Tissue Architecture Tunes Active Chloroplast Positioning in Plants

Plants thrive under fluctuating sunlight by combining fast photochemistry with organelle-scale reconfiguration and organism-level tropisms. We show that chloroplasts (which can be seen as disk-shaped active particles) execute a collective mesoscale response that solves a constrained disk packing problem: dense monolayers under dim light, rapid sidewall relocation under high light. This response exhibits a light-dependent, phase-transition-like switch with hallmarks of jamming/unjamming in confined active matter. We further find that high-light patterns depend on tissue architecture and internal light paths, tying leaf development to intracellular positioning. Collectively, these results link plant photoprotection to core soft-matter concepts like collective motion, confinement, and optimal packing, and suggest principles for light-responsive materials.