High temperature stress increases the expression of wheat leaf ribulose-1,5-bisphosphate carboxylase/oxygenase activase protein

The effect of high temperature stress on the expression of ribulose-1,5-bis phosphate carboxylase/oxygenase (Rubisco) activase was examined in wheat (T riticum aestivum L.) leaves, which normally possess 46- and 42-kDa activase forms. Heat stress at 38 degreesC significantly reduced total activase mRN A levels compared to controls, and recovery of activase transcription was o nly marginal 24 h after alleviating heat stress. In contrast to transcript abundance, immunoblot analysis indicated that heat stress increased the acc umulation of the 42-kDa activase and induced a putative 41-kDa form. Heat s tress did not affect the amounts of the 46- and 42-kDa activase forms (pres ent as 51- and 45-kDa preproteins) recovered after their immunoprecipitatio n from in vitro translation products. De novo protein synthesis in vivo in the presence of [S-35]Met/Cys showed an increase in the amount of newly syn thesized 42-kDa subunit after 4 h of heat stress, and synthesis of the puta tive 41-kDa activase was apparent. In contrast to activase, heat stress led to a rapid and large reduction in the de novo synthesis of the large and s mall subunits of Rubisco, Long-term (48-h) heat stress further increased th e amounts of de novo synthesized 42- and 41-kDa activase forms. After 24 h of recovery from heat stress, de novo synthesis of the 42-kDa activase retu rned to control levels, while a small amount of 41-kDa protein was still ex pressed. Southern analysis suggested the presence of a single activase gene , These results indicate that heat stress alters activase expression, most likely posttranscriptionally, and suggest that the heat-induced expression of the 42- and 41-kDa subunits of wheat leaf Rubisco activase may be relate d to the maintenance and acclimation of photosynthetic CO2 fixation during high temperature stress in wheat. (C) 2001 Academic Press.