03-P049 Control of mitotic spindle orientation and position during epiboly

We are using the maize leaf as an experimental system to ask how positional information establishes the proximal/distal axis. A mature maize leaf has three regions, the distal blade that functions in photosynthesis, the proximal sheath that wraps the stalk, and the ligule, marking a sharp boundary between blade and sheath. The recessive liguleless mutants remove the ligule, but the distinction of sheath and blade remains. When combined with a dominant mutant, Wavy auricle in blade, regions of the leaf are only sheath and leaves are very narrow. A similar phenotype is seen in the single mutant, Liguleless narrow (Lgn). Lgn heterozygotes have narrow leaves and no ligule at the margins. Lgn homozygotes leaves lack distinction of blade and sheath, and have no stem or reproductive parts. Lgn encodes a serine threonine kinase and the mutated version fails to autophosphorylate, suggesting that correct signaling from LGN is needed for plant architecture as well as proximal/distal patterning of the leaf. In contrast with these mutants, the dominant homeobox mutation, Knotted1, recreates proximal distal patterning due to misexpression in the blade. Normally, kn1 is expressed only in the meristem. We propose that kn1 establishes the proximal end of the leaf at its inception. The gain of function Kn1 phenotype is suppressed in the Lgn background. This result suggests that Lgn may provide or transmit a distal signal that interacts with that of kn1.