Glass-like dynamics in plant cells

Photosynthesis is essential for all life on earth. Plants have evolved multiple mechanisms to adapt their photosynthetic performance to ever-changing light conditions: from the active motion of their leaves during the day to directed growth towards light. However, such adaptation could also occur on a cellular scale, via the motion of chlorophyll-containing organelles - chloroplasts. A plant-specific light-controlled actin-binding mechanism enables chloroplasts to individually move towards or away from light, depending on the light intensity. This mechanism enables chloroplasts to collectively self-organize into different light-adapted configurations. 

Here, we study the collective re-arrangements of chloroplasts in the water plant Elodea densa. We uncover a state close to a glass transition when chloroplasts arrange into a single layer within the plant cell to increase the light uptake. We show that the vicinity to a glass transition allows chloroplasts to easily fluidize the configuration and re-arrange into an aggregate as a light-avoidance response under strong light exposure. The high degree of self-organization in this system of disk-like light-responsive organelles renders the motion of chloroplasts an in-vivo model system for active matter in confinement at high densities while showcasing an interesting role of organelle motion in cells.