Abstract
Superoxide dismutases (SODs) protect against reactive oxygen species (ROS) by detoxifying superoxide. Three types of SOD are present in plants: FeSOD, CuSOD, and MnSOD. The Arabidopsis thaliana genome contains three FeSOD genes, in which two (FSD2, and FSD3) are targeted to chloroplast thylakoids. Loss of FSD2 or FSD3 expression impairs growth and causes leaf bleaching. FSD2 and FSD3 form heterocomplexes present in chloroplast nucleoids, raising the question of whether FSD2 and FSD3 are functionally interchangeable. In this study, we examined how loss of FSD2 or FSD3 expression affects photosynthetic processes and whether overexpression of one compensates for loss of the other. Whereas loss of the cytosolic FSD1 had little effect, an fsd2 mutant exhibited increased superoxide production, reduced chlorophyll levels, lower PSII efficiency, a lower rate of CO2 assimilation, but elevated non-photochemical quenching (NPQ). In contrast, fsd3 mutants failed to survive beyond the seedling stage and overexpression of FSD2 could not rescue the seedlings. Overexpression of FSD3 in an fsd2 mutant, however, partially reversed the fsd2 mutant phenotype resulting in improved growth characteristics. Overexpression of FSD2 or FSD3, either individually or together, had little effect. These results indicate that, despite functioning as FeSODs, FSD2 and FSD3 are functionally distinct.
Highlights
Photosynthesis is critical to most plants to capture and convert absorbed light energy into chemical energy, an inevitable consequence of this process is the generation of reactive oxygen species (ROS)
The three types of superoxide dismutases (SODs) present in plants based on their metal cofactors are iron SOD (FeSOD), copper–zinc SOD (Cu/ ZnSOD) and manganese SOD (MnSOD) each located in specific cellular locations including the cytosol, mitochondria, and chloroplasts [3]
As each protein lacks a transmembrane domain, they were predicted to be attached to the stromal side of thylakoid membranes. Because of their association in heterocomplexes and co-localization in within chloroplast nucleoids [11], we examined whether overexpression of FSD3 (FSD3OE) could compensate for loss of FSD2 expression or whether overexpression of FSD2 (FSD2OE) could compensate for loss of FSD3 expression
Summary
Photosynthesis is critical to most plants to capture and convert absorbed light energy into chemical energy, an inevitable consequence of this process is the generation of reactive oxygen species (ROS). As ROS can be highly damaging, plants have evolved several means to manage ROS levels that include enzymatic and nonenzymatic mechanisms [1]. One such mechanism involves superoxide dismutases (SODs) which reduce superoxide to hydrogen peroxide (H2O2) that in turn is converted to water by catalase [2]. The three types of SOD present in plants based on their metal cofactors are iron SOD (FeSOD), copper–zinc SOD (Cu/ ZnSOD) and manganese SOD (MnSOD) each located in specific cellular locations including the cytosol, mitochondria, and chloroplasts [3]. Three FeSOD genes (FE SUPEROXIDE DISMUTASE) are present in the genome of Arabidopsis thaliana (FSD1, FSD2, and FSD3) [4].
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