Identification of a maize root transcript expressed in the primary response to nitrate: characterization of a cDNA with homology to ferredoxin-NADP+ oxidoreductase

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Abstract
To more fully understand the biochemical and molecular events which occur in plants exposed to nitrate, cDNAs whose accumulation was enhanced in nitrate- and cycloheximide-treated maize (Zea mays L. W64A × W182E) roots were isolated. The 340 bp Zmrprn 1 (for Zea mays root primary response to nitrate) cDNA also hybridized with a probe enriched for nitrate-induced sequences, and was characterized further. Sequence analysis of a near full-length cDNA (Zmrprn 1A) showed strong homology (>90% amino acid identity) with a root ferredoxin-NADP+ oxidoreductase (FNR) of rice, and 45–50% amino acid identity with leaf FNR genes. When expressed in Escherichia coli, the Zmrprn 1A cDNA produced a protein with NADPH: ferricyanide reductase activity, consistent with the enzymatic properties of an FNR. The Zmrprn 1 cDNA hybridized with a 1.4 kb transcript which was expressed in the maize root primary response to nitrate. That is, mRNA levels in roots increased rapidly and transiently in response to external nitrate, and low levels of nitrate (10 μM) induced transcript accumulation. The accumulation of the Zmrprn 1 transcript was not prevented by cycloheximide, indicating that the cellular factor(s) required for expression were constitutively present in maize roots. The Zmrprn 1 mRNA accumulated specifically in response to nitrate, since neither K+ nor NH4 + treatment of roots caused transcript accumulation. Maize leaves had about 5% of the transcript level found in roots, indicating a strong preference for expression of Zmrprn 1 in roots. Analysis of maize genomic DNA indicated the presence of only a single gene or very small gene family for the Zmrprn 1. Together, the data indicate that Zmrprn 1A encodes a nitrate regulated maize root FNR.