Physiology, genetics and genomics of drought adaptation in Populus

Abstract

As the demand for energy rises, Populus species are increasingly grown as bioenergy crops. Meanwhile, due to global change, predictions indicate that summer droughts will increase in frequency and intensity over Europe. This study was carried out to evaluate the adaptation to drought in Populus, at different levels: genetic, genomics and physiology. Forests trees such as poplar are very important ecologically and economically but the Populus genus is known to be drought sensitive. Consequently, it is essential to understand drought response and tolerance for those trees. Two populations of poplar were used for this study, a mapping population (Family 331) and a natural population of Populus nigra. The F2 mapping population obtained from a cross of Populus deltoides and Populus trichocarpa, showed differences in stomatal conductance and carbon isotope composition in both clones and the F2 progeny. It was also used to discover QTL related to water use efficiency highlighting interesting areas of the genome. Combining QTL discovery and microarray analysis of the two clones in response to drought, a list of candidate genes was defined for water use efficiency. The natural population of Populus nigra consisting of 500 genotypes of wild black poplar showed variation in numerous physiological measurements such as leaf development and carbon isotope discrimination in well-watered conditions depending on their latitude of origin. The drier genotypes (from Spain and South France) had the smallest leaf area which could be linked to an adaptation to drought. Physiological measurements of extreme genotypes in leaf size of this population revealed differences in response to water depending on their latitude of origin. Stomatal conductance rapidly decreased and water use efficiency improved for Spanish genotypes after a slow and moderate drought stress. Direct comparisons between the transcriptome of extreme genotypes from Spain and North Italy in well watered and drought conditions provided an insight into the genomic pathways induced during water deficit. Six candidate genes were selecting for further analysis using real-time PCR: two stomatal development genes (ERECTA and SPEECHLESS), two ABA related genes (ATHVA22A and CCD1), a second messenger (IP3) and a NAC transcription factor (RD26)

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