Spatial transcriptomics of a lycophyte root sheds light on root evolution.

Plant roots originated independently in lycophytes and euphyllophytes, whereas early vascular plants were rootless. The organization of the root apical meristem in euphyllophytes is well documented, especially in the model plant Arabidopsis. However, little is known about lycophyte roots and their molecular innovations during evolution. In this study, spatial transcriptomics was used to detect 97 root-related genes in the roots of the lycophyte Selaginella moellendorffii. A high number of genes showed expression patterns similar to what has been reported for seed plants, supporting the idea of a highly convergent evolution of mechanisms to control root development. Interaction and complementation data of SHORTROOT (SHR) and SCARECROW (SCR) homologs, furthermore, support a comparable regulation of the ground tissue (GT) between euphyllophytes and lycophytes. Root cap formation, in contrast, appears to be differently regulated. Several experiments indicated an important role of the WUSCHEL-RELATED HOMEOBOX13 gene SmWOX13a in Selaginella root cap formation. In contrast to multiple Arabidopsis WOX paralogs, SmWOX13a is able to induce root cap cells in Arabidopsis and has functionally conserved homologs in the fern Ceratopteris richardii. Lycophytes and a part of the euphyllophytes, therefore, may share a common mechanism regulating root cap formation, which was diversified or lost during seed plant evolution. In summary, we here provide a new spatial data resource for the Selaginella root, which in general advocates for conserved mechanisms to regulate root development but shows a clear divergence in the control of root cap formation, with a novel putative role of WOX genes in root cap formation in non-seed plants.

Single-cell transcriptomics sheds light on the identity and metabolism of developing leaf cells.

As the main photosynthetic instruments of vascular plants, leaves are crucial and complex plant organs. A strict organization of leaf mesophyll and epidermal cell layers orchestrates photosynthesis and gas exchange. In addition, water and nutrients for leaf growth are transported through the vascular tissue. To establish the single-cell transcriptomic landscape of these different leaf tissues, we performed high-throughput transcriptome sequencing of individual cells isolated from young leaves of Arabidopsis (Arabidopsis thaliana) seedlings grown in two different environmental conditions. The detection of approximately 19,000 different transcripts in over 1,800 high-quality leaf cells revealed 14 cell populations composing the young, differentiating leaf. Besides the cell populations comprising the core leaf tissues, we identified subpopulations with a distinct identity or metabolic activity. In addition, we proposed cell-type-specific markers for each of these populations. Finally, an intuitive web tool allows for browsing the presented dataset. Our data present insights on how the different cell populations constituting a developing leaf are connected via developmental, metabolic, or stress-related trajectories.

An in situ sequencing approach maps PLASTOCHRON1 at the boundary between indeterminate and determinate cells.

The plant shoot apex houses the shoot apical meristem, a highly organized and active stem-cell tissue where molecular signaling in discrete cells determines when and where leaves are initiated. We optimized a spatial transcriptomics approach, in situ sequencing (ISS), to colocalize the transcripts of 90 genes simultaneously on the same section of tissue from the maize (Zea mays) shoot apex. The RNA ISS technology reported expression profiles that were highly comparable with those obtained by in situ hybridizations (ISHs) and allowed the discrimination between tissue domains. Furthermore, the application of spatial transcriptomics to the shoot apex, which inherently comprised phytomers that are in gradual developmental stages, provided a spatiotemporal sequence of transcriptional events. We illustrate the power of the technology through PLASTOCHRON1 (PLA1), which was specifically expressed at the boundary between indeterminate and determinate cells and partially overlapped with ROUGH SHEATH1 and OUTER CELL LAYER4 transcripts. Also, in the inflorescence, PLA1 transcripts localized in cells subtending the lateral primordia or bordering the newly established meristematic region, suggesting a more general role of PLA1 in signaling between indeterminate and determinate cells during the formation of lateral organs. Spatial transcriptomics builds on RNA ISH, which assays relatively few transcripts at a time and provides a powerful complement to single-cell transcriptomics that inherently removes cells from their native spatial context. Further improvements in resolution and sensitivity will greatly advance research in plant developmental biology.