OVEREXPRESSION OF THE MAIZE HOMEO BOX GENE, KNOTTED-1, CAUSES A SWITCH FROM DETERMINATE TO INDETERMINATE CELL FATES

The KNOTTED-1 (KN1) locus of maize is defined by dominant mutations that affect leaf cell fates. Transposon tagging led to the isolation of the gene and the discovery that KN1 encodes a homeo domain. Immunolocalization studies showed that in wild-type maize plants, KN1 protein is present in nuclei of apical meristems and immature shoot axes but is down-regulated as lateral organs, such as leaves, are initiated. The protein is not immunohistochemically detectable in wild-type leaves at any stage. In developing leaves of plants carrying the dominant Kn1 mutation, temporally and spatially restricted ectopic expression of KN1 causes the mutant phenotype. To better understand the function of KN1 in plant development, we sought to determine the phenotype of plants in which KN1 was constitutively expressed. We find that tobacco plants transformed with the KN1 cDNA driven by the CaMV 35S promoter have a dramatically altered phenotype. The phenotypes are variable and depend on the level of KN1 protein. Plants expressing moderate levels of KN1 are reduced in stature with rumpled or lobed leaves. Plants with relatively high levels of KN1 lack apical dominance and are severely dwarfed in overall height and leaf size. Small shoots originate from the surface of these diminutive leaves. On the basis of phenotypes in maize and tobacco, we propose that the KN1 homeo box gene plays a role in determining cell fate. The consequences of KN1 overexpression appear to depend on the concentration of KN1 and the timing of its expression during organogenesis.