Ectopic expression of the maize kn1 gene phenocopies the Hooded mutant of barley

The homeobox gene, knotted1, (kn1) is expressed in shoot meristems and is required for maintaining indeterminacy and preventing cellular differentiation. Awns, extensions of the bract-like lemma found in all grass inflorescences, are normally determinate structures. We show that ectopic expression of kn1 in the barley awn is sufficient to direct the development of ectopic meristems, forming inflorescence-like structures. This homeotic transformation is similar to the phenotype produced by misexpression of the barley hvknox3 gene, associated with the dominant Hooded mutant (Muller, K. J., Romano, N., Gerstner, O., Garcia-Maroto, F., Pozzi, C., Salamini, F. and Rohde, W. (1995) Nature 374, 727–730). We suggest that the inverse polarity of the ectopic flowers seen in Hooded and transgenic kn1 plants results from the transformation of the awn into reiterative inflorescence axes. We observed that the protein and mRNA localization of the transgene, driven by a constitutive promoter, is similar to the expression pattern of hvknox3 in awns of Hooded mutants, suggesting posttranscriptional regulation.