Agriculture Reference
In-Depth Information
6
Lessons from the vegetative shoot apex
John F. Golz
6.1
Introduction
Post-embryonic growth and development of plants is largely determined by the
activity of meristems, structures that contain self-renewing populations of undif-
ferentiated cells. During embryogenesis, the primary meristems of the plant are
formed at opposite poles of the embryo. The shoot apical meristem (Fig. 6.1A),
which forms all above-ground organs, such as leaves, bracts and the tissue of the
stem, is located at the apical pole; whereas the root apical meristem, which is the
source of all subterranean organs, forms at the basal pole. Several different types
of meristems also form during post-embryonic shoot growth. The most common
are the bud-forming axillary meristems that arise in the axils of leaves. These buds
may either continue to develop into organ-bearing branches or remain dormant in-
definitely until growth is triggered. Floral meristems, which are specialised axillary
meristems, arise from the axils of bracts following the transition to flowering. They
produce a limited number of floral organs before ceasing activity. Other meris-
tems, which do not form lateral organs, are also active in the shoot. The intercalary
meristem drives shoot elongation whereas the lateral meristem causes an increase
in the width of the shoot axis. In most dicots and some monocots, the activity
of the lateral meristem is usually associated with secondary growth such as the
formation of wood and bark. This chapter focuses on the vegetative shoot apical
meristem and the role that signalling plays in its function. Descriptions of the root
apical meristem and floral meristem can be found in Chapters 7 and 8 of this vol-
ume.
Studies over the last decade have shown that cell signalling plays important roles
in regulating the size of the stem cell population at the summit of the shoot apical
meristem, determining the position of organ formation and patterning the emerging
organs. Not surprisingly the types of signals involved in these processes are varied
and involve some of the molecules and pathways that have been mentioned in
previous chapters of this topic. How these and other pathways are integrated into a
functioning shoot apical meristem is the focus of this chapter.
6.2
Structure of the angiosperm shoot apical meristem
In order to place signalling within the context of meristem function, the structure of
the shoot apex is briefly considered.
