Temperature dependence of photosynthesis in Arabidopsis plants with modifications in Rubisco activase and membrane fluidity.

Net photosynthesis (Pn) is reversibly inhibited at moderately high temperature. To investigate this further, we examined the effects of heat stress on Arabidopsis plants in which Rubisco activase or thylakoid membrane fluidity has been modified. During heating leaves from 25 to 40 degrees C at 250 ppm CO2 and 1% O2, the wild-type (WT), plants expressing the 43 kDa isoform only (rwt43), and plants accumulating activase 40% of WT (R100) exhibited similar inhibitions in the Pn and Rubisco activation state. Despite better membrane integrity than WT, plants having less polyunsaturation of thylakoid lipids (fad7/8 double mutant) failed to maintain greater Pn than the WT. Plants expressing the 46 kDa isoform only (rwt46) exhibited the most inhibition, but plants expressing a 46 kDa isoform incapable of redox regulation (C411A) were similar to the WT. The null mutant (rca) exhibited a continuous decline in Pn. As measured by fluorescence, electron transport activity decreased concomitantly with Pn but PSII was not damaged. Following a quick recovery to 25 from 40 degrees C, whereas most lines recovered 90% Pn, the rwt46 and rca lines recovered only to 59 and <10%, respectively. As measured by NADP-malate dehydrogenase activation, after an initial increase at 30 degrees C, stromal oxidation in the WT and rwt46 plants did not increase further as Pn decreased. These results provide additional insight into the role of Rubisco activation and activase in the reversible heat inhibition of Pn.