KNOX homeodomain protein directly suppresses the expression of a gibberellin biosynthetic gene in the tobacco shoot apical meristem

To identify genes targeted by the tobacco KNOX homeodomain protein, Nicotia na tabacum homeobox 15 (NTH15), we have generated an inducible system using the human glucocorticoid receptor. In this system, steroid treatment stric tly induced NTH15 function and immediately suppressed the expression of a g ibberellin (GA) biosynthetic gene encoding GA 20-oxidase (Ntc12) and also r esulted in a decrease in bioactive GA levels. The repression of Ntc12 was o bserved even when indirect effects were blocked by cycloheximide. NTH15 mRN A was present in corpus cells of the shoot apical meristem (SAM), whereas N tc12 mRNA was observed in leaf primordia and rib meristem but not in the co rpus. Recombinant NTH15 protein strongly bound to a 5-bp dyadsymmetric sequ ence, GTGAC, in the first intron of Ntc12 in vitro. Mutation of this sequen ce in the Ntc12 gene abolished the NTH15-dependent suppression of Ntc12 in the corpus of the SAM. Our results indicate that NTH15 directly represses N tc12 expression in the corpus of the wild-type SAM to maintain the indeterm inate state of corpus cells. The suppression of NTH15 within cells at the f lanks of the SAM permits GA biosynthesis, which promotes organized cell pro liferation and consequently induces the determination of cell fate.