Integration Analysis of Circle-Sequencing and Transcriptome Reveals Extrachromosomal Circular DNA Is Involved in the Regulation of Vascular Cambium Annual Cycle in Chinese Pine

Extrachromosomal circular DNA (eccDNA) has been reported to play important roles in regulating genome replication, immune response and cellular communications in humans and animals. Recently, the presence of eccDNA has also recently been discovered in Arabidopsis, Amaranthus palmeri and Oryza sativa. Nevertheless, whether eccDNA exists and has roles in woody plants remains enigmatic. Here, we conducted a comprehensive analysis of morphological imaging, transcriptome and eccDNA expression profiles during different development stages of vascular cambium in Chinese pine (Pinus tabuliformis (P. tabuliformis)). It was found that eccDNA existed in the different development stages of vascular cambium and derived from each chromosome of P. tabuliformis. Further analysis revealed that eccDNA was not entirely random but rather exhibited a certain preference in exon regions. We also identified a high frequency of AA/AT/TT/TA dinucleotide repeats at the junctions of eccDNA and found that the length distribution of eccDNA was clustered between 158 and 316 bp. Notably, integration analysis revealed that differentially expressed eccDNAs and their annotated genes exhibited more significant dynamic changes in the dormant stage as compared with other stages. Taken together, our results provide new insights into the important mechanisms by which eccDNA influences vascular cambium development, enhancing our understanding of its role in tree plasticity.