The apical hook is a structure in dicotyledonous plants that protects the fragile shoot meristem from damage as the seedling penetrates the soil. Its formation requires an asymmetric auxin distribution in the hypocotyl from which it arises, mediated by a polarised auxin transport machinery. The question of how this early auxin asymmetry is established has, however, been unclear, and is the subject of Eva Benková and colleagues’ investigation. The authors first show that root bending in response to gravity – in a short developmental time window – is an essential cue to apical hook development, and that this dependence on gravity is irrespective of light. A transient local peak in auxin response is observed at the site of apical hook formation, and then at the concave side as the hook region closes. The auxin transport machinery is established early in germination to aid root gravitropic bending, and also functions later in the hypocotyl. Seedlings germinated from green embryos show normal gravitropism but do not form an apical hook, even though they initially respond normally to gravity. Finally, excess abscisic acid (ABA) leads to loss of the apical hook, and a balance between ABA and gibberellin signalling is required for proper hypocotyl growth and apical hook formation. This paper thus provides a novel link between root gravitropism and plant hormone signalling in apical hook development.
