Fig. 5.

Stromules are initiated by signals within the chloroplast. (Left) Stromule frequency increases in the light (daytime) in both chloroplasts and leucoplasts. (Center) ROS generated from the pETC trigger stromule formation in chloroplasts. (Right) Sucrose promotes stromule formation in leucoplasts, but not chloroplasts. Sucrose is synthesized in the cytosol from products of the Calvin–Benson cycle in chloroplasts and then moves into neighboring heterotrophic pavement cells via plasmodesmata. For simplicity of presentation, only photosynthetic mesophyll cells are shown (and not photosynthetic guard cells), because there is no evidence suggesting that stromules in these cell types behave differently.