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Asteroxylon mackiei

How Earth’s Plants Developed Roots – 407-million-year-old fossil reveals transitional root formation

A window into prehistoric times: 407-million-year-old plant fossils reveal for the first time what the roots of early land plants looked like and how they evolved. In contrast to today’s plants, these first roots emerged from unequal branching: one of the contestants showed the features of the axis of the stem with leafy roots, but the second was a leafless root. This represents a transitional form of root development that is now extinct – a kind of missing link in plant evolution.

of plants about 500 million years ago conquered earth, they had to develop completely new structures. The algae-like plants initially adapted to life outside of the water and formed roots, releasing axes and leaves in order to confirm their presence in the new habitat. Paleontologists suggest that there was a symbiosis with mushrooms The first wild plants could have helped.

A complete ecosystem in the world

407-million-year-old fossils from the Scottish region now reveal what these first wild plants looked like and how their leaves, axes and roots grew Rainy Tshirt. This fine grain formation from the Early Devonian period is known for its good preservation of fossils. “Rhynie Chert maintains the entire ecosystem around the hot springs – plants, animals, fungi and microbes are conserved here on site,” Alexander Hetherington of the University of Edinburgh and colleagues explain.

Finds at Rhynie Chert also include fossils of the extinct plant species Asteroxylon mackiei. Researchers have now, for the first time, reconstructed in detail how this early relative of the bear moss family formed its members. This 3D reconstruction opens a glimpse into a key moment in plant development – the formation of the first plant roots.

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Unequal branching of axes

Analyzes reveal: Asteroxylon mackiei did indeed have leaf axes, shoot axes, and a root-like runner. But its roots appeared in a completely different way from the roots of today’s plants. In these, the roots and other axons of the plant grow through a similar branching: two runners of the same variety appear from a main branch – two roots or axons.

Uneven branching of the stem and root axis in the model and in the cross section of the fossa. © Hetherington et al./ eLife, CC-by-sa 4.0.0 Update

But it was different with prehistoric pioneer Bärlapp: “Asteroxylon roots developed when a bud-like axis formed a thorn, with one thorn retaining the identity of the bud and the second developing its root identity,” explains senior author Liam Dolan of Gregor Mendel’s Institute of Molecular Plant Biology in Vienna. The nature of the root itself in the absence of leaf and epidermal appendages, as well as vascular bundles of various shapes.

Transitional form of root formation

These results show that early Earth plants still formed their root-like rootstock through divergent cleavage – an unequal division. Thus Asteroxylon represents a transitional form from primitive land plants without the typical division into leaves, buds and roots to higher plants with clearly defined plant parts. “Our results indicate that divergent cleavage was likely key to developing a truly complex scheme for Asteroxylon mackiei,” says Hetherington and his team.

In principle, the prehistoric relative of Barlap is a kind of missing link in the evolution of the plant. “Understanding the structure and evolution of these early Devonian plants gives us a glimpse into the key period in Earth’s history, shortly after plants colonized the arid surfaces of the continents,” Dolan says. (eLife, 2021; doi: 10.7554/eLife.69447)

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Source: Gregor Mendel Institute of Molecular Plant Biology (GMI)