The Questions of Developmental Biology

ole of KN1 in shoot meristem and leaf development, however, is consistent with the hypothesis
that compound leaves are modified shoots.
A second gene, LEAFY, that is essential for the transition from vegetative to reproductive
development also appears to play a role in compound leaf development. It was identified in
Arabidopsis and snapdragon (in which it is called FLORICAULA), and has homologues in other
angiosperms. The pea homologue (UNIFOLIATA) has a mutant phenotype in which compound
leaves are reduced to simple leaves (Hofer and Ellis 1998). This finding is also indicative of a
regulatory relationship between shoots and compound leaves.
In some compound leaves, developmental decisions about leaf versus tendril formation are also
made. Mutations of two leaf-shape genes can individually and in sum dramatically alter the
morphology of the compound pea leaf (Figure 20.26). The acacia mutant (tl) converts tendrils to
leaflets; afilia (af) converts leaflet to tendrils (Marx 1987). The af tl double mutant has a complex
architecture and resembles a parsley leaf.
At a more microscopic level, the patterning of stomata (openings for gas and water exchange) and
trichomes (hairs) across the leaf is also being investigated. In monocots, the stomata form in
parallel files, while in dicots the distribution appears more random. In both cases, the patterns
appear to maximize the evenness of stomata distribution . Genetic analysis is providing insight
into the mechanisms regulating this distribution. A common gene group appears to be working in
both shoots and roots, affecting the distribution pattern of both trichomes and root hairs (Benfey
1999).
*The similarities between plant meristem cells and animal stem cells may extend to the molecular
level, indicating that stem cells existed before plants and animals pursued separate phylogenetic
pathways. Homology has been found between genes required for plant meristems to persist and
genes expressed in Drosophila germ line stem cells (Cox et al. 1998).
This phenomenon, called fasciation, is found in many species, including peas and tomatoes.
The Vegetative-to-Reproductive Transition
Unlike some animal systems in which the germ line is set aside during early embryogenesis, the
germ line in plants is established only after the transition from vegetative to reproductive
development that is, flowering. The vegetative and reproductive structures of the shoot are all
derived from the shoot meristem formed during embryogenesis. Clonal analysis indicates that no
cells are set aside in the shoot meristem of the embryo to be used solely in the creation of
reproductive structures (McDaniel and Poethig 1988). In maize, irradiating seeds causes changes
in the pigmentation of some cells. These seeds give rise to plants that have visually
distinguishable sectors descended from the mutant cells. Such sectors may extend from the
vegetative portion of the plant into the reproductive regions (Figure 20.27), indicating that maize
embryos do not have distinct reproductive compartments.
Maximal reproductive success depends on the timing of flowering and on balancing the number
of seeds produced with resources allocated to individual seeds. As in animals, different strategies
work best for different organisms in different environments. There is a great diversity of