The molecular basis of wood formation in Pinus radiata

Abstract

Gene discovery and genetic mapping approaches were utilised to study wood formation and wood density in Pinus species. Firstly, a method was developed for genetic mapping expressed sequence tags (ESTs), which is semi-automatable and utilises the highly polymorphic untranslated regions (UTR) within genes. On average, 75% of ESTs assayed could be genetically mapped in both P. radiata and P. taeda pedigrees using this approach. Mapping of orthologous loci enabled the alignment of genetic framework maps between Pinus species. Subsequently, a 3’UTR-based transcript profiling technique was developed and utilised to resolve gene expression differences between wood-forming tissues of P. radiata. Wood formation was compared between the base and the crown of two trees. Microscopic examination showed that 33% of developing tracheids at the base of the tree were producing secondary cell walls, reducing to 3% at the crown. All other cells remained in the cell division/cell expansion phase. Functional classification of differentially expressed transcripts revealed close parallels with the microscopy findings. Genes involved in cell division and expansion tended to be more highly expressed in the crown, while transcripts involved in secondary wall formation were more abundant at the base. Two genes were isolated that showed allele-specific regulation, namely, a dehydrin and a glycine-rich protein. Dehydrins act like antifreeze in cars, and protect tissues from extreme cold and drought stress. Sequence polymorphisms were identified in the 5’ regulatory region of the two dehydrin alleles that may alter their expression in response to stress. Sequence analysis of this gene in larger populations revealed that the locus was under strong natural selection pressure in two Pinus species (P. radiata and P. pinaster}, suggesting that this gene is important to the survival of these species. Genetic mapping revealed that the gene was associated with wood density in a full-sib pedigree and in a large unstructured NZ population. These findings have immediate applications for tree breeding and the NZ forestry industry, with an estimated gain of up to $450/hectare based on increases in wood density alone. Further gains may be possible based on an improved understanding of the connection between allelic forms of dehydrin and their response to abiotic stress.