Distinct regulatory patterns of seed dormancy release and germination commencement

According to the general opinion, seed germination is a hormone-regulated process, being a continuation of hormone-controlled dormancy release. This concept is based on dormancy testing by germination percentage and rate. Here, quite a different approach to the problem of seed transition from dormancy to germination is taken. In seeds with non-deep physiological dormancy (Arabidopsis thaliana), dormancy is relieved due, on the one hand, to decrease in abscisic acid (ABA) content and attenuation of its signalling, and, on the other hand, to gibberellin (GA) synthesis and elimination of GA repressors. Dormancy termination manifests in GA-induced transcription of genes encoding proteins participating in wall loosening. Such seeds become quiescent. Initiation of germination needs water as a trigger if temperature and oxygen concentration permit it, i.e., it is a hydration-driven but not a hormone-regulated process. Water uptake results in a sequential activation of primary metabolic processes by reaching one by one the threshold hydration levels. This metabolic activation is automatic below the level of saturated matric potential if water is available. Later water inflow is provided by accumulation of endogenous osmotica in embryo axes, restoration of vacuoles and cell wall loosening. These pregerminative events culminate in initiation of cell elongation, i.e., in radicle emergence and growth commencement.