Plants first colonised land ~470–515 million years ago. Prior to this the Earth’s terrestrial surface was an extremely inhospitable place, largely devoid of life. The arrival of plants on land from aquatic algal ancestors changed both the Earth’s surface and atmosphere, enabling a huge explosion in terrestrial life. The expansion of land plants initiated the development of soils on a previously barren landscape and led to atmospheric oxygen concentrations that could support animal life. The ‘greening’ of Earth changed the course of evolution and was essential to the emergence of vertebrate animals including humans. Understanding the nature of early land plants is thus fundamental to understanding our own evolutionary history.
The land plants we live among and depend on today look very different to the ancestral green alga that first colonised the land. Green algae have basic body plans that are usually restricted to two-dimensions with limited apical growth (i.e. directional proliferative growth facilitated by the self-renewing activity of undifferentiated stem cells). Furthermore, the diploid stage of the life cycle is brief with meiosis occurring immediately following fertilisation. On the other hand, land plants grow in complex 3D structures with extensive apical growth and prolonged diploid development prior to meiosis. What, then, were the critical innovations that led to the establishment of plants on land? With this fundamental question in mind, the aim of this project was to elucidate the ancestral role of a key genetic network in land plants.
To do this we carried out a comparative analysis of gene function in a vascular plant (the flowering plant Arabidopsis) and a bryophyte (the moss Physcomitrium), which arose following an ancient divergence in land plant evolution. Sustained apical growth is a defining feature of vascular plants and the establishment of elaborate shoot systems with delayed reproductive development enabled their extraordinary evolutionary success. In contrast, mosses undergo more transient apical growth prior to rapid reproductive development. In Arabidopsis, a well-studied genetic network regulates apical growth and promotes developmental transitions. However, the role of this module in plant evolution remains to be determined.
Excitingly, early results suggest that this network has an ancestral role in regulating developmental transitions in plants and that the appearance of the network shortly after plants colonised the land was a critical event in plant evolution.
