#1: Vegetative Apical Growth
Index & Bookmarks
Click HERE to return to the Factfile List
In many species, there is little difference in the apparent rate of vegetative cell division on opposite sides of the shoot apex. Most apices exhibit high rates of cell division and growth rate in the apical regions of the shoot apex itself. However, it has been suggested that increased growth does not change the shape or size of the apex itself during primordium differentiation. This suggests that the main cause of the formation of the leaf primordium could be related to the change in the direction rather than the rate of cell division. As we will see later on, it has now been shown that plasmodesmal regulation plays an important part in this process. This page is linked to The Black Box.htm, which explains how plasmodesmata may be involved in gating (opening, or closing) stomata, thus allowing or stopping signals between particular groups of cells.
The apex must be able to bulge out at the site of leaf initiation. The removal of constraints and restrictions (changing communication patterns for example can be set up by gating plasmodesma open or closed) which can thus result in the ability of the apex to develop in a multidirectional way.
|Thus for a leaf to be able to form at all the
epidermis must be
capable of controlled and regulated processes that will enable the
surface to deform, and these process must be coupled with an associated
increased plasticity of the surface. Deformation and plasticity
are essential irrespective of the plane of orientation of the microfibrils within the
walls of the underlying epidermal and sub-epidermal
cell layers and irrespective of the growth rates
of the underlying cells. Without periclinal cell division at the early stages
of primordium development, bulging would not be possible.
The occurrence of periclinal cell divisions, coupled to an appropriate cell division rate at the potential leaf primordium site are regarded as prerequisites for leaf formation. Plasticity and deformability are essential for the formation of a primordium as well. Changes in microfibrillar orientation may facilitate the change in growth direction and may help to determine the shape of the emergent organ when it starts to form.
An obvious candidate for the role of outer restraining layer, must be the protoderm which, even at this stage, has thickened outer wall layers. In addition, the protoderm responds positively to changes in the endogenous auxin concentration. Leaf primordia thus form at discontinuities within the cellulose re-enforcement pattern within the protoderm of the shoot apex.
Positioning of Leaf Primordia.
Whatever the mechanism of primordium initiation and formation, it must account not only for exactly how the primordia are formed, but also to when and where they are formed within the shoot apex. Studies clearly indicate that leaf primordia are formed only at the side of the apex, never on top or near the apex of the primordium itself. Phylotaxis, or the the arrangement of primordial tissue and hence leaves on the stem, is under strong genetic control (see Hofer and Ellis, 1998).
Various growth promoters and inhibitors have been shown to have an effect on the initiation, shape, size and position of leaf primordia, as well as on the general and specific structure of the shoot apex as well. As examples:
NPA (Naphthyl phthalamic acid) is an inhibitor of auxin transport, addition of small quantities of this substance, causes the apex to occupy a larger area. It has recently been shown to inhibit auxin response in Arabidopsis roots (see Oono et al., 2003) TIBA - (Triiodobenzoic acid) is also an inhibitor of auxin transport in the meristem. TIBA results in raised auxin concentrations (movement is inhibited) as it cannot be transported away from the apex. Addition of TIBA causes the formation of a raised `collar' of primordial tissues, and a rosette-like leaf primordial layer.
Exogenously-applied PCB (p-chlorophenoxyisobutyric acid) apparently inhibits auxin action, and may cause the primordium to be smaller. 2,4-D (2,4-dichlorophenoxyacetic acid, an auxin analogue) causes an increase in the number of leaves when applied to apices. This suggests a promotional role for endogenous auxins. TIBA has also been shown to be able to alter and influence phyllotaxis, by making the apex narrower. Avsian-Kretchmer et al., (2002) used an anti-indole acetic acid (IAA or auxin) monoclonal antibody-based immunocytochemical procedure to monitor IAA level in Arabidopsis tissues. Using immunocytochemistry and the IAA-driven β-glucuronidase (GUS) activity of Aux/IAA promoter::GUS constructs to detect IAA distribution, the authors investigated the role of polar auxin transport in vascular differentiation during leaf development in Arabidopsis. They concluded that shoot apical cells normally contain high levels of IAA and that IAA decreases in concentration as the leaf primordia expanded. Avsian-Kretchmer et al., (2002) determined that when seedlings were grown in the presence of IAA transport inhibitors, they showed a very low IAA signal in the shoot apical meristem (SAM) as well as in the youngest pair of leaf primordia.
According to Reinhardt, Mandel, and Kuhlemeier (2000), whilst the mechanisms controlling the switch between meristem propagation and leaf initiation are being identified by genetic and molecular analyses, the radial positioning of leaves, known as phyllotaxis, remains poorly understood. The arrangement of leaves at the apex, or Phyllotaxis, is such that it usually gives rise to consistent and predictable patterns. Perhaps the most obvious and striking are those where leaves are produced exactly opposite each other. In many species, the successive-formed pairs of leaves are placed at exactly 90o apart. The most striking patterns are those of distichous leaves, which occur in two ranks, and in this case, the successive leaves are exactly 180o away from their neighbours, as in Pisum, and the opposite and decussate arrangement in the Labiatae, in which each leaf of an opposite pair is 180o from its twin, and each pair is lies at 90o from the next pair, up or down the stem and the arrangement is thus described as four-ranked. In addition, a helical pattern may form, in which each successive leaf is displaced from its immediate neighbour at a divergence of 137o.
Leaf development is a regulated process that involves the re-arrangement of the cellulose microfibrils within the outer walls of epidermal cells. It also involves subtle changes that may result in discontinuities; including a change in direction of cell division (anticlinal to periclinal) plasmodesmal gating, and at a molecular level, precise regulation of hormonal balance, especially IAA.
The concept of domains in apical development is not new, but has been explained in a series of elegant papers, which are based on some equally elegant research by Rinnie and van der Schoot (1998).
A MUST READ article!
PLH Rinne and C van der Schoot 1998. Symplasmic fields in the tunica of the shoot apical meristem coordinate morphogenetic events. Development 144: 1477-1485.
Other reference of importance: