Gibberellin transport affects lateral root growth through HY5 in response to far-red light.

Plants compete for light by growing taller than their nearest competitors. This is part of the shade avoidance syndrome and is a response to an increase in far-red (FR) light reflected from neighboring leaves. The root responds to this shoot-sensed FR cue by reducing lateral root emergence. It is well established that the phytohormone gibberellic acid (GA) is involved in supplemental FR-induced shoot elongation. Although GA is transported from shoot to root, its role in regulating lateral root growth is unclear. Here, we chemically and genetically manipulated GA and showed that GA modulates the lateral root reduction induced by shoot-sensed FR enrichment in Arabidopsis (Arabidopsis thaliana). Using the FRET-based GA biosensor GPS1 (GIBBERELLIN PERCEPTION SENSOR 1), we observed detailed GA changes in the root upon shoot exposure to FR enrichment and upon GA application to the shoot. Supplying GA to the shoot mitigated the FR-enrichment-induced root phenotype, indicating a functional link between GA and changes in root development in response to shoot-sensed FR. The regulatory role of GA in root growth appears to be partially dependent on ELONGATED HYPOCOTYL 5 (HY5), a light-responsive transcription factor that regulates root growth. Shoot-to-root transported GA4 led to increased HY5 protein levels in the lateral root primordia. HY5 then repressed auxin signaling, which inhibited lateral root growth. Our data reveal a gibberellin-dependent mechanism through which above-ground FR light signals modulate lateral root growth, whereby phytohormone and light signaling coordinate development across spatial scales.