Characterization of three homologous basic leucine zipper transcription factors (bZIP) of the ABI5 family during Arabidopsis thaliana embryo maturation.

The Arabidopsis thaliana genome contains approximately 80 genes encoding basic leucine zipper transcription factors, divided into 11 groups. Abscisic Acid-Insensitive 5 (ABI5) is one of the 13 members of group A and is involved in ABA signalling during seed maturation, and germination. Seven other members of this group are expressed during seed maturation, but only one of them (Enhanced Em Level, EEL) has been functionally characterized during this developmental phase. Since EEL and two other group A genes, AtbZIP67 and AREB3 (ABA-Responsive Element Binding protein 3), display similar mRNA temporal expression in whole siliques, it is suspected that they might share some overlapping functions. To address this question, the proteins' tissular and subcellular localization in transgenic Arabidopsis were precisely characterized, using translational fusions with a green fluorescent protein (GFP) expressed under the corresponding bZIP promoter. It was found that the three fusion proteins were expressed with a largely overlapping pattern and constitutively localized in the nuclei. An RNA interference approach (RNAi) was then used to knock out the expression of all three genes simultaneously. Endogenous EEL, AREB3, and AtbZIP67 transcripts could be specifically reduced, but no visible defects could be observed during seed maturation.