The evolutionary origin of higher plants

Algae

The transformation of green, photosynthetic algae from their original simple aquatic life to advanced terrestrial plants was a key event in the history of planet Earth, ranking in importance with the origin of photosynthesis. One of the earliest achievements to come out of RSBS was the Identification of the extant representatives of the algae that accomplished this major evolutionary advance.

Foundation Professor Denis Carr appointed Jeremy Pickett-Heaps to a Research Fellowship in his Department of Developmental Biology in the one-year-old School in 1968. Jeremy embarked on extensive surveys of ultrastructural aspects of algae, often collecting material from Lake Burley Griffin and Sullivans Creek. His electron microscope studies disclosed for the first time that some members of one sub-group, the Charophytes, had acquired a mode of cell division similar to that found in land plants, in which “phragmoplasts” (precursors of new cell walls) arise between daughter nuclei after mitosis. He showed that by contrast the great majority of algae, consisting of many evolutionary lines with simpler organisation, use a quite different structure which he termed a “phycoplast”. He knew from his earlier work that higher plants possess a mechanism for inserting phragmoplasts in specific locations in dividing cells, and saw this as a truly fundamental feature of plant development, essential for formation of functional tissue geometries and therefore likely to be stable in evolutionary terms once it had been established. Thus his discovery that some Charophytes, alone among the algae, employ phragmoplasts, pointed to this group being ancestral.

From this and many other observations he proposed an entirely new picture of phylogeny in the green algae, including that the progenitors of the land flora were of the Charophyte type. His seminal early papers from RSBS (“The phylogeny of the green algae: a new proposal” and “The evolution of the mitotic apparatus: an attempt at comparative ultrastructural cytology in dividing plant cells”) have been hugely cited and his conclusions have been amply confirmed by subsequent biochemical and sequencing data.

Now distinguished with a special taxonomic title based on the spiral, twisted form of their sperm (where present), the Streptophyta (including the Charophytes) are firmly placed at the root of the higher plant branch of the Tree of Life, where, 450-500 million years ago, they initiated the development of all of the complex multicellular plants that are so familiar and important to us today.

For a contemporary view, see Domozych et al, Frontiers in Plant Science, 7: 1470, 2016
http://journal.frontiersin.org/article/10.3389/fpls.2016.01470/full

In 2017, we celebrate 50 years of Biology at ANU. This article is one of a set featuring the achievements and memorable occasions of ANU biologists those first 50 years.
Read more at Biology at ANU – the first 50 years.

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