Abstract
Paulownia (Paulownia elongata) is a fast-growing, multipurpose deciduous hardwood species that grows in a wide range of temperatures from –30 °C to 45 °C. 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. An 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 the highest homology (64.8%) with Erythranthe guttata. KEGG annotation of 35,471 unigenes identified pathways enriched in metabolic activities. Transcriptome-wide DEG analysis showed that 2688 and 7411 genes were upregulated and downregulated, respectively, in spring tissues compared to winter. Interestingly, several transcripts encoding heat shock proteins were upregulated in the spring season. RT-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.
Highlights
To obtain the candidate genes associated with cambium development of the empress tree during seasonal growth, transcriptome sequencing analysis for tissues representing winter and spring seasons (Figure 1A,B)
A total of 40,814 unigenes were greater than 1 Kbs, and 58,654 unigenes were greater than 500 nucleotides in length
The paired-end reads resulted in longer unigenes than those reported in previous transcriptome studies on trees [45,46]
Summary
In order to survive multiple growing seasons, perennial plant species have adapted a dormancy regulation system that allows active growth during the desirable time of year and vegetative dormancy when climatic conditions are unfavorable for growth [15]. Paulownia harvest is not limited to a small seasonal window but can be conducted year-round with proper management practices. Transcriptome analyses from various tree species indicate that the putative role of gene families belonging to receptor kinases, transcription factors, and secondary wall biosynthesis are highly expressed in wood-forming cells [17,18,19,20,21]. A transcriptome-wide profiling study in P. elongata identified a subset of candidate genes that contribute to the production of wood [30] by investigating the differential expression of transcripts of the vascular cambium.
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