A major event in the evolution of land plants was the invention of their vascular systems, which help them conduct water, nutrients, and food throughout their bodies. These systems consist of two tissues: the xylem and the phloem.
While the xylem allows the transport of water to the stems and leaves, the phloem helps to transport sugars, resulting from photosynthesis, to the rest of the plant. In addition, only the xylem cells are lined with lignin, structural support materials that provide the rigidity of wood and bark. The researchers wanted to understand how these vascular systems evolve in ferns and how lignin is made.
“Ferns are the first vascular plants, and lignified cell walls were a key innovation during the evolution of these plants,” said Ray Ming (GEGC), professor of plant biology. “This study has improved our understanding of how vascular tissues developed in ferns and other terrestrial plant species.”
For this study, researchers sequenced the genome of a flying tree fern. Alsophila spinulosa and studied how its vascular tissues are constructed. They found that two vasculature-related Mac-Domain genes were highly expressed in xylem compared to other tissues, indicating that they may be key regulators in xylem-specific cell formation.
Using microscopy and biochemical methods, the researchers also measured levels of lignin and secondary metabolites — compounds that aren’t necessary for growth or reproduction, but confer certain benefits — in ferns. They discovered that lignin made up 40% of the stem cell wall. In comparison, wood usually contains 25%. They also discovered a new secondary metabolite mainly made in the xylem, which they named “alsophilin”.
“This new compound is abundant in the xylem, likely as one of the cavity-filling compounds of non-functional tracheid cells. We also identified the genes involved in alsophilin biosynthesis in the genome,” Ming said.
To understand the evolution of ferns, researchers compared the genomic sequence of A. spinulosa to other members of the same species in nine locations in China. To their surprise, they discovered that there were six distinct populations, differing in their genomic sequences. Based on their sequencing results, the researchers pieced together the population history of ferns and found that there were two times when these species suffered drastic population declines. The first occurred 35.6 to 34.5 million years ago, and the second 2.5 to 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,” Ming said. “In our future studies, we hope to increase the number of locations and sample size for genomic analysis.”
Reference: Huang X, Wang W, Gong T, et al. The flying spider monkey tree fern genome provides insight into the evolution and tree fern structure. Nat. Plants. 2022;8(5):500-512. do I: 10.1038/s41477-022-01146-6.
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