The Questions of Developmental Biology

flowering patterns among the over 300,000 angiosperm species, yet there appears to be an
underlying evolutionary conservation of flowering genes and common patterns of flowering
regulation.
A simplistic explanation of the flowering process is that a signal from the leaves moves to the
shoot apex and induces flowering. In some species, this flowering signal is a response to
environmental conditions. The developmental pathways leading to flowering are regulated at
numerous control points in different plant organs (roots, cotyledons, leaves, and shoot apices) in
various species, resulting in a diversity of flowering times and reproductive architectures. The
nature of the flowering signal, however, remains unknown.
Some plants, especially woody perennials, go through a juvenile phase, during which the plant
cannot produce reproductive structures even if all the appropriate environmental signals are
present (Lawson and Poethig 1995). The transition from the juvenile to the adult stage may
require the acquisition of competence by the leaves or meristem to respond to an internal or
external signal (McDaniel et al. 1992; Singer et al. 1992; Huala and Sussex 1993).
Grafting and organ culture experiments, mutant analyses, and molecular analyses give us a
framework for describing the reproductive transition in plants (Figure 20.28). Grafting
experiments have identified the sources of signals that promote or inhibit flowering and have
provided information on the developmental acquisition of meristem competence to respond to
these signals (Lang et al. 1977; Singer et al. 1992; McDaniel et al. 1996; Reid et al. 1996).
Analyses of mutants and molecular characterization of genes are yielding information on the
mechanics of these signal-response mechanisms (Hempel et al. 2000; Levy and Dean 1998).
Leaves produce a graft-transmissible substance that induces flowering. In some species, this
signal is produced only under specific photoperiods (day lengths), while other species are dayneutral
and will flower under any photoperiod (Zeevaart 1984). Not all leaves may be competent
to perceive or pass on photoperiodic signals. The phytochrome pigments transduce these signals
from the external environment. The structure of phytochrome is modified by red and far-red light,
and these changes can initiate a cascade of events leading to the production of either floral
promoter or floral inhibitor (Deng and Quail 1999). Leaves, cotyledons, and roots have been
identified as sources of floral inhibitors in some species (McDaniel et al. 1992; Reid et al. 1996).
A critical balance between inhibitor and promoter is needed for the reproductive transition.
In some species, meristems change in their competence to respond to flowering signals during
development (Singer et al. 1992). Vernalization, a period of chilling, can enhance the
competence of shoots and leaves to perceive or produce a flowering signal. The reproductive
transition depends on both meristem competence and signal strength (Figure 20.29). Shoot tip
culture experiments in several species (including tobacco, sunflower, and peas) have
demonstrated that determination for reproductive function can occur before reproductive
morphogenesis (reviewed in McDaniel et al. 1992). That is, isolated shoot tips that are
determined for reproductive development but are morphologically vegetative will produce the
same number of nodes before flowering in situ and in culture (Figure 20.30).
The "black box" between environmental signals and the production of a flower is vanishing
rapidly, especially in the model plant Arabidopsis. The signaling pathways from light via
different phytochromes to key flowering genes are being elucidated. Molecular explanations are
revealing redundant pathways that ensure that flowering will occur. Light-dependent, gibberellin-