Mechanisms underlying variation in barley hull adhesion

本研究探讨了植物生物学中一个核心但很少被研究的问题——植物表面如何影响植物结构？我们知道，覆盖在植物体上的保护角质层，除了防止水分流失和病原体入侵外，还确保紧密生长的植物器官保持分离，在器官脱水中起着重要作用。然而，人们对植物界面融合的生物学原理知之甚少，比如管状花，或者像大麦颗粒那样粘在一起的器官。在后者中，一种特定的途径导致一种特殊的胶结层分泌到外果皮角质层。这一层的丧失是由单一的主要调控因子NUDUM （NUD）的突变引起的，在大麦种植过程中发生过一次，导致稻壳粘附性完全丧失或“裸”籽粒，这是人类消费的理想选择。然而，在英国种植的大多数大麦用于动物饲料和麦芽，其中“覆盖”的谷物，保留附着的外壳，是首选，因为外壳保护胚芽，并有助于过滤后的麦芽。因此，谷壳与籽粒之间的变化关系是决定大麦下游用途的关键品质。尽管如此，我们实际上对NUD表达和果皮上胶凝物质挤压之间的步骤一无所知，也不了解解释果皮粘附特性的化学反应。较新的优质大麦麦芽品种的遗传变异与一种非常不受欢迎的中间表型的发生率增加有关，即谷物在收获或加工期间部分脱落外壳，这种现象称为“剥皮”。然而，由于剥皮对环境的敏感性和缺乏强有力的筛选方法，确定剥皮背后的因果变异被证明是非常困难的，因此迄今为止，育种者没有遗传标记来帮助控制这种性状。除了解决这些重要的农艺问题外，我们还对确定剥皮的基因和遗传机制感兴趣，因为这些等位基因可能代表未解决的nudd驱动途径中的缺陷步骤。此外，通过表征蒙皮中发生的分子和化学变化，我们可以揭示胶结层的关键特征和/或其他影响外壳粘附的颗粒特征，这些特征在蒙皮突变体中丢失。为了避免研究栽培种质定量剥皮变异的问题，我们组装了一组具有稳定剥皮表型的突变体。我们在一个单一的近等基因背景中筛选了蜡缺失突变体，并确定了一小部分表现出船体粘附缺陷的突变体。这项基础工作提供了一个强大的和遗传学上强大的平台来解剖分子，化学和遗传机制，解释船体粘附的变化。我们的研究小组认为，表面脂质调控途径的特定成分可能在谷物剥皮中存在缺陷。在本提案中，我们试图定义与船体粘附相关的化学和超微结构表面变化，以及它们是如何通过蒙皮位点改变的。我们还将揭示基因表达的相关变化，无论是在全球还是在组织特异性水平上，促进船体粘附并评估其对皮肤表型的重要性。此外，我们将识别控制剥皮的单个基因，并评估这些基因座在栽培种质中的多样性，这将是一个关键的里程碑，它将解决哪些等位基因变异与剥皮的发生率有关，并使我们能够开发用于育种的标记。综上所述，我们的工作将揭示导致大麦粘附壳部分丧失的化学和遗传成分，缩小关于关键谷物品质性状的巨大知识差距，并为改进种质选择提供途径

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