Cambium-specific Transcriptome Analysis of Paulownia to Study the Molecular Impacts of Winter and Spring Seasons on Tree Growth

Paulownia (Paulownia elongata) is a fast growing, multipurpose deciduous hardwood species that grows in a wide range of temperatures from minus 30 degrees celsius to 45 degrees celsius. Seasonal cues influence the secondary growth of tree stems, including cambial activity, wood chemistry, and transition to latewood formation. In this study, a de novo transcriptome approach was conducted to identify the transcripts expressed in vascular cambial tissue from senescent winter and actively growing spring seasons. Illumina paired-end sequenced cambial transcriptome generated 297,049,842 clean reads which finally yielded 61,639 annotated unigenes. Based on non-redundant protein database analyses, Paulownia cambial unigenes shared highest homology (64.8%) with Erythranthe guttata. A total of 35,471 unigenes resulted from KEGG annotation that were mapped to 128 pathways with metabolic pathways dominated among all. Additionally, DEG analysis showed that 2,688 and 7,411 genes were significantly upregulated and downregulated, respectively in spring compared to winter. Interestingly, quite a number of transcripts belonging to heat shock proteins were upregulated in spring season. qPCR expression results of fifteen wood-forming candidate genes involved in hemicellulose, cellulose, lignin, auxin and cytokinin pathways showed that the hemicellulose genes (CSLC4, FUT1, AXY4, GATL1, and IRX19) were significantly upregulated in spring season tissues when compared to winter tissues. In contrast, lignin pathway genes CCR1 and CAD1 were upregulated in winter cambium. Finally, a transcriptome-wide marker analysis identified 11,338 Simple Sequence Repeat (SSRs). The AG/CT dinucleotide repeat predominately represented all SSRs. Altogether, the cambial transcriptomic analysis reported here highlights the molecular events of wood formation during winter and spring. The identification of candidate genes involved in the cambial growth provides a roadmap of wood formation in Paulownia and other trees for the seasonal growth variation.