The mechanism by which the mitochondrial alternative oxidase (AOX) pathway 32 contributes to PSII photoprotection is in dispute. The current consensus is that the 33 AOX pathway protects photosystems by dissipating excess reducing equivalents that 34 are exported from chloroplasts through the malate/oxaloacetate (Mal/OAA) shuttle, 35 preventing the over-reduction of chloroplasts. Here, using the aox1a Arabidopsis 36 mutant and nine other C3 and C4 species, an additional model of the AOX pathway 37 in PSII photoprotection is proposed. Although the AOX pathway contributes to PSII 38 photoprotection in C3 leaves treated with high light, this contribution was observed to 39 disappear when photorespiration was suppr... More
The mechanism by which the mitochondrial alternative oxidase (AOX) pathway 32 contributes to PSII photoprotection is in dispute. The current consensus is that the 33 AOX pathway protects photosystems by dissipating excess reducing equivalents that 34 are exported from chloroplasts through the malate/oxaloacetate (Mal/OAA) shuttle, 35 preventing the over-reduction of chloroplasts. Here, using the aox1a Arabidopsis 36 mutant and nine other C3 and C4 species, an additional model of the AOX pathway 37 in PSII photoprotection is proposed. Although the AOX pathway contributes to PSII 38 photoprotection in C3 leaves treated with high light, this contribution was observed to 39 disappear when photorespiration was suppressed. AOX pathway inhibition (mutant or 40 inhibitor) significantly decreased photorespiration in C3 leaves. Moreover, the AOX 41 pathway did not respond to high light or contribute to PSII photoprotection in C4 42 leaves possessing a highly active Mal/OAA shuttle with little photorespiration. These 43 results demonstrate that the AOX pathway contributes to PSII photoprotection in C3 44 plants by maintaining photorespiration to detoxify glycolate and via the indirect export 45 of excess reducing equivalents from chloroplasts by the Mal/OAA shuttle. This new 46 model explains why the AOX pathway does not contribute to PSII photoprotection in 47 C4 plants.
