OVER-EXPRESSION OF CHLOROPLAST-TARGETED MN SUPEROXIDE-DISMUTASE IN COTTON (GOSSYPIUM-HIRSUTUM L., CV COKER-312) DOES NOT ALTER THE REDUCTION OF PHOTOSYNTHESIS AFTER SHORT EXPOSURES TO LOW-TEMPERATURE AND HIGH LIGHT-INTENSITY

Transgenic cotton plants from several independently-transformed lines expressing a chimeric gene encoding a chloroplast-targeted Mn superoxi de dismutase (SOD) from tobacco exhibit a three-fold increase in the t otal leaf SOD activity, strong Mn SOD activity associated with isolate d chloroplasts, and a 30% and 20% increase in ascorbate peroxidase and glutathione reductase activities, respectively. The Mn SOD plants did exhibit a slightly enhanced protection against light-mediated, paraqu at-induced cellular damage but only at 0.3 mu M paraquat. In addition, photosynthetic rates at 10 degrees C and 15 degrees C were similar to those of controls, and the immediate recovery of photosynthesis after a 35-min exposure to 5 degrees C and full sun was only slightly bette r than that for wild-type plants. The recovery for longer exposure tim es was comparable for both genotypes as was the deactivation of the H2 O2-sensitive, Calvin-cycle enzyme, stromal fructose 1,6-bisphosphatase (FBPase). Compared to the controls, Mn SOD plant leaves in full sun p rior to chilling stress had a lower activation of FBPase, a higher rat io of oxidized to reduced forms of ascorbate, and a higher total gluta thione content. After 35 min at 5 degrees C in full sunlight, total gl utathione had risen in control leaves to 88% of the Mn SOD plant value s, and oxidized to reduced ascorbate ratios were higher for both genot ypes. However, an 80% increase in the ratio of oxidized to reduced glu tathione occurred for Mn SOD plant leaves with no change for controls. This increased demand on the ascorbate-glutathione cycle is circumsta ntial evidence that high Mn SOD activity in the chloroplast leads to i ncreased H2O2 pools that could, in some manner, affect photosynthetic recovery after a stress period. We postulate that the pool sizes of re duced ascorbate and glutathione may restrict the ability of the ascorb ate-glutathione cycle to compensate for the increased activity of SOD in cotton over-producing mitochondrial Mn SOD in chloroplasts during s hort-term chilling/high light stress.