Sapwood and Heartwood Variation Pattern and Candidate Gene Discovery in Eucalyptus cloeziana

Abstract:

In order to provide the genetic resource for accelerating the breeding in Eucalytpus, the variation of heartwood and the related candidate genes were explored in E. cloeziana. Wood discs were cut at intervals of 1 meter along the stem of felled tree in two 18-year-old families with contrasting high and low heartwood width, and each family has three individuals. The diameter outside and inside bark, overall ring number, sapwood ring number, sapwood width at two directions of northeast and southwest were measured. Radial and axial analysis were analyzed and the DNA from developing xylem tissues were harvested at breast height and used for bulk DNA sequencing, the SNP loci and the functional genes highly related with the variation of heartwood were discovered. The variation of sapwood width and heartwood radius among orientations were the highest in family 2, the average difference in family 1 and 2 were 0.7 and 5.5 cm, respectively. The decrease rate of heartwood radius and ring number with tree height were 0.40 and 0.64 in family 1, and 0.43 and 0.36 in family 2. The average basic density of E. cloeziana was significantly different between two families, with family 1 ranged from 0.80 to 0.82 g/cm³, and family 2 from 0.75 to 0.78 g/cm³. The basic density was negatively correlated with tree height, cross-section radius and heartwood radius, and positively correlated with tensile strength parallel to the grain and the hardness of tangential section, along some mechanical properties. Using Bulk DNA sequencing, a total of 1842 SNPs with high difference allele frequency were screened. Approximately 55.8% of these SNPs were distributed between genes, 18.3% in the upstream region, 16.3% in the downstream region, 5.1% in exons and 4.4% in introns using SnpEff annotation. The identified SNPs were used to locate the genes belonging to 50 terms, mainly related to plant cell division, plant cell membrane and plant protein kinase by GO enrichment analysis. By conducting the radial and axial analysis of these two families with contrasting high and low heartwood width, the trend of sapwood and heartwood variation were discussed. The SNPs and candidate genes were screened by bulk DNA sequencing, and the functions of genes involved to the process of sapwood formation were explored.