Proanthocyanidins in Cereals and Brassicaceae: A Cross-Species Approach on their Roles for Seed-Coat Biophysical Properties, Dormancy and Germination

种子是所有高等植物生命周期的开始和结束。许多野生物种表现出高度的休眠性，使种子在脱落后不能立即发芽，并推迟发芽，直到遇到更有利于植物生长的条件。从这些野生物种驯化作物植物往往导致休眠的显着丧失，因为史前农民选择了快速发芽的种子。然而，一定程度的休眠是许多作物的重要品质性状，因为它可以防止种子在仍然附着在母体植物上时过早发芽。这在谷物如面包小麦中是一个严重的问题，因为接近收获期的潮湿天气会导致种子在穗内发芽（"收获前发芽"）。这导致酶的释放，部分降解淀粉（面粉），从而对面包制作质量产生负面影响。发芽诱导的种子产生如此多的酶，以至于作物中一小部分这样的谷物可能导致整个收获仅适合作为动物饲料，从而给农民带来经济损失。因此，太少的休眠可能导致收获前发芽，而太多的休眠可能导致田间发芽不均匀。这对于来自卷心菜科（菊科）的阔叶作物物种的高质量种子也是重要的。作物种子播种后发芽的速度和均匀性是同样重要的种子质量性状，并且与休眠一起直接影响作物产量。显然，了解决定休眠水平和发芽速度的过程是至关重要的，使我们能够设计和培育即使在压力环境下（气候变化下）也表现良好的作物新品种。在许多物种中，一个明显的促成因素是种皮，损坏或去除种皮可导致完全丧失休眠和更快的发芽。这似乎与种皮中存在红棕色单宁有关;这些种皮单宁的重要性从失去制造化合物的能力的植物中显而易见，从而导致具有较低休眠水平和更快发芽的浅色种子。例如，白粒小麦比红粒小麦更容易在收获前发芽，因此很难在英国潮湿的气候中成功种植。与红粒小麦相比，白粒小麦有几个优势，包括白色面粉产量更高，生产的"白色全麦"具有白色面包的味道，但具有正常红色全麦的纤维和营养特性。种皮中的单宁可以通过几种方式影响休眠和发芽：它们可以增加种皮的物理强度以防止发芽，它们可以影响种皮对发芽所需的水、激素或氧气的渗透性，或单宁的前体或代谢产物可能直接抑制种子萌发。这些不同的假设以前没有被直接测试过，但我们的合作团队已经开发了材料和方法，通过这些材料和方法，我们可以依次检查每个假设。我们计划研究两个物种，水芹和小麦，分别作为真双子叶植物（阔叶）和单子叶植物（谷类）物种的模型。我们已经开发了这两个物种的品种，在种皮中单宁生产的后期步骤中受损，因此我们可以研究单宁对种皮性质，休眠和发芽速度的影响。与此同时，我们还开创了测量分离种皮的强度、延展性和渗透性的方法。我们将这些属性与环境因素，如温度的相互作用，提供了一个全面的了解单宁的作用，种子的外套强加的休眠和发芽速度。

Seeds are at the beginning and end of the life cycle of all higher plants. Many wild species show a high degree of dormancy that prevents seeds from germinating immediately after shedding, and delays germination until more favourable conditions for plant growth are encountered. Domestication of crop plants from such wild species has often resulted in significant loss of dormancy as prehistoric farmers selected in favour of seeds with rapid germination. However, some degree of dormancy is an important quality trait in many crops, as it prevents seeds from germinating prematurely while still attached to the mother plant. This is a serious problem in cereals such as bread wheat, because wet weather near harvest time can cause seeds to germinate within the ear ("pre-harvest sprouting"). This causes release of enzymes that partially degrade of the starch (flour) and thereby impacts negatively on bread-making quality. Sprout-induced seeds produce so much enzyme that a small proportion of such grains within a crop can result in the entire harvest being suitable only as animal feed, with consequent economic loss to the farmer.Thus, too little dormancy can result in pre-harvest sprouting, while too much dormancy may lead to non-uniform germination in the field. This is important for high-quality seeds also of broadleaf crop species from the cabbage family (Brassicaceae). The speed and uniformity of germination of crop seeds after sowing is an equally important seed quality trait and together with dormancy directly affects crop production. Clearly, an understanding of the processes that determine the level of dormancy and the speed of germination are essential to enable us to design and breed new varieties of crop plants which perform well even in stressful environments (under climate change). One clear contributory factor in many species is the seed coat, damage to which or removal can result in complete loss of dormancy and in faster germination. This appears to be associated with the presence of reddish-brown tannins in the seed coat; the importance of these seed coat tannins is clear from plants that have lost the ability to make the compounds, resulting in pale-coloured seeds with lower levels of dormancy and faster germination. For example, white-grained wheat is much more prone to pre-harvest sprouting than a red-grained variety and is therefore difficult to grow successfully in the wet UK climate. White-grained wheat has several advantages over red-grained types, including a higher yield of white flour and the production of "white wholemeal" with the taste of white bread but the fibre and nutrient properties of normal red wholemeal.There are several ways in which the tannins in the seed coat could affect dormancy and germination: they may increase the physical strength of the coat to prevent germination, they may affect the permeability of the coat to water, hormones or oxygen, which are required for germination, or precursors or metabolites of the tannins might directly suppress seed germination. These different hypotheses have not previously been directly tested, but our collaborative team has developed the materials and the methods through which we can examine each in turn. We plan to look at two species, cress and wheat, as models for eudicot (broadleaf) and monocot (cereal) species, respectively. We have developed varieties of both species that are impaired in the late steps in tannin production in the seed coat, so that we can examine the effects on tannins for seed coat properties, on dormancy and the speed of germination. At the same time we have pioneered methods for measuring the strength, extensibility and permeability of the isolated seed coats. We will relate these properties to the interaction with environmental factors such as temperature, to provide a comprehensive understanding of the roles of tannins in coat-imposed dormancy and germination speed of seeds.