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Gene expression data support the hypothesis that Isoetes rootlets are true roots and not modified leaves
Alexander J Hetherington1,2, David M Emms1, Steven Kelly1
1Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, OX1 3RB, UK.
Scientific Reports
|December 10, 2020
Summary
Rhizomorphic lycopsid rootlets are confirmed as true roots, not modified leaves. Gene expression analysis in Isoetes echinospora supports their evolutionary origin as roots, aligning with other land plants.
Area of Science:
- Plant evolutionary biology
- Molecular genetics
- Paleobotany
Background:
- Rhizomorphic lycopsids, including ancient giant trees and modern Isoetes, exhibit unique terminal structures called rootlets.
- Two competing hypotheses exist regarding the evolutionary origin of these rootlets: they are either true roots or modified leaves.
Purpose of the Study:
- To investigate the evolutionary origin of rootlets in Isoetes echinospora.
- To differentiate between the hypotheses that rootlets are true roots versus modified leaves using gene expression analysis.
Main Methods:
- De novo transcriptome assembly of axenically cultured Isoetes echinospora.
- Comparative gene expression analysis between Isoetes echinospora rootlets and leaves.
- Comparison of Isoetes echinospora gene expression with that of true roots and leaves from Selaginella moellendorffii and Arabidopsis thaliana.
- Analysis of Root-Specific Leucine-Rich Repeat (RSL) genes expression patterns.
Main Results:
- Significant differences were observed in gene expression profiles between Isoetes echinospora rootlets and leaves.
- Gene expression signatures of Isoetes echinospora rootlets closely resembled those of true roots in Selaginella moellendorffii and Arabidopsis thaliana.
- Root-Specific Leucine-Rich Repeat (RSL) genes, crucial for root cell differentiation, showed exclusive or preferential expression in the rootlets of Isoetes, Selaginella, and Arabidopsis, but not in their respective leaves.
Conclusions:
- The gene expression data strongly support the hypothesis that Isoetes rootlets are homologous to true roots found in other lycopsids and land plants.
- This study provides molecular evidence clarifying the evolutionary nature of lycopsid rootlets, resolving a long-standing debate in plant evolution.
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