Mechanisms of ozone tolerance in rice: characterization of two QTLs affecting leaf bronzing by gene expression profiling and biochemical analyses

High surface ozone concentration is increasingly being recognized as a factor that negatively affects crop yields in Asia. However, little progress has been made in developing ozone-tolerant genotypes of rice-Asia's major staple crop. This study aimed to identify possible tolerance mechanisms by characterizing two quantitative trait loci (QTLs) that were previously shown to influence visible leaf symptoms under ozone exposure (120 nl l(-1), 7 h d(-1), 13 d). Two chromosome segment substitution lines (SL15 and SL41) that carried introgressions of the QTLs OzT3 and OzT9, respectively, were exposed to ozone at 120 nl l(-1) along with their parent Nipponbare. In accordance with the expected QTL effect, SL15 showed stronger visible symptoms of ozone damage than Nipponbare, whereas SL41 had fewer symptoms. Gene expression profiling by microarray hybridization yielded 470 probes that were differentially expressed in SL15 and 314 in SL41. Potential tolerance mechanisms were evaluated by investigating changes in gene expression in three general categories. (i) Processes involved in programmed cell death, in which a number of genes related to ethylene or jasmonic acid metabolism or general disease resistance were identified that were differentially regulated in one of the substitution lines. (ii) Biosynthesis of antioxidants. Testing this hypothesis did not reveal any genes differentially regulated between genotypes, and it was thus rejected. (iii) Turnover of antioxidants and enzymatic detoxification of radical oxygen species (ROS), in which a number of differentially regulated genes were also identified. Genes encoding antioxidant enzymes (catalase and peroxidases) tended to be more strongly expressed in SL15. A potential tolerance gene which encodes a putative ascorbate oxidase was identified within the QTL introgression in SL41. This gene showed consistently lower expression in SL41 under ozone exposure across different points in time within independent experiments. Its expression may be involved in mechanisms leading to enhanced ascorbic acid status in SL41 under ozone exposure, and may be linked to a higher concentration of total apoplastic ascorbic acid in SL41 that was observed in an independent experiment.