The analysis of plant development by genetic, molecular, and surgical approaches
has accumulated a large body of data, and yet it remains a challenge to uncover the
basic mechanisms that are operating. Early steps of development, when the zygote
and its daughter cells organize the embryonic plant, are poorly understood despite
considerable efforts toward the identification of relevant genes. Reported cases of genetic
redundancy suggest that the difficulty in uncovering patterning genes may reflect
overlapping gene activities. Our current knowledge on plant embryo development
still leaves open whether mechanisms for axis formation and subsequent pattern
formation are fundamentally different in animals and plants. Axis formation may follow
the general principle of establishing a peripheral asymmetric cue and mobilizing
the cytoskeleton toward this cue-in the case of plants possibly located in the
cell wall-but the molecules involved may be entirely different. Embryonic pattern
formation involves the establishment of different domains, but although there are candidates, it is not clear whether genes that define these domains are identified yet.
Pattern formation continues postembryonically in the meristem, and the flexibility
of this process may be explained by a feed-forward system of patterning cues originating
from more mature cells. Control of cell division and differentiation, which is
important in the meristems-regions of continuous development-has been studied
intensively and appears to involve short-range signaling and transmembrane receptor
kinase activation. Finally, although high importance of control of cell division
rates and planes for plant morphogenesis have been often inferred, recent genetic
studies as well as comparative morphological data point to a less decisive role of cell
division and to global controls of as yet unknown nature
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