Water uptake and distribution in germinating tobacco seeds investigated in vivo by nuclear magnetic resonance imaging

Arabidopsis ABA INSENSITIVE4 regulates lipid mobilization in the embryo and reveals repression of seed germination by the endosperm

Regulation of seed germination requires coordinate action by the embryo and surrounding endosperm. We used Arabidopsis thaliana to establish the relative roles of embryo and endosperm in the control of seed germination and seedling establishment. We previously showed that endospermic oil reserves are used postgerminatively via gluconeogenesis to fuel seedling establishment and that lipid breakdown is repressed by abscisic acid (ABA) in embryo but not endosperm tissues. Here, we use RNA amplification to describe the transcriptome of the endosperm and compare the hormone responses of endosperm and embryo tissues. We show that the endosperm responds to both ABA and gibberellin but that ABA in particular regulates nuclear but not plastid-encoded photosynthetic gene expression in the embryo. We also show that ABA INSENSITIVE4 (ABI4) expression is confined to the embryo, accounts for the major differences in embryo response to ABA, and defines a role for ABI4 as a repressor of lipid breakdown. Furthermore, ABI5 expression in the endosperm defines a second region of altered ABA signaling in the micropylar endosperm cap. Finally, embryo and endosperm ABA signaling mutants demonstrate the spatial specificity of ABA action in seed germination. We conclude that the single cell endosperm layer plays an active role in the regulation of seed germination in Arabidopsis.

Seed dormancy and the control of germination

Seed dormancy is an innate seed property that defines the environmental conditions in which the seed is able to germinate. It is determined by genetics with a substantial environmental influence which is mediated, at least in part, by the plant hormones abscisic acid and gibberellins. Not only is the dormancy status influenced by the seed maturation environment, it is also continuously changing with time following shedding in a manner determined by the ambient environment. As dormancy is present throughout the higher plants in all major climatic regions, adaptation has resulted in divergent responses to the environment. Through this adaptation, germination is timed to avoid unfavourable weather for subsequent plant establishment and reproductive growth. In this review, we present an integrated view of the evolution, molecular genetics, physiology, biochemistry, ecology and modelling of seed dormancy mechanisms and their control of germination. We argue that adaptation has taken place on a theme rather than via fundamentally different paths and identify similarities underlying the extensive diversity in the dormancy response to the environment that controls germination.

beta-1,3-Glucanase gene expression in low-hydrated seeds as a mechanism for dormancy release during tobacco after-ripening

An air-dry developmental state with low-hydrated tissues is a characteristic of most plant seeds. Seed dormancy is an intrinsic block of germination and can be released during after-ripening, that is air-dry storage of mature seeds. Both seed-covering layers, testa and endosperm, cause the coat-imposed dormancy of tobacco (Nicotiana tabacum). After-ripening and over-expression of class I beta-1,3-glucanase (betaGlu I) confer maternal effects on testa rupture and dormancy release. Very little is known about the molecular mechanisms of after-ripening and whether gene expression is possible in low-hydrated seeds. Transient, low-level betaGlu I transcription and translation was detected during tobacco seed after-ripening. (1)H NMR 2D micro-imaging showed uneven distribution of proton mobility in seeds. betaGlu I gene expression is associated spatially with the inner testa and temporally with the promotion of testa rupture. Local elevation in moisture content seems to permit local, low-level betaGlu I gene transcription and translation in the maternal tissues of air-dry, low-hydrated seeds. De novo gene expression is therefore proposed to be a novel molecular mechanism for the release of coat-imposed dormancy during oilseed after-ripening.