How a common weed builds up explosive force
Hairy bittercress has explosive fruit that fire seeds in all directions. MPI researchers discover how these seed pods power their own explosion.
In a new study published in Current Biology, Dr. Angela Hay and colleagues - from the Max Planck Institute for Plant Breeding Research in Cologne, Germany - investigate the exploding seed pods of hairy bittercress (Cardamine hirsuta). These pods consist of two, long valves. When the seeds are ready for dispersal, these valves rapidly coil back, accelerating seeds at astonishing speeds.
The study’s lead authors, Dr. Gabriella Mosca and Dr. Ryan Eng, discovered that the plant exploits the growth process of its own cells to produce tissue-wide contraction in the pods, building up enough tension to make them explode. "It seems contradictory that expansive growth should lead to tissue contraction, but in this context, one leads to the other," says Mosca.
During growth, cells change not only in size, but also shape. The shape of a cell is influenced by the arrangement of cellulose fibers in its walls. “These fibers are like steel cables that resist being stretched. So, cells must grow at right angles to these cables,” says Eng. As explosive seed pods mature, cells reorient their cellulose fibers and grow into a specific shape. As Hay explains, “This shape, combined with the internal pressure of each cell, causes tissue-wide contraction, acting almost like a muscle.”
The process is optimized by the criss-cross pattern of cellulose fibers in the cell wall. “When the cells grow with crossed cellulose fibers instead of parallel ones, this adds an extra stretch,” Mosca explains. "The criss-crossing of cellulose fibers may appear random. But in fact, the pattern is important for cells to contract.” This way of contracting is analogous to an artificial muscle, called a McKibben actuator, but here the cells are using growth to increase the pulling force.
The researchers used live cell imaging and quantitative techniques to measure cellular growth. In addition, Mosca, in collaboration with Dr. Richard Smith from the John Innes Centre in Norwich, UK, developed a computer model of the multi-layered structure of plant’s cell walls for this study. This model is part of a software called MorphoMechanX that they developed to model plant mechanics and growth. The model may now be used in further studies on the biomechanics of plants and in cell wall research.
By studying growth in this particular context, the authors also provide general insights about how expansive growth of plant cells works. Their work suggests that growth acts mainly on the softer component of the cell wall and if the wall is highly reinforced in a certain direction, it won’t yield to growth in this direction. These findings are in line with the theory of strain-based growth, according to which, the more the cell wall is stretched by turgor pressure in a certain direction and location, the more it will be able to respond to hormonal and enzymatic growth stimulating factors.
For the researchers Angela Hay and Gabriella Mosca, the results also raise further questions. “We want to understand how microtubules switch their orientation in a coordinated manner during fruit growth. Our data doesn’t fit the current dogma of microtubules as stress sensors, so we want to dig deeper into this,” Dr. Hay explains. Dr Mosca notes that, “All valves coil with the same chirality when they explode from the fruit, suggesting a coordinated symmetry breaking mechanism. I am adopting a multi-scale modelling approach to investigate this in my new research group at the ZMBP, University of Tübingen.
(AH)
