Exome resequencing and GWAS for growth, ecophysiology, and chemical and metabolomic composition of wood of Populus trichocarpa.

BACKGROUND: Populus trichocarpa is an important forest tree species for the generation of lignocellulosic ethanol. Understanding the genomic basis of biomass production and chemical composition of wood is fundamental in supporting genetic improvement programs. Considerable variation has been observed in this species for complex traits related to growth, phenology, ecophysiology and wood chemistry. Those traits are influenced by both polygenic control and environmental effects, and their genome architecture and regulation are only partially understood. Genome wide association studies (GWAS) represent an approach to advance that aim using thousands of single nucleotide polymorphisms (SNPs). Genotyping using exome capture methodologies represent an efficient approach to identify specific functional regions of genomes underlying phenotypic variation. RESULTS: We identified 813 K SNPs, which were utilized for genotyping 461 P. trichocarpa clones, representing 101 provenances collected from Oregon and Washington, and established in California. A GWAS performed on 20 traits, considering single SNP-marker tests identified a variable number of significant SNPs (p-value < 6.1479E-8) in association with diameter, height, leaf carbon and nitrogen contents, and δ15N. The number of significant SNPs ranged from 2 to 220 per trait. Additionally, multiple-marker analyses by sliding-windows tests detected between 6 and 192 significant windows for the analyzed traits. The significant SNPs resided within genes that encode proteins belonging to different functional classes as such protein synthesis, energy/metabolism and DNA/RNA metabolism, among others. CONCLUSIONS: SNP-markers within genes associated with traits of importance for biomass production were detected. They contribute to characterize the genomic architecture of P. trichocarpa biomass required to support the development and application of marker breeding technologies.

Breeding progress and preparedness for mass-scale deployment of perennial lignocellulosic biomass crops switchgrass, miscanthus, willow and poplar.

Genetic improvement through breeding is one of the key approaches to increasing biomass supply. This paper documents the breeding progress to date for four perennial biomass crops (PBCs) that have high output-input energy ratios: namely Panicum virgatum (switchgrass), species of the genera Miscanthus (miscanthus), Salix (willow) and Populus (poplar). For each crop, we report on the size of germplasm collections, the efforts to date to phenotype and genotype, the diversity available for breeding and on the scale of breeding work as indicated by number of attempted crosses. We also report on the development of faster and more precise breeding using molecular breeding techniques. Poplar is the model tree for genetic studies and is furthest ahead in terms of biological knowledge and genetic resources. Linkage maps, transgenesis and genome editing methods are now being used in commercially focused poplar breeding. These are in development in switchgrass, miscanthus and willow generating large genetic and phenotypic data sets requiring concomitant efforts in informatics to create summaries that can be accessed and used by practical breeders. Cultivars of switchgrass and miscanthus can be seed-based synthetic populations, semihybrids or clones. Willow and poplar cultivars are commercially deployed as clones. At local and regional level, the most advanced cultivars in each crop are at technology readiness levels which could be scaled to planting rates of thousands of hectares per year in about 5 years with existing commercial developers. Investment in further development of better cultivars is subject to current market failure and the long breeding cycles. We conclude that sustained public investment in breeding plays a key role in delivering future mass-scale deployment of PBCs.

Association genetics of chemical wood properties in black poplar (Populus nigra).

Black poplar (Populus nigra) is a potential feedstock for cellulosic ethanol production, although breeding for this specific end use is required. Our goal was to identify associations between single nucleotide polymorphism (SNP) markers within candidate genes encoding cellulose and lignin biosynthetic enzymes, with chemical wood property phenotypic traits, toward the aim of developing genomics-based breeding technologies for bioethanol production. Pyrolysis molecular beam mass spectrometry was used to determine contents of five- and six-carbon sugars, lignin, and syringyl : guaiacyl ratio. The association population included 599 clones from 17 half-sib families, which were successfully genotyped using 433 SNPs from 39 candidate genes. Statistical analyses were performed to estimate genetic parameters, linkage disequilibrium (LD), and single marker and haplotype-based associations. A moderate to high heritability was observed for all traits. The LD, across all candidate genes, showed a rapid decay with physical distance. Analysis of single marker-phenotype associations identified six significant marker-trait pairs, whereas nearly 280 haplotypes were associated with phenotypic traits, in both an individual and multiple trait-specific manner. The rapid decay of LD within candidate genes in this population and the genetic associations identified suggest a close relationship between the associated SNPs and the causative polymorphisms underlying the genetic variation of lignocellulosic traits in black poplar.

Association genetics of traits controlling lignin and cellulose biosynthesis in black cottonwood (Populus trichocarpa, Salicaceae) secondary xylem.

• An association genetics approach was used to examine individual genes and alleles at the loci responsible for complex traits controlling lignocellulosic biosynthesis in black cottonwood (Populus trichocarpa). Recent interest in poplars as a source of renewable energy, combined with the vast genomic resources available, has enabled further examination of their genetic diversity. • Forty candidate genes were resequenced in a panel of 15 unrelated individuals to identify single nucleotide polymorphisms (SNPs). Eight hundred and seventy-six SNPs were successfully genotyped in a clonally replicated population (448 clones). The association population (average of 2.4 ramets per clone) was phenotyped using pyrolysis molecular beam mass spectrometry. Both single-marker and haplotype-based association tests were implemented to identify associations for composite traits representing lignin content, syringyl : guaiacyl ratio and C6 sugars. • Twenty-seven highly significant, unique, single-marker associations (false discovery rate Q < 0.10) were identified across 40 candidate genes in three composite traits. Twenty-three significant haplotypes within 11 genes were discovered in two composite traits. • Given the rapid decay of within-gene linkage disequilibrium and the high coverage of amplicons across each gene, it is likely that the numerous polymorphisms identified are in close proximity to the causative SNPs and the haplotype associations reflect information present in the associations between markers.