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

We examined the effect of nitrate on the expression of the NADPH produ cing enzymes of the oxidative pentose phosphate pathway, glucose-6-pho sphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PG DH) in maize seedlings (Zea mays L. W64A x A182E). In extracts of 5 da y old maize roots and leaves treated with 10 mM KNO3, G6PDH and 6PGDH activities increased by 44 and 53%, respectively, relative to untreate d roots. In isolated plastids from KNO3 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 6PGD H protein in root plastid extracts from KNO3 treated plants. The data suggest that KNO3 specifically affects the plastidic forms of G6PDH an d 6PGDH. Three classes of 6PGDK cDNA were identified in maize roots. O f these, one cDNA hybridized with a transcript that accumulated rapidl y and transiently in response to low concentrations of external nitrat e. The accumulation of this transcript was not affected by pretreating plants with 50 mu g/ml cycloheximide, which was previously shown to i nhibit cytoplasmic protein synthesis in maize roots by more than 85% ( Gowri et al., Plant Mol. Biol. 26 (1998) 679). Neither NH4+ nor K+ tre atment affected transcript accumulation. The data indicate coordinated regulation of genes and enzymes required for NO3- assimilation and NA DPH production in root plastids. (C) 1998 Elsevier Science Ireland Ltd . All rights reserved.