Abstract
Eucalyptus species constitutes the most widely planted hardwood trees in temperate and subtropical regions. In this study, we compared the transcript levels of genes involved in lignocellulose formation such as cellulose, hemicellulose and lignin biosynthesis in two selected 3-year old hybrid Eucalyptus (Eucalyptus urophylla × Eucalyptus grandis) genotypes (AM063 and AM380) that have different lignin content. AM063 and AM380 had 20.2 and 35.5% of Klason lignin content and 59.0 and 48.2%, α-cellulose contents, respectively. We investigated the correlation between wood properties and transcript levels of wood formation-related genes using RNA-seq with total RNAs extracted from developing xylem tissues at a breast height. Transcript levels of cell wall construction genes such as cellulose synthase (CesA) and sucrose synthase (SUSY) were almost the same in both genotypes. However, AM063 exhibited higher transcript levels of UDP-glucose pyrophosphorylase and xyloglucan endotransglucoxylase than those in AM380. Most monolignol biosynthesis-related isozyme genes showed higher transcript levels in AM380. These results indicate monolignol biosynthesis-related genes may regulate wood composition in Eucalyptus. Flavonoids contents were also observed at much higher levels in AM380 as a result of the elevated transcript levels of common phenylpropanoid pathway genes, phenylalanine ammonium lyase, cinnamate-4-hydroxylase (C4H) and 4-coumarate-CoA ligase (4CL). Secondary plant cell wall formation is regulated by many transcription factors. We analyzed genes encoding NAC, WRKY, AP2/ERF, and KNOX transcription factors and found higher transcript levels of these genes in AM380. We also observed increased transcription of some MYB and LIM domain transcription factors in AM380 compared to AM063. All these results show that genes related to monolignol biosynthesis may regulate the wood composition and help maintain the ratio of cellulose and lignin contents in Eucalyptus plants.
Highlights
Eucalyptus trees are ubiquitous in tropical/subtropical and temperate zones and are well known for their rapid growth rate and adaptability
We evaluated wood properties of 918 elite candidate plants using a traditional breeding program, and selected two hybrid Eucalyptus genotypes AM380 and AM063, which exhibited the highest (35.5%) and the lowest (20.2%) Klason lignin content at the progeny evaluation stage (Figure 1)
Collected wood core samples were grounded by the mill, and basic density, α-cellulose, and Klason lignin were estimated by NIR analysis
Summary
Eucalyptus trees are ubiquitous in tropical/subtropical and temperate zones and are well known for their rapid growth rate and adaptability. The Eucalyptus genus is composed of more than 600 species with its origin center based in Oceania (Ladiges et al, 2003). Eucalyptus wood is utilized in many ways, such as pulp, paper, civil construction, and furniture and energy production. Eucalyptus plantations are increasing around the world. Brazil has more than 600,000 hectares of Eucalyptus plantation. Understanding the mechanism of regulation of the individual components of wood (such as cellulose, hemicellulose, and lignin) is of great commercial importance. Predominance of cellulose and hemicellulose is optimal for the kraft pulp industry while higher lignin content is preferable for the combustible energy production industry (White, 1987). Xylem transcription profiles obtained using novel sequencing technologies in three Eucalyptus species revealed many genes in wood formation (Salazar et al, 2013)
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