Dynamic cortical behavior of plant protoplasts reveals unexpected similarities between plant and animal cells

In contrast to animal cells with their contractile cortical cytoskeleton, plant cells are encased in walls and are usually thought to behave like stiff, pressurized vessels. When digesting the cell wall, plant protoplasts form spherical shapes, further consolidating this model. Here we challenge this apparent opposition between animal and plant cells. We show that plant protoplasts can form protrusions. More precisely, using plasma membrane markers, we reveal that 27 {+/-} 1% of the protoplasts show dot-like protrusions and 16 {+/-} 7% of protoplasts show long (typically 1-20 m, up to 200 m) protrusions that we name "filopods". We demonstrate that this behavior is independent of the plant species, the membrane marker or the osmolyte. Protrusions can host large and long structures, such as microtubule bundles, which in turn can impact the mechanical behavior of the protrusions. We find that filopods can form de novo when increasing osmolarity, that they most likely move passively, and that they can attach to artificial surfaces. Altogether, these observations show that forming and retaining protrusions is not exclusive to animal cells. This calls for revisiting the dynamics and probing ability of the plant cell cortex in different osmotic and mechanical environments.