The colonization of land by plants nearly half a billion years ago was a pivotal moment in Earth’s history. It marked the transition from aquatic life to the vast terrestrial ecosystems we see today. Recent research has unveiled a fascinating relationship between ancient liverworts and Mucoromycotina fungi, shedding light on how plants first conquered the land.

Liverworts and their symbiotic relationships

Liverworts are among the closest living relatives to the earliest land plants. These small, simple plants lack the complex structures of more evolved flora and primarily grow in damp environments. One such example is Hooker’s flapwort, or Haplomitrium hookeri, a rare leafy liverwort that thrives in the misty, rocky habitats of New Hampshire’s Mt. Washington.

For many years, scientists have recognized the symbiotic relationship between liverworts and a group of fungi known as Glomeromycotina. These fungi form branched structures within the root cells of their host plants, helping them to access vital nutrients like phosphorus and nitrogen in exchange for carbohydrates. This mutualistic relationship is thought to have been crucial for the survival of the earliest land plants, allowing them to thrive in nutrient-poor soils.

An ancient alliance

However, a groundbreaking discovery by botanist Jeff Duckett and his collaborators has revealed that liverworts also partner with another group of fungi, the Mucoromycotina, or “mucs.” This ancient lineage of fungi was previously believed to be made up solely of free-living decomposers. Through meticulous electron microscopy and molecular analysis, Duckett and his team found that mucs form symbiotic relationships with liverworts, providing them with essential nutrients in exchange for carbon1.

This discovery is significant because it suggests that mucs played a crucial role in the colonization of land by plants. Unlike the Glomeromycotina, which are more specialized, mucs have finer hyphae and can form a variety of structures within their host plants’ cells. This versatility may have offered early plants unique advantages as they adapted to life on land, including access to additional sources of nitrogen due to the mucs’ role as decomposers.

The presence of mucs in liverworts and other ancient plant lineages like hornworts, lycopods, and ferns indicates that these fungi were part of a broader ancestral symbiosis that supported the early colonization of terrestrial environments. Researchers have identified a wide variety of non-Glomeraceae fungi in liverworts, many of which are novel and specific to these ancient plants2. This finding challenges the previously held notion that liverworts’ symbiotic relationships with fungi were acquired from flowering plants and supports the idea that these partnerships are deeply rooted in the history of land plants.

Implications for the evolution of plant life

Understanding the relationship between liverworts and Mucoromycotina fungi could be very important for unraveling the complexities of plant evolution. It raises intriguing questions about how different fungal symbioses contributed to the success of early land plants. Did the presence of both Glomeromycotina and Mucoromycotina fungi provide plants with complementary benefits that enhanced their ability to thrive in new environments? How did these relationships evolve over time, and what role did they play in the diversification of plant life on Earth? The story of liverworts and mucs is a reminder of the remarkable adaptability of life and the enduring impact of these early symbioses on the natural world we see today. 

References

  1. Field, Katie J., et al. “Functional Complementarity of Ancient Plant–Fungal Mutualisms: Contrasting Nitrogen, Phosphorus and Carbon Exchanges between Mucoromycotina and Glomeromycotina Fungal Symbionts of Liverworts.” New Phytologist, vol. 223, no. 2, 5 May 2019, pp. 908–921, https://doi.org/10.1111/nph.15819. Accessed 5 May 2021. ↩︎
  2. Rimington, William R., et al. “Ancient Plants with Ancient Fungi: Liverworts Associate with Early-Diverging Arbuscular Mycorrhizal Fungi.” Proceedings of the Royal Society B: Biological Sciences, vol. 285, no. 1888, 10 Oct. 2018, p. 20181600, https://doi.org/10.1098/rspb.2018.1600. ↩︎