METHIONYL SULFOXIDE CONTENT AND PROTEIN-METHIONINE-S-OXIDE REDUCTASE-ACTIVITY IN RESPONSE TO WATER DEFICITS OR HIGH-TEMPERATURE

Cellular injury resulting from partially reduced oxygen species (super oxide, peroxides and/or hydroxyl radicals) or singlet oxygen frequentl y increases during environmental stress. Because protein methionine re sidues are susceptible to oxidation, we investigated the effects of wa ter-deficit stress and high temperature stress on the content of oxidi zed methionyl residues [Met(O)] in leaves. Leaf proteins from water-de ficit-stressed cotton (Gossypium hirsutum L. cv. Paymaster HS-26), pea (Pisum sativum L. cv. Progress No. 9), wheat (Triticum aestivum L. em . Thell. cv. Len) and potato (Solanum tuberosum L. cv. Norgold M) and from the leaves of high-temperature-stressed pea seedlings were evalua ted. The activity of protein methionine-S-oxide reductase (PrMSR), an enzyme responsible for re-reducing oxidized methionyl residues, was al so determined. Protein Met(O) content did not change in response to ei ther water-deficit or high temperature stress. PrMSR activity decrease d in pea and cotton leaves, remained unchanged in potato leaves and si gnificantly increased in leaves of water-deficit-stressed wheat. The f indings demonstrate that these plants have developed protection system s that effectively maintain stable levels of oxidized methionyl residu es in leaf proteins despite exposure to severe water and high temperat ure stress. The findings also suggest that changes in PrMSR activity d o not fully account for the observed maintenance of protein methionyl sulfoxide content at constant levels.