Gibberellin transport affects (lateral) root growth through HY5 during Far-Red light enrichment.

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 of Far-Red light (FR) reflected from neighboring leaves. The root responds to this shoot-sensed FR cue by reducing lateral root emergence. It is well-established that the plant hormone Gibberellic Acid (GA) is involved in supplemental FR-induced shoot elongation. Although GA is also transported from shoot to root, its role in regulating lateral root growth is unclear. We show via GA manipulations, both chemical and genetic, that GA modulates the lateral root reduction induced by shoot-sensed FR enrichment. Using the FRET-based GA biosensor GPS1, we observed detailed GA changes in the root upon shoot exposure to FR enrichment and when GA was supplied to the shoot. Supplying GA to the shoot also mitigated the FR-enrichment 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 upon the role of ELONGATED HYPOCOTYL 5 (HY5), a light-responsive transcription factor that regulates root growth. Shoot-to-root transported GA4 led to an increase in HY5 protein levels in the lateral root primordia. HY5 then repressed auxin signaling to repress lateral root growth. Our data unveil a novel way in which hormone and light signaling coordinate development across spatial scales by adjusting (lateral) root growth from above-ground FR light signals.