Land plants evolved 470 million years ago from algae and, since then, they fundamentally reshaped our planet. During their evolution, ferns have undergone several changes that helped them survive and thrive on land. Now, a team of researchers led by the Chinese Academy of Forestry in Beijing and the University of Illinois at Urbana-Champaign has managed for the first time to comprehensively study the genomic arrangement of tree ferns, offering new insights into how ferns evolved.
According to the scientists, a major breakthrough in the evolution of land plants involved the “invention” of their vascular systems, which helped them conduct nutrients and water through their bodies. Such systems consist of two tissues: the xylem and the phloem. While the xylem facilitates the transport of water to the stems and leaves, the phloem helps transporting sugars created through photosynthesis to the rest of the plant. Moreover, xylem cells are lined with lignin, a supportive structural material that provides rigidity to wood and bark.
“Ferns are the earliest vascular plants, and lignified cell walls were a key innovation during the evolution of these plants,” said study senior author Ray Ming, a professor of Plant Biology at the University of Illinois. “This study has improved our understanding of how vascular tissues developed in ferns and other land plant species.”
By sequencing the genome of the flying spider-monkey tree fern Alsophila spinulosa, Professor Ming and his colleagues discovered that two vascular-related Mac-Domain genes were highly expressed in this fern’s xylem compared to other tissues, suggesting that they might be key regulators in the formation of xylem-specific cells. The scientists also measured the levels of lignin and secondary metabolites – compounds that are not required for growth and reproduction, but nevertheless offer some benefits – in ferns, and found that lignin makes up 40 percent of the stem cell wall.
Moreover, they identified a new secondary metabolite primarily made in xylem, which they called alsophilin. “This new compound is abundant in the xylem, likely as one of the compounds filling up the cavity of non-functional tracheid cells. We also identified the genes involved in the biosynthesis of alsophilin in the genome,” Professor Ming reported.
To understand how ferns evolved, the researchers compared the genome of A. spinulosa to other members of the same species spread across China. Surprisingly, they identified six distinct populations, with different genomic sequences. Based on these findings, the experts reconstructed the evolutionary history of ferns and discovered that these species underwent a drastic decline twice (35.6 – 34.5 million years ago and 2.5 – 0.7 million years ago).
“This analysis of genomes and lignin composition from a broader collection of ferns will help us understand the role of lignin in the early lineage of vascular plants. In our future studies, we hope to increase the number of locations and the sample sizes for the genomic analysis,” Professor Ming concluded.
The study is published in the journal Nature Plants.