Differential response to elevated NaCl by antioxidant enzymes and gene transcripts in two contrasting lettuce genotypes.

Enzyme activities and transcript profiles of major antioxidant enzymes in two contrasting cultivars of lettuce seedlings (Lactuca sativa L.) were profiled after a 15 day period growing in 100 mM and 200 mM NaCl. Total superoxide dismutase activity (SOD) was enhanced in the more saline-tolerant‘Verte de Cobham’ cultivar in response to NaCl, and this was accompanied by increased expression of Fe-SOD and Mn-SOD genes at the two salt concentrations. Transcripts for these two genes were limited in the more salt sensitive ‘Romaine’ cultivar, while CuZn-SOD transcription remained consistent across all salinity levels in both cultivars. Ascorbate peroxidase (APX) enzyme activity was more than 1000-fold lower than total SOD activity in both cultivars in roots and leaves, even though APX enzyme activity increased with salinity in ‘Verte’ leaves. Catalase (CAT) and gaiacol peroxidase (GPX) enzyme activity were ~100-fold lower than total SOD enzyme activity in both leaves and roots, but did not appear to be limiting in salt-sensitive ‘Romaine’ at the lower NaCl concentration. The pattern of root activity for these two latter enzymes differed between the two cultivars, and CAT transcripts disappeared in ‘Romaine’ roots at the higher salt concentration. Our findings suggest that lettuce could benefit from enhancement of antioxidant enzymes through molecular or traditional plant breeding, although clearly such an effort would be of greater benefit to ‘Romaine’.