Role of Vrs1 and Hox2 in Barley Spikelet Development; New Findings and Insights

The spike architecture (row phenotype) of barley is controlled by the VRS1, a HD-ZIP I family transcription factor. Vrs1 (HvHox1) is a paralog of HvHox2 which is conserved among cereals, whereas Vrs1 might have acquired its current function during the evolution of barley. Vrs1 expression was found only in the lateral spikelets of two-rowed barley and might negatively regulate its development, whereas in six-rowed barley either the gene is absent or mutated, resulting in a non-functional protein. Contrastingly, HvHox2 expression was found in leaves, roots and vascular bundles of developing spikes. Though the expression locations of both genes were discovered, the functional significance of these genes in spikelet development is still unclear.

We fused the Vrs1 and Hox2 promoter to eGFP and studied their expression pattern during spikelet development of two-rowed transgenic barley. Surprisingly, we found Vrs1 promoter activity in both central and lateral spikelet meristems. The results were confirmed by quantifying the Vrs1 transcripts in qRT-PCR from both tissues of Hordeum vulgare cv. Bonus (two-rowed barley). Also, we observed Vrs1 promoter activity in stomata and vascular bundles of transgenic barley leaves. Similarly, we observed Hox2 promoter activity in both central and lateral spikelet meristems and matured reproductive organs. This result has also been confirmed by qRT-PCR. Additionally, we swapped both promoters to identify the regulatory targets and their significance in spikelet fertility. These findings pave way for the new insights into the role of Vrs1 and Hox2 in barley spikelet development.