Der bZIP-Transkriptionsfaktor BZI-1 aus Nicotiana tabacum: Analyse der in vivo Funktion durch Modulation der BZI-1- Aktivierungseigenschaften in transgenen Pflanzen

Abstract

The tobacco (Nicotiana tabacum) bZIP protein BZI-1 exhibits all typical characteristics of a transcription factor. BZI-1 binds to G-box and C-box cis-elements in vivo, and the N-terminus of BZI-1 functions as an activation domain in plant cells. With reference to amino acid sequence, conservation of protein domains, genomic exon-intron structure and expression pattern, BZI-1 is closely related to CPRF2, OHP1/2, BLZ1 or REB, a group of bZIP proteins which have been described in a number of dicot and monocot species. In order to analyse the function of BZI-1 in detail, transgenic tobacco plants ectopically overexpressing various derivatives of BZI-1 were engineered. Since BZI-1-related dicot transcription factors have been isolated by in vitro binding to chalcone synthase (CHS) G-box promoter elements, it has been suggested that phenylpropanoid pathway genes, such as CHS and PAL (phenylalanine ammonia-lyase), are in vivo target genes. However, no changes in CHS or PAL expression were observed in transgenic plants expressing increased levels of BZI-1 or a dominant negative BZI-1-DN protein. Plants expressing BZI-1-DN protein develop various phenotypical changes which implicate an effect of BZI-1 on plant hormone response. Auxin-induced root- or callus-development is strongly inhibited in BZI-1-DN plants whereas cytokinin mediated shoot-induction is unaffected. In BZI-1-DN plants the induction of the auxin-inducible genes PARA, PARC, and GH3 is reduced and delayed. Hence, BZI-1 is involved in plant development by regulating the response to auxin-mediated hormone signalling in planta.