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The blade and sheath of a maize leaf are separated by a linear epidermal fringe, the ligule, and two wedge-like structures, the auricles. In plants homozygous for the null mutation, liguleless2-reference (lg2-R), the ligule and auricles are often absent or positioned incorrectly and the blade-sheath boundary is diffuse. This phenotype is in contrast to that of liguleless1-reference (lgl-R) mutant plants, which have a more defined boundary even in the absence of the ligule and auricles. Additionally, mosaic analysis indicates the lg2-R phenotype is cell-nonautonomous and the lg1-R phenotype is cell-autonomous. Using scanning electron microscopy we show that lg2-R mutant plants are affected before the first visible sign of ligule and auricle formation. We have cloned the Lg2+ gene through a Mutator-8 transposon insertion allele, and verified it with five independently derived alleles. The comparison of genomic DNA and cDNA sequences reveals an open reading frame encoding a protein of 531 amino acids with partial homology to a subclass of plant basic leucine zipper (bZIP) transcription factors. Although a large body of molecular and biochemical characterization exists on this subclass of bZIP proteins, our work represents the first report of a mutant phenotype within this group. A specific reverse transcriptase (RT)-PCR assay shows LG2 mRNA expression in meristem/developing ligule regions. RT-PCR also shows that LG2 mRNA accumulation precedes that of LG1 mRNA. The mutant phenotype and expression analysis of lg2 suggest an early role in initiating an exact blade-sheath boundary within the young leaf primordia.
Walsh et al. (Thu,) studied this question.