Developmental landscape and asymmetric gene expression in the leaf vasculature of <i>Brassica rapa</i> revealed by single-cell transcriptome

Leaf vasculature not only acts as a channel for nutrients and signaling information but also influences leaf morphology. It consists of several distinct cell types with specialized functions. Cell type-specific characterizations based on single-cell RNA sequencing technology could aid in understanding the identities of vascular tissues and their roles in leaf morphogenesis in <i>Brassica rapa</i>. Here, we generated a single-cell transcriptome landscape of the Chinese cabbage leaf vasculature. A total of 12 cell clusters covering seven known cell types were identified. Different vascular cell types were characterized by distinct identities. The xylem parenchyma and companion cells exhibited an active expression pattern of amino acid metabolism genes. Tracheary elements and sieve elements were enriched in many genes related to cell wall biosynthesis, and the phloem parenchyma was enriched in many sugar transporter-encoding genes. Pseudo-time analyses revealed the developmental trajectories of the xylem and phloem and the potential roles of auxin and ethylene in xylem development. Furthermore, we identified key candidate regulators along the differentiation trajectory of the sieve elements and companion cells. Most of the homoeologous genes in the syntenic triads from the three subgenomes showed asymmetric gene expression patterns in different vascular cell types. Collectively, our study revealed that Chinese cabbage leaf vasculature cells had highly heterogeneous transcriptomes, providing new insights into the complex processes of leaf vasculature development in <i>B. rapa</i> leafy vegetables and other <i>Brassica</i> crops.