Artificial seed germination and premature ageing
Farmers expect sugar-beet seeds to be of the highest quality. Seed producers such as Einbeck-based KWS SAAT AG use complex technologies to ensure that seeds start germinating immediately after sowing, even under unfavourable environmental conditions. One of the techniques used by KWS SAAT AG involves a special method of seed preparation to ensure immediate germination. However, the disadvantage of this special type of pretreatment is that the seeds are likely to suffer damage when stored over the long term. Dr. Annette Büttner-Mainik, a post-doc, and Dr. Gerhard Leubner, a department head from the University of Freiburg, are working with KWS SAAT AG in a project aimed at finding ways to prevent undesired reactions from occurring in the seeds or to halt an undesired process once it has started. This project is of major economic importance as well as of interest to seed banks and basic researchers. Gerhard Leubner is head of the “The Seed Biology Place” (www.seedbiology.de) work group within the Department of Botany/Plant Physiology at the Institute of Biology II at the University of Freiburg. He is also Dr. Annette Büttner-Mainik’s supervisor.
Around 1.6 million tons of sugar are used globally every year. Sugar beet provides thirty percent of the worldwide demand, and is thus the most important source of sugar after sugar cane. KWS SAAT AG based in Einbeck, Germany, is one of the three largest international seed companies working on the optimisation of the quality of sugar-beet seeds. Farmers want seeds to germinate quickly and reliably - even when they are faced with unfavourable environmental conditions. After harvest, the sugar-beet seeds, which have a star-shaped pericarp with five projections, need to be processed in order to prevent irregular germination. Non-germinating pieces of skin are removed in a complex process that leads to the creation of pellets. During this process, up to eight percent of the seeds are discarded for quality reasons. The remaining high-quality seeds are specifically coated to make the seeds round and hence compatible with farmers’ seed drills. The pellets are also treated with fungicides and insecticides. One of the principal goals of this complex process is the uniform germination of seeds. “Seed producers invest a lot of money and energy in the effort to produce seeds that germinate homogeneously and reliably,” said Dr. Büttner-Mainik.
Young outer appearance and mature inside
Dr. Büttner-Mainik is involved in a cooperative project with KWS to investigate the physiological processes inside seeds. The priming of the seeds can be compared to the red-amber functions of traffic lights, which tell the seeds that germination is imminent. The implementation of a controlled uptake of moisture during the priming process triggers metabolic processes in a swelling seed that prepare it for germination, the first step in this process being the switching on of the energy metabolism. Shortly after, molecular repair mechanisms that repair DNA damage are then switched on, the mitochondria divide, and the entire protein biosynthesis machinery is ignited. However, the germination process is stopped “at the last minute” by drying the seeds. All the seeds will then germinate in the field immediately and synchronously after they are sown.
“Determining the right moment to halt the germination process and ensure that the seeds do not lose their dehydration tolerance is an art form,” said Dr. Leubner. “In addition, the priming of seeds is associated with a particular problem - seeds can normally be stored for many years without being damaged; however, depending on the treatment method and intensity, pretreated seeds might display a higher sensitivity to ageing and, due to specific environmental influences, they may become damaged within twelve months of storage.”
Farmers tend to buy more seeds than they are able to sow in a given year. The remaining seeds are stored under controlled temperature and moisture conditions for use the following year. However, many farmers are then dismayed to discover that the quality of the seeds has suffered. The seed tissue can be damaged to the extent that germination is impaired or even prevented. This is because the molecular structures that are introduced during the pretreatment of the seeds in order to guarantee immediate germination after sowing are very sensitive to free radicals, which attack the proteins, DNA and lipids of the seeds. In a dry state, during storage, for example, all repair mechanisms are halted and the seeds are defenceless against oxidative processes, which eventually leads to age-related damage. “In cooperation with KWS, we are now investigating the physiological and molecular mechanisms that trigger such ageing processes,” said Dr. Büttner-Mainik. The project, which also involves several other research institutions, will run for two years during which Büttner-Mainik and Leubner are planning to investigate the effects reactive oxygen species have on seed enzymes and hormones. Leubner’s research group has already shown in a previous cooperative project with KWS that hormones play a key role during germination.
A model that explains the ageing of seeds
Büttner-Mainik and her KWS cooperation partners have developed an ageing model aimed at achieving an in-depth understanding of how sugar-beet seeds age. The researchers exposed the seeds to a broad range of different combinations of moisture and temperature over different periods of time in order to see how the various factors affect the ageing process in sugar-beet seeds. They let the seeds germinate and monitored the germination process over time. The researchers’ goal is to find out which seeds germinate slowly and which seeds do not. They are also investigating the environmental influences that could affect the germination process. Over the next 18 months, Büttner-Mainik and Leubner will take a closer look at the seeds in their attempt to find answers to the question as to what happens at a molecular level when seeds age artificially. The researchers will use modern microarray analyses to identify the genes that play a key role in these processes. In addition, the researchers want to find out how the hormone system reacts and what role reactive oxygen species play during the artificial ageing process. The work requires equal measures of practical and theoretical work and requires the researchers to remove the skin from many hundreds of seeds. Real finger work!
But all the hard work is worth it. “Our quantitative approach is a world first,” said Büttner-Mainik. The results of the project will be used by KWS to optimise processes, and Leubner and Büttner-Mainik hope to be able to elucidate the general mechanisms of seed ageing. “Such information might be of importance for gene banks that collect the seeds of wild-type and cultivated plants,” said Leubner. “We could not have chosen a better time for our project. The complete sequence of the sugar-beet genome will be available in a few months' time, which will enable phylogenetic comparisons with other plant groups to be carried out. This will put what we find out into a greater perspective.” In times of changing climate, fluctuating temperature and moisture conditions will most likely become the rule, at least in developing countries. And a greater understanding of the conditions that affect the quality of seeds will be of great help to seed companies working on the optimisation of their product.
Dr. Annette Büttner-Mainik
University of Freiburg
The Seed Biology Place
Gerhard Leubner Lab