Bundle sheath proteins are more sensitive to oxidative damage than those of the mesophyll in maize leaves exposed to paraquat or low temperatures

In maize leaves growth at low temperatures causes decreases in maximum cata lytic activities of photosynthetic enzymes and reduced amounts of proteins, rather than effects on regulation or co-ordination of the photosynthetic p rocesses. To test the hypothesis that differential localization of antioxid ants between the different types of photosynthetic cell in maize leaves is a major determinant of the extreme sensitivity of maize leaves to chilling damage, oxidative damage to proteins, induced by incubation of maize leaves with paraquat, has been measured and compared with the effects incurred by growth at low temperatures. While the increase in protein carbonyl groups caused by paraquat treatment was much greater than that caused by low tempe rature growth conditions, most carbonyl groups were detected on bundle shea th proteins in both stress conditions, With one or two exceptions proteins located in the mesophyll tissues were free of protein carbonyl groups in bo th situations. Paraquat treatment caused a complete loss of the psaA gene p roducts, modified the photosystem II reaction centre polypeptide, D1, and i ncreased the number of peptides arising from breakdown of ribulose 1,5-bisp hosphate carboxylase oxygenase (Rubisco), In contrast, growth at 15 degrees C increased the abundance (but not number) of Rubisco breakdown products a nd decreased that of the psaB gene product while the psaA gene product and PEP carboxylase were largely unaffected. Since bundle sheath proteins are m ore susceptible to oxidative damage than those located in the mesophyll cel ls, strategies for achieving a more balanced system of antioxidant defence may be effective in improving chilling tolerance in maize.