Characterization of Proanthocyanidins as a Biomarker for Wood Quality in the Native Timber-Wood Tree Acacia koa.

Abstract

Acacia koa (koa) is an economically valuable timber wood tree playing an important role for Hawaiian culture and ecology. Wood color of koa can range from blond to dark red while its grain figure ranges from plain to curly. Given that koa populations are highly heterozygous and heterogeneous due to cross-pollinating nature, there is no current screening method to select seedlings for superior wood quality. Proanthocyanidins (PA), a subclass of tannins, are shown to be involved in wood quality. The goal of this study was to quantify PAs and identify PA biosynthesis genes as a biomarker for wood color. PA biosynthesis genes in koa were identified and isolated including dihydroflavonol reductase, anthocyanidin synthase, anthocyanidin reductase, and leucoanthocyanidin reductase. To correlate wood color, PAs, and expression of PA biosynthesis genes, koa seedlings expressing green to red hues were compared from Hawai‘i, O‘ahu, Kaua‘i, and Maui. Measurement of total tannin content was analyzed by the Folin-Ciocalteau method and PA content was measured by acid butanol assay in dry matter. Red-hued koa from Maui contained the highest total tannin and PA content. Distribution of the total tannin and PA content was assessed within koa seedlings expressing either green or red hues (O‘ahu and Maui). Total tannin content varied significantly among tissue type (leaf, stem, root). Aside from PA content also varying among tissue type, an opposing trend of lowest to highest PA content dependent on the hue expression of the family was observed. This suggests PA content is correlated to the overall hue of each family. qRT-PCR analysis of PA biosynthesis gene expression showed an upregulation in red-hued koa (Maui) compared to greenhued koa (Hawai‘i and O‘ahu). This suggests PA biosynthesis gene expression is also correlated to the overall hue of each family. Liquid chromatography-mass spectrometry (LC-MS) confirmed procyanidin B2 as the type of proanthocyanidin in koa. Characterizing proanthocyanidins as a biomarker for wood quality will be useful as a rapid screening method among progenies of different families in koa improvement programs.