An Integrated Functional Genomics Consortium to Increase Carbon Sequestration in Poplars: Optimizing Aboveground Carbon Gain

Abstract

Executive Summary:This project used gene expression patterns from two forest Free-Air CO 2 Enrichment (FACE) experiments (Aspen FACE in northern Wisconsin and POPFACE in Italy) to examine ways to increase the aboveground carbon sequestration potential of poplars (Populus). The aim was to use patterns of global gene expression to identify candidate genes for increased carbon sequestration. Gene expression studies were linked to physiological measurements in order to elucidate bottlenecks in carbon acquisition in trees grown in elevated CO 2 conditions. Delayed senescence allowing additional carbon uptake late in the growing season, was also examined, and expression of target genes was tested in elite P. deltoides x P. trichocarpa hybrids.In Populus euramericana, gene expression was sensitive to elevated CO 2 , but the response depended on the developmental age of the leaves. Most differentially expressed genes were upregulated in elevated CO 2 in young leaves, while most were downregulated in elevated CO 2 in semi-mature leaves. In P. deltoides x P. trichocarpa hybrids, leaf development and leaf quality traits, including leaf area, leaf shape, epidermal cell area, stomatal number, specific leaf area, and canopy senescence were sensitive to elevated CO 2. Significant increases under elevated CO 2 occurred for both above-and belowground growth in the F-2 generation. Three areas of the genome played a role in determining aboveground growth response to elevated CO 2 , with three additional areas of the genome important in determining belowground growth responses to elevated CO 2 . In Populus tremuloides, CO 2 -responsive genes in leaves were found to differ between two aspen clones that showed different growth responses, despite similarity in many physiological parameters (photosynthesis, stomatal conductance, and leaf area index). The CO 2 -responsive clone shunted C into pathways associated with active defense/response to stress, carbohydrate/starch biosynthesis and subsequent growth. The CO 2 -unresponsive clone partitioned C into pathways associated with passive defense and cell wall thickening. These results indicate that there is significant variation in gene expression patterns between different tree genotypes. Consequently, future efforts to improve productivity or other advantageous traits for carbon sequestration should include an examination of genetic variability in CO 2 responsiveness.