Abstract
BackgroundPlant secondary growth depends on the activity of the vascular cambium, which produces xylem and phloem. Wood derived from xylem is the most abundant form of biomass globally and has played key socio-economic and subsistence roles throughout human history. However, despite intensive study of vascular development, the full diversity of cell types and the gene networks engaged are still poorly understood.ResultsHere, we have applied an optimized protoplast isolation protocol and RNA sequencing to characterize the high-resolution single-cell transcriptional landscape of highly lignified poplar stems. We identify 20 putative cell clusters with a series of novel cluster-specific marker genes and find that these cells are highly heterogeneous based on the transcriptome. Analysis of these marker genes’ expression dynamics enables reconstruction of the cell differentiation trajectories involved in phloem and xylem development. We find that different cell clusters exhibit distinct patterns of phytohormone responses and emphasize the use of our data to predict potential gene redundancy and identify candidate genes related to vascular development in trees.ConclusionsThese findings establish the transcriptional landscape of major cell types of poplar stems at single-cell resolution and provide a valuable resource for investigating basic principles of vascular cell specification and differentiation in trees.
Highlights
Plant secondary growth depends on the activity of the vascular cambium, which produces xylem and phloem
We found that the suberin biosynthetic gene, ASFT (ALIPHATIC SUBERIN FERULOYL TRANSFERASE) [56, 57], and several genes involved in biosynthesis of lignin and very long-chain fatty acids, including PEROXIDASE 52 (PRX52), PRX72, 3-KETOACYL-COA SYNTHASE 11 (KCS11), LONG-CHAIN ACYL-COA SYNTHASE 1 (LACS1), and LACS2 [58, 59], were expressed in cluster 11 (Additional file 1: Fig. S10)
We found that biosynthesis genes of most hormones, including gibberellin acid (GA), abscisic acid (ABA), and strigolactone (SL), were mainly overrepresented in cortex and endodermal cells, while auxin, cytokinin, and brassinosteroid (BR) biosynthesis genes were enriched in xylem cells, especially parenchyma cells (Additional file 1: Fig. S18)
Summary
Plant secondary growth depends on the activity of the vascular cambium, which produces xylem and phloem. The evolution of vascular tissue played key roles in plants’ colonization and domination of terrestrial environments by enabling long-distance transport of water and nutrients and providing mechanical strength that supports their vertical growth [1, 2] These tissues mainly consist of xylem and phloem cells, usually respectively produced by inward and outward division of cells in the vascular cambium, namely a process of secondary growth [3]. [5], and a hierarchical regulatory network involving several types of transcription factors that regulate cell specification and differentiation during vascular development has been proposed [6,7,8] Those studies were mostly based on roots and inflorescence stems of the model plant Arabidopsis [9] and stems of the model tree poplar [10]. Partly due to reliance on information and markers from Arabidopsis, the molecular-level regulation of vascular tissue development in trees, especially the SCW regulatory network involved, has not been fully elucidated [10, 13]
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