The maize duplicate genes narrow sheath1 and narrow sheath2 encode a conserved homeobox gene function in a lateral domain of shoot apical meristems

Abstract

The narrow sheath (ns) phenotype of maize is a duplicate factor trait conferred by mutations at the unlinked loci ns1 and ns2. Recessive mutations at each locus together confer the phenotypic deletion of a lateral compartment in maize leaves and leaf homologs. Previous analyses revealed that the mediolateral axis of maize leaves is comprised of at least two distinct compartments, and suggest a model whereby NS function is required to recruit leaf founder cells from a lateral compartment of maize meristems. Genomic clones of two maize homeodomain-encoding genes were isolated by homology to the WUSCHEL-related gene PRESSED FLOWER (PRS). PRS is required for lateral sepal development in Arabidopsis, although no leaf phenotype is reported. Co-segregation of the ns phenotype with multiple mutant alleles of two maize PRS homologs confirms their allelism to ns1 and ns2. Analyses of NS protein accumulation verify that the us-R mutations are null alleles. ns transcripts are detected in two lateral foci within maize meristems, and in the margins of lateral organ primordia. Whereas ns1 and ns2 transcripts accumulate to equivalent levels in shoot meristems of vegetative seedlings, ns2 transcripts predominate in female inflorescences. Previously undiscovered phenotypes in the pressed flower mutant support a model whereby the morphology of eudicot leaves and monocot grass leaves has evolved via the differential elaboration of upper versus lower leaf zones. A model implicating an evolutionarily conserved NS/PRS function during recruitment of organ founder cells from a lateral domain of plant meristems is discussed.