Spread out – Explosive seed pods

Researchers discover what powers explosive seed pods

Driven by passion and curiosity, Angela Hay and her team are eager to unravel the secrets behind this explosive mechanism, digging deeper into development, mechanics and evolution.

Dispersal is a fundamental process in biology and vital for plants to move from one location to another. So, it is not surprising that different plants have developed diverse strategies to disperse their seeds effectively. Some rely on wind or animals, while others surprise with exploding seed pods reminiscent of New Year's party poppers.

This explosive strategy gives the plant remarkable independence from environmental factors like wind or animals and enables the plant to effortlessly colonize new territories and expand their presence.

Unravelling the mechanism of exploding seed pods

Given that plants lack muscles, it is not obvious how they can move their organs. It is even more puzzling how they can generate the type of rapid movements that fire seeds several meters away with enormous force and speed.

The hairy bittercress, also known as popping cress (scientific name: Cardamine hirsuta), is a cosmopolitan weed that can be found in many gardens and green areas. The seeds sit inside long, thin pods. When ready, the seed pod walls quickly roll up, causing the seeds to shoot out several meters away.

This plant is the focus of many different types of research carried out in the Department of Comparative Development and Genetics at MPIPZ. With its exploding seed pods, it is an excellent model to gain a closer look at what lies behind this remarkable and fascinating mechanism.

Making the invisible visible – speed up!

The seed pods of popping cress explode in the blink of an eye, such that normal cameras can't even catch it. The coiling pods accelerate to a speed of 10 meters per second in an instant, ejecting all the seeds in just a few milliseconds. That's where high-speed cameras come to the rescue to capture the event.

An intricate design of cameras, laser beams, and electric circuits paved the way for an extraordinary journey, allowing researchers to witness the rapid movements driving seed dispersal. This innovative setup marked the beginning of an exploration that led to exciting and unexpected insights into this fundamental biological process.

How plants build up such energetic force

The energy to explode comes from tension that builds up in the fruit. Tension is commonly generated in exploding seed pods as they mature and dry, causing the plant tissues to shrink (or contract). This is what happens in the pods of lupins or geraniums.

In popping cress however, the seed pods explode before they dry. They had to be using a different mechanism to generate tension.

Harnessing growth for contraction

Plants don’t have muscle tissues that contract like in humans or animals. Yet popping cress generates tension in its pod walls by tissue contraction. The question is, how does it do it?

The scientists discovered an important role for growth and cell shape. The driving force for plant cells to grow comes from turgor pressure – the pressure from fluid within the cell pushing against the cell wall. When turgor pressure comes together with the specific shape of cells in the seed pods, it causes the cells to bulge out and contract. This is similar to how an air mattress pulls inwards as it inflates. Curiously, it is the way the cells grow into a larger shape that allows them to contract.

The arrangement of cellulose fibres in the cell wall adds another piece to the puzzle. The arrangement of these fibres is carefully controlled to ensure that cells grow into an optimal shape for contraction. When the fibres are arranged in a crisscrossed pattern they work almost like a muscle. This is very different to what was known before.

Copper is key for the explosive release mechanism to work

Popping cress is not the only plant with exploding seed pods. Many other species in the Cardamine group share this trait. What all of these plants have in common is a special cell wall in the seed pod that provides a rapid release mechanism.

Lignin is a cell wall material that stiffens the woody part of the fruit that protects the seeds. In Cardamine species, the lignin is deposited in a unique pattern that allows ultrafast coiling of the pod walls – powered by the rapid release of tension.

The researchers were keen to delve deeper into understanding the genes that control lignin formation in the seed pod walls. They found three important genes. These genes have instructions for making enzymes called laccases, which are needed to make lignin. When popping cress doesn’t have these genes, the woody layer in their seed pods doesn't have lignin. Without lignin, seed pods are unable to spread their seeds very far.

Curiously, they identified another gene called SPL7 that is essential to form lignin in the seed pod walls. This gene does not contain instructions to make an enzyme, but rather to make a protein that turns on and off other genes, in order to regulate the amount of copper present in plants.

Interestingly, laccases are enzymes that need copper to work properly. Copper availability is often limited in the soil, so plants depend on SPL7 to help them out to keep up the level of copper within the plant.

Only with enough copper around, are the laccases active and able to deposit lignin in the cells of the seed pod walls. Thus, the explosive release of seeds relies on SPL7 controlling the levels of copper.

An array of secrets waiting to be unravelled

Angela Hay's team is currently working on understanding how the rapidly coiling pods pick up and launch the seeds, trying to figure out the cellular and genetic mechanisms behind this process. They are also exploring seed dispersal in some of the other Cardamine plants related to popping cress, in the hope of discovering more of nature’s secrets.

So next time you're out exploring, keep an eye out for popping cress plants. You might just witness their explosive talent.

 

References:

Miguel Pérez-Antón, Ilsa Schneider, Patrizia Kroll, Hugo Hofhuis, Sabine Metzger, Markus Pauly, and Angela Hay

Explosive seed dispersal depends on SPL7 to ensure sufficient copper for localized lignin deposition via laccases

PNAS, June 6, 2022

 

Mosca, G., Eng, R.C., Adibi, M., Yoshida, S., Lane, B., Bergheim, L., Weber, G., Smith, R.S., Hay, A.

Growth and tension in explosive fruit

Current Biology, February 15, 2024

 

Morphomechanical Innovation Drives Explosive Seed Dispersal.

Hofhuis H, Moulton D, Lessinnes T, Routier-Kierzkowska AL, Bomphrey RJ, Mosca G, Reinhardt H, Sarchet P, Gan X, Tsiantis M, Ventikos Y, Walker S, Goriely A, Smith R, Hay A.

Cell. 2016 Jun 30;166(1):222-33.