The emphasis of modern dormancy research is almost entirely on the form of dormancy that is acquired during seed development, primary dormancy. Abscisic acid (ABA) appears to be intimately involved in its regulation. The action of abscisic acid has also been implied in many other developmental processes. The coincidence of developmental events, such as dehydration and completion of maturation, with the acquisition of primary dormancy suggests that dormancy is influenced by these processes. Germinability, both during development and after maturation, is sometimes directly correlated with ABA content. The lack of such a correlation may be explained by assuming a decisive role for the responsiveness to ABA or other overriding factors. ABA has been detected in all seed components. The different seed tissues may all contribute, to various extents, to the degree of whole seed dormancy. It is concluded that ABA action in dormancy regulation is not restricted to the embryo but is also located in endospermic tissue. In addition, a role of ABA in the morphological development of germination modifying seed tissues is proposed. The mechanism for ABA action appears to be associated with cell wall properties.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0960258500002634

A critical update on seed dormancy. I. Primary dormancy

Cell wall storage polysaccharides (CWSPs) are found as the principal storage compounds in seeds of many taxonomically important groups of plants. These groups developed extremely efficient biochemical mechanisms to disassemble cell walls and use the products of hydrolysis for growth. To accumulate these storage polymers, developing seeds also contain relatively high activities of noncellulosic polysaccharide synthases and thus are interesting models to seek the discovery of genes and enzymes related to polysaccharide biosynthesis. CWSP systems offer opportunities to understand phenomena ranging from polysaccharide deposition during seed maturation to the control of source-sink relationship in developing seedlings. By studying polysaccharide biosynthesis and degradation and the consequences for cell and physiological behavior, we can use these models to develop future biotechnological applications.

Seed Cell Wall Storage Polysaccharides: Models to Understand Cell Wall Biosynthesis and Degradation

The addition of abscisic acid (ABA) to mature non-dormant seeds inhibits their germination. This effect of ABA might be related to its natural function as an endogenous inhibitor of precocious germination during seed formation. In this work, we studied how ABA affects the germination of mature seeds and the growth of nascent seedlings of Arabidopsis thaliana (L.) Heynh. Our findings were as follows: (i) inhibition by ABA was gradual, dose-dependent, and did not disappear after germination; (ii) inhibition of germination was relieved by the addition of metabolizable sugars or amino acids to the plating media; (iii) the effect of sugars and amino acids was cooperative, indicating that these two groups of metabolites relieve different deficiencies; (iv) ABA caused appreciable alterations in energy and nitrogen metabolism; and (v) ABA prevented the degradation of the seed storage proteins. In summary, ABA appears to inhibit seed germination by restricting the availability of energy and metabolites. This mechanism seems consistent with other known effects of ABA.

Abscisic acid inhibits germination of mature Arabidopsis seeds by limiting the availability of energy and nutrients

Two new aspartic proteinases have been isolated from stigmas of the cardoon Cynara cardunculus L. by a two-step purification procedure including extraction at low pH, gel filtration on Superdex 200, and ion-exchange chromatography on Mono Q. To follow the conventional nomenclature for aspartic proteinases, we have named these proteinases cardosin A and cardosin B. On SDS/PAGE, cardosin A migrated as two bands with apparent molecular masses of 31 000 Da and 15000 Da where as the chains of cardosin B migrated as bands of 34000 Da and 14000 Da. The partial amino acid sequences of the two cardosins revealed that they are similar but not identical, and that they differ horn the previously reported cardoon proteinases named cynarases, which were assumed to be derived from a common precursor. Although the cardosins show some degree of similarity to each other, we could detect no immunological cross-reactivity between them. Both cardosins were active at low pH and were inhibited by pepstatin, with Ki values of 3 nM for cardosin A and 1 nM for cardosin B, indicating that they belong to the class of aspartic proteinases. Significant differences between the two enzymes were also found for the Kcat/Km values for the hydrolysis of two chromophoric synthetic peptides. The active-site ionization constants, pKe1 and pKe2, for cardosin A are 2.5±0.2 and 5.3±20.2, whereas for cardosin R they are 3.73±10.09 and 6.7±50.1. The results herein described on the structural and kinetic properties of the cardosins indicate that they are the products of distinct genes which have probably arisen by gene duplication. A scheme for the proteolytic processing of the two enzymes is also proposed.

/pdf/purification-characterization-and-partial-amino-acid-56mcag77l3.pdf

Purification, characterization and partial amino acid sequencing of two new aspartic proteinases from fresh flowers of Cynara cardunculus L.

Rupture of the seed coat and rupture of the endosperm are separate events in the germination of Nicotiana tabacum L. cv Havana 425 seeds. Treatment with 10–5 M abscisic acid (ABA) did not appreciably affect seed-coat rupture but greatly delayed subsequent endosperm rupture by more than 100 h and resulted in the formation of a novel structure consisting of the enlarging radicle with a sheath of greatly elongated endosperm tissue. Therefore, ABA appears to act primarily by delaying endosperm rupture and radicle emergence. Measurements of [beta]-1,3-glucanase activity, antigen content, and mRNA accumulation together with reporter gene experiments showed that induction of class I [beta]-1,3-glucanase genes begins just prior to the onset of endosperm rupture but after the completion of seed-coat rupture. This induction was localized exclusively in the micropylar region of the endosperm, where the radicle will penetrate. ABA treatment markedly inhibited the rate of [beta]-1,3-glucanase accumulation but did not delay the onset of induction. Independent of the ABA concentration used, onset of endosperm rupture was correlated with the same [beta]-1,3-glucanase content/seed. These results suggest that ABA-sensitive class I [beta]-1,3-glucanases promote radicle penetration of the endosperm, which is a key limiting step in tobacco seed germination.

Class I [beta]-1,3-Glucanases in the Endosperm of Tobacco during Germination

A high-molecular-weight aspartyl proteinase complex, sensitive to pepstatin A was purified to near electrophoretic homogeneity from dry seeds ofjack pine (Pinus banksiana, Lamb.). Two partial activities, endo-and exo-proteinolytic, were associated with the complex, judged by the analysis of haemoglobin hydrolysis products. The high activity of this enzyme in dry jack pine seed suggests a possible function in early germination.

Purification and characterization of an aspartyl proteinase from dry jack pine seeds

Galactomannan hydrolysis in isolated endosperms of fenugreek seeds is promoted by the presence of the embryo. Incubation in a large volume in the absence of the axis also results in endosperm mobilization and an increase in endo-β-mannanase activity. This is prevented when endosperms are incubated in a small volume, or when abscisic acid is present in the large volume. Fenugreek endosperms contain abscisic acid which is present in greater concentrations when these tissues have been incubated in a small volume rather than in a large volume. Antibodies prepared against purified tomato endo-β-mannanase detect the fenugreek enzyme on Western blots. Endosperms contain equal amounts of this enzyme whether they are incubated in a large volume, or in a small volume, or in the presence of abscisic acid. Thus, increased enzymic activity may not be related to increased synthesis, but rather to activation or release from the cells into the galactomannan-containing cell walls.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0960258500000623

Endo-β-mannanase activity and reserve mobilization in excised endosperms of fenugreek is affected by volume of incubation and abscisic acid

Jack pine (Pinus hanksinna Lamb.) seeds were primed in 10% (w/w) polyethylene glycol8000 for 2-6 days and their germination, as well as soluble protein, amino acid, and ATP contents compared with unprimed germinating seeds. Primed seed germinated about 2 days earlier than unprimed seed and had a high ATP content early in the priming period. Pepstatin-sensitive acid proteinase activity, which was high even in dry seeds, correlated poorly with germination in primed seed.

L. Malek

Papers

Purification and characterization of an aspartyl proteinase from dry jack pine seeds

Endo-β-mannanase activity and reserve mobilization in excised endosperms of fenugreek is affected by volume of incubation and abscisic acid

Metabolic changes related to the acceleration of jack pine germination by osmotic priming