Molecular Mechanisms for Covered vs. Naked Caryopsis in Barley

Typical barley cultivars have caryopses with adhering hulls at maturity, known as covered (hulled) barley. However, a few barley cultivars are a free-threshing variant called naked (hulless) barley. The covered vs. naked caryopsis is controlled by a single locus (nud) on chromosome arm 7HL. Positional cloning identified that an ERF (ethylene response factor) family transcription factor gene controls the covered vs. naked caryopsis phenotype. This conclusion was further supported by (1) fixation of the 17-kb deletion, harboring the ERF gene, among all 100 naked cultivars studied; (2) five induced nud alleles with a DNA lesion at a different site, each affecting the putative functional motif; and (3) gene expression strictly localized to the testa. Survey of natural variation at the nud locus indicates that naked barley has monophyletic origin but that covered barley is classified into some clusters, suggesting plural lineages. The Nud gene has homology to the Arabidopsis WIN1/SHN1 transcription factor gene, whose deduced function is control of a lipid biosynthesis pathway. Staining with a lipophilic dye (Sudan Black B) detected a lipid layer on the pericarp epidermis only in covered barley. This observation indicates that in covered barley, lipids on the surface of caryopses act as a glue for their tight adhesion with hulls. Separation of hulls in naked barley is due to the absence of surface lipids on caryopses. Genetic complementation experiment is in progress toward functional validation of the Nud gene.