title: Homo- and heterodimers of tobacco bZIP proteins counteract as positive or negative regulators of transcription during pollen development.
abstract: Expression of BZI-1 Delta N, a dominant-negative form of the tobacco (Nicotiana tabacum) basic leucine zipper (bZIP) transcription factor BZI-1 leads to severe defects in pollen development which coincides with reduced transcript abundance of the stamen specific invertase gene NIN88 and decreased extracellular invertase enzymatic activity. This finding suggests a function of BZI-1 in regulating carbohydrate supply of the developing pollen. BZI-1 heterodimerises with the bZIP factors BZI-2, BZI-3 and BZI-4 in vitro and in planta. Whereas BZI-1 exhibits only weak activation properties, BZI-1/BZI-2 heterodimers strongly activate transcription. Consistently, approaches leading to reduced levels of functional BZI-1 or BZI-2 both significantly interfere with pollen development, auxin responsiveness and carbohydrate partitioning. In situ hybridisation studies for BZI-1 and BZI-2 confirmed temporal and spatial overlapping expression patterns in tapetum and pollen supporting functional cooperation of these factors during pollen development. Plants over-expressing BZI-4 produce significantly reduced amounts of intact pollen and are also impaired in NIN88 transcription and enzymatic activity. BZI-4 homodimer efficiently binds to a G-box located in the NIN88 promoter but exhibits almost no transcriptional activation capacity. As BZI-4 does not actively repress transcription, we propose that its homodimer blocks G-box mediated transcription. In summary, these data support a regulatory model in which BZI-4 homodimers and BZI-1/BZI-2 heterodimers perform opposing functions as negative or positive transcriptional regulators during pollen development.
authors.affiliation: Julius-von-Sachs-Institut, Pharmazeutische Biologie, Julius-Maximilians-Universität Würzburg, Julius-von-Sachs-Platz 2, D-97082 Würzburg, Germany.
authors.forename: Tim
authors.initials: T
authors.lastname: Iven
authors.forename: Anne
authors.initials: A
authors.lastname: Strathmann
authors.forename: Stefan
authors.initials: S
authors.lastname: Böttner
authors.forename: Thomas
authors.initials: T
authors.lastname: Zwafink
authors.forename: Thorsten
authors.initials: T
authors.lastname: Heinekamp
authors.forename: Anne
authors.initials: A
authors.lastname: Guivarc'h
authors.forename: Thomas
authors.initials: T
authors.lastname: Roitsch
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Dröge-Laser
mesh_terms: D050976:Basic-Leucine Zipper Transcription Factors / Q000235:genetics / Q000378:metabolism*; D018744:DNA, Plant / Q000235:genetics; D018507:Gene Expression Regulation, Developmental; D018506:Gene Expression Regulation, Plant; D007210:Indoleacetic Acids / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D011058:Pollen / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D034622:RNA Interference; D014026:Nicotiana / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D014158:Transcription, Genetic
chemical_list: D050976:Basic-Leucine Zipper Transcription Factors; D018744:DNA, Plant; D007210:Indoleacetic Acids; D010940:Plant Proteins