Thermal stress induces differential degradation of Rubisco in heat-sensitive and heat-tolerant rice

Degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC .4.1.1.39), due to elevation in atmospheric temperature, and extent of enzy me damage, due to varietal differences, were studied by [S-35]-methionine p ulse-chase, transcript level of the large subunit (LSU), and protease activ ity in two rice cultivars. The cultivars N 22 and IR 8 are certified as the rmotolerant and thermosensitive, respectively. Differential responses were observed in both cultivars, with the N 22 showing greater thermostability o f the Rubisco protein up to 45 degrees C, whereas IR 8 was thermolabile. El evation of the temperature to 50 degrees C favored the degradation of the p rotein in both cultivars, The transcript level of the LSU, as studied by No rthern blot hybridization, also showed thermostability in N 22, whereas it was thermosensitive in IR 8, Protease activity, as measured by Western blot analysis using purified Rubisco, revealed the same trend in both cultivars and was correlated with protein turnover by [S-35]-methionine and Northern blot analysis. The results indicate that genetic differences exist in two rice cultivars and that the heat-tolerant cultivar has a protective mechani sm against thermal degradation of Rubisco.