The Questions of Developmental Biology

etween nodes is called an internode (see Figure 20.20). In a simplistic sense, the mature
sporophyte is created by stacking node/internode units together. Phyllotaxy, the positioning of
leaves on the stem, involves communication among existing and newly forming leaf primordia.
Leaves may be arranged in various patterns, including a spiral, 180-degree alternation of single
leaves, pairs, and whorls of three or more leaves at a node (Jean and Barabé 1998).
Experimentation has revealed a number of mechanisms for maintaining geometrically regular
spacing of leaves on a plant, including chemical and physical interactions of new leaf primordia
with the shoot apex and with existing primordia (Steeves and Sussex 1989).
It is not clear how a specific pattern of phyllotaxy gets started. Descriptive mathematical models
can replicate the observed patterns, but reveal nothing about the mechanism. Biophysical models
(e.g., of the effects of stress/strain on deposition of cell wall material, which affects cell division
and elongation) attempt to bridge this gap. Developmental genetics approaches are promising, but
few phyllotactic mutants have been identified. One candidate is the terminal ear mutant in maize,
which has irregular phyllotaxy. The wild-type gene is expressed in a horseshoe-shaped region,
with a gap where the leaf will be initiated (Veit et al. 1998). The plane of the horseshoe is
perpendicular to the axis of the stem.
Leaf development
Leaf development includes commitment to become a leaf, establishment of leaf axes, and
morphogenesis, giving rise to a tremendous diversity of leaf shapes. Culture experiments have
assessed when leaf primordia become determined for leaf development. Research on ferns and
angiosperms indicates that the youngest visible leaf primordia are not determined to make a leaf;
rather, these young primordia can develop as shoots in culture (Steeves 1966; Smith 1984). The
programming for leaf development occurs later. The radial symmetry of the leaf primordium
becomes dorsal-ventral, or flattened, in all leaves. Two other axes, the proximal-distal and lateral,
are also established. The unique shapes of leaves result from regulation of cell division and cell
expansion as the leaf blade develops. There are some cases in which selective cell death
(apoptosis) is involved in the shaping of a leaf, but differential cell growth appears to be a more
common mechanism (Gifford and Foster 1989).
Leaves fall into two categories, simple and compound (Figure 20.24; see review by Sinha 1999).
There is much variety in simple leaf shape, from smooth-edged leaves to deeply lobed oak leaves.
Compound leaves are composed of individual leaflets (and sometimes tendrils) rather than a
single leaf blade. Whether simple and compound leaves develop by the same mechanism is an
open question. One perspective is that compound leaves are highly lobed simple leaves. An
alternative perspective is that compound leaves are modified shoots. The ancestral state for seed
plants is believed to be compound, but for angiosperms it is simple. Compound leaves have arisen
multiple times in the angiosperms, and it is not clear if these are reversions to the ancestral state.
Developmental genetic approaches are being applied to leaf morphogenesis. The Class I KNOX
genes are homeobox genes that include STM and the KNOTTED 1 (KN1) gene in maize. Gain-offunction
mutations of KN1 cause meristem-like bumps to form on maize leaves. In wild-type
plants, this gene is expressed in meristems. When KN1, or the tomato homologue LeT6, has its
promoter replaced with a promoter from cauliflower mosaic virus and is inserted into the genome
of tomato, the gene is expressed at high levels throughout the plant, and the leaves become "super
compound" (Figure 20.25; Hareven et al. 1996; Janssen et al. 1998). Simple leaves become more
lobed (but not compound) in response to overexpression of KN1, consistent with the hypothesis
that compound leaves may be an extreme case of lobing in simple leaves (Jackson 1996). The