Pre-harvest sprouting of cereal seeds in the field is directly linked to the seed’s dormancy level. Plants produce different compounds that regulate physiological processes, including seed germination and dormancy. When seeds are dormant, even if they have adequate moisture, heat and oxygen, they simply won’t germinate.
One of the factors which prevents seeds from germinating is a plant hormone called abscisic acid (ABA). Seeds with a high level of this hormone, or that are very sensitive to it, are dormant and will need some kind of additional treatment to kick start the germination process. Dry storage at room temperature is the most economical treatment to release seeds from the state of dormancy. When seeds are stored for an extended period of time, the amount of ABA or seed sensitivity to this hormone decreases rapidly when the seeds come into contact with water, so they have a better chance of germinating when planted.
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Dr. Belay Ayele, associate professor in the department of plant science at the University of Manitoba, is investigating what genes in wheat and barley seeds produce and degrade ABA. “We have identified some of the genes involved in the production and the degradation of this compound, and tested the functionality of those genes to see if they can alter the germination phenotype or the germination behaviour of seeds,” says Ayele.
Making the genes work
Ayele’s team has been working on this project for a couple of years. Progress would have been much faster if the wheat genome sequence, which scientists announced they had sequenced in early 2016, had been available at the start. Instead, Ayele’s team started from scratch, using Arabidopsis, rice, barley and maize genomes as a template to help identify the genes in wheat that degrade ABA or control the amount of ABA that the seed produces.
“Wheat has three genomes, which makes it more complex than other crops,” says Ayele. “When we had identified the ABA degrading genes, with our basic research program, we had to test them in another plant, Arabidopsis, the genome of which was sequenced 16 years ago. For testing the function of ABA degrading genes, we got some plant material where its ABA degrading genes were knocked out. We tested our wheat ABA degrading gene by putting it back into that plant material, and we saw a reversal on the germination phenotype.
“The original plant material was dormant because of the accumulation of ABA due to removal of the inactivating gene from the seed. But when we put our wheat gene back into that plant material, it started again inactivating the ABA, so the seeds germinated. We now know that we have identified the functional wheat gene which can control the accumulation of ABA in wheat seeds.”
The next phase of Ayele’s research involves testing the genetic material in wheat plants, and to figure out the level of dormancy that they want to achieve using the gene. “It’s a balancing act,” says Ayele. “We don’t want too much dormancy in seed because it may require an extended period of storage, which affects germination and seedling establishment, and in malt barley if can also affect the malting process. At the same time, we don’t want low dormancy because it causes pre-harvest sprouting that causes the production of low quality seed, and if there is too high a level of sprouted seed in the seed lot, the market doesn’t accept it for human food, it will go to animal feed. So there is a huge economic implication to producers if dormancy is too low.”
Ayele is now working to find the genetic markers that can balance the dormancy in seeds. “We are working on breeding lines and germplasm collections of wheat to find alleles which give that balance so we can find optimum or intermediate dormancy,” he says. To this end, the University of Manitoba and Agriculture and Agri-Food Canada’s Cereal Research Centre are collaborating on projects funded through the Western Grains Research Foundation, Manitoba Wheat and Barley Growers Association, and the Agri-Food Research and Development Initiative as part of Manitoba’s Growing Forward 2 programs.
Sequenced genome helps
Now that Ayele and other researchers are beginning to have access to the wheat genome sequence, their work is progressing faster. “It has helped us identify target gene sequences and which chromosome those genes are located on, and from now on we don’t have to go through long and complicated processes to do that,” he says. “Now we can just go to the databases and get the information we want. It is very easy. It’s a very good resource and it really facilitates our work.”
Ayele’as goal is to identify marker genes that control field sprouting in wheat and barley, to help breeders transfer of sprouting tolerance from germplasm into commercial cultivars that have high yield but are susceptible to sprouting.
“If we incorporate such genes to elite cultivars, we can help producers achieve high yields, and the quality will be also maximized because it will not be susceptible to sprouting in the field. The application of such genomic tools significantly reduces the time required to breed cultivars with improved sprouting resistance,” says Ayele.
When a seed sprouts in the field, starches, which ultimately become bread or pasta, convert to sugars, which make end products, like bread or pasta too sticky. In North America, wheat or barley that contains more than four per cent sprouted grain is downgraded and rejected for human consumption. In countries like Japan, the acceptable level is even lower.
“Each region has its own market requirement, and with climate change, and the unpredictability of precipitation, and temperature, this kind of problem will be more and more prevalent,” says Ayele. “If we can develop the tools for breeders to use that will improve or mitigate pre-harvest sprouting it would be very helpful for producers.”