Seed Science Research 13: 17-34 (2003)

Invited review:
Functions and regulation of ß-1,3-glucanases during seed germination, dormancy release and after-ripening

Gerhard Leubner-Metzger

Albert-Ludwigs-Universität Freiburg, Institut für Biologie II, Botanik, Schänzlestr. 1, D-79104 Freiburg i. Br., Germany

Received: 20 February 2002 / Accepted: 8 November 2002

Abstract.  ß-1,3-Glucanase (ßGlu) expression in seeds plays important roles in the regulation of seed germination and dormancy and in the defense against seed pathogens. A thick ß-1,3-glucan layer is typical for the seed envelope of Cucurbitaceous species, confers seed semipermeability and is degraded during germination. In many species with coat-imposed dormancy, the seed envelope confers a physical constraint to radicle emergence. In the Solanaceous species the micropylar endosperm and testa have this function and endosperm weakening appears to be a prerequisite for germination. Class I ßGlu is transcriptionally induced in the micropylar endosperm of tobacco, tomato and other Solanaceous seeds just prior to radicle emergence. ßGlu induction and germination are tightly linked in response to plant hormones and environmental factors, e.g. they are both promoted by gibberellins and inhibited by abscisic acid (ABA). Sense and antisense transformation of tobacco reveals two sites of ßGlu action: after-ripening-mediated release of testa-imposed dormancy and endosperm rupture during germination. The use of an ABA-inducible chimeric sense-transgene resulted in over-expression of class I ßGlu in seeds and provided direct evidence that ßGlu contributes to endosperm rupture. A model integrating ßGlu, seed dormancy, after-ripening and germination is presented and possible mechanisms for ßGlu action are discussed. It is proposed that ßGlu not only helps defend seeds against pathogens, but is also a key factor in regulating coat-imposed dormancy and germination of seeds in response to environmental and hormonal cues.

Table of Contents:
    1. Introduction
    2. Structure, regulation and function of plant ß-1,3-glucanases (ßGlu I)
    3. Nicotiana seeds as a model system for the hormonal regulation of dormancy, after-ripening and germination
    4. ßGlu I contributes to endosperm rupture
    5. Occurrence and functions of ß-1,3-glucans in seeds
    6. Effects of ßGlu I and after-ripening on testa rupture and photodormancy
    7. ß-1,3-Glucanases during the germination of non-endospermic dicot seeds
    8. ß-1,3-Glucanases during the germination of cereal caryopses
    9. Seed pathogen-related functions of ß-1,3-glucanase
  10. A speculative model for tobacco seed dormancy and germination
  11. References

Key words: Abscisic acid - After-ripening - Gibberellin - ß-1,3-Glucanase - Nicotiana seeds - Seed dormancy - Seed germination

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