Nitrate regulation of the oxidative pentose phosphate pathway in maize ( Zea mays L.) root plastids: induction of 6-phosphogluconate dehydrogenase activity, protein and transcript levels

Abstract

We examined the effect of nitrate on the expression of the NADPH producing enzymes of the oxidative pentose phosphate pathway, glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) in maize seedlings ( Zea mays L. W64A×A182E). In extracts of 5 day old maize roots and leaves treated with 10 mM KNO 3, G6PDH and 6PGDH activities increased by 44 and 53%, respectively, relative to untreated roots. In isolated plastids from KNO 3 treated roots, G6PDH and 6PGDH specific activities were more than 25- and 12-fold higher than in the untreated control. Western blot analysis showed higher levels of 6PGDH protein in root plastid extracts from KNO 3 treated plants. The data suggest that KNO 3 specifically affects the plastidic forms of G6PDH and 6PGDH. Three classes of 6PGDH cDNA were identified in maize roots. Of these, one cDNA hybridized with a transcript that accumulated rapidly and transiently in response to low concentrations of external nitrate. The accumulation of this transcript was not affected by pretreating plants with 50 μg/ml cycloheximide, which was previously shown to inhibit cytoplasmic protein synthesis in maize roots by more than 85% (Gowri et al., Plant Mol. Biol. 26 (1998) 679). Neither NH 4 + nor K + treatment affected transcript accumulation. The data indicate coordinated regulation of genes and enzymes required for NO 3 − assimilation and NADPH production in root plastids.