My research was aimed at profiling of metabolites in Glycine max (soybean; cultivar: Pioneer-93B15). Specific characteristics of identified and unidentified analytes in the profiles were tabulated. Possible effects of elevated atmospheric carbon dioxide (CO2) and ozone (O3) on soybean metabolites were investigated. Soybeans exposed to elevated CO2 may provide higher levels of C-sources to nodule bacteria, thereby increasing the N-compounds synthesized by Rhizobium sp. and transported to the host plant.
Metabolite profiling was conducted with leaf tissue extracts, and with phloem- and xylem-exudates of soybeans grown in a field in Illinois during the summers of 2005 and 2006 under Free Air Concentration Enrichment (FACE). Atmospheric CO2 and O3 levels were elevated to 1.5- and 1.2-times the current concentrations, respectively, to mimic the expected conditions in 2050. Responses in metabolite concentrations were compared with those observed in ambient air. Leaf tissues collected were extracted and phase-separated into polar and apolar fractions. Vascular exudates and the polar fraction of leaf metabolites were profiled using gas chromatography coupled with mass spectrometry. Metabolites were identified and quantified using commercial, public, and personal mass-spectral libraries, and reference curves of standards. All the analytes detected in the profiles of each tissue type were tabulated. Elements of soybean metabolism were monitored: mono- and di-saccharides, sugar alcohols and other sugar derivatives, amino acids, organic acids, secondary metabolites, and products of nodule metabolism such as ureides.
Over 300 analytes were detected in the profiles. There were distinct differences in the metabolite profiles among leaf extracts, phloem exudates and xylem exudates. Pinitol, together with sucrose, glucose and fructose, seemed to be a major transportation form of carbohydrates in soybeans. The host plant provided higher levels of C-sources in the forms of pinitol and citrate through phloem to the nodules under elevated CO2. Nodule bacteria in return supplied the host with increased levels of N-products including allantoin, urea, aspartate, asparagine, glutamine and glutamate via the xylem. Under elevated O3, glucose, fructose, sucrose, ortho-phosphate, salicylate and fumarate were higher in the petiole phloem exudates, both in the field and in growth chambers. Possibly redued partitioning of photosynthate into structural and storage carbohydrates, compared with ambient conditions, may have increased the levels of sugars transported for increased energy, signalling and regulatory requirements under oxidative stress, potencially retarding the plant growth in long-term exposure to O3.
Exudate metabolomics combined with leaf biochemistry provided novel information and an understanding on inflow and outflow of metabolites in the leaves for a better explanation of source-sink relationships. It is hoped that the results will facilitate the phenotyping of C3 species allowing for the breeding of better adapted crops.