Creating an explosion: how growth and tension interact in explosive fruit

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Seed dispersal is a physical process in which seeds are moved from one location to another. And evolution has shaped a multitude of ingenious adaptations for plants to disperse their seeds. Perhaps most intriguing from a mechanical perspective, are the fruits that generate their own explosive force to eject their seeds. We study this trait of explosive seed dispersal in the Arabidopsis relative, Cardamine hirsuta. In this project, we address how the tension required for explosion is produced in C. hirsuta fruit. We previously showed that tension is generated by differential contraction of tissues in the fruit valve. An active, outer tissue layer contracts while an inextensible inner layer, stiffened by lignin, does not. Cells in the outer exocarp tissue layer use turgor pressure to contract in length by expanding in other dimensions. We used finite element modelling to show that exocarp cell shape and anisotropy are important for this response. The specific question addressed in phase one of this project was how do exocarp cells acquire these properties through growth? Using live cell imaging and a morphodynamics approach, we quantified cortical microtubule orientations together with the amount, direction and duration of cellular growth in the exocarp layer of C. hirsuta fruit. Our findings revealed a distinctive pattern of cellular growth, underpinned by predictive reorientations of cortical microtubules, that partitions directional growth into two developmental phases. A late phase of exclusively anisotropic growth in the exocarp, produces the cell shape and anisotropy required for contraction. By incorporating this information into a 3-D model of growing cells, we could mimic the differential contraction found in explosive fruit of C. hirsuta. In phase two, we will capitalize on these methodologies and findings to further understand the role of cortical microtubules in directing the generation and release of tension in explosive coiling and twisting of C. hirsuta fruit valves.