Comparative rhizotaxy of fossil and living isoetalean rhizomorphs reveals development through rootlet intercalation within a triangular lattice

Abstract

BACKGROUND AND AIMS: Isoetales is a clade of lycopsids that evolved colossal arborescent forms during their Palaeozoic prime but today are represented solely by the small, herbaceous monogeneric Isoetes. Despite the differences in scale of taxa in the clade, the rooting system of all members consists of two parts; rootlets develop from a rhizomorph in a regular pattern termed rhizotaxy. Rhizomorphs are highly diverse in morphology, leading to different terms being used to describe aspects of rhizotaxy in contrasting lineages. Here we set out to investigate the degree to which rhizotaxy was conserved among taxa, aiming to provide a standard geometric definition and developmental interpretation of rhizotaxy. METHODS: We developed a pipeline to quantitatively describe rhizotaxy. This pipeline allowed rootlet arrangement to be captured in 3D, before being visualized on a 2D lattice to which Delaunay triangulation could be applied. This approach offers a standard quantitative method of comparing rhizotaxy across disparate rhizomorphs. Next, to investigate the evolution and development of rhizotaxy we applied our pipeline to 3D reconstructions we generated of the rooting system of the extinct Carboniferous lycopsid, Oxroadia. Finally, we made direct observations of rootlet development in Isoetes using time-course imaging. KEY RESULTS: We demonstrate that rhizotaxy can be described as an equilateral triangular lattice for all members of the Isoetales, including Oxroadia. By combining evidence from direct observation of rootlet development in Isoetes with inferences of rootlet development and the early stages of sporophyte ontogeny of Oxroadia, we conclude that the conserved rhizotaxy developed through the process of rootlet intercalation. CONCLUSIONS: We provide a single geometric definition and predicted developmental mechanism for rhizotaxy that applies to all Isoetales. Our findings call into question the literal interpretation that the rhizomorph is a modified shoot.

Plant phenotyping relevance

植物の根器官配置（rhizotaxy）を3Dで定量化し、2D格子化・Delaunay三角測量によって比較する解析パイプラインを開発した研究であり、形態表現型の取得・抽出法が中心です。

Code and data availability

The article describes paper-specific phenotyping assets — 3D photogrammetry models of Stigmaria and Isoetes, SPIERS 3D reconstructions of Oxroadia, and images of all serial peels — stated to be uploaded to Edinburgh Datashare (DOI: 10.7488/ds/7972). However, no URLs are present in allowed_urls, so a public URL cannotbe

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