Abstract

The phytohormone gibberellin (GA) is a key regulator of plant growth and development. An ensemble of enzymatic and transport steps control the cellular distribution of GA and these steps are themselves influenced by myriad endogenous and exogenous signals. Although the upstream regulation and downstream responses to GA vary across cells and tissues, developmental stages, and environmental conditions, the spatiotemporal distribution of gibberellin in vivo remains unclear. We engineered a high-affinity optogenetic biosensor, Gibberellin Perception Sensor 1 (GPS1), that senses nanomolar levels of bioactive gibberellins (Kd = 24 nM for GA4 ). Arabidopsis thaliana plants expressing a nuclear localised GPS1 report on gibberellins at the cellular level. In rapidly elongating tissues in which GA promotes rapid cell elongation (i. e. root tips and dark-grown hypocotyls), GA levels correlated with cell length resulting in longitudinal gradients of GA in these organs. In roots, exogenous GA accumulation was also correlated with cell length, suggesting that a root GA distribution gradient can be generated independent of GA biosynthesis. In hypocotyls, GA levels were reduced in a phytochrome interacting factor (PIF) quadruple mutant in the dark and increased in a phytochrome double mutant in the light, suggesting that PIFs elevate GA in the dark and that phytochrome inhibition of PIFs lowers GA levels in the light. This result is in marked contrast to regulation of GA in seeds, where PIFs lower GA levels in the dark and phytochrome inhibition of PIFs elevates GA in the light. Better knowledge of cellular GA distributions and how they are determinedwill enable a deeper understanding of the signal integration upstream and growth programming downstream of GA, a small, mobile signalling molecule with substantial influence over plant growth and development.