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, fum2.1 , were grown for 8 weeks at a daytime temperature of 20°C, before being 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 1a). Prior to transfer to low temperature, Pmax of Col-0 was slightly higher than that offum2.1 . Following one day at low temperature, the capacity for photosynthesis in Col-0 increased. Pmax continued to increase over the following days, rising to a new steady state approx. 50% higher than the starting value by Day 7 of treatment. This indicates that dynamic acclimation of photosynthesis is occurring in response to cold, with a new steady-state being reached within 7 days under our conditions. In contrast, the Pmax offum2.1 did not vary over the course of the experiment, confirming previous evidence that mutants lacking FUM2 are defective in cold acclimation (Dyson et al. , 2016).
Measurements of the rate of gas exchange in plants under growth conditions , performed towards the end of the first day of cold, showed that 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 1b,c). In Col-0, acclimation of both parameters occurred, such that values recorded at 4°C in plants exposed to cold for 7 days did not differ significantly from those recorded at 20°C prior to acclimation. In contrast, infum2.1 , no recovery occurred. This shows that either fumarate accumulation or FUM2 protein, is essential not only for the acclimation of photosynthesis but also of respiration to cold.