The maize gene liguleless2 encodes a basic leucine zipper protein involved in the establishment of the leaf blade–sheath boundary

Abstract

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 (lg1-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 Mutator8 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.