Faculty of Liberal Arts and Sciences, Osaka Prefecture University Sakai, Osaka, Japan.
Faculty of Science, Osaka Prefecture University Sakai, Osaka, Japan.
Front Plant Sci. 2014 Apr 8;5:137. doi: 10.3389/fpls.2014.00137. eCollection 2014.
The formation of the apical hook in dicotyledonous seedlings is believed to be effected by gravity in the dark. However, this notion is mostly based on experiments with the hook formed on the hypocotyl, and no detailed studies are available with the developmental manners of the hook, particularly of the epicotyl hook. The present study aims at clarifying the dynamics of hook formation including the possible involvement of gravity. Time-course studies with normal Alaska pea (Pisum sativum L., cv. Alaska) and an agravitropic pea mutant, ageotropum, under the 1-g conditions and on a 3-D clinostat revealed that (1) the apical hook of the epicotyl forms by the development of the arc-shaped plumule of the embryo existing in the non-germinated seed. The process of formation consists of two stages: development and partial opening, which are controlled by some intrinsic property of the plumule, but not gravity. Approximately when the epicotyl emerges from the seed coat, the hook is established in both pea varieties. In Alaska the established hook is sustained or enhanced by gravity, resulting in a delay of hook opening compared with on a clinostat, which might give an incorrect idea that gravity causes hook formation. (2) During the hook development and opening processes the original plumular arc holds its orientation unchanged to be an established hook, which, therefore, is at the same side of the epicotyl axis as the cotyledons. This is true for both Alaska and ageotropum under 1-g conditions as well as on the clinostat, supporting finding (1). (3) Application of auxin polar transport inhibitors, hydroxyfluorenecarboxylic acid, naphthylphthalamic acid, and triiodobenzoic acid, suppressed the curvature of hook by equal extents in Alaska as well as ageotropum, suggesting that the hook development involves auxin polar transport probably asymmetrically distributed across the plumular axis by some intrinsic property of the plumule not directly related with gravity action.
双子叶植物幼苗的顶端弯钩的形成被认为是在黑暗中受重力影响的结果。然而，这一概念主要基于对下胚轴形成的弯钩的实验，而关于弯钩的发育方式，特别是上胚轴弯钩的发育方式，尚无详细的研究。本研究旨在阐明弯钩形成的动力学，包括重力可能的参与。在 1g 条件下和三维回旋仪上对正常的阿拉斯加豌豆（Pisum sativum L.，cv. Alaska）和一种非向地性豌豆突变体 ageotropum 进行的时程研究表明：（1）上胚轴顶端弯钩是通过存在于未萌发种子中的弧形胚芽的发育形成的。形成过程包括两个阶段：发育和部分张开，这两个阶段由胚芽的内在特性控制，但不受重力影响。大约在上胚轴从种皮中伸出时，两种豌豆品种都形成了弯钩。在阿拉斯加，由于重力的作用，已建立的弯钩得以维持或增强，导致弯钩张开的时间延迟，这可能会产生一个错误的概念，即重力导致弯钩的形成。（2）在弯钩发育和张开过程中，原始的胚芽弧形保持其方向不变，形成一个已建立的弯钩，因此，弯钩位于上胚轴轴的同一侧，与子叶相对。这在 1g 条件下以及在回旋仪上，对于阿拉斯加和 ageotropum 都是如此，支持了结论（1）。（3）应用生长素极性运输抑制剂羟基芴羧酸、萘基邻氨甲酰苯甲酸和三碘苯甲酸，同样程度地抑制了阿拉斯加和 ageotropum 中弯钩的弯曲，表明弯钩的发育涉及生长素极性运输，可能通过胚芽的内在特性不对称地分布在胚芽轴上，与重力作用没有直接关系。
