Large-scale analyses of gene expression and metabolite levels reveal the contrasting sensitivity of two winter wheat cultivars exposed to ozone under field conditions

Two winter wheat cultivars ( Triticum aestivum L., cv Premio and Soissons) were exposed to ozone in field conditions by means of a linear free-air ozone enrichment system at the INRA research station of Grignon (30 km west of Paris). The experi mental set up was designed in such a way that plants located do wnwind were exposed to decreasing ozone levels as their distance from the ozone source increased. As ozone concentrations and water fluxes were monitored at th e study site, the Phytotoxic Ozone Dose (POD) were calculated following the instructi ons given in the EMEP mapping manual. At the end of the experiment, after 2 months of ozone enrichment, flag-leaves of plants at 2m, 3.5m and 5m from the ozone source were e xposed to POD 6 of 5.42, 4.49 and 4.03 mmol.m -2 , respectively. Control plants, which were located 15m upwind from the ozone source, were subjected to a POD 6 of 3.5 mmol.m -2 . Grain yield was negatively correlated to the POD 6 index for both cultivars. At 2m, grain yield was redu ced by 12 % in Prem io and 18 % in Soissons as compared to the control. For Premio, the linear re lationship between POD 6 and grain yield agreed well with the impact function for wheat described in the EMEP mappi ng manual, but the olde r cultivar Soissons appeared to be more sensitive. To unravel early res ponses to ozone, large-scale analyses of gene expression and metabolite levels were carried out on leaf samples harvested after 34 days of ozone fumigation, before the flowering stage, on control plants (POD 6 = 1.46 mmol.m -2 ) and on plants located at 2m (POD 6 = 3.26 mmol.m -2 ) and 3.5m (POD 6 = 2.23 mmol.m -2 ) from the ozone source. Overall, the ozone treatment hardly modified gene expression in Premio while the older cultivar Soissons showed a higher degree of deregulation that was correlated to the POD 6 index. Similarly, me tabolite levels were modified onl y in response to the highest ozone exposure in Premio, whereas So issons showed stronger respon ses that were related to the POD 6 index. The identification of differentially e xpressed genes in Sois sons revealed that approximately 20 % of the down-regulated genes were transcription factors, notably ethylene- responsive ones, while 20 % of the up-regulated genes were linked to intracellular signal transduction and calcium homeostasis. These results emphasize the prominen t role of cellular signaling processes in early res ponses to ozone. In addition, 6 % of the up-regulated genes were associated to plant-pathogens interactions , which underlines the similarity between early events triggered by oz one and pathogens. The identification of metabol ites which abundance was affected by oz one showed features of the impact of the polluta nt on plants , such as reduced levels of pho tosynthetates and plas tid membrane lipids. Large-scale analyses of gene expressi on and metabolite levels highlighted the differential responses to ozone of two wheat cultivars with contrast ing sensitivity. Such analyses are helpfu l in dissecting molecular and cellular aspects of the im pact of ozone on plants and in finding re levant indicators of early respon ses to ozone