Phase III of germination can either be a one-step process in which the radicle emerges immediately after the seed coat (testa) is ruptured, or it may involve two steps in which the endosperm must undergo weakening before the radicle can emerge. In the one-step process, there is no residual endosperm in the mature seed, so testa rupture plus the initial radicle elongation result in the completion of germination. Although abscisic acid (ABA) does not inhibit testa rupture, it can inhibit radicle emergence (Web Figure 18.4.A, upper part). The two-step process is more complex because a residual endosperm is present in the mature seed that can physically block radicle emergence (see Web Figure 18.4.A, lower part). Dormancy release occurs either during seed after-ripening or via the phytochrome-regulated gibberellin (GA) pathway during imbibition (Phase I). ABA inhibits endosperm rupture but not testa rupture, while GA, ethylene, and brassinosteroids (BR) promote endosperm weakening and counteract the inhibitory effects of ABA (Web Figure 18.4.A, lower part).

Web Figure 18.4.A Hormone interactions regulate seed dormancy release and germination of Nicotiana (A) and Brassica (B) model species. (A) Nicotiana sp. seed germination occurs in two-steps: testa rupture and then endosperm rupture. Dormancy release and promotion of germination occur during seed after-ripening (dry storage at room temperature for several months) or by the phytochrome-gibberellin (GA) pathway during imbibition. Abscisic acid (ABA) inhibits endosperm rupture, but not testa rupture. GA, ethylene, and brassinosteroids (BR) promote endosperm rupture and counteract the inhibitory effects of ABA. Endosperm rupture and gene expression of cell wall remodeling proteins are promoted by light, GA, and ethylene, and are inhibited by darkness, ABA, and osmotica. (B) Brassica napus seed germination occurs in one-step. The mature seeds lack an endosperm, therefore testa rupture and the initial radicle elongation complete the germination process. While ABA does not inhibit testa rupture, ABA inhibits subsequent radicle growth (Schopfer and Plachy 1984). (A after Leubner-Metzger 2003; B after Kucera et al. 2005.)