Abstract
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.
Highlights
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
We describe a new kn1 allele, Kn1-DL, which results from alterations in the promoter region. kn1 misexpression at margins of the developing blade leads to flaps of sheath tissue at blade margins. kn1 misexpressed at the midrib and tip of the blade leads to a bifurcated leaf, which is highly unusual for grass species
Using a probe spanning the two BclI sites within the coding region, we found that the 0.5-kb fragment that spans the fourth exon and the 3.1-kb fragment that extends into the 3# end are the same in mutants and normal siblings, but the 9.1-kb fragment that extends into the 5# end is replaced by two fragments, of 6.2 and more than 12 kb (Fig. 4B)
Summary
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. Vegetative development is normal but plants have reduced axillary branching, and ectopic leaves form in the axils of leaves (Kerstetter et al, 1997; Lunde and Hake, 2009) Both recessive and dominant mutations have been found that cause misexpression of knox genes in leaves (Hake et al, 2004). The Kn1-DL phenotype indicates that kn reinitiates new proximal/distal patterns, suggesting that it may play a role in establishing proximal/distal polarity during normal leaf development
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