ALUMINUM RESISTANCE IN WHEAT (TRITICUM-AESTIVUM) IS ASSOCIATED WITH RAPID, AL-INDUCED CHANGES IN ACTIVITIES OF GLUCOSE-6-PHOSPHATE-DEHYDROGENASE AND 6-PBOSPHOGLUCONATE DEHYDROGENASE IN ROOT APICES

We have investigated the effect of aluminum (Al) on the activity of gl ucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluc onate dehydrogenase (6PGDH; EC 1.1.1.44) isolated from 5-mm root apice s of 4-day-old wheat (Triticum aestivum) cultivars differing in resist ance to Al. Rapid increases in G6PDH and 6PGDH activities were observe d in Al-resistant cultivars (PT741 and Atlas 66) during the first 10 h of treatment with 100 mu M Al, while no change in the activity of eit her enzyme was observed in Al-sensitive cultivars (Katepwa and Neepawa ) during a 24-h exposure to Al. The Al-induced increases in enzyme act ivities observed in the Al-resistant PT741 appear to reflect an induct ion of protein synthesis since the increases were completely abolished by 1 mM cycloheximide. No differences in G6PDH and 6PGDH activities w ere observed between the Al-sensitive and the Al-resistant genotypes w hen Al was supplied in vitro. Under these conditions, an increase in A l concentration from 0 to 1.4 mM caused a gradual decrease in activity of both enzymes, irrespective of the Al-resistance of whole seedlings . Aluminum-sensitive and aluminum-resistant cultivars also differed in the rate and extent of accumulation of slowly-exchanging Al in 5-mm r oot apices. During the first 6 h of Al treatment, Al accumulation was only 10% more rapid in Katepwa than in PT741. After 24-h exposure, acc umulation in the Al-sensitive Katepwa, was two-fold higher. A decline in Al accumulation in a slowly-exchanging compartment as well as a dec rease in activities of G6PDH and 6PGDH were found in the Al-resistant PT741, when seedlings were transferred to Al-free treatment solutions after 16-h exposure to 100 mu M Al. These results suggest that rapid i nduction of G6PDH and 6PGDH in the Al-resistant line PT741 by Al may p lay a role in the mechanism of Al resistance, possibly by regulation o f the pentose phosphate pathway.