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

The plant hormone gibberellin (GA ₄ ) regulates numerous developmental processes. Within the root, GA ₄ controls growth, in part, by controlling the extent of cell elongation. The nlsGPS1 FRET biosensor revealed a GA ₄ gradient within the Arabidopsis root growth zones, with GA ₄ levels correlating with cell length. We developed a multiscale mathematical model to understand how biosynthesis, catabolism and transport create the GA ₄ distribution within the root growth zones. The model showed that phloem delivery of the biosynthetic intermediate GA ₁₂ contributes to higher levels of bioactive GA ₄ in the elongation zone, with the GA ₄ synthesis pattern being further modified by local GA ₁₂ synthesis in the quiescent centre region and the spatial distribution of biosynthesis enzymes (GA20ox and GA3ox). Model predictions revealed that whilst GA20ox and GA3ox transcript is present throughout the growth zones, these enzymes are inactive in the dividing cells, which explains steep GA ₄ gradients observed in GA20ox and GA3ox over-expression lines, and improves agreement between model predictions and data in wildtype, ga20ox and ga3ox lines. The model revealed that the GA ₄ gradient also depends a balance of diffusion through plasmodesmata and catabolism. Both model predictions and biosensor data demonstrated that plasmodesmatal diffusion enables a more gradual GA ₄ gradient, with higher diffusion antagonizing the GA ₄ gradient. Model predictions suggested that catabolism limits GA ₄ levels, which we validated via biosensor imaging in the ga2oxhept mutant. We concluded that GA ₄ distribution mediates root growth programming via local GA ₄ synthesis combining with diffusion and catabolism to create a spatial gradient that provides positional information and patterns cell elongation.