Shoot development in land plants is a remarkably complex process that
gives rise to an extreme diversity of forms. Our current understanding
of shoot developmental mechanisms comes almost entirely from studies of
angiosperms (flowering plants), the most recently diverged plant
lineage. Shoot development in angiosperms is based around a layered
multicellular apical meristem that produces lateral organs and/or
secondary meristems from populations of founder cells at its periphery.
In contrast, non-seed plant shoots develop from either single apical
initials or from a small population of morphologically distinct apical
cells. Although developmental and molecular information is becoming
available for non-flowering plants, such as the model moss Physcomitrella patens,
making valid comparisons between highly divergent lineages is extremely
challenging. As sister group to the seed plants, the monilophytes
(ferns and relatives) represent an excellent phylogenetic midpoint of
comparison for unlocking the evolution of shoot developmental
mechanisms, and recent technical advances have finally made transgenic
analysis possible in the emerging model fern Ceratopteris richardii.
This review compares and contrasts our current understanding of shoot
development in different land plant lineages with the aim of
highlighting the potential role that the fern C. richardii could play in shedding light on the evolution of underlying genetic regulatory mechanisms.

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