Thermal stress responses and temperature-dependent expression of glutamate dehydrogenase isozymes in a sterile mutant of Ulva pertusa (Chlorophyta)

The thermal stress responses of a sterile mutant of Ulva pertusa were investigated at 20 C and 30 C. The amounts of the photosynthetic pigments and the major free amino acids increased 1.4-2.4 and 1.9-10.5 fold, respectively, in response to thermal stress. Total carbon and nitrogen contents also increased in the 30 C-cultivated alga. The changes in chemical components due to thermal stress account for the morphological changes such as darkening of chloroplasts, thickened cell walls, and enlarged cytoplasm in the 30 C-cultivated alga. Isozyme assay showed that both temperature-treated algae possessed NAD+ and NADP+-specific glutamate dehydrogenase (NAD/NADP-GDH) and that the 30 C-cultivated alga had an additional NADP+-specific GDH (NADP-GDH). Two cDNA clones encoding GDH-L and GDH-S were isolated from the 30 C-cultivated alga. mRNA encoding GDH-L was constitutively expressed in the algae, whereas expression of that encoding GDH-S was induced by thermal stress. These results suggest that NADP-GDH compensates for the drop in NAD/NADP-GDH activity under thermal stress conditions and that NAD/NADP-GDH and NADP-GDH in the alga correspond to GDH-L and GDH-S, respectively.