IDENTIFICATION OF A MAIZE ROOT TRANSCRIPT EXPRESSED IN THE PRIMARY RESPONSE TO NITRATE - CHARACTERIZATION OF A CDNA WITH HOMOLOGY TO FERREDOXIN-NADP(+) OXIDOREDUCTASE

To more fully understand the biochemical and molecular events which oc cur in plants exposed to nitrate, cDNAs whose accumulation was enhance d in nitrate- and cycloheximide-treated maize (Zea mays L. W64A x W182 E) roots were isolated. The 340 bp Zmrprn1 (for Zea mays root primary response to nitrate) cDNA also hybridized with a probe enriched for ni trate-induced sequences, and was characterized further. Sequence analy sis of a near full-length cDNA (Zmrprn1A) showed strong homology ( > 9 0 % amino acid identity) with a root ferredoxin-NADP(+) oxidoreductase (FNR) of rice, and 45-50 % amino acid identity with leaf FNR genes. W hen expressed in Escherichia coli, the Zmrprn1A cDNA produced a protei n with NADPH: ferricyanide reductase activity, consistent with the enz ymatic properties of an FNR. The Zmrprn1 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 transient ly in response to external nitrate, and low levels of nitrate (10 mu M ) induced transcript accumulation. The accumulation of the Zmrprn1 tra nscript was not prevented by cycloheximide, indicating that the cellul ar factor(s) required for expression were constitutively present in ma ize roots. The Zmrprn1 mRNA accumulated specifically in response to ni trate, 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 Zmrprn1 in roots. Analysis of maize genomic DNA indicated the presence of only a single gene or very small gene family for the Zmrprn1. Together, the data indicate that Zmrprn1A encodes a nitrate regulated maize root FNR .