Six-rowed spike4 (Vrs4) Mediates Spikelet Architecture and Row-Type Identity in Barley

Row-type is the most distinguishable spike phenotype in barley that groups it into two cultivated forms two-rowed and six-rowed. Six-rowed barleys have increased grain yield per spike compared to two-rowed barleys. Thus far five different loci were identified which convert two-rowed barley to six-rowed barley; they include vrs1 (2HL), vrs2 (5HL), vrs3 (1HL), vrs4 (3HS) and Int-c (4HS). Komatsuda et al. (PNAS 2007, 104:1424–1429) identified Vrs1 as being an HD-ZIP transcription factor and Ramsay et al. found Int-c as the barley orthologue of maize Teosinte Branched1 (Nat Genet 2011, 43:169-172). Among other vrs loci (vrs2, vrs3 and vrs4) vrs4 produces the most prominent six-rowed phenotype with many completely fertile spikelets. Besides, vrs4 mutants develop additional spikelet/floret primordia at the triple spikelet meristem. In our study we isolated the gene encoding VRS4 via genetic mapping and mutant analysis. We found that Vrs4 encodes a protein that controls the determinacy of spikelets and functions upstream of Vrs1. Our microarray gene expression analyses in two vrs4 deletion mutants supported previous transcript results of Vrs1. Tissue localization of vrs4 transcripts through in-situ hybridizations in immature spikes during triple mound stage and later showed that Vrs1 and Vrs4 express in highly overlapping domains, further strengthening the idea of direct physical interaction. Our microarray analyses also revealed other important target genes probably involved in establishing spikelet determinacy and the Hordeum-specific row-type pathway in barley. In summary our data suggests that Vrs4 plays a central role in mediating spikelet determinacy and row-type identity in barley.