Tansley Review No. 74 Control of organogenesis at the shoot apex

Abstract

summary The processes involved in the formation of primordia on the shoor apex are those controlling (1) growth rate, (2) division plane. (3) surface microstructure, and (4) extensibility of the surface. Changes in growth rate and division planes may accompany primordium formation but are considered as probably not in themselves being causal. Changes in surface microstructure may be necessary to delimit the position and area occupied by an incipient primordium. However, attention is directed to changes in surface extensibility as perhaps being the overriding factor in primordium formation. Nevertheless, the position and form of the primordia will also depend on growth rate, division plane, and surface microstructurc being permissive. The relative importance of these four sets of processes may differ from species to species and from one stage of development to another. Chemical and metabolic changes within the apex may first be necessary to determine whether the surface can extend sufficiently for any primordia to form at all, but their positions and time of initiation may depend more on the other factors. The surface microstructure may become more important when patterning is detailed and precise as it is in the developing flower, whereas a less precise mechanism dependent on localized induction of synthesis of a morphogen (auxin?) may provide sufficient information to determine the general position and liming of primordium initiation in vegetative apices. In determining the pattern of primordia on the apex, primordial area at initiation is important and reasons for believing that auxin may be involved in determining this are summarised. The different developmental pathways of primordia seem to diverge from the moment of initiation. Developmental fate of primordia is determined by the hamcotic genes which may in fact be heterochronic genes. How these regulatory genes control the processes involved in differentiation of different types of primordia is so far unknown.

The Polar Auxin Transport InhibitorN-1-Naphthylphthalamic Acid Disrupts Leaf Initiation, KNOX Protein Regulation, and Formation of Leaf Margins in Maize

“…A role for PAT in organ separation is well documented in many dicot species. Inhibition of auxin transport is correlated with the formation of fused cotyledons (Hadfi et al, 1998) and collar-like leaf primordia (Lyndon, 1994). Likewise, mutations in PIN-FORMED1, which encodes the putative auxin efflux transporter of Arabidopsis (Galweiler et al, 1998), cause fusion of lateral organ primordia and altered expression of the organ boundary marker CUP-SHAPED COTYLEDON2 (Vernoux et al, 2000).…”

Organ Formation at the Vegetative Shoot Meristem.

“…These studies have investigated the diversity, morphology, histology, cell division patterns, and cell lineages of the SM. Because this work has been summarized in detail elsewhere (Steeves and Sussex, 1989; Lyndon, 1990Lyndon, , 1994, this review focuses on recent advances in understanding the genetic control of organ formation at the SM. …”

Branch apices, heterochrony, and inflorescence morphology in some mimosoid legumes (Leguminosae: Mimosoideae)

“…Sattler (1975b) provided a generalized discussion, and later (1978) examined heterotopy in the context of fusion and floral morphology. In none of these works is the relationship between heterotopy and heterochrony mentioned, though the idea that home os is is a result of heterochronic change is becoming widely accepted (d. Coen 1991;Hill & Lord 1989;Lord 1991;Lyndon 1994). As the taxa studied here are widely distributed on the published cladogram (Grimes 1995), little may be said about the polarity of the heterotopic change in stipule inception until a better resolved cladogram becomes available, and morphology of the stem-apex of more species is studied.…”

“…Heterotopy is the change in the position of inception of organs (Sattler 1978). Several authors argue that homeosis is a special case of heterochrony (Coen 1991;Hill & Lord 1989;Lord 1991;Lyndon 1994), and Lyndon (1994) points out that while homeotic genes are often assumed to be positional genes, their role in regulating timing (heterochrony) should be distinguished from any role in affecting position. The role of heterochrony in affecting or effecting heterotopic changes remains unstudied, and the relationships between heterochrony, heterotopy and homeosis require theoretical discussion.…”