Twilight length in Arabidopsis
The plant circadian clock governs growth and development by syncing biological processes to periodic environmental changes. Decades of research has shown how the clock enables plants to respond to two environmental variables that change at different latitudes and over different seasons: photoperiod and temperature. However, a third variable, twilight length, has so far gone unstudied. The duration of twilight changes across the planet, lengthening with latitude and changing across seasons, and yet most circadian experiments are performed in lab environments with no twilight. Here, using controlled growth setups, we show that twilight length optimizes plant growth and changes flowering time through the LHY/CCA1 morning module of the Arabidopsis circadian clock. Further, using a series of progressively truncated no-twilight photoperiods, we found that plants respond differently to twilight compared to solely changes in photoperiod. Further, transcriptome and proteome analyses showed that twilight length alters plant biology via protein-level regulation, potentially via proteasomal and translational control of proteins involved in reactive oxygen species metabolism, photosynthesis, and carbon metabolism. Overall, our findings demand more nuanced models of daylength perception in plants and posit that twilight length is an important determinant of plant growth, development, and circadian function.