Figure 1. The time course of events associated with seed germination and subsequent postgerminative seedling growth. The time required for the events to be completed varies from several hours to many weeks, depending upon inherent genetic factors and the prevailing germination conditions, particularly temperature and water availability. Based on Bewley (1997) and Nonogaki et al. (2007).

Figure 2. Hypotheses related to seed germination mechanisms. The occurrence of radicle emergence is determined by the balance between the mechanical resistance of the covering tissues such as the testa and the endosperm and the growth potential of the embryo. In Hypothesis 1, the increase in the growth potential of the embryo is associated with the effect of gibberellin (GA) during germination. The two restrictive factors in the covering tissues – the mechanical resistance of the testa (brown arrow) and the endosperm (blue arrow) and another hypothetical restrictive factor imposed by abscisic acid (ABA) in the embryo (pink arrow) are equal to or greater than the initial growth potential of the embryo (three yellow arrows). Additional increases in the embryo growth potential (black arrow) induced by the action of GA synthesized in this tissue change the global balance of forces in seed to induce radicle protrusion. Arabidopsis GA-deficient mutant ga1 seeds cannot germinate due to the lack of final embryo growth potential increase which can be compensated by exogenous GA. In contrast, the ga1 aba1 (GA- and ABA-deficient) double mutants are capable of germinating without GA application, since the absence of ABA alleviates its negative effect in the embryo and removes the GA requirement. In the testa pigmentation mutant transparent testa 4 (tt4), the GA requirement is also removed because of the lack of restriction by the testa in this mutant. In Hypothesis 2, additional increase in the embryo growth potential is not required. Instead, the reduction in the mechanical resistance of the endosperm occurs, for example, by degradation of the cell walls of this tissue. The weakening of the endosperm is assumed to be inducible by GA in this hypothesis. Lack of germination in the ga1 mutant can be explained by the lack of endosperm weakening. The seeds of the ga1 aba1 and ga1 tt4 double mutants are still capable of germinating in the absence of GA, since another restrictive factor in the embryo and the testa, respectively, is missing in these mutants. These are not conflicting hypotheses; the increase in the embryo growth potential and the decrease in the mechanical resistance can occur simultaneously. Endo: endosperm, GP: growth potential. From Nonogaki (2006) Seed germination – the biochemical and molecular mechanisms. Breeding Science 56: 93–105, reproduced by permission of Japanese Society of Breeding.

Figure 3. Diagrammatic representation of the major events taking place during mobilization in a young cereal (barley) seedling following germination. The plant hormone GA is released from the scutellum and diffuses to the living cells of the aleurone layer where it promotes the synthesis of several hydrolytic enzymes (Jones and Armstrong, 1971). These are secreted into the nonliving cells of the starchy endosperm where the starch and protein reserves are stored. -Amylase and maltase are key enzymes in the degradation of starch (see Figure 4) to Glc, and the proteinases hydrolyse proteins to short peptides and amino acids. The hydrolytic products are absorbed by the scutellum, which is part of the growing embryo. There the Glc is converted to sucrose, and the products of protein mobilization are converted to the amino acid glutamine (Gln) and asparagine (Asn). These are transported throughout the seedling via the vascular system as a supply of nutrients to support growth.

Figure 4. Hydrolysis of starch grains in cereals by amylolysis. In legumes amylose and amylopectin may be converted initially to glucose-1-phosphate (Glc-1-P), maltose and limit dextrins by starch phosphorylase and -amylase, and then further to Glc by limit dextrinase, -amylase and maltase.

Figure 5. Pathways of triacylglycerol (TAG) catabolism and hexose assimilation. Enzymes: 1, lipases; 2, fatty acid thiokinase; 3, acyl-CoA dehydrogenase; 4, enoyl-CoA hydratase (crotonase); 5, -hydroxyacyl-CoA dehydrogenase; 6, -ketoacyl thiolase; 7, citrate synthetase; 8, aconitase; 9, isocitrate lyase; 10, malate synthetase; 11, malate dehydrogenase; 12, catalase; 13, succinate dehydrogenase; 14, fumarase; 15, malate dehydrogenase; 16, phosphoenolpyruvate carboxykinase; 17, enolase; 18, phosphoglycerate mutase; 19, phosphoglycerate kinase; 20, glyceraldehyde-3-phosphate dehydrogenase; 21, aldolase; 22, fructose-1,6-bisphosphatase; 23, phosphohexoisomerase; 24, phosphoglucomutase; 25, UDPGlc pyrophosphorylase; 26, sucrose synthetase or sucrose-6-P synthetase and sucrose phosphate. (i) Glycerol kinase; (ii) -glycerol phosphate oxidoreductase. Substrates: TAG, triacylglycerol; MAG, monoacylglycerol; Gly, glycerol; FFA, free fatty acid; PEP, phosphoenolpyruvate; 2PGA, 2-phosphoglyceric acid; 3PGA, 3-phosphoglyceric acid; DPGA, 1,3-diphosphoglyceric acid; G3P, glyceraldehyde 3-phosphate; FruDP, fructose-1,6-bisphosphate; Fru-6-P, fructose-6-phosphate; Glc-6-P, glucose-6-phosphate; Glc-1-P, glucose-1-phosphate; UDPGlc, uridine diphosphoglucose; -Gly P, -glycerol phosphate; DHAP, dihydroxyacetone phosphatase. Coenzymes and energy suppliers: FAD(H), flavin adenine dinucleotide (reduced); NAD(H), nicotinamide adenine dinucleotide (reduced); GTP, guanosine triphosphate; ATP, adenosine triphosphate; UTP, uridine triphosphate; GDP, guanosine diphosphate; ADP, adenosine diphosphate; AMP, adenosine monophosphate; CoA, coenzyme A. Based on Bewley and Black (1994).

Scheme 1. Hydrolysis of storage proteins to their constituent amino acids by proteinases.

Scheme 2. Initial TAG hydrolysis by lipases.