After completing embryogenesis and seed maturation, most seeds enter into a state of dormancy, which can be thought of as something like “suspended animation”. Dormancy is characterized by extremely slow metabolic activity brought about by mechanical and/or chemical signals. At this stage, the environment may be sealed out (in the case of coat-imposed dormancy), or the embryo may be under the influence of chemical messages that maintain dormancy. Chief among these chemical messengers is the plant hormone abscisic acid (ABA), a small organic molecule classified as an sesquiterpene and synthesized in plastids.
ABA in Dormancy
ABA plays a number of diverse roles in plants, and its name reflects its discovery based on its involvement in fruit abscission (detachment from the parent plant). ABA also plays a critical role in plant drought response, in part by regulating stomatal pore size. In seeds, ABA concentrations increase with embryo maturation due to its synthesis by the embryo, and results in several responses that promote dormancy. ABA promotes expression of seed storage proteins associated with seed maturity, known as Late Embryogenesis Abundant (LEA) proteins. ABA also promotes the synthesis of proteins associated with desiccation (drying) tolerance, known as heat-shock proteins (HSPs). In addition, ABA also prevents the premature germination of the embryo, known as vivipary.
The release of embryo dormancy is associated both with a decrease in ABA synthesis and concentration and an increase in the concentration of a second plant hormone, gibberellic acid (GA). As levels of ABA decrease, the inhibitory effects of this hormone begin to decline. At the same time, this decline promotes the synthesis of GA by the embryo. The increase in GA concentration actively promotes germination, with perhaps its most prominent influence occurring on the expression of several hydrolytic enzymes, including α- and β-amylase. α-amylase is known as an endoglucanase, meaning it hydrolyzes starch chains in mid-chain, while β-amylase is an exoglucanase, acting on the ends of the starch chain to release maltose units (a disaccharide of glucose). As starches are digested by these hydrolytic enzymes, the resulting sugars can be taken up by the embryo and used to fuel growth.
If high levels of ABA promote dormancy, and high levels of GA promote germination, how does the embryo “know” when to make the switch? There are a number of factors, both intrinsic and external, that influence this decision. External cues such as water availability and chilling exposure have been shown to influence germination, and internal cues related to seed maturation, known as afterripening, are also important in some species. Because plants are photosynthetic autotrophs, waiting to germinate until the light conditions were optimal would seem like an adaptive response, and in fact many plants use light as a cue to regulate germination. In particular, plants use the phytochrome photoreceptor to determine the quality of light available in the environment. Locations that are rich in high-energy light promote germination through the phytochrome signaling pathway, while locations predominated by shade inhibit germination.