Ronald, J.; Lock, S. C. L.; Claydon, W.; Zhu, Z.; McCarthy, K.; Pendlington, E.; Redmond, E. J.; Vong, G. Y. W.; Stanislas, S. P.; Davis, S. J.; Quint, M.; Ezer, D.

Sensing and responding to photoperiod changes is essential for plants to adapt to seasonal progression. Most of our understanding of how plants sense photoperiodic changes is through studies on flowering time. However, other aspects of plant development are regulated by the photoperiod, including hypocotyl elongation. Unlike flowering, hypocotyl elongation displays a greater plasticity to changes in the photoperiod with increases in daylength causing greater inhibition of growth until a threshold is met. Although we have a good understanding of how the photoperiod regulates hypocotyl elongation, previous investigations have only looked at hypocotyl development under a constant photoperiod. It is unknown if changes in the photoperiod during development influence hypocotyl elongation and whether the initial photoperiod exposure continues to impact subsequent development. Here, we developed a physiological assay to investigate this question. We have discovered that hypocotyl elongation is influenced by a memory of past photoperiod exposure in Arabidopsis and Brassicaceae cultivars used for microgreen agriculture. Photoperiodic memory persisted for multiple days, although it weakened over time, and the strength of the memory was dependent on the genetic background. We identified that phyB and ELF3, key regulators of hypocotyl development, were required for photoperiodic memory. Finally, we identified that the circadian clock is unlikely to function as a repository for photoperiodic memory as circadian rhythms quickly re-aligned with the new photoperiod. In summary, our work highlights for the first-time evidence of a photoperiodic memory that can control plant development.