When the plants conquered the land, they had to develop completely new structures – including the roots. How these developed is now revealed by the fossil of a primeval Bärlapp precursor that paleontologists discovered in Scotland. The 407 million year old fossil shows that the first root-like outgrowths arose through a mechanism that is now extinct: A leaf shoot formed a branch, part of which remained a shoot with leaf attachments, the other a morphologically clearly distinguishable preform of the root became. Such branches in two different axis types no longer exist in today’s plants, but they could have been an evolutionarily important transition form to real roots.
For billions of years there was only aquatic life on our planet – the land masses were bare and uninhabited. But this changed almost 500 million years ago: The first algae-like plants began to colonize the country. Researchers suspect that a symbiosis with fungus-like organisms allowed them to develop protective layers against dehydration and more stable cell walls. Over time, this first vegetation adapted better and better to the conditions on land, until the first forerunners of real land plants developed a good 400 million years ago. For the first time they formed typical plant parts such as leaves, shoot axes or roots.
A complete ecosystem in the chert
What these early representatives of the land plants looked like is particularly well preserved in the so-called Rhynie Chert in Scotland. These chert formations from the early Devonian are so fine-grained that even the finest details have been preserved. “The Rhynie Chert conserves the entire ecosystem around a hot spring, plants, animals, fungi and microbes are preserved here in situ,” explain Alexander Hetherington from the University of Edinburgh and his colleagues. Using fossils of the fossil plant species Asteroxylon mackiei, they have now reconstructed how the first preforms of the roots of these early land plants developed. Asteroxylon belongs to the lycophytes, a group to which only a few plants belong today, including the bear moss family. The 3D reconstruction of this plant from the Devonian is the first from this period that is based solely on fossil evidence.
“Understanding the structure and development of these early Devonian plants gives us an insight into the key period in Earth’s history, shortly after the plants colonized the dry surfaces of the continents and began to spread across the land,” says senior author Liam Dolan from Gregor Mendel Institute for Molecular Plant Biology in Vienna. The reconstruction of the anatomy of Asteroxylon mackiei reveals that these plants already had leaf axes, shoot axes and root-like runners. But their roots were formed in a completely different way than in today’s plants. With these, roots and other plant axes always arise through a similar branching: Two roots of the same variety always emerge from a main branch – two roots or two shoot axes.
Uneven branching
But it was different with the prehistoric Bärlapp forerunner: “The roots of Asteroxylon developed when a sprout-like axis formed a fork, where one prong retained its sprout identity and the second developed its root identity,” explains Dolan. One extension of this branch had leaf attachments, stomata and a cuticle as is typical for shoot axes, the other, on the other hand, had neither leaf attachments nor a cuticle and the vascular bundles had a different shape. These features visible in the fossils show that this early land plant still formed its root-like runners through an anisotomous dichotomy – an uneven division. “Roots do not develop this way in living plants, which shows that this mechanism of root formation is now extinct,” says Dolan.
According to the researchers, Asteroxylon thus represents a transitional form from primitive land plants without the typical division into leaves, shoots and roots to the higher plants with clearly defined plant parts. “Our results suggest that the anisotomic dichotomy was likely key to developing the already complex blueprint for Asteroxylon mackiei,” state Hetherington and his team.
Source: Alexander Hetherington (University of Edinburgh) et al., ELife, doi: 10.7554 / eLife.69447