Darwin Review - Journal of Experimental Botany 62: 3289-3309 (2011)

First off the mark: early seed germination

Abstract
Most plant seeds are dispersed in a dry, mature state. If these seeds are non-dormant and the environmental conditions are favourable, they will pass through the complex process of germination. In this review we summarize recent progress made with state-of-the-art techniques including genome-wide gene expression analyses that provided deeper insight into the early phase of seed germination, which includes imbibition and the subsequent plateau phase of water uptake in which the metabolism is reactivated. The physiological state of a seed is determined, at least in part, by the stored mRNAs that are translated upon imbibition. Very early upon imbibition massive transcriptome changes occur, which are regulated by ambient temperature, light conditions and plant hormones. The hormones abscisic acid and gibberellins play a major role in regulating early seed germination. The early germination phase of Arabidopsis thaliana culminates in testa rupture, which is followed by the late germination phase and endosperm rupture. We provide an integrated view on the early phase of seed germination and show that it is characterized by dynamic biomechanical changes together with very early alterations in transcript, protein, and hormone levels that set the stage for the later events. Early seed germination thereby contributes to seed and seedling performance important for plant establishment in the natural and agricultural ecosystem.

Contents:
Introduction
The dry seed stage: moisture content, after-ripening, and the stored transcriptome
Physical, morphological and physiological aspects of imbibition and testa rupture
Hormonal and temperature regulation of early gene expression in imbibed seeds
Reactivation of the metabolism: transcription and translation
Reactivation of the metabolism: energy production
Novel directions and techniques for studying early seed germination

Key words: dry seed state • stored mRNAs • imbibition • testa rupture • gibberellins • abscisic acid • cold stratification • transcriptome • translation • energy metabolism

"… I have had one experiment some little time in progress which will, I think, be interesting, namely, seeds in salt water, immersed in water of 32°-33° […]
I have in small bottles out of doors, exposed to variation of temperature, cress, radish, cabbages, lettuces, carrots, and celery, and onion seed—four great families.
These, after immersion for exactly one week, have all germinated, which I did not in the least expect (and thought how you would sneer at me);
for the water of nearly all, and of the cress especially, smelt very badly, and the cress seed emitted a wonderful quantity of mucus
(the 'Vestiges' would have expected them to turn into tadpoles), so as to adhere in a mass; but these seeds germinated and grew splendidly.
The germination of all (especially cress and lettuces) has been accelerated, except the cabbages, which have come up very irregularly, and a good many, I think, dead.
One would have thought, from their native habitat, that the cabbage would have stood well. The Umbelliferae and onions seem to stand the salt well."
(April 13th, 1855, cited from: Darwin, 1887).

Financial support: Our work is supported by the ERA-NET Plant Genomics grant vSEED (grant no. DFG LE720/8) to GLM and by a postdoctoral fellowship of the Deutsche Forschungsgemeinschaft to KM (grant no. MU3114/1-1).