Ferns: the missing link in shoot evolution and development.

Andrew R G Plackett,Verónica S Di Stilio,Jane A Langdale

doi:10.3389/fpls.2015.00972

Andrew R G Plackett, Verónica S Di Stilio + Show 1 more

Open Access

https://doi.org/10.3389/fpls.2015.00972

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Abstract

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.

Highlights

Land plants evolved from aquatic green algae ∼470 million years ago, with phylogenetic analyses consistently positioning charophytic algae as the closest extant sister group (Karol et al, 2001; Lewis and McCourt, 2004; Wodniok et al, 2011; Ruhfel et al, 2014)
Shoots and organs develop from the co-ordinated activity of multiple cells, requiring complex intercellular communication to co-ordinate development
With little known about the genetic pathways underlying apical cell (AC) function, based on morphology they were considered to be functionally divergent from the shoot apical meristem (SAM)

Summary

Introduction

Land plants (embryophytes) evolved from aquatic green algae ∼470 million years ago, with phylogenetic analyses consistently positioning charophytic (streptophyte) algae as the closest extant sister group (Karol et al, 2001; Lewis and McCourt, 2004; Wodniok et al, 2011; Ruhfel et al, 2014). (A–G) Shoots and lateral organs of representative model land plant species; (A) Physcomitrella patens gametophore; (B) Selaginella kraussiana shoot and microphylls, showing unequal apical branching; (C–E) Ceratopteris richardii sporophyte (C), showing emergence of new frond from the shoot apex (D) and a fully developed reproductive frond with lateral pinnae (E); (F,G) Arabidopsis thaliana sporophyte with axillary branches emerging (F), showing a rosette leaf (G).

Results

Conclusion