Stromule geometry allows optimal spatial regulation of organelle interactions in the quasi-2D cytoplasm

In plant cells, plastids form elongated extensions called stromules, the regulation and purposes of which remain unclear. Here we quantitatively explore how different stromule structures serve to enhance the ability of a plastid to interact with other organelles: increasing the effective space for interaction and biomolecular exchange between organelles. Strikingly, electron microscopy and confocal imaging showed that the cytoplasm in Arabidopsis thaliana and Nicotiana benthamiana epidermal cells is extremely thin (around 100 nm in regions without organelles), meaning that inter-organelle interactions effectively take place in 2D. We combine these imaging modalities with mathematical modelling and new in planta experiments to demonstrate how different the elongation of stromules (single or multiple, linear or branching) could be employed to optimise different aspects of inter-organelle interaction capacity in this 2D space. Stromule formation and branching is shown to provide a proportionally higher benefit to interaction capacity in 2D than in 3D. Additionally we find this benefit depends on optimal plastid spacing. We hypothesize that cells can promote the formation of different stromule architectures in the quasi-2D cytoplasm to optimise their interaction interface to meet specific requirements. These results provide new insight into the mechanisms underlying the transition from low to high stromule numbers during biotic stress, the consequences for interaction with smaller organelles, how plastid access and plastid to nucleus signalling is balanced, as well as the impact of plastid density on organelle interaction.