Acclimation of both photosynthesis and respiration to cold are impaired in plants lacking FUM2
Plants of the wild-type Arabidopsis, accession Col-0 and a mutant in the same background, fum2.2 , were grown for 8 weeks at a daytime temperature of 20°C. Plants were then transferred to a growth cabinet with the same light conditions, but with a temperature of 4°C. Photosynthetic capacity (Pmax; measured at 20°C in saturating light and CO2) of these plants was measured over the following 9 days (Figure 1 a). Prior to transfer to low temperature, Pmax of Col-0 was slightly higher than that of fum2.2 . Following one day at low temperature, the capacity for photosynthesis (Pmax) in Col-0 increased. Pmax continued to increase over the following days, rising to a new steady state approximately 50% higher than the starting value by the sixth day of cold treatment. This indicates that, under our experimental conditions, dynamic acclimation of photosynthesis occurs in response to cold and that a new steady-state is reached within 7 days. In contrast, the Pmax of fum2.2 did not vary over the course of the experiment, confirming previous evidence that bothfum2.1 and fum2.2 mutants lacking FUM2 are unable to acclimate their photosynthetic capacity in response to cold (Dysonet al. , 2016).
Measurements of the rate of gas exchange achieved by plants in growth cabinets, with ambient CO2, light and temperature, were performed in the last hour of the first day of cold. Transfer to cold resulted in a small but significant inhibition of both photosynthesis and respiration at the end of the first day of exposure to cold (ANOVA, P<0.05; Figure 1 b,c). In Col-0, acclimation of both parameters had occurred after 7 days, such that in situ rates of gas exchange recovered and did not differ significantly from those recorded at 20°C prior to acclimation. In contrast, in fum2.2 , no recovery occurred in either photosynthesis or respiration. This suggests that either fumarate accumulation or FUM2 protein, is essential for the acclimation of both photosynthesis and respiration to cold.