Unearthing the complex evolutionary origins of root branching

The roots of extant vascular plants branch in two fundamentally different ways. The roots of living clubmosses (lycophytes) branch in a process termed dichotomous branching. By contrast, the roots of living ferns, horsetails and seed plants (euphyllophytes) branch by a process called lateral root branching. These two different modes of branching result in very different root architectures, with predicted different functions. However, it was not previously known when lateral root branching first evolved.

A new study, published in Nature Plants today, examined the fossil record of early diverging euphyllophytes to establish when lateral root branching evolved and to test if lateral root branching was a defining feature of all euphyllophytes.

The roots of all living euphyllophytes branch laterally, and so it was predicted that roots of early diverging euphyllophytes, like L. goense, would also branch laterally. However, it was found that the roots of L. goense were described as dichotomously branched. To verify the original description and to ensure that root branching had not been misinterpreted, lead author Dr Sandy Hetherington, travelled to Liège in Belgium to examine the original specimens of L. goense.

By re-examining the original specimen and taking new images of the 380 million year old roots, Hetherington verified that root branching was dichotomous, based not only on morphology (the overall shape of the roots) which had been used by the previous authors but also on its anatomy (the internal tissues which were preserved in this fossil).

“385 million years ago when roots were first evolving in the ferns, horsetails and seed plants, root branching was fundamentally different from what we see in their living relatives today”, said Hetherington. “A plants ability to explore the soil and mine for water and nutrients will depend on the type of root branching, dichotomous or lateral.”

The re-description of L. goense indicated that not all euphyllophyte roots branched laterally and the roots of some species branched dichotomously. However, was L. goense the exception to the rule or was dichotomous branching common in early euphyllophytes? To answer this question a thorough review of the literature was needed. Evidence for fossil roots from 50 fossil taxa from the Devonian and Carboniferous were identified from 80 papers, the oldest dating back to 1895, and covering worked published in multiple languages.

Dichotomous root branching, a character not present in extant euphyllophytes, was common among Devonian and Carboniferous euphyllophytes. The team are now confident that lateral root branching was not a defining character of the roots of euphyllophytes. But when did lateral root branching evolve? To investigate this question the team mapped root branching types to the ages of known groups of euphyllophytes. Their analysis indicated that lateral root branching evolved at different times in the major groups of euphyllophytes, ferns, horsetails and seed plants, which in turn indicates that lateral branching may have evolved independently multiple times in euphyllophytes.

Of this discovery, Professor Liam Dolan said: “This discovery gives scientists the first coherent framework for the sequence of events that occurred during the evolution of roots, which were key adaptations that transformed the terrestrial environment as plants colonised and spread across the continental surfaces for the first time.”

Dr Christopher Berry of Cardiff University added: “It has been amazing to see how Sandy’s work has transformed the way that we think about early rooting systems over the last few years. Roots are critical in the way that plants and biology interact with the solid Earth and its atmosphere. There is no more important time to get to grips with this than during the Devonian, the time when rapidly evolving plants were pushing terrestrial environments towards a state that we might recognise as familiar to us today. Understanding the biology of this transition is fundamentally interesting and important.”