Organ-specific expression of glutamate dehydrogenase (GDH) subunits in yellow lupine

Glutamate dehydrogenase (GDH, EC 1.4.2-4) is present in yellow lupine (Lupinus luteus cv. Juno) in many isoforms. The number and banding pattern of isoenzymes varies with respect to plant organ and developmental stage. To better understand the complex nature of GDH regulation in plants, the levels of GDH transcripts, enzyme activity and isoenzyme patterns in germinating seeds and roots of yellow lupine were examined. The analysis of GDH cDNA sequences in lupine revealed three mRNA types, of which two encoded the β-GDH subunit and one encoded the α-GDH subunit (corresponding to the GDH1(GDH3) and GDH2 genes, respectively). The relative expression of GDH1 and GDH2 genes was analyzed in various lupine organs by using quantitative real-time PCR. Our results indicate that different mRNA types were differently regulated depending on organ type. Although both genes appeared to be ubiquitously expressed in all lupine tissues, the GDH1 transcripts evidently predominated over those of GDH2. Immunochemical analyses confirmed that, during embryo development, varied expression of two GDH subunits takes place. The α-GDH subunit (43 kDa) predominated in the early stages of germinating seeds, while the β-GDH subunit (44 kDa) was the only GDH polypeptide present in lupine roots. These results firmly support the hypothesis that isoenzyme variability of GDH in yellow lupine is associated with the varied expression of α and β subunits into the complexes of hexameric GDH forms. The presence of several isogenes of GDH in yellow lupine may explain the high number (over 20) of its molecular forms in germinating lupine.