Genetic Control of Spikelet Determinacy and Lateral Spikelet Fertility in Barley

The barley (Hordeum vulgare L.) inflorescence is an indeterminate spike, which bears three single-flowered spikelets at each rachis internode, one central and two lateral spikelets. Fertility in lateral spikelets provides row-type identity to barley spikes. Six-rowed spikes show fertile lateral spikelets, compared to two-rowed spikes with sterile lateral spikelets. Thus far, two loci governing the row-type phenotype were isolated in barley that include Six-rowed spike1 (Vrs1) (PNAS 2007, 104(4):1424–1429) and Intermedium-C (Nat. Genet. 2011, 43(2):169-172). We isolated Vrs4, a barley ortholog of the maize inflorescence architecture gene RAMOSA2 (RA2) through genetic mapping and mutant analysis (PNAS 2013, 110(32):13198-13203). mRNA in situ hybridization and microarray analyses showed that Vrs4 (HvRA2) controls the row-type pathway through Vrs1, which is a negative regulator of lateral spikelet fertility in barley. Moreover, microarray data revealed that Vrs4 might also regulate transcripts of barley SISTER OF RAMOSA3 (HvSRA), a putative trehalose-6-phosphate phosphatase (T6PP), involved in T6P homeostasis implicated to control spikelet determinacy. Our expression data illustrated that although RA2 is conserved among different grass species, its down-stream target genes appear to be modified in barley and possibly other species of tribe Triticeae. Based on spatial and temporal expression pattern of RA2 in barley we hypothesize RA2 as being one of the prime regulators of spikelet number per rachis internode in Triticeae species. Currently we are testing this hypothesis by swapping regulatory regions (promoter and UTR’s) of wheat and sorghum RA2 orthologs with barley RA2 coding sequence.

*RK & NA contributed equally to the work.