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

Abstract

Under high light, the rates of photosynthetic CO2 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 NO3 (-) or NH4 (+) were used to determine (i) the response of NO3 (-) uptake and assimilation to the disruption of mAET, and (ii) the interaction of N source (NO3 (-) versus NH4 (+)) and mAET on photosynthetic CO2 assimilation and electron transport. The results showed that foliar NO3 (-) assimilation was enhanced in both aox1a and ucp1 compared with the wild-type, suggesting that foliar NO3 (-) 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 CO2 assimilation (A net) and quantum yield of PSII (ϕPSII) under NO3 (-) feeding compared with NH4 (+) feeding. Additionally, under NO3 (-) feeding, A net and ϕPSII were decreased in aox1a and ucp1 compared with the wild type; however, under NH4 (+) they were not significantly different between genotypes. This indicates that NO3 (-) 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.

Keywords: Alternative oxidase; ammonium; energy balancing; nitrate assimilation; reductant; uncoupling protein..

Fig. 1.

Rates of foliar NO₃ ^– uptake and assimilation, and free NO₃ ^– content in wild-type, aox1a and ucp1 shoots of A. thaliana fed with ¹⁵NO₃ ^– for 6h. Plants were exposed to either growth (A, 160 μmol quanta m^–2 s^–1) or saturating (B, 1000 μmol quanta m^–2 s^–1) light conditions during the feeding. Shown are the means ±SE of measurements made on five plants. An asterisk denotes a significant difference (P<0.05) between genotypes.

Fig. 2.

Changes in the content of 19 different amino acids in aox1a and ucp1 leaves of A. thaliana fed with either NO₃ ^– or NH₄ ⁺ as sole N source and exposed to either growth (160 μmol quanta m^–2 s^–1) or saturating (1000 μmol quanta m^–2 s^–1) irradiance for 6h. Values represent the log2 of the mutant to WT ratio. Values in bold indicate differences statistically significantly different between the WT and mutants. (This figure is available in colour at JXB online.)

Fig. 3.

Net CO₂ assimilation rate (A _net), quantum yield (ϕ_PSII), and excitation pressure (1–qP) in response to light intensity in wild-type, aox1a, and ucp1 leaves of A. thaliana fed either NO₃ ^– or NH₄ ⁺ as sole N source. Shown are the means of measurements made on three plants. The inset box within each panel presents the grand mean with the standard error, estimated from the MSE term in the ANOVA. Significant differences (P<0.05) were denoted by an asterisk either in the inset box when the genotype effect was significant or on the graph to indicate a significant genotype×irradiance interaction.

Fig. 4.

Net CO₂ assimilation rate (A _net), quantum yield (ϕ_PSII), and excitation pressure (1–qP) in response to O₂ atmospheric partial pressure in wild-type, aox1a, and ucp1 leaves of A. thaliana fed with either NO₃ ^– or NH₄ ⁺ as sole N source. Shown are the means ±SE of measurements made on three plants. The inset box within each panel presents the grand mean with the standard error, estimated from the MSE term in the ANOVA. An asterisk denotes a significant difference (P<0.05) between genotypes.

Fig. 5.

Correlation between the electron transport rate through PSII (J _f) and the electron transport rate required for CO₂ assimilation and photorespiration (J _g) in wild-type, aox1a, and ucp1 leaves of A. thaliana fed with either NO₃ ^– or NH₄ ⁺ as sole N source. Correlation was established in response to light intensity (A; from 2000 to 20 μmol quanta m^–2 s^–1) and O₂ partial pressure (B; from 46.6 to 1.9 kPa pO₂).