Identification of Gene Sets with Potential Roles in Biotic Stress Defenses in Western Redcedar by Comparative RNA-Seq Analysis

Western redcedar (Thuja plicata) trees can live for more than 1000 years, indicating exceptionally strong biotic resistance. An example of that is the naturally rot resistant heartwood, popular for various outdoor applications. Nevertheless, reforestation is inefficient and expensive due to extensive deer and elk browsing, and second-growth trees are frequently afflicted by extensive heart rot, jeopardizing the reputation of durable cedar lumber. We used RNA-seq to identify candidate genes and chemical biosynthesis pathways contributing to biotic resistance in both foliage and heartwood-forming tissues. First, a comparison of transcriptomes from wildtype foliage and a natural variant lacking resin glands yielded +500 genes, including complete biosynthetic pathways of foliar terpenoids, compounds that are linked to browsing resistance. Functional assays identified sabinene synthase and hydroxylases, key enzymes in the biosynthesis of the deer repellant a-thujone. Secondly, a comparison of transcriptomes from the sapwood to heartwood transition zone with other wood fractions provided candidates for complete biosynthetic pathways for tropolones and lignans, compounds that are linked to heartwood rot resistance, and surprisingly also pathways for flavonoid biosynthesis, compounds that hitherto have not been linked to rot resistance in this species. Finally, the bark-forming region also expressed genes in lignan, terpenoid and flavonoid biosynthesis, suggesting that the bark and heartwood have similar biotic defenses, but also expression indicating production of bark-specific toxins such as phenazines. Identified genes are now being explored for use in multi-trait Genomic Selection in this economically important species.