Apical hook and related phenomena---Their mechanism, function and significance
Uozaki Life Science Laboratory, Japan
Kobe University, Japan
When the seeds of dicotyledonous plants germinate, the hypocotyl or epicotyl forms an arch-shaped structure at the apex, which is referred to as the apical hook. Traditionally, the apical hook was believed to form in the dark under the control of gravity and to open in the light where it served to protect the apical meristem from injury when emerging through the soil. While very appealing, this is not necessarily correct. In many seed species the apical hook is formed just after germination in the light, and further, in tomato and some other species, it is even exaggerated by light. The apical hook of the pea epicotyl is found on a clinostat or under a micro-gravity environment on a satellite. Some species such as corn poppy (Papavea rhoeas), Erigeron phyladephicus and thickhead (Crassocephlum crepidioides) form an apical hook-like structure at the top part of the flower-bearing stalks or stems which grow in the open air. Thus, the apical hook is not only important for protecting the shoot apex from possible injury in soil, but also likely to be a crucial event for the subsequent development of the seedling or flower stalk. Being located just below the apical meristem, the apical hook is a center of cell translocation and differentiation. The tissues constituting the hook are continuingly replenished by new cells supplied from the apical meristem, i.e. new cells reaching the hook part must shape themselves to fit the hook (differentiating the outer and inner flanks of the hook) and after passing the hook, the cells must switch to the phase of vigorous elongation. These events of morphogenesis involve the coordinated action of phytochrome and several hormones. In addition to ethylene, which has been well studied, the involvement of gibberellins, cytokinins and auxins are now being evaluated. An unidentified endosperm factor and micro-RNAs are also claimed to be involved. Thus, as new concepts and knowledge on the apical hook are emerging, it is timely to bring together and publish collectively as an e-book the new information on the apical hook and its related phenomena from a wide variety of disciplines ranging from histology to molecular biology. This will provide the foundation for further enhancement of studies on this important plant structure. We welcome you to participate in this Frontiers Research Topic either with original research articles, method articles, reviews, mini-reviews or perspective articles.