Transcriptional roadmap to seasonal variation in wood formation of Norway spruce
Plant Physiol. 2018, 176(4):2851-2870

Jokipii-Lukkari S, Delhomme N, Schiffthaler B, Mannapperuma C, Prestele J, Nilsson O, Street NR, Tuominen H

Abstract
Seasonal cues influence several aspects of the secondary growth of tree stems, including cambial activity, wood chemistry, and transition to latewood formation. We investigated seasonal changes in cambial activity, secondary cell wall formation, and tracheid cell death in woody tissues of Norway spruce (Picea abies) throughout one seasonal cycle. RNA sequencing was simultaneously performed in both the xylem and cambium/phloem tissues of the stem. Principal component analysis revealed gradual shifts in the transcriptomes that followed a chronological order throughout the season. A notable remodeling of the transcriptome was observed in the winter, with many genes having maximal expression during the coldest months of the year. A highly co-expressed set of monolignol biosynthesis genes showed high expression during the period of secondary cell wall formation as well as a second peak in mid-winter. This mid-winter peak in the expression did not trigger lignin deposition, as determined by pyrolysis-gas chromatography/mass spectrometry. Co-expression consensus network analyses suggested the involvement of transcription factors belonging to the AS2/LOB and MYB-HB families in seasonal control of secondary cell wall formation of tracheids. Interestingly, the lifetime of the latewood tracheids stretched beyond the winter dormancy period, correlating with a lack of cell death-related gene expression. Our transcriptomic analyses combined with phylogenetic and microscopic analyses also identified the cellulose and lignin biosynthetic genes and putative regulators for latewood formation and tracheid cell death in Norway spruce, providing a toolbox for further physiological and functional assays of these important phase transitions.

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