To protect their newly formed fragile organs, dark-grown dicotyledonous plants form an apical hook when penetrating through the soil. The apical hook of pifq (pif1 pif3 pif4 pif5) mutant was fully opened, even in complete darkness, suggesting that PIF proteins are required for maintaining the apical hook in the darkness and are involved in regulation of the apical hook opening. But the underlying mechanism for PIF proteins mediated apical hook development remains elusive.