A balance of metabolism and diffusion articulates a gibberellin hormone gradient in the Arabidopsis root

The plant hormone gibberellin (GA₄) regulates numerous developmental processes. Within the root, GA₄ controls growth, in part, by controlling the extent of cell elongation. The nlsGPS1 FRET biosensor revealed a GA₄ gradient within the Arabidopsis root growth zones, with GA₄ levels correlating with cell length. We developed a multiscale mathematical model to understand how biosynthesis, catabolism, and transport create the GA₄ distribution within the root growth zones. The model predicted that phloem delivery of the biosynthetic intermediate GA₁₂ contributes to higher levels of bioactive GA₄ in the elongation zone, with the GA₄ synthesis pattern being further modified by local GA₁₂ synthesis in the quiescent center region and the spatial distribution of biosynthesis enzymes (GA20ox and GA3ox). Model predictions suggested that while GA20ox and GA3ox transcript is present throughout the growth zones, these enzymes are inactive in the dividing cells, which explains steep GA₄ gradients observed in the GA3ox overexpression line and improves agreement between model predictions and data in wildtype. The model suggested that the GA₄ gradient also depends on a balance of diffusion through plasmodesmata and catabolism. Both model predictions and biosensor data demonstrated that plasmodesmatal diffusion enables a more gradual GA₄ gradient, with higher diffusion antagonizing the GA₄ gradient. Model predictions suggested that catabolism limits GA₄ levels, which we validated via biosensor imaging in the ga2oxhept mutant. In conclusion, our results suggest that local GA₄ synthesis combines with diffusion and catabolism to create a spatial GA₄ gradient that provides positional information and patterns cell elongation.