Disruption of the mitochondrial alternative oxidase (AOX) and uncoupling protein (UCP) alters rates of foliar nitrate and carbon assimilation in Arabidopsis thaliana

Under high light, the rates of photosynthetic CO(2) assimilation can be influenced by reductant consumed by both foliar nitrate assimilation and mitochondrial alternative electron transport (mAET). Additionally, nitrate assimilation is dependent on reductant and carbon skeletons generated from both the chloroplast and mitochondria. However, it remains unclear how nitrate assimilation and mAET coordinate and contribute to photosynthesis. Here, hydroponically grown Arabidopsis thaliana T-DNA insertional mutants for alternative oxidase (AOX1A) and uncoupling protein (UCP1) fed either NO(3) (–) or NH(4) (+) were used to determine (i) the response of NO(3) (–) uptake and assimilation to the disruption of mAET, and (ii) the interaction of N source (NO(3) (–) versus NH(4) (+)) and mAET on photosynthetic CO(2) assimilation and electron transport. The results showed that foliar NO(3) (–) assimilation was enhanced in both aox1a and ucp1 compared with the wild-type, suggesting that foliar NO(3) (–) assimilation is probably driven by a decreased capacity of mAET and an increase in reductant within the cytosol. Wild-type plants had also higher rates of net CO(2) assimilation (A (net)) and quantum yield of PSII (ϕ(PSII)) under NO(3) (–) feeding compared with NH(4) (+) feeding. Additionally, under NO(3) (–) feeding, A (net) and ϕ(PSII) were decreased in aox1a and ucp1 compared with the wild type; however, under NH(4) (+) they were not significantly different between genotypes. This indicates that NO(3) (–) assimilation and mAET are both important to maintain optimal rates of photosynthesis, probably in regulating reductant accumulation and over-reduction of the chloroplastic electron transport chain. These results highlight the importance of mAET in partitioning energy between foliar nitrogen and carbon assimilation.