Brassinosteroids promote seed germination (2003)
Gerhard Leubner-Metzger
Universität Freiburg, Institut für Biologie II, Botanik, Schänzlestr.
1, D-79104 Freiburg, Germany
Book Chapter 5 in: "Brassinosteroids", Hayat S, Ahmad A (Eds.),
Kluwer Academic Publishers, The Netherlands, pp. 119-128 (2003)
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Abstract. Seed germination of Arabidopsis thaliana, Nicotiana
tabacum, and of parasitic angiosperms (Orobranche and Striga
species) is determined by the balance of forces between the growth potential
of the embryo and the mechanical restraint of the micropylar testa and/or
endosperm tissues. Brassinosteroids (BR) and gibberellins (GA) promote seed
germination of these species and counteract the germination-inhibition by
abscisic acid (ABA). Severe mutations in GA biosynthetic genes in Arabidopsis,
such as ga1-3, result in a requirement for GA application to germinate,
but germination in this phenotype can also be rescued by BR. Germination
of both the BR biosynthetic mutant det2-1 and the BR-insensitive
mutant bri1-1 is more strongly inhibited by ABA than is germination
of wild type. In contrast to GA, BR does not release tobacco photodormancy;
i.e. seed germination in darkness remains blocked. BR promotes germination
of non-photodormant tobacco seeds, but did not appreciably affect the induction
of class I ß-1,3-glucanase (ßGlu I) in the micropylar endosperm.
BR and GA promote tobacco seed germination by distinct signal transduction
pathways and distinct mechanisms. Xyloglucan endo-transglycosylase (XET)
enzyme activity accumulates in the embryo and the endosperm of germinating
tobacco seeds and this appears to be partially controlled of BR. GA and
light seem to act in a common pathway to release photodormancy, whereas
BR does not release photodormancy. Induction of ßGlu I in the micropylar
endosperm and promotion of release of ‘coat-imposed’ dormancy
seem to be associated with the GA-dependent pathway, but not with BR signaling.
It is proposed that BR promote seed germination by directly enhancing the
growth potential of the emerging embryo in a GA-independent manner.
Key words: abscisic acid - Arabidopsis - brassinosteroids - gibberellins
- ß-1,3-glucanase (ßGlu I) - Nicotiana - Orobranche
- parasitic angiosperms - seed germination - Striga - xyloglucan
endo-transglycosylase (XET)
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