The apical hook of the hypocotyl or epicotyl forms in the dark and opens in the light. This was widely accepted notion until recently, and many sophisticated studies have been made on the basis of this notion. However, recent extensive survey, triggered by the discovery with tomato that the hook is markedly exaggerated by light instead of being opened, has shown that many seed species exhibit similar hook exaggeration by light, while many others do not. Experiments with tomato show: The light-induced hook exaggeration (LIHE), mediated by phytochrome in cooperation with the endosperm remaining unconsumed when the hypocotyl emerges from the seed coat, occurs at the same site and orientation (plane of hook) as the original hook which forms in response to gravity, but is no more likely to respond to gravity and seems merely to follow the direction of the original hook. Thus, it may be suspected whether the exaggerated hook is an entity distinct from the original hook despite sharing the site and orientation or the original hook loses the responsiveness to gravity after being once formed.
Observations of long radish (Raphanus sativus) and persimmon (Diospiros kaki) seedlings, species lacking LIHE, show that the original hook can form not only in the dark but even in the light, and is likely to be programmed to form once before allowing the cotyledons to unfold. Thus, the formation of the hypocotyl hook is not only important for protecting the shoot apex in germination in soil, but might be probably a crucial event for the subsequent development of the seedling. Besides in germinating seedlings, apical hooks are seen in flower bud-bearing stalks or shoots of some plants such as corn poppy (Papavea rhoeas), Erigeron phyladephicus and thickhead (Crassocephlum crepidioides). Such hooks may have some function for the development of the plants, which must be considered in the framework of seedling apical hook.
So far, research efforts have mostly been devoted to elucidating the mechanism of hook opening and exaggeration in terms of phytochrome and plant hormone actions as well as histology, and earned deserved results. Unfortunately, however, recognition of the importance of the hypocotyl hook has not been so great as to be included in many plant physiology textbooks. At the time when a new concept on the apical hook is emerging, it is considered significant for broadening the scope of studies and promoting comprehensive understanding of hook formation to bring together a variety of current information, knowledge, and concept on the apical hook and its related phenomena from a wide variety of expertise ranging from histology to molecular biology involving light and gravity actions. We welcome you to participate in this Frontiers program with original research articles, method articles, reviews, mini-reviews or perspective articles.