Further Studies on O2-Resistant Photosynthesis and Photorespiration in a Tobacco Mutant with Enhanced Catalase Activity

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Abstract
The increase in net photosynthesis in M4 progeny of an O2-resistant tobacco (Nicotiana tabacum) mutant relative to wild-type plants at 21 and 42% O2 has been confirmed and further investigated. Self-pollination of an M3 mutant produced M4 progeny segregating high catalase phenotypes (average 40% greater than wild type) at a frequency of about 60%. The high catalase phenotype cosegregated precisely with O2-resistant photosynthesis. About 25% of the F1 progeny of reciprocal crosses between the same M3 mutant and wild type had high catalase activity, whether the mutant was used as the maternal or paternal parent, indicating nuclear inheritance. In high-catalase mutants the activity of NADH-hydroxypyruvate reductase, another peroxisomal enzyme, was the same as wild type. The mutants released 15% less photorespiratory CO2 as a percent of net photosynthesis in CO2-free 21% O2 and 36% less in CO2-free 42% O2 compared with wild type. The mutant leaf tissue also released less 14CO2 per [1-14C]glycolate metabolized than wild type in normal air, consistent with less photorespiration in the mutant. The O2-resistant photosynthesis appears to be caused by a decrease in photorespiration especially under conditions of high O2 where the stoichiometry of CO2 release per glycolate metabolized is expected to be enhanced. The higher catalase activity in the mutant may decrease the nonenzymatic peroxidation of keto-acids such as hydroxypyruvate and glyoxylate by photorespiratory H2O2.