant Â鶹ÒùÔºiology, UmeÃ¥ University and group leader at UmeÃ¥ Plant Science Center, UPSC.

He explains that each pollen grain contains a tiny capsule, a cell that carries the male genetic material necessary for the next generation of plants.

When a pollen grain encounters a female plant of the same species, fertilization may happen and can give rise to a new generation. But immediate contact is not always a given. To survive harsh environments, pollen grain has developed a specific outer structure that protects the cell, allowing it to travel long distances with the wind or pollinators, such as insects, birds or reptiles.

"This tough outer wall is largely made of one of the most resistant biomaterials known on this planet. This makes the pollen grain wall highly resistant to environmental damage and some pollen grains can remain preserved in sedimentary rocks for millions of years," says Keech.

That a pollen grain can survive for such a long time span is thanks to an in-house energy production—the mitochondria. "It's a tiny compartment of the cell that is essential for its survival," says Keech.

The mitochondria have its own genetic material, essential for its biological activity, and notably for producing the energy that keeps the cell alive. But to study the pollen mitochondria, they had to break open the protective wall.

A surprising discovery and a collaborative success

The idea to study pollen mitochondria germinated at a conference in 2019. Keech and his colleague at UPSC, Clément Boussardon, presented a new technique developed in Umeå, that enables trapping and isolation of mitochondria. This innovative technique intrigued collaborators who studied pollen cells.

However, from the birth of the idea, it took a few years to reveal the secrets of pollen, quite literally. "Breaking up pollen grains and isolating intact mitochondria was truly challenging. These are biological structures, a million times smaller than a meter, encapsulated in a tiny safe—dynamite was not an option!" says Boussardon, staff scientist in Keech's group and first author of the study in Current Biology.

Boussardon, together with their collaborator Matthieu Simon from INRAE in France, spent more than four years perfecting their method to open the pollen grains while preserving the cell. What they discovered, was not what they expected.

"What we found was quite surprising," says Keech. "We discovered that the proteins that are associated with maintenance and the expression of the genetic material in mitochondria, essential for keeping it alive, were nowhere to be found."

"This is a bit like mitochondria were ready to produce energy but were not equipped for any repairs if needed. This discovery may explain why a pollen grain in the end has a limited lifetime, and why it is fine tuned to survive for the duration of its unique mission—fertilization," says Keech.

Keech and Boussardon credit their success to the multidisciplinary nature of the study, which brought together researchers from Germany, France, New Zealand, and of course Umeå.

"Combining the expertise of our diverse research teams has been a great pleasure and was key to this success," concludes Boussardon.