Homeobox Genes in the Functioning of Plant Meristems

The maize homeobox gene knotted1 (kn1) is expressed in vegetative and floral meristems and is down-regulated at the site of primordia formation. kn1-related genes from maize and other species also show meristem-specific expression and offer additional tools for studying the activities of shoot meristems. Members of this gene family are expressed early in embryogenesis, providing molecular markers for meristem initiation. Ectopic expression of either kn1 or a related Arabidopsis gene, KNAT1, causes dramatic alterations in Arabidopsis and tobacco leaf morphology. Most significantly, meristems form on the leaf, producing small shoots. We discuss whether the phenotypes can be interpreted as changes in positional information or timing of determination.