Seeds are a vital component of the world’s diet. Cereal grains alone, which comprise -90% of all cultivated seeds, contribute up to half of the global per capita energy intake. Not surprisingly then, seed biology is one of the most extensively researched areas in plant physiology. Even in relation to the topics reviewed here, a casual perusal of the Agricola database reveals that well over 5000 publications on seed germination and 700 on seed dormancy have appeared in the last decade. Yet we still cannot answer two fundamental questions: how does the embryo emerge from the seed to complete germination, and how is embryo emergence blocked so that seeds can be maintained in the dormant state? Obviously, with such a large literature on the subject, this review is far from comprehensive. Nevertheless, it provides both an overview of the essential processes that are associated with germination and a description of the possible impediments thereto that may result in dormancy. With the seed, the independence of the next generation of plants begins. The seed, containing the embryo as the new plant in miniature, is structurally and physiologically equipped for its role as a dispersa1 unit and is well provided with food reserves to sustain the growing seedling until it establishes itself as a self-sufficient, autotrophic organism. Because the function of a seed is to establish a new plant, it may seem peculiar that dormancy, an intrinsic block to germination, exists. But it may not be advantageous for a seed to germinate freely, even in seemingly favorable conditions. For example, germination of annuals in the spring allows time for vegetative growth and the subsequent production of offspring, whereas germination in similar conditions in the fall could lead to the demise of the vegetative plant during the winter. Thus, dormancy is an adaptive trait that optimizes the distribution of germination over time in a population of seeds. Seed dormancy is generally an undesirable characteristic in agricultural crops, where rapid germination and growth are required. However, some degree of dormancy is advantageous, at least during seed development. This is particularly true for cereal crops because it prevents germination of grains while still on the ear of the parent plant (preharvest sprouting), a phenomenon that results in major losses to the

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Seed Germination and Dormancy.

The proposal is made that seed scientists need an internationally acceptable hierarchical system of classification for seed dormancy. Further, we suggest that a modified version of the scheme of the Russian seed physiologist Marianna G. Nikolaeva be adopted. The modified system includes three hierarchical layers – class, level and type; thus, a class may contain levels and types, and a level may contain only types. The system includes five classes of dormancy: physiological dormancy (PD), morphological dormancy (MD), morphophysiological dormancy (MPD), physical dormancy (PY) and combinational dormancy (PY + PD). The most extensive classification schemes are for PD, which contains three levels and five types (in the non-deep level), and MPD, which contains eight levels but no types. PY is not subdivided at all but probably should be, for reasons given. Justifications are presented for not including mechanical dormancy or chemical dormancy in the modified scheme. PD (non-deep level) is the most common kind of dormancy, and occurs in gymnosperms (Coniferales, Gnetales) and in all major clades of angiosperms. Since, first, this is the class and level of dormancy in seeds of wild populations of Arabidopsis thaliana and, secondly, Type 1 (to which seeds of A. thaliana belong) is also common, and geographically and phylogenetically widespread, it seems that biochemical, molecular and genetic studies on seed dormancy in this model species might have rather broad application in explaining the basic mechanism(s) of physiological dormancy in seeds.

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A classification system for seed dormancy

This review focuses mainly on eudicot seeds, and on the interactions between abscisic acid (ABA), gibberellins (GA), ethylene, brassinosteroids (BR), auxin and cytokinins in regulating the interconnected molecular processes that control dormancy release and germination. Signal transduction pathways, mediated by environmental and hormonal signals, regulate gene expression in seeds. Seed dormancy release and germination of species with coat dormancy is determined by the balance of forces between the growth potential of the embryo and the constraint exerted by the covering layers, e.g. testa and endosperm. Recent progress in the field of seed biology has been greatly aided by molecular approaches utilizing mutant and transgenic seeds of Arabidopsis thaliana and the Solanaceae model systems, tomato and tobacco, which are altered in hormone biology. ABA is a positive regulator of dormancy induction and most likely also maintenance, while it is a negative regulator of germination. GA releases dormancy, promotes germination and counteracts ABA effects. Ethylene and BR promote seed germination and also counteract ABA effects. We present an integrated view of the molecular genetics, physiology and biochemistry used to unravel how hormones control seed dormancy release and germination.

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Plant hormone interactions during seed dormancy release and germination

Seed dormancy is an important component of plant fitness that causes a delay of germination until the arrival of a favourable growth season. Dormancy is a complex trait that is determined by genetic factors with a substantial environmental influence. Several of the tissues comprising a seed contribute to its final dormancy level. The roles of the plant hormones abscisic acid and gibberellin in the regulation of dormancy and germination have long been recognized. The last decade saw the identification of several additional factors that influence dormancy including dormancy-specific genes, chromatin factors and non-enzymatic processes. This review gives an overview of our present understanding of the mechanisms that control seed dormancy at the molecular level, with an emphasis on new insights. The various regulators that are involved in the induction and release of dormancy, the influence of environmental factors and the conservation of seed dormancy mechanisms between plant species are discussed. Finally, expected future directions in seed dormancy research are considered.

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Molecular mechanisms of seed dormancy

Microbial mannanases have become biotechnologically important since they target the hydrolysis of complex polysaccharides of plant tissues into simple molecules like manno-oligosaccharides and mannoses The role of mannanases in the paper and pulp industry is well established and recently they have found application in the food and feed technology, coffee extraction, oil drilling and detergent industry Mannanses are enzymes produced mainly from microorganisms but mannanases produced from plants and animals have also been reported Bacterial mannanases are mostly extracellular and can act in a wide range of pH and temperature, though acidic and neutral mannanases are more common This review will focus on complex mannan structure and the microbial enzyme complex involved in its complete breakdown, mannanase sources, production conditions and their applications in the commercial sector The reference to plant and animal mannanases has been made to complete the overview However, the major emphasis of the review is on the microbial mannanases

Microbial mannanases: an overview of production and applications.

A new assay for quantifying endo-β-d-mannanase activity using congo red dye

Intact wild-type tomato (Lycopersicon esculentum cv. Moneymaker) seeds do not complete germination to the same percentage or at the same speed as intact ABA-deficient sitiens (sit w ) mutant seeds when seeds of both genotypes are imbibed on polyethylene glycol (PEG) solutions of -0.3 to -1.5 MPa osmotic potential. However, if the thicker testas of wild-type seeds are removed (stripped) from the micropyle without damaging the endosperm, both the percentage and speed of germination at low external water potential are similar to that of sit w mutant seeds. Removing the micropylar end of the testa from sit w seeds did not enhance either the speed or percentage of germination on PEG solution. Despite similar germination percentage and speed between stripped wild-type seeds and either stripped or intact sit w seeds under osmotic stress, some differences in seed metabolism are evident between genotypes. The activity of endo-β-mannanase was greater in the endosperm of sit w mutant seeds compared to the endosperm of wild-type seeds when seeds were exposed to osmotic stress. Although α-galactosidase activities were similar between the two genotypes, monosaccharide, sucrose and raffinose contents were greater in the sit w mutant embryos than the wild-type embryos when tomato seeds were exposed to -0.3MPa external water potentials. Neutral sugar contents actually increased above the amounts of these sugars in 1 h imbibed seeds during germination on -0.3 MPa PEG solution. However, neither greater endo-β-mannanase activity nor greater amounts of sugar in the sit w seeds result in significant differences in germination parameters when stripped wild-type and stripped or intact sit w seeds were germinated on PEG solutions of -0.3 MPa. The ability of intact, ABA-deficient sit w mutant tomato seeds to complete germination at much lower external osmotic conditions relative to the intact wild-type Moneymaker cultivar was solely dependent on a structural alteration in the mutant testa, making it more delicate and lesssening its resistance to penetration by the radicle.

Primary dormancy in tomato (Lycopersicon esculentum cv. Moneymaker): studies with the sitiens mutant

Multiple isozymes of endo-β-D-mannanase in dry and imbibed seeds

The IDS (incubation–desiccation–separation) technique, designed to upgrade germinability and vigour of conifer seed lots, was tested on various seed lots of white spruce (Piceaglauca (Moench) Voss), lodgepole pine (Pinuscontorta var. latifolia Engelm.), and jack pine (Pinusbanksiana Lamb.). The effect on germinability of redrying separated seeds to storage moisture content (approx. 5% fresh weight) and of accelerated ageing was also investigated. Separation resulted in significant increases in germinability for four of five lodgepole pine, three of four jack pine, and two of five white spruce seed lots. Redrying usually had no significant effect on germinability of separated seeds, and its effect was not predictable. Accelerated ageing decreased the mean germinability of all seed lots and fractions. Although the mean index of ageing was usually not significantly different between the control and bottom fraction (live seeds) for any species, the surface fraction (dead and dying seeds) usually had a signifi...

Upgrading germinability and vigour of jack pine, lodgepole pine, and white spruce by the IDS technique

The germination of intact, moist-chilled white spruce (Picea glauca [Moench.] Voss.) seeds and liquid–nitrogen–decoated (LN2), unchilled seeds is significantly more than of intact, unchilled seeds and intact, unchilled seeds exposed to LN2. The testa is largely responsible for imposition of dormancy in these seeds, although the megagametophyte and/or nucellus also play a role. One or both of these tissues undergoes significant weakening during moist chilling. Excised embryos from dormant and nondormant white spruce seeds elongate when placed on solid Murashige and Skoog minimal organics medium supplemented with a carbon source (sucrose, glucose or galactose), but elongate very little on medium without sugar. They are not killed by rapid imbibition on unsupplemented media. Thus, embryos of white spruce do not exhibit innate dormancy, but are dependent on a carbon source for elongation, and have dormancy imposed on them by their surrounding structures.

Bruce Downie

Papers

A new assay for quantifying endo-β-d-mannanase activity using congo red dye

Primary dormancy in tomato (Lycopersicon esculentum cv. Moneymaker): studies with the sitiens mutant

Multiple isozymes of endo-β-D-mannanase in dry and imbibed seeds

Upgrading germinability and vigour of jack pine, lodgepole pine, and white spruce by the IDS technique

Dormancy in white spruce (Picea glauca [Moench.] Voss.) seeds is imposed by tissues surrounding the embryo