The radiation of life on Earth depended on the colonisation of land by plants. Plants’ photosynthetic activity enriched oxygen in the atmosphere, plants made the first soils and plants make food and homes for animals. The greening of the land started by the formation of thin soil crusts growing fungi, lichens and algae, and land plants originated from these algae[1-3]. The first land plants had a single tiny stem capped in a reproductive structure, as in modern mosses and other bryophytes. In contrast, most of today’s plants have large and elaborate branching forms nourished by vascular transport[4,5]

My lab’s research aims to identify genes and developmental changes that enabled such vascular plant forms to arise during evolution. Whilst this question is simple to articulate, it is hard to address because bryophytes and vascular plants have such disparate overall forms and patterns of development. A handful of tiny fossils suggest that plants gained a capacity to branch before they started to make shoots with indefinitely proliferative shoot tips and leaves[4,5].

Using liverwort[6], moss[7-10] and lycophyte[11,12] model systems we have identified likely roles for PIN[8,9], CLAVATA[10] and KNOX[11] genes in the origin of branching and shoot growth. I wish to build on this work to show the pattern of development in primitive branching forms and identify ancestral mechanisms for branching in the vascular plants. I also wish to identify the molecular basis for the origin of stem cell functions and proliferation in vascular plant shoot tips.

If you would like to join my lab please get in touch to discuss your ideas.

Figure 1: The line diagram represents the evolutionary past of six extant land plant groups. Whilst bryophytes and vascular plants have widely divergent shoot forms (shown in green), fossil intermediaries similar to (A) Partitatheca and (B) Cooksonia suggest that branching was an early innovation in plant evolution[4,5]. PIN genes regulate branching in angiosperms, and perturbation of PIN function in a moss induces branching (C) to give plants a similar form to Partitatheca fossils[8].

References

1.        Edwards and Kenrick (2015). The early evolution of land plants, from fossils to genomics: a commentary on Lang (1937) ‘On the plant-remains from the Downtonian of England and Wales'. Philosophical Transactions of the Royal Society B 370.

2.       Wellman and Strother (2015). The terrestrial biota prior to the origin of land plants (embryophytes): a review of the evidence. Palaeontology 58, 601-627.

3.       Harholt et al. (2016). Why plants were terrestrial from the beginning. Trends in Plant Science 21, 96-101.

4.       Harrison (2017). Development and genetics in the evolution of land plant body plans. Philosophical Transactions of the Royal Society B 372, e20150490.

5.      Harrison and Morris (2018). The origin and early evolution of vascular plant shoots and leaves. Philosophical Transactions of the Royal Society B.

6.      Solly et al. (2016). Regional Growth Rate Differences Specified by Apical Notch Activities Regulate Liverwort Thallus Shape. Current Biology 27, 1-11.

7.       Harrison, C.J., Roeder, A.H.K., Meyerowitz, E.M., and Langdale, J.A. (2009). Local cues and asymmetric cell divisions underpin body plan transitions in the moss Physcomitrella patens. Current Biology 19, 461-471.

8.       Bennett et al. (2014). Plasma membrane-targeted PIN proteins drive shoot development in a moss. Current Biology 24, 2776-2785.

9.       Coudert et al. (2015). Three ancient hormonal cues co-ordinate shoot branching in a moss. eLIFE 4, e06808.

10.     Whitewoods et al. (2018). CLAVATA Was a Genetic Novelty for the Morphological Innovation of 3D Growth in Land Plants. Current Biology 28, 2365-2376.

11.      Harrison et al. (2005). Independent recruitment of a conserved developmental mechanism during leaf evolution. Nature 434, 509-514.

12.    Harrison et al. (2007). Growth from two transient apical initials in the meristem of Selaginella kraussiana. Development 134, 881-889.