Photorespiratory metabolism differs between gymnosperm conifers and angiosperms

In C₃ photosynthetic plants, which include most herbaceous and woody species, the CO₂-fixing enzyme Rubisco reacts with atmospheric O₂ in a process known as photorespiration. Photorespiration produces CO₂, which reduces net CO₂ fixation rates, but consumes significant amounts of ATP and reducing power, thereby acting as an electron sink that prevents the production of harmful reactive oxygen species. It also produces NH₃, which is assimilated by chloroplast-localized glutamine synthetase (GS2) in the leaves of angiosperm C₃ species such as broad-leaved trees and herbaceous plants. In contrast, GS2 is absent from the leaves of gymnosperm conifers and is functionally replaced by a cytosolic enzyme, GS1a. A recent study suggested that the activity of catalase (CAT), a peroxisome-localized enzyme involved in angiosperm C₃ photorespiratory metabolism, is remarkably low in conifer peroxisomes. It also indicated that low peroxisomal CAT activity gives rise to a low electron sink capacity and increases CO₂ production in conifer photorespiratory metabolism. The subcellular localization of enzymes such as GS and CAT may have played a crucial role in the evolution of the photorespiratory functions in angiosperms. Additionally, conifers possess an enzyme, flavodiiron (FLV), which may function as an additional electron sink, whereas angiosperms lack FLV. An analysis of a global leaf Δ¹³C dataset revealed a significant difference in Δ¹³C values between angiosperm C₃ and conifer species, suggesting an inherent difference in gas exchange and metabolism between these two groups. Given these findings, the implicit assumption of a common photorespiratory metabolism in C₃ angiosperms and conifers should be revised.