Seed development, dormancy and germination. Annual Plant Reviews, Volume 27.

Two major types of dormancy mechanisms exist: embryo dormancy where the agents inhibiting germination are inherent to the embryo, and coat-imposed dormancy where inhibition is conferred by the seed envelopes (Bewley, 1997). Generally, complex interactions between the embryo and covering structures determine whether a seed will germinate. As a consequence, many intermediate situations are encountered due to varying contributions by the embryo and envelopes to dormancy. Seed dormancy is a typical quantitative genetic character involving many genes and being substantially influenced by environmental effects (Koornneef et al., 2002; Alonso-Blanco et al., 2003). It is an adaptative trait allowing germination to occur during the most suitable period for seedling establishment and life cycle completion. Embryo growth potential and characteristics of the seed envelopes that determine theintrinsic capacity of a seed to germinate are established during development. The purpose of this review is to analyze the role of the seed envelopes, particularly the testa (seed coat), in dormancy and germination. The developmental events leading to the formation of the testa in Arabidopsis are presented. Special attention is paid to the roles played by flavonoids, particularly proanthocyanidins (condensed tannins), in determining the physicochemical characteristics of the testa that influence seed dormancy, germination and longevity in various species. In particular, the recent progress made in this field using the model plant Arabidopsis, which also illustrates the power of molecular genetics combined with physiology, is emphasized to dissect the mechanisms of seed coat-imposed dormancy