Modification of Rubisco and Altered Proteolytic Activity in O3-Stressed Hybrid Poplar (Populus maximowizii x trichocarpa)

Exposing hybrid poplar (Populus maximowizii x trichocarpa) plants to ozone (O3) resulted in an acceleration of the visual symptoms of senescence and a decrease in the activity and quantity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Whole plants, crude leaf extracts, and isolated intact chloroplasts of hybrid poplar clone 245 were used to test the hypothesis that O3-induced structural modifications of Rubisco affect the activity of this key photosynthetic enzyme. Proteolytic activity, per se, could not account for losses in Rubisco; acidic and alkaline protease activities declined or were unaffected in foliage of O3-treated poplar saplings. In vitro treatment of leaf extracts with O3 decreased total Rubisco activity and binding of the enzyme's transition-state analog, 2-carboxyarabinitol bisphosphate. Additionally, O3 increased the loss of Rubisco large subunit (LSU) when extracts were incubated at 37deg]C. Treatment of isolated intact chloroplasts with O3 accelerated both the loss of the 55-kD Rubisco LSU and the accumulation of Rubisco LSU aggregates, as visualized by immunoblotting. The time-dependent modification in Rubisco structure was the primary response of the isolated organelles to O3 treatment, with little proteolytic degradation of the LSU detected.