THE IBP GENES OF MAIZE ARE EXPRESSED IN NON-MERISTEMATIC, ELONGATING CELLS OF THE SEEDLING AND IN ABORTIVE FLORAL ORGANS

The transcription start site of the maize Shruken-1 (Sh-1) gene is suf ficient for transcriptional initiation in the absence of other promote r elements and is recognized in vitro by the Initiator Binding Protein (IBP). We describe here in situ hybridization experiments performed o n various maize tissues to quantify IBP transcription at the cellular level. IBP transcripts are found in the endosperm and in differentiati ng, enlarging cells of the shoot and the root of the maize seedling. T his expression pattern overlaps with that of the Sh-1 gene and is ther efore compatible with the hypothesis that the Sh-1 transcription start site is a target for IBP. In the developing spikelets of male and fem ale inflorescences IBP transcript levels are very high in those organs that are later aborted when flowers become unisexual. Overexpression of the maize IBP1 gene product in transgenic tobacco causes a reductio n in internodal elongation and effects gibberellin hormonal balance. T he cellular expression pattern described here establishes IBP transcri pts as an interesting molecular marker for enlarging, and presumably d ifferentiating, cells released from the root or shoot apex.