Expression of two HOOKLESS genes in peas (Pisum sativum L.).

The apical hook of dark-grown dicotyledonous plants results from asymmetric growth of its inner and outer sides. It is a protective structure that prevents damage to the shoot apical meristem and the young leaves as the seedling pushes through the soil. Two phytohormones, ethylene and auxin, are thought to be involved in regulating apical hook formation. HOOKLESS1 (HLS1) of Arabidopsis was recognized as an ethylene-response gene whose product is required for hook formation. We cloned two cDNAs from peas, Ps-HLS1 and Ps-HLS2, whose products are functional homologs of HLS1. Both Ps-HLS1 and Ps-HLS2 complement the hls1 mutation in Arabidopsis. Expression of Ps-HLS1 is enhanced by ethylene and by IAA. Because the effect of ethylene is counteracted by 2,5-norbornadiene, an inhibitor of ethylene action, it appears that the primary factor in apical hook formation in peas is ethylene.