Wild barleys, the original domesticated forms and the majority of current elite UK cultivars produce two rows of grain bearing spikelets either side of the inflorescence, or spike. However, soon after the domestication, barleys with six rows of grain emerged that ultimately dominated early barley cultivation. Mutations in a single gene called SIX-ROWED SPIKE 1 (VRS1) have been identified as responsible for this important developmental switch. In two-rowed types, VRS1 mRNA is expressed in progenitor cells of the lateral spikelets which remain sterile presumably because VRS1 protein represses expression of genes that are required for development of ‘lateral fertility’. Inactivating VRS1 via mutation would de-repress expression of lateral fertility genes, resulting in a six-rowed spike. While VRS1 is core to this process, we also know from historical studies with barley mutants that at least 11 different SIX-ROWED SPIKE genes influence the degree of fertility of the lateral spikelets. For example, we recently identified SIX-ROWED SPIKE 5 (VRS5), and showed that different versions of this gene (that we call ‘alleles’) are always paired with different versions of VRS1 in commercial two- and six- rowed barleys. This pairing is important because in lines that have the six-row version of VRS1 (denoted as vrs1), a two-row VRS5 allele (Vrs5) causes the development of small grain from the lateral spikelets. In contrast, the six-row version of VRS5 (vrs5) causes the lateral spikelets to develop fully, with important consequences on yield. This observation demonstrates that getting the correct combination of alleles at VRS genes is extremely important. While mutant studies have identified many VRS genes, we recently showed that natural variation in only four genes is associated with determining whether current elite UK barley cultivars are genetically optimal two- or six-row-types. As expected, one of these was VRS1 and another VRS5. We recently identified the third gene as VRS3 and are trying to identify the fourth, which does not coincide with the location of any of the eleven VRS mutants. In parallel, VRS4 has been identified by German collaborators.
While we now know these genes are intrinsically linked by their involvement in the developmental pathway that restores fertility to a nascent floral organ (i.e. the lateral spikelets (LS), see Figure 1) at the moment we have no idea if or how these components interact, what other genes/proteins are involved or how six-rowed types evolved over the 10,000 years since the domestication of the species. These are the issues we plan to address in this project. We believe that by employing a wide range of approaches, for instance investigating and comparing early stages of barley inflorescence development (Fig.1), we will gain a better understanding of this fundamental developmental process and will be able to provide insights into how we can exploit variation in genes controlling plant morphology and architecture to ultimately improve plant yield.
Figure 1 shows scanning electron microscopy of immature Bowman wild-type inflorescence demonstrating four (A-D) spike maturation zones in 20 days-old barley. Panel A shows the inflorescence meristem giving rise to the first reproductive stage, double ridge with upper- spikelet, and lower- leaf ridge stage (B). The spikelet ridge differentiate into the triple spikelet meristem (C) which carry a single central (CS) and two lateral spikelet (LS) meristems. Fertility of the lateral spikelets at triple spikelet meristem gives row-type identity to barley ears. Early floret differentiation stage with hallmarks glume (G), lemma (L) and floret (F) primordia is shown in the last (D) part. Scale bar =0.2 mm. M.Zwirek, unpublished data.
For further information on this project please contact Robbie Waugh (firstname.lastname@example.org), Hazel Bull (email@example.com) or Monika Zwirek (firstname.lastname@example.org) from The James Hutton Institute.