Single-nuclei sequencing of Moricandia arvensis reveals bundle sheath cell function in the photorespiratory shuttle of C3-C4 intermediate Brassicaceae.

Spatially confined gene expression determines cell identity and is fundamental to complex plant traits. In the evolutionary transition from C3 to the more efficient C4 photosynthesis, restricting the glycine decarboxylase reaction to bundle sheath cells initiates a carbon concentrating mechanism via the photorespiratory glycine shuttle. This evolutionary step is generally thought to play an essential role in the progression from ancestral C3 to C4 photosynthesis. Plants operating this shuttle are often referred to as C3-C4 intermediates or C2 species. Within the Brassicaceae family, which includes model and crop plants, such species have evolved independently at least five times. However, research on the biochemistry of C3-C4 intermediates in the Brassicaceae has been limited to a few case studies of differentially localized proteins between mesophyll and bundle sheath cells. Here, we leverage recent advances in single-cell transcriptome sequencing to better understand how cellular specialization affects interconnected pathways. We generated a single-nuclei RNA sequencing dataset for Moricandia arvensis, a Brassicaceae with C3-C4 intermediate characteristics, and compared it to a publicly available single-cell transcriptome of leaf tissue of the C3  Arabidopsis thaliana. We confirmed the localization of selected photorespiratory proteins by electron microscopy of immunogold-labelled leaf sections. Our analysis revealed a M. arvensis-specific shift in expression of genes directly associated with the photorespiratory reactions, including components of the glycine decarboxylase complex, glutamate:glyoxylate aminotransferase, and glycolate oxidase, suggesting a shuttle of several C2 metabolites to the bundles sheath. Additionally, associated pathways, such as ammonium assimilation, synthesis of specific amino acids, redox regulation, and transport, also showed enhanced abundance in the M. arvensis bundle sheath.