In the first part, I will describe our study on ferns sporangia where negative pressures lower than -100 bar are used in this catapult-like elastic beam.

                  To ensure good dispersal of their spores, ferns use a fast ejection mechanism (10 m / s) of these particles measuring 20-30 microns. The sporangium, a capsule containing the spores, acts as a spring which is stretched by evaporation-induced deformation and then suddenly coils back like a catapult when cavitation bubbles appear. One of the specificities of this movement, observed using fast imaging, is that it is realized with different characteristic times, which allows a stop halfway of the arm of this catapult. A poroelastic model can explain these results. The catapult mechanism is triggered by a very fast collective nucleation of bubbles in most cells. I will present then our study on the same mechanism observed in hydrogels-based biomimetic devices. We found that the nucleation of one bubble, that comes out randomly, can trigger subsequently the nucleation of several (up to hundreds) bubbles. I will present high speed imaging and acoustical measurements that we could record in such experiments, and finally our modelling approach to better understand this phenomenon. Our results explain why the fern sporangium catapult can be so efficient since all the cells can cavitate in a few microseconds, and they give insights in a potential “new” way cavitation could propagate in the plants xylem networks.

In the second part, I will present our ongoing project on filamentous growth in fungi.Hyphal formation is important for fungi to invade host cell layers, but little is known about how these filaments invade solid or viscoelastic materials. This is especially true for the human pathogen Candida albicans, which is normally a harmless commensal that is found on mucosal surfaces of the gastrointestinal and urogenital tract in most healthy individuals. This organism can cause superficial as well as life-threatening systemic infections in response to alterations of its environment, and is particularly aggressive in immuno-compromised individuals. Candida albicans, as an opportunistic pathogen, invades a wide range of surfaces within its human host, such as soft tissue and medical devices and is responsible for most predominant fungal nosocomial infections. The ability of this fungus to switch from an ovoid form to a filamentous form is critical for its pathogenicity, in particularly its ability to invade and penetrate into host tissues and evade and burst out of host immune cells. Little is known about the forces involved in the penetration of this organism into solid surface. We aim at determining quantitative relationships between physical forces and cell growth. We monitor invasive growth rates in individual fungal filaments of C. albicans on / in an elastic substrate in PDMS using live-cell imaging. These defined PDMS substrates allow us to infer biomechanical properties (such as turgor pressure) of fungal filaments during growth without cell manipulation and to compare the growth rate in and out of the PDMS. To test whether the cell wall is important for mechanically-driven invasive growth, we examined in particular penetrative hyphae in the mutant rho1, a Rho-GTPase critical for β-1,3 glucan synthesis in the C. albicans cell wall.