Post-transcriptional regulation prevents accumulation of glutathione reductase protein and activity in the bundle sheath cells of maize

Glutathione reductase (GR; EC 1.6.4.2) activity was assayed in bundle sheat h and mesophyll cells of maize (Zea mays L.. var H99) from plants grown at 20 degrees C, 18 degrees C, and 15 degrees C. The purity of each fraction w as determined by measuring the associated activity of the compartment-speci fic marker enzymes, Rubisco and phosphoenolpyruvate carboxylase, respective ly. GR activity and the abundance of GR protein and mRNA increased in plant s grown at 15 degrees C and 18 degrees C compared with those grown at 20 de grees C. In all cases GR activity was found only in mesophyll fractions of the leaves, with no GR activity being detectable in bundle sheath extracts. Immunogold labeling with GR-specific antibodies showed that the GR protein was exclusively localized in the mesophyll cells of leaves at all growth t emperatures, whereas GR transcripts (as determined by in situ hybridization techniques) were observed in both cell types. These results indicate that post-transcriptional regulation prevents GR accumulation in the bundle shea th cells of maize leaves. The resulting limitation on the capacity for rege neration of reduced glutathione in this compartment may contribute to the e xtreme chilling sensitivity of maize leaves.