BZI-1 specifically heterodimerises with the tobacco bZIP transcription factors BZI-2, BZI-3/TBZF and BZI-4, and is functionally involved in flower development

The tobacco (Nicotiana tabacum) protein BZI-1 is closely related to the plant bZIP transcription factors CPRF2, G/HBF-1 and OHP1. Using the C-terminal part of BZI-1, which includes the bZIP domain, as a bait in a yeast two-hybrid screen, three BZI-1 interacting bZIP transcription factors, referred to as BZI-2, BZI-3/TBZF and BZI-4, were isolated. The observed interactions are due to the leucine zipper dimerisation domain and have been found to be specific, in so far as other bZIP transcription factors do not interact with BZI-1. The formation of heterodimers is favoured to homodimerisation. Furthermore, physical protein-protein interaction was confirmed by in vitro binding studies. Expression analysis reveals that BZI-2 mRNA is predominantly located in the stems and parts of the flower. BZI-3/TBZF expression has been observed predominantly in flowers and, to a lesser extent, in the vegetative parts of the plant. In particular, BZI-4 is transcribed specifically in the stamen, the petals and the pistils of the tobacco flower. Since BZI-1 is expressed ubiquitously in tobacco plants, co-localisation with BZI-2, BZI-3/TBZF or BZI-4 might influence BZI-1 heterodimerisation and consequently target gene selection. Analysis of transgenic plants displaying changes in BZI-1 protein level revealed that BZI-1 regulates BZI-4 expression. Moreover, a reduction in functional BZI-1 protein resulted in flowers having a reduced size; in particular, the stamen and the petals are affected. Consequently, BZI-1 homo- or heterodimers are involved in the control of the size of the organs of flowers. Based on these data, we discuss a model postulating BZI transcription factor heterodimerisation as a mechanism determining target gene selection and regulating processes involved in plant development.