Seed germination: definition and reviews 

 

• Seed germination: Germination of seeds is a complex physiological process triggered by imbibition of water after possible dormancy mechanisms have been released by appropriate triggers (see webpage "Seed Dormancy").Under favorable conditions rapid expansion growth of the embryo culminates in rupture of the covering layers and emergence of the radicle. Radicle emergence is considered as the completion of germination. The definition that a visible protrusion of radicle tip is the completion of germination is not only a definition issue of seed physiologists. This transition point is also characterized by the loss of dessication tolerance and this is a molecular checkpoint (in Arabidopsis regulated by ABI5), a developmental molecular switch from the germination program to the seedling program.

• Non-dormant seeds: A completely non-dormant seed has the capacity to germinate over the wides range of normal physical environmental factors possible for the genotype (Finch-Savage and Leubner-Metzger, 2006) Besides the basic requirement for water, oxygen and an appropriate temperature the seed may also be sensitive to light and/or nitrate. Germination commences with the uptake of water by imbibition of the dry seed, followed by embryo expansion. This usually culminates in rupture of the covering layers and emergence of the radicle, generally considered as the completion of germination. Radicle protrusion at the completion of seed germination therefore depends on embryo growth driven by water uptake.

• Cell elongation is necessary and is generally accepted to be sufficient for the completion of radicle protrusion, cell division is not essential. Thus, germination is a process and the completion of germination is an event; visible protrusion of the radicle tip through all covering structures is the typical criterion for the completion of seed germination.

• Uptake of water by a mature dry seed is triphasic (see webpage "Water Relations"), with a rapid initial uptake (phase I, imbibition) followed by a plateau phase (phase II, metabolic preparation for germination). Phase III is a further increase in water uptake which occurs directly after germination is completed. Phase-III water uptake causes hydraulic growth of the embryo and the emerged seedling. The plant hormone ABA inhibits phase III-water uptake.

• In many species the covering tissues, e.g. endosperm and testa (seed coat), act as a physical barriers which must be overcome by the growth potential of the embryo if the seed is to complete its germination (Linkies et al., 2010). In coat-dormancy, the seeds are prevented from completing germination because the embryo is constrained by the covering layers. The micropylar endosperm is known since a long time as a constraining structure in members of the Asteraceae (e.g. lettuce) and Solanaceae (e.g. tomato, tobacco and Datura spp.). Endosperm weakening, at least limited cell wall hydrolysis, by the action of specific hydrolases seems to be needed for the completion of germination by endosperm rupture. In addition to the endosperm, the micropylar testa also confers a hindrance for radicle protrusion of of tobacco and tomato seeds.

• Arabidopsis thaliana and Lepidium sativum are Brassicaceae seeds with distinct testa rupture and endosperm rupture. Müller et al. (2006) demonstrated that endosperm weakening occurs prior toLepidium sativum endosperm rupture and is controlled by the GA-ABA ratio. Linkies et al. (2009) demonstrated that ethylene promotes endosperm cap weakening of Lepidium sativum and endosperm rupture of the close Brassicaceae relatives Lepidium sativum and Arabidopsis thaliana. Ethylene counteracts the inhibitory action of abscisic acid (ABA) on these two processes.

• A comprehensive table of Arabidopsis homone mutants summarizes the altered phenotypes regarding germination and dormancy.

• Recommended reviews on seed germination: