Age-dependent microRNAs in regulation of vascular cambium activity in Chinese fir ( Cunninghamia lanceolata )

High-throughput sequencing and Degradome sequencing revealed that miRNAs play crucial roles during the juvenile-to-mature stages in Chinese fir. In woody plants, the vascular cambium is a continuous cylinder of meristematic cells in the stem that produces secondary xylem (wood) and secondary phloem via cell division, expansion, secondary wall formation, lignification, and programmed cell death. MicroRNAs (miRNAs) are endogenous single-stranded non-coding RNAs that play important roles in plant growth and development. However, little is known about the roles of miRNAs in age-related changes in the vascular cambium, especially in conifers. Here, we compared age-related miRNAs in the vascular cambium of 3-(juvenile), 13-(transition), and 35-year-old (mature) Chinese fir (Cunninghamia lanceolata) trees. Transmission electron microscopy revealed changes in the morphology and cell wall thickness of the vascular cambium during the juvenile-to-mature phase transition. In addition, high-throughput sequencing and subsequent analyses identified 700 miRNAs, including 651 known and 49 novel miRNAs. We identified 105, 120, and 94 differentially expressed miRNAs at the three stages of growth. These miRNAs included various differentially expressed development-related and phytohormone-related miRNAs involved in cambium activity, cell division, and cell wall modification. Degradome sequencing revealed 38 target genes that were cleaved by 88 known miRNAs and 16 target genes that were cleaved by seven novel miRNAs. Additionally, miR159, miR164, and miR166 were highly expressed in the 13-transition stage; these miRNAs are involved in the auxin-signaling pathway. Moreover, miR156 levels were abundant in VC13 along with miR172 levels decreased during the juvenile-to-mature stages. These results provide insight into the roles of age-specific miRNAs in the growth of C. lanceolata and shed light on vascular cambium development in conifers.