Mechanisms underlying variation in barley hull adhesion

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This research explores a central but little studied problem in plant biology - how do plant surfaces influence plant architecture? We know that the protective cuticle covering the plant body, beyond preventing water loss and pathogen invasion, also ensures that closely growing plant organs remain separate and plays important roles in organ absicssion. However, less is known about the biology underlying plant interfaces that fuse, such as in tubular flowers, or organs that stick together as in the barley grain. In the latter, a species-specific pathway leads to secretion of a special cementing layer onto the outer pericarp cuticle. Loss of this layer, caused by mutations in a single master regulatory factor, NUDUM (NUD), occurred once during barley cultivation, leading to a complete loss of hull adherence or 'naked' grain, ideal for human consumption. However, most barley grown in the UK is used for animal feed and malt where 'covered' grain, retaining the adherent hull, is preferred as hulls protect the germ and aid in filtration after malting. Thus, the changing relationship of the hull to the grain is a critical quality that largely determines barleys downstream uses. Despite this, we understand practically nothing about the steps between NUD expression and the extrusion of the cementing material on the pericarp or the chemistry explaining the adherent properties of the layer.Genetic variation in newer elite barley malting cultivars is linked to an increasing incidence of a highly undesirable, intermediate phenotype whereby grain partially sheds its hull during harvest or processing, a phenomenon called 'skinning'. However, identifying the causal variation underlying skinning has proved very difficult, due to its environmental sensitivity and lack of robust screening methods, so to date breeders do not have genetic markers to help control this trait. In addition to addressing these important agronomic concerns, we are interested in identifying the genes and genetic mechanisms underlying skinning since these alleles may represent defective steps along unresolved NUD-driven pathway(s). Moreover, by characterising the molecular and chemical changes occuring in skinning, we may reveal the critical features of the cementing layer and/or other grain characters which influence hull adhesion that are lost in skinning mutants.To circumvent issues of studying the cultivated germplasm for quantitative skinning variation, we have assembled a panel of mutants with stable skinning phenotypes. We screened a wax-deficient collection of mutants in a single near-isogenic background and identified a small subset that show defective hull adhesion. This foundation work provides a robust and genetically powerful platform to dissect the molecular, chemical and genetic mechanisms that explain variation in hull adhesion.Our panel suggests that specific components of the surface lipid regulatory pathway may be defective in grain that skins. In this proposal, we seek to define the chemical and ultrastructural surfaces changes associated with hull adhesion and how they are altered by skinning loci. We will also reveal related changes in gene expression, both globally and on a tissue-specific level, that promote hull adherence and assess their importance to the skinning phenotype. Furthermore, we will identify individual genes that control skinning and evaluate diversity at these loci in cultivated germplasm, a critical milestone which will resolve which allelic variants track with incidence of skinning and allow us to develop markers for use in breeding. Taken together, our work will reveal the chemical and genetic components that cause partial loss of adherent hulls in barley, closing a vast knowledge gap about a critical grain quality trait, and delivering routes to improved germplasm selection