Effect of activase level and isoform on the thermotolerance of photosynthesis in Arabidopsis.

Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activation decreases under moderate heat stress. This decrease is caused by an impairment of activase function, which is exacerbated by faster rates of Rubisco deactivation at elevated temperatures. To determine if stromal oxidation causes inhibition of activase, transgenic Arabidopsis plants expressing suboptimal amounts of either the redox-regulated 46 kDa alpha- or non-redox regulated 43 kDa beta-isoform of activase were examined. Photosynthesis, as measured by gas exchange and chlorophyll fluorescence, and Rubisco activation were inhibited to a much greater extent by moderately high temperatures in the two transgenic lines expressing suboptimal levels of the individual isoforms of activase compared with wild-type plants or transgenic plants expressing levels of the beta-isoform sufficient for wild-type rates of photosynthesis. Net photosynthesis and Rubisco activation in transgenic plants expressing suboptimal amounts of the beta-isoform of activase from the Antarctic hairgrass were even more sensitive to inhibition by moderate heat stress than in the transgenic plants containing Arabidopsis activase. The results demonstrate that photosynthesis exhibits a similar sensitivity to inhibition by moderately high temperature in plants expressing either of the two different isoforms of activase. Thus, impairment of activase function under heat stress is not caused by oxidation of the redox-sensitive sulphydryls of the alpha-isoform of activase. Instead, the results are consistent with thermal denaturation of activase under moderate heat stress, the effects of which on Rubisco activation would be enhanced when activase levels are suboptimal for photosynthesis.