Copyright © Physiologia Plantarum 1999 PHYSIOLOGIA PLANTARUM 105: 89 – 94. 1999 Printed in Ireland —all rights resered ISSN 0031-9317 Thermal stress induces differential degradation of Rubisco in heat-sensitive and heat-tolerant rice Anindita Bose, BS Tiwari, Manas K Chattopadhyay, Sudhiranjan Gupta and Bharati Ghosh* Department of Botany, Bose Institute, 93 /1 Acharya Prafulla Chandra Road, Calcutta 700 009, India *Corresponding author, e -mail: bharati@boseinst.ernet.in Received 12 August 1998 temperature to 50°C favored the degradation of the protein in Degradation of ribulose-1,5-bisphosphate carboxylase/oxyge- nase (Rubisco, EC.4.1.1.39), due to elevation in atmospheric both cultivars. The transcript level of the LSU, as studied by temperature, and extent of enzyme damage, due to varietal Northern blot hybridization, also showed thermostability in N 22, whereas it was thermosensitive in IR 8. Protease activity, differences, were studied by [ 35 S]-methionine pulse-chase, as measured by Western blot analysis using purified Rubisco, transcript level of the large subunit (LSU), and protease revealed the same trend in both cultivars and was correlated activity in two rice cultivars. The cultivars N 22 and IR 8 are with protein turnover by [ 35 S]-methionine and Northern blot certified as thermotolerant and thermosensitive, respectively. analysis. The results indicate that genetic differences exist in Differential responses were observed in both cultivars, with the N 22 showing greater thermostability of the Rubisco protein two rice cultivars and that the heat-tolerant cultivar has a up to 45°C, whereas IR 8 was thermolabile. Elevation of the protective mechanism against thermal degradation of Rubisco. Introduction Temperature is one of the major environmental constraints affecting photosynthetic efficiency and limiting the yield of crop plants (Ghosh et al. 1989, Bose and Ghosh 1995b). Temperature adversely affects ribulose-1,5-bisphosphate car- boxylase/oxygenase (Rubisco, EC 4.1.1.39) (Grover et al. 1986). The synthesis and maintenance of Rubisco are gov- erned in a complex manner by the cooperation of both the nuclear and chloroplast genomes (Miziorko and Lorimer 1983). The Rubisco level in the leaf is balanced by its synthesis and degradation (Mae et al. 1983), which is di- rectly related to photosynthetic efficiency (Wittenbach et al. 1982). The synthesis of Rubisco or either of its subunits appears to be very sensitive to environmental factors, such as infection by pathogens (Higgins et al. 1985), heat shock (Vierling and Key 1985), low temperature (Meza-Basso et al. 1986), and oxidative stress (Mehta et al. 1992). The drastic reduction in the synthesis of the large and small subunits (LSU and SSU) of Rubisco, as well as its mRNA level, was reported in rice seedlings under low temperature stress by Hahn and Walbot (1989). One of the major changes induced by high temperature is the denaturation of proteins by endogenous proteases (Mas- carenhas 1984). Robinson and Ellis (1984a,b) have purified one of the proteases from pea chloroplasts stroma. In rye seedlings, in plastids lacking ribosomes due to growth at 32°C, unassembled SSUs were degraded by proteolytic ac- tivity at pH 2–3 (Feierabend et al. 1990). Another protease, EP1, has been isolated from pea chloroplasts (Bushnell et al. 1993). In vitro, this stromally localized metalloprotease de- grades the LSU of Rubisco to a smaller polypeptide of 36 kDa, suggesting that Rubisco is an in vivo substrate of this protease (Bushnell et al. 1993). Rice is a staple crop and its yield is also affected by fluctuations in temperature. We reported earlier that there is greater inhibition of Rubisco activity and its holozyme in a sensitive cultivar (IR 8) than in a tolerant cultivar (N 22) under high temperature (Bose and Ghosh 1995a). The quan- titative changes of Rubisco under stress is thought to be associated with the degradation of this protein by endoge- neous proteases, coupled with loss of synthesis. Apart from Abbreiations – LSU: large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase; Rubisco: ribulose-1,5-bisphosphate carboxylase/ oxygenase; SSU: small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase. Physiol. Plant. 105, 1999 89