Distal expression of knotted1 in maize leaves leads to reestablishment of proximal/distal patterning and leaf dissection.

Maize (Zea mays) leaves provide a useful system to study how proximal/distal patterning is established because of the distinct tissues found in the distal blade and the proximal sheath. Several mutants disrupt this pattern, including the dominant knotted1-like homeobox (knox) mutants. knox genes encode homeodomain proteins of the TALE superclass of transcription factors. Class I knox genes are expressed in the meristem and down-regulated as leaves initiate. Gain-of-function phenotypes result from misexpression in leaves. We identified a new dominant allele of maize knotted1, Kn1-DL, which contains a transposon insertion in the promoter in addition to a tandem duplication of the kn1 locus. In situ hybridization shows that kn1 is misexpressed in two different parts of the blade that correlate with the different phenotypes observed. When kn1 is misexpressed along the margins, flaps of sheath-like tissue form along the margins. Expression in the distal tip leads to premature termination of the midrib into a knot and leaf bifurcation. The gain-of-function phenotypes suggest that kn1 establishes proximal/distal patterning when expressed in distal locations and lead to the hypothesis that kn1 normally participates in the establishment of proximal/distal polarity in the incipient leaf.