Redox and translational regulation of glutamate dehydrogenase α subunits in Brassica napus under wounding stress

Abstract

Glutamate dehydrogenase (GDH) is a ubiquitous enzyme that catalyzes the reversible amination of 2-oxoglutarate to glutamine. In Brassica napus, GDH isoenzymes 1 and 7 are hexamers of β and α subunits, respectively. When leaves of B. napus were sliced into narrow strips, seven GDH isoforms appeared with increases in both amination and deamination activity. Isoform profiles ceased changing from 1.5 to 2.5 h after wounding. Treatment with jasmonic acid (JA), salicylic acid (SA), ethylene, and oligosaccharides (OGs) changed the profiles. Ammonium ions at the same concentration as the signal substances (5 mM) caused only slight induction of the GDH7 isoenzyme. Ethylene was not released until 2 h after wounding, whereas JA and OGs were generated soon after wounding and peaked at 30 and 60 min after wounding, respectively. Accordingly, the wounding of leaves first releases OGs. Transcription of GDH2 , which encodes the GDH α subunit, was activated by wounding, whereas translation of the GDH2 mRNA was repressed. The GDH7 isoform was activated by the reduction of disulfide bonds in the GDHα subunits, but wounding did not cause synthesis of new α subunits. In leaves, wounding changed the redox state; it decreased NAD(P)H, ascorbic acid, and glutathione levels and increased dehydroascorbic acid. These results indicate that the GDH2 gene is regulated both at the transcriptional and translational levels and that the α subunit proteins are regulated posttranslationally via redox mechanisms. Thus, GDH isoenzyme profiles may reflect the redox state of leaves under oxidative stress.