The genetics, genomics and physiology of the response of Populus to elevated ozone

Tucker, James (2007) The genetics, genomics and physiology of the response of Populus to elevated ozone. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The genetic basis of susceptibility of Populus to ozone damage was investigated using a combination of phenotyping, TQL mapping and microarray analysis on an F₂ mapping population, Family, 331, bred from a cross between Populus trichocarpa (93-968) and P. deltoides Bart (ILL-129).

P. trichocarpa and P. deltoides exposed to an acute ozone treatment in growth chambers showed highly divergent physical symptoms, with P. trichocarpa developing necrotic lesions within 9 hrs of treatment and P. deltoides remaining undamaged. This provided the basis for a large-scale QTL mapping and phenotyping experiment in open-top chambers on 164 individuals of the F₂ mapping population, and also for gene expression studies in the grandparental species. A total of 20 ozone-specific QTL were mapped for numerous physiological traits, several of which displayed marked transgressive segregation. This wide range of sensitivity allowed selection of two groups of ozone sensitive and tolerant genotypes with contrasting patterns of visible damage. cDNA microarrays were used to uncover genes that showed altered expression in response to an acute ozone treatment, and included those encoding antioxidants and also those involved in photosynthesis and the phenypropanoid pathway. Furthermore, genes that were differentially expressed between the grand-parental species and between the two sensitivity groups of F₂ genotypes were identified. The recent completion of the physical genome sequence allowed the position of these candidate genes to be compared to QTL locations, a field termed "genetical genomics". Differentially expressed genes that co-located with QTL for visible damage to ozone were considered to be encouraging candidates for mediating the response. These included genes encoding arginine decarboxylase, SAM synthetase, and a plasma membrane intrinsic protein, which can be targeted for further investigation.