INHIBITION OF THE IRON-INDUCED ZMFER1 MAIZE FERRITIN GENE-EXPRESSION BY ANTIOXIDANTS AND SERINE THREONINE PHOSPHATASE INHIBITORS/

Two pathways have been implicated in the regulation of maize ferritin synthesis in response to iron. One of them involves the plant hormone abscisic acid (ABA) and controls the expression of ZmFer2 gene(s). Ano ther pathway, ABA-independent, has been characterized in a de-rooted m aize plantlet system and involves an oxidative step, The ZmFer1 maize ferritin gene is not regulated by ABA, and it is shown in this paper t hat the corresponding mRNA accumulates in de-rooted maize plantlets an d EMS (Black Mexican Sweet) maize cell suspension cultures in response to iron via the oxidative pathway described previously. To investigat e ZmFer1 gene regulation further, the EMS cell system has been used to develop a transient expression assay using a ZmFer1-beta-glucuronidas e fusion, Both iron induction and antioxidant inhibition of ZmFer1 gen e expression were observed in this system. Using Northern blot analysi s and transient expression experiments, it was shown that both okadaic acid and calyculin A, two serine/threonine phosphatase inhibitors, sp ecifically inhibit ZmFer1 gene expression, These data indicate that an okadaic acid-sensitive protein phosphatase activity is involved in th e regulation of the ZmFer1 ferritin gene in maize cells, and this acti vity is required for iron-induced expression of this gene.