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

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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.