LaMotte Clifford E, Pickard Barbara G
Gladys Levis Allen Laboratory of Plant Sensory Physiology, Biology Department, Washington University, St. Louis, MO 63130-4899, USA.
Funct Plant Biol. 2004;31(2):109-20. doi: 10.1071/fp03089.
Gravitropism of vascular plants has been assumed to require a single gravity receptor mechanism. However, based on the evidence in Part I of this study, we propose that maize roots require two. The first mechanism is without a directional effect and, by itself, cannot give rise to tropism. Its role is quantitative facilitation of the second mechanism, which is directional like the gravitational force itself and provides the impetus for tropic curvature. How closely coupled the two mechanisms may be is, as yet, unclear. The evidence for dual receptors supports a general model for roots. When readiness for gravifacilitation, or gravifacilitation itself, is constitutive, orthogravitropic curvature can go to completion. If not constitutively enabled, gravifacilitation can be weak in the absence of light and water deficit or strong in the presence of light and water deficit. In either case, it can decay and permit roots to assume reproducible non-vertical orientations (plagiogravitropic or plagiotropic orientations) without using non-vertical setpoints. In this way roots are deployed in a large volume of soil. Gravitropic behaviours in shoots are more diverse than in roots, utilising oblique and horizontal as well as vertical setpoints. As a guide to future experiments, we assess how constitutive v. non-constitutive modes of gravifacilitation might contribute to behaviours based on each kind of setpoint.
维管植物的向重力性一直被认为需要单一的重力受体机制。然而，基于本研究第一部分的证据，我们提出玉米根需要两种机制。第一种机制没有方向效应，仅凭自身不能产生向性。它的作用是对第二种机制进行定量促进，第二种机制与重力本身一样具有方向性，并为向性弯曲提供动力。这两种机制的耦合程度究竟如何，目前尚不清楚。双受体的证据支持了一个关于根的通用模型。当重力促进准备就绪或重力促进本身是组成型时，正向重力性弯曲可以完成。如果不是组成型启用，在没有光照和水分亏缺的情况下，重力促进作用可能较弱，而在有光照和水分亏缺的情况下则可能较强。在这两种情况下，它都可能衰减，并允许根在不使用非垂直设定点的情况下呈现可重复的非垂直方向（斜向重力性或斜向性方向）。通过这种方式，根可以分布在大量土壤中。茎的向重力性行为比根更多样化，利用倾斜、水平以及垂直设定点。作为未来实验的指导，我们评估重力促进的组成型与非组成型模式如何可能基于每种设定点对行为产生影响。
