The genetics of inflorescence architecture in Hordeum

Abstract

The barley (Hordeum vulgare) spike exists in two alternative forms: six-rowed or two rowed. The genetic basis of this difference lies in the single gene Vrs1, the recessive allele of which is responsible for the six-rowed phenotype. The gene encodes a homeodomain-leucine zipper I class transcription factor. Its paralog HvHox2 is well conserved among the cereals, whereas Vrs1 acquired its function after the speciation of barley. To determine whether it is the mode of expression or neofunctionalization which accounts for the action of Vrs1, a comparative analysis was carried out between HvHox2 and Vrs1. This revealed that HvHOX2 and VRS1 share the same transcriptional activation activity. In situ hybridization analysis showed that HvHox2 mRNA is concentrated in the vascular bundles of the immature spike, whereas Vrs1 message is deposited exclusively in the pistil, lemma, palea, and lodicule of the lateral spikelets. The transcript abundance of Vrs1 was over 10-fold higher than that of HvHox2 during the pistil developmental stage, suggesting that the key function of Vrs1 is to inhibit gynoecial development. An analysis of six-rowed spike mutants which are non-allelic to vrs1 showed that Vrs1 was up-regulated by Vrs4, whereas HvHox2 expression was not. These data demonstrate that it is most likely a divergence in gene expression which has determined the function of Vrs1.