Time-lapse images of Arabidopsis thaliana photoreceptor mutants under darkness and blue-light conditions

Rapid cell expansion pushes the Arabidopsis hypocotyl (juvenile stem)
through the soil until blue light acting first through phototropin 1
(phot1) and then through cryptochrome 1 (cry1) suppresses elongation to
produce the short hypocotyl characteristic of established,
photosynthetically capable seedlings. To determine where these two
different blue light receptors act to suppress hypocotyl elongation, we
measured relative elemental growth rate specifically along the hypocotyl
midline at 5-minute intervals before and during blue light using a machine
learning-based image analysis pipeline designed specifically for this
kinematic analysis of growth. In darkness, hypocotyl material expanded
most rapidly (approximately 4% h⁻¹) in a zone from 0.4 to 1.5 mm below the
apical terminus of the hypocotyl (cotyledonary node). Blue light
acting through phot1 rapidly inhibited expansion in this zone, while
simultaneously stimulating unexpanded cells in a more apical region only
0.1–0.3 mm below the cotyledonary node. Nuclear cry1, and not its
cytoplasmic pool, counteracted the phot1-initiated expansion of the small
cells in this apical region, preventing them from entering the more basal
elongation zone. In a cry1 mutant, activation of these apical
cells proceeded unchecked, reaching rates as high as 6% h⁻¹ to produce the
iconic cry1 long-hypocotyl phenotype. In addition to showing where future
cell and molecular studies of cry1 and phot1 signaling mechanisms should
focus, the new spatial information indicates that a seedling may use an
apical reservoir of elongation potential to reenter a lit environment
should a natural darkening event such as soil disturbance deactivate
cry1.