The development of a hook-like structure at the apical part of the soil-emerging organs has fasci-nated botanists for centuries,but how it is initiated remains unclear.Here,we demonstrate with high-throughput infrared imaging and 2-D clinostat treatment that,when gravity-induced root bending is absent,apical hook formation still takes place.In such scenarios,hook formation begins with a de novo growth asymmetry at the apical part of a straightly elongating hypocotyl.Remarkably,such de novo asymmetric growth,but not the following hook enlargement,precedes the establishment of a detectable auxin response asymmetry,and is largely independent of auxin biosynthesis,trans-port and signaling.Moreover,we found that functional cortical microtubule array is essential for the following enlargement of hook curvature.When microtubule array was disrupted by or-yzalin,the polar localization of PIN proteins and the formation of an auxin maximum became im-paired at the to-be-hook region.Taken together,we propose a more comprehensive model for apical hook initiation,in which the microtubule-dependent polar localization of PINs may mediate the instruction of growth asymmetry that is either stochastically taking place,induced by gravitropic response,or both,to generate a significant auxin gradient that drives the full development of the apical hook.