The flying spider-monkey tree fern, scientifically known as Alsophila spinulosa, is a species of tree fern widely distributed across Asia.
Alsophila spinulosa. Image credit: Quanzi Li.
Land plants evolved 470 million years ago from aquatic algae and have since transformed the terrestrial ecosystem.
The body plan of land plants has undergone a series of developmental, biochemical and physiological adaptations, one of which is the appearance of vascular tissues.
In seed plants, xylem, with thickened cell walls, provides the trunk with high water-conducting efficiency and strong structural support.
Lignin is an essential component of xylem secondary cell walls — it not only gives mechanical support in fiber cells but also forms a hydrophobic surface in vessels to aid water transport.
Outside of seed plants, tree ferns — members of the order Cyatheales — are one of the few lineages having tree-like trunks.
These plants have high ornamental values and are regarded as a resource for natural products with pharmaceutical applications.
Some molecules have been identified as having antitumor and antibacterial activities in the tree fern Alsophila spinulosa, but they probably represent only a small fraction of the total natural product diversity.
Many tree fern species are also being overexploited, which, in combination with climate change, poses serious threats to their survival.
A better understanding of their recent demographic history will help guide future conservation efforts.
“Ferns are the earliest vascular plants, and lignified cell walls were a key innovation during the evolution of these plants,” said Professor Ray Ming, a researcher in the Department of Plant Biology at the University of Illinois at Urbana-Champaign.
“This study has improved our understanding of how vascular tissues developed in ferns and other land plant species.”
In the study, Professor Ming and colleagues sequenced the genome of Alsophila spinulosa and investigated how its vascular tissues are constructed.
They found that two Vascular-related Mac-Domain genes were highly expressed in xylem compared to other tissues, indicating that these might be key regulators in the formation of xylem-specific cells.
Using microscopy and biochemical methods, they also measured the levels of lignin and secondary metabolites in ferns.
They found that lignin made up 40% of the stem cell wall. In comparison, wood generally contains 25%.
They also discovered a new secondary metabolite primarily made in the xylem, which they named alsophilin.
“This new compound is abundant in the xylem, likely as one of the compounds filling up the cavity of non-functional tracheid cells,” Professor Ming said.
“We also identified the genes involved in the biosynthesis of alsophilin in the genome.”
To understand how ferns evolved, the researchers compared the genomic sequence of Alsophila spinulosa to other members of the same species across 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 authors reconstructed the history of the fern population and saw that there were two times that these species underwent a drastic decrease in population numbers.
The first one occurred 35.6-34.5 million years ago and the second occurred 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,” Professor Ming said.
“In our future studies, we hope to increase the number of locations and the sample sizes for the genomic analysis.”
The results were published in the journal Nature Plants.
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X. Huang et al. The flying spider-monkey tree fern genome provides insights into fern evolution and arborescence. Nat. Plants 8, 500-512; doi: 10.1038/s41477-022-01146-6
