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Sound asleep seeds

Plants are very choosy, especially when it comes to germination. Embryos enclosed within the seed coat can survive for a long time in soil, even up to several decades, and ‘wake’ up when the environmental conditions are right. Dr. Gerhard Leubner and his colleagues from the Institute of Biology II at the University of Freiburg are investigating the regulation of seed dormancy and the onset of germination. The scientists’ insights are extremely valuable for companies that are working on the optimisation of seeds for agricultural purposes.

Logo of the Seed Biology website maintained by Gerhard Leubner that has the highest number of visitors of websites dealing with seed biology worldwide (www.seedbiology.de) (Figure: Work group Gerhard Leubner) © AG Gerhard Leubner
Seeds are central to a plant’s life cycle. Seeds are not only used by plants to spread into new habitats through birds or on the wind, but they are also perfect as protection against drought. Plant embryos are able to survive in seeds even with a water content as low as 5 to 10%. Plants tend to germinate only under optimal environmental conditions and they go through a resting stage while they are waiting for such conditions to occur. This stage is referred to as dormancy. Dormancy is a physiological barrier that prevents germination.

The sum of temperatures is important

“The onset of germination is impossible during dormancy,” said Dr. Gerhard Leubner, group leader at the Institute of Biology II, Botany/Plant Physiology, Department of Molecular Plant Sciences. “Even if it is warm enough and sufficient quantities of water are available, the seeds will remain in the dormant stage.” The barrier to germination is established during the development of a plant embryo by the plant hormone abscisic acid and the genetic programmes governed by this hormone.
The embryo emerges from dormancy once certain environmental conditions have been met. For example, the germination of tobacco seeds heavily depends on temperature, or to be more precise, temperature history. Tobacco embryos register cold and warm phases. The embryo emerges from dormancy when the sum of all temperature fluctuations has reached a certain threshold value. In this way, the plant ensures that germination occurs in spring rather than on a single warm day in November.

“However, we still do not know how tobacco seeds measure the accumulation of certain temperatures,” said Leubner. “But we do know that the end of the dormancy period is regulated by certain plant hormones.” The gibberellins are such hormones. During cold nights, the concentration of gibberellin molecules increases in the embryo. In addition, the sensitivity of the embryonic tissue to these hormones increases. The concentration of abscisic acid, which inhibits endosperm weakening and hence initiates embryo dormancy, decreases. Differences in the endogenous levels of abscisic acid and gibberellins are one of the major factors that enable the plants to change from dormancy to activity.

Through two layers

The radicle protrudes little by little from the tip of the tobacco seed – at first, the brown seed coat ruptures, followed by the endosperm (white) located further down. (Figure: www.seedbiology.de)
When dormancy comes to an end, germination commences with the uptake of water, followed by embryo expansion; the embryo elongates and breaks through the covering layers. Leubner and his team are also carrying out a detailed investigation of this process using garden cress seeds that are considerably bigger than tobacco seeds, and Arabidopsis seeds, a popular laboratory plant that is well characterised on the molecular level. The investigations into these two seed types clearly show that the process of germination is very complicated. The embryo is surrounded by two types of envelopes – an outer coat (testa) and an inner protective and nutritive tissue (endosperm) layer. The radicle has to break through these two layers.

Leubner, his doctoral students Ada Linkies and Kerstin Müller and his degree students are mainly interested in how the radicles are able to bore through the inner envelope. This is mainly made possible because the endosperm weakens during the germination process, induced by enzymes that degrade the cell walls of the surrounding tissue. The scientists are now using molecular biology and genetic methods to look for the genes. They are using a biomechanical tool to show that the weakening is a prerequisite for the germination of the seeds: specially prepared plant envelopes are fixed into a clamp and a metal rod pressed against the tissue with a specific amount of force. This enables the scientists to measure the point at which the radicle is able to break through the endosperm. “These different approaches have helped us to obtain increasing amounts of information on the overall process,” said Leubner.
The radicle protrudes little by little from the tip of the tobacco seed – at first, the brown seed envelope ruptures, followed by the endosperm (white) located further down. (Figure: www.seedbiology.de)
The radicle protrudes little by little from the tip of the tobacco seed – at first, the brown seed coat ruptures, followed by the endosperm (white) located further down. (Figure: www.seedbiology.de)

A challenge for agriculture

The findings are of great interest for the seed industry. Many companies are hoping to breed optimised seeds, i.e. seeds that germinate more rapidly, that are stronger and that can be handled more effectively with agricultural machines. In another project, Leubner and his colleagues are investigating how wheat seeds can be released from dormancy through dry storage, and how to make them more sensitive to gibbellerins. Seeds that have been treated like this not only germinate more rapidly but also more synchronously. “The market for this type of research is enormous,” said Leubner assuming that this will still be the case even in times of climate change. The envisaged temperature fluctuations will not only be a great challenge for seeds stored in the earth’s natural seed banks, but also for seeds that are sown by the farmers in their fields.

mn – 10th June 2008
© BIOPRO Baden-Württemberg GmbH

Further information:
Dr. Gerhard Leubner
Institute of Biology II, Botany/Plant Physiology
Schänzlestr. 1
University of Freiburg
Tel.: +49 (761) 203 2936
Fax: +49 (761) 203 2612
E-mail: gerhard.leubner@biologie.uni-freiburg.de

Website address: https://www.biooekonomie-bw.de/en/articles/news/sound-asleep-seeds