, Volume 199, Issue 2, pp 282-288

Effects of gibberellins, darkness and osmotica on endosperm rupture and class I β-1,3-glucanase induction in tobacco seed germination

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Abstract

The “Havana 425” cultivar of Nicotiana tabacum L. is photodormant. Gibberellins (e.g. 10−5 M GA4 or GA7) can substitute for light in releasing dormancy. Measurements of β-1,3-glucanase activity, mRNA accumulation and the activity of the class I β-1,3-glucanase B promoter indicated that class I β-1,3-glucanases are induced by GA4 in the dark in association with germination. As in the light, this induction occurred prior to endosperm rupture and was localized exclusively in the micropylar region of the endosperm where the radicle will penetrate. Abscisic acid (ABA, 10−5 M) did not appreciably affect GA-induced release of photodormancy or seed-coat rupture, but it delayed endosperm rupture and inhibited the rate of class I β-1,3-glucanase accumulation. Seeds imbibed in the light in the presence of osmotica, e.g. 0.04 M polyethylene glycol 6000, showed delayed seed-coat and endosperm rupture, delayed onset of β-1,3-glucanase induction, and decreased rates of β-1,3-glucanase accumulation. These delays were shortened by GA4 treatment. Our results suggest that GAs and ABA act at two distinct sites during germination and that expansive growth of the embryo acts in two ways by triggering β-1,3-glucanase induction and by providing force for endosperm penetration. This provides further support for our working hypothesis that class I β-1,3-glucanases promote endosperm weakening and facilitate radicle penetration.

We thank our colleague W. Schmidt-Puchta, R. Fluhr (Department of Plant Genetics, Weizmann Institute of Science, Rehovot, Israel) and E. Ward (Agrobiotechnology, Ciba Corp., Research Triangle Park, NC, USA) for providing hybridization probes, M. Thomas for technical assistance, M. Briker for care of plants, and our colleagues Arnd Sturm and Alain Meyer for critical reading of the manuscript.