Eight distinct cellulose synthase catalytic subunit genes from Betula luminifera are associated with primary and secondary cell wall biosynthesis

Cellulose, a significant proportion of the plant cell wall, is the main determining factor of the wood qualities of a timber tree. Cellulose synthase (CESA), an essential catalyst for cellulose biosynthesis, is encoded by the CesA gene family. We isolated eight full-length CesA cDNAs from Betula luminifera on the basis of transcriptome sequencing. The predicted proteins comprised 985–1,103 amino acids, sharing 85–92 % identity with the corresponding PtiCESAs from Populus trichocarpa. Each protein contained typical motifs of plant CESA proteins. Phylogenetic analysis showed that BlCESA1, BlCESA3 and BlCESA4 belonged to three clades linked to secondary cell wall synthesis, and BlCESA2, BlCESA5, BlCESA6, BlCESA7 and BlCESA8 belonged to three clades associated with primary cell wall synthesis. Quantitative reverse transcription PCR and in situ mRNA hybridization indicated that transcripts of BlCesA1, BlCesA3 and BlCesA4 were most abundant in the xylem of the lignified stem and had similar expression patterns when treated with hormones and sucrose. Thus, the proteins encoded by these three genes might be subunits of the secondary wall CesA complex. However, the expression patterns of BlCesA1, BlCesA3 and BlCesA4 were different in response to mechanical stress. These three genes showed much higher expression levels in the tension wood than in the control, and they showed different expression patterns in the opposite wood, suggesting that these genes play different roles during secondary wall formation. BlCesA2, BlCesA5, BlCesA6 and BlCesA7 were predominantly expressed in the leaf and phloem. BlCesA8, which is most similar to BlCesA6, was strongly expressed in the leaf, phloem and cambium. Interestingly, these two genes showed distinct expression patterns when treated with hormones, sucrose and bending. Thus, in addition to biosynthesis of the primary cell wall, BlCesA8 might have other functions.