REGULATION OF CYTOSOLIC PYRUVATE, ORTHOPHOSPHATE DIKINASE EXPRESSION IN DEVELOPING MAIZE ENDOSPERM

Pyruvate orthophosphate dikinase (PPDK, E.C. 2.7.9.1) is an abundant e nzyme in the leaves of C4 plants associated with the dicarboxylic acid pathways of CO2 fixation in the dark. PPDK activity has also been det ected in the seeds of maize and other, non-C4 cereals, where its role has yet to be established. Using an anti-PPDK serum, two cross-reactin g species of M(r) close to 90 000 were detected in developing maize en dosperm of wild-type plants. In two independent opaque-2 mutant lines, one of the polypeptides was absent and the other was reduced in level . Similarly, endosperm PPDK mRNA levels were greatly reduced in the op aque-2 maize lines compared to wild type, suggesting that endosperm PP DK gene expression is under Opaque-2 control. However, a low level of PPDK mRNA could still be detected in these mutants, indicating that PP DK gene expression is not absolutely dependent on Opaque-2 but rather can be modulated by it. This interpretation was reinforced by the demo nstration that the distribution of PPDK transcripts is not affected in o2 mutants, although the level is reduced, and that PPDK mRNA is dete ctable prior to 02 mRNA during the maturation of wild-type maize endos perm. Using oligonucleotides specific for the different maize PPDK gen es, the 02 mutations were shown to affect only cyPPDKZml gene expressi on in maize line A69Y.