Sensitivity of Superoxide Dismutase Transcript Levels and Activities to Oxidative Stress Is Lower in Mature-Senescent Than in Young Barley Leaves

Antioxidant enzyme activities are inducible by oxidative stress and decrease during senescence. To determine if the age-dependent decrease of superoxide dismutase (SOD) activities is due to decreased sensitivity to oxidative stress, we have investigated the changes in steady-state levels of transcripts and activities of mitochondrial Mn-SOD (SOD1), chloroplastic Fe-SOD (SOD2), and cytoplasmic Cu-Zn-SOD (SOD3) in young and mature-senescent detached barley (Hordeum vulgare L.) leaves in response to incubation in darkness, growth light (20 W m-2), and photooxidative stress conditions (100 W m-2 with 21 or 100% O2). For a comparison, changes in the mRNA for ribulose bisphosphate carboxylase were also measured. After leaf detachment, the abundance of all three SOD mRNAs increased, then decreased and eventually stabilized after 6 h of incubation. After 20 h of incubation under darkness SOD transcripts decreased in both young and mature-senescent leaves. While under strong photooxidative stress the levels of the three SOD transcripts significantly increased in young leaves; in mature-senescent leaves SOD2 and, to lesser extent, SOD1 and SOD3 transcripts decreased. Generally, SOD activity changes were similar to those of mRNAs. It is proposed that oxidative damage during senescence could be favored by the inability of senescing leaves to modulate the steady-state level of SOD mRNA, and probably those of other antioxidant enzymes, concomitant with the rate of oxyradical formation.