An increase of ornithine δ-aminotransferase-mediated proline synthesis in relation to high-temperature injury in Gracilaria tenuistipitata (Gigartinales, Rhodophyta)

Abstract

Changes of proline biosynthesis in relation to high-temperature (35°C) injury were investigated in Gracilaria tenuistipitata var. liui Zhang et Xia. On exposure to 35°C, the specific growth rate decreased after 5 days while free proline levels increased gradually after 2 days and reached the maximal level on days 4-6 but decreased at day 7. The repair ability of thalli treated at 35°C by measuring the growth rate after transfer to 25°C for another 5 days decreased in thalli that had been grown at 35°C for more than 2 days, and the extent increased as treatment at 35°C was prolonged. After 4 days of treatment at 35°C, the activities of both ornithine δ-aminotransferase (δ-OAT; EC 2.6.11.3) and Δ1-pyrroline-5-carboxylate reductase (P5CR; EC 1.5.1.2) increased, but that of γ-glutamyl kinase (γ-GK; EC 2.7.2.11) remained unchanged, and that of glutamate-5-semialdehyde dehydrogenase (GSAd; EC 1.4.1.3) decreased. The application of 10 μM gabaculine, an irreversible inhibitor of δ-OAT, at 35°C recovered the growth ability but inhibited the increase of both δ-OAT activity and free proline level; its effects were reversed by 1 mM proline. G. saliconia, which is relatively tolerant to high temperature, showed a decrease of both 8-OAT activity and free proline level at 35°C. It seems that a stimulation of proline synthesis from the ornithine pathway via an increase in both δ-OAT and P5CR activities might be associated with high-temperature injury in G. tenuistipitata.