Gibberellin Perception Sensors 1 and 2 reveal cellular GA dynamics articulated by COP1 and GA20ox1 that are necessary but not sufficient to pattern hypocotyl cell elongation

The phytohormone gibberellin (GA) is critical for environmentally sensitive plant development including germination, skotomorphogenesis and flowering. The FRET biosensor GIBBERELLIN PERCEPTION SENSOR1, which permits single-cell GA measurements in vivo, was previously used to observe a GA gradient correlated with cell length in dark-grown but not light-grown hypocotyls. We sought to understand how light signalling integrates into cellular GA regulation. Here we show how the E3 ligase COP1 and transcription factor HY5 play central roles in directing cellular GA distribution in skoto- and photomorphogenic hypocotyls, respectively. We demonstrate that the expression pattern of biosynthetic enzyme GA20ox1 is the key determinant of the GA gradient in dark-grown hypocotyls and is a target of COP1 signalling. We engineered a second generation GPS2 biosensor with improved orthogonality and reversibility to show the cellular pattern of GA depletion during the transition to growth in the light. This GA depletion partly explains the resetting of hypocotyl growth dynamics during photomorphogenesis. Achieving cell-level resolution has revealed how GA distributions link environmental conditions with morphology and morphological plasticity and the GPS2 biosensor is an ideal tool for GA studies in further conditions, organs and plant species.