Root gravity response module guides differential growth determining both root bending and apical hook formation

Abstract

The apical hook is a transiently formed structure that plays a protective role when the germinating seedling penetrates towards the soil surface. Critical for proper bending is the local auxin maxima, which defines the concave (inner) side of the hook curvature. As no sign of asymmetric auxin distribution has been reported in embryonic hypocotyls prior to hook formation, the question about how auxin asymmetry is established in the early phases of seedling germination, is largely unanswered. Here, we analyzed the auxin distribution and expression of PIN auxin efflux carriers from early phases of germination, and show that bending of the root in response to gravity is the crucial initial cue that governs the hypocotyl bending required for the apical hook formation. Importantly, polar auxin transport machinery is established gradually after germination starts as a result of tight root-hypocotyl interaction and a proper balance between abscisic acid and gibberellins.