diversity Article Transcriptome Analysis of Cambium Tissue of Paulownia Collected during Winter and Spring Zachary D. Perry 1 , Thangasamy Saminathan 2 , Alok Arun 3 , Brajesh N. Vaidya 1 , Chhandak Basu 4 , Umesh K. Reddy 2 and Nirmal Joshee 1, *   Citation: Perry, Z.D.; Saminathan, T.; Arun, A.; Vaidya, B.N.; Basu, C.; Reddy, U.K.; Joshee, N. Transcriptome Analysis of Cambium Tissue of Paulownia Collected during Winter and Spring. Diversity 2021, 13, 423. https://doi.org/10.3390/d13090423 Academic Editor: Michael Wink Received: 14 July 2021 Accepted: 27 August 2021 Published: 1 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Agricultural Research Station, Fort Valley State University, Fort Valley, GA 31030, USA; Zachary.Perry@usda.gov(Z.D.P.); braz_v@hotmail.com (B.N.V.) 2 Gus R. Douglass Institute and Department of Biology, West Virginia State University, Institute, WV 25112, USA; tsaminathan@wvstate.edu (T.S.); ureddy@wvstateu.edu (U.K.R.) 3 Institute of Sustainable Biotechnology, Inter American University of Puerto Rico, Barranquitas, PR 00794, USA; alok_arun@br.inter.edu 4 Department of Biology, California State University, Northridge, Los Angeles, CA 91330, USA; chhandak.basu@csun.edu * Correspondence: josheen@fvsu.edu 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 an- notation 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 candi- date 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. Keywords: Paulownia; cambium; transcriptome; winter season; spring season; tree growth 1. Introduction Paulownia (Paulownia elongata) is an extremely fast-growing woody plant growing up to 20 feet in one year when young. Some Paulownia spp., when in plantation, can be harvested for saw timber in as little as five years. The genus Paulownia consists of nine species of deciduous, fast-growing, multi-purpose, hardwood trees [1] that have long been shown to be extremely adaptive to wide environmental variations in both edaphic and climatic factors, as well as being capable of growing on marginal lands [2,3]. Species of Paulownia are native to Asia and are widely cultivated in China, Laos, Vietnam, Japan, and Korea. It has now been introduced and cultivated in Australia, Europe, and Diversity 2021, 13, 423. https://doi.org/10.3390/d13090423 https://www.mdpi.com/journal/diversity