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Unlocking a mystery of seed development promises fatter, oilier oilseeds

Expression of the gene UPL3 was found to limit seed development in Brassica napus plants, pictured here heavy with the long, green fruits that yield oily seeds. Image credit: Peng Gao/Aquatic and Crop Resource Development Research Center (ACRD), National Research Council Canada

Expression of the gene UPL3 was found to limit seed development in Brassica napus plants, pictured here heavy with the long, green fruits that yield oily seeds. Image credit: Peng Gao/Aquatic and Crop Resource Development Research Center

The canola cooking oil lining supermarket shelves comes from the seeds of Brassica napus, a weedy-looking plant in the mustard family. Farmers of this crop understandably want varieties that yield big, oily seeds. A recent study in The Plant Cell found a gene controlling both the oil content and the size of these seeds. The findings from this stepwise investigation, led by geneticist Charlotte Miller at the John Innes Center in Norwich, England, could improve selective breeding to help farmers produce more oil.

“I’m hoping a bunch of people will use this,” Miller says. “If this technology was used widely in canola breeding, you could expect to see a large increase in yield.”

Canola, developed in Canada, is one variety of the rapeseed or oilseed rape plant, which is among the most popular vegetable oil crops worldwide, second only to palm oil and soybean oil in world supply and distribution, according to the US Department of Agriculture’s August 2019 oilseed trade report. Canada is the leading producer worldwide. Miller chose to study B. napus because it’s so popular in agriculture, its genetics are already well characterized, and it’s closely related to the classic plant model Arabidopsis thaliana, she says.

For the new work, she and colleagues grew about 100 rapeseed varieties, each of which had been previously sequenced at the transcriptome level—revealing which genes each variety expressed, and at what levels—and showing huge variation in gene activity across varieties. Miller also harvested and weighed seeds from each variety, finding some heavier than others, a proxy for their oil content.

Comparing differences in seed weight to differences in the transcriptome across varieties, the team found that one gene in particular, called UPL3, was highly active in plants with light seeds, and less so in plants with heavy seeds. The pattern suggested that UPL3‘s protein might be involved in seed development. When the researchers analyzed mutants with nonfunctional UPL3 in an Arabidopsis model, sure enough, mutant seeds were 10% heavier than wild-type seeds.

To figure out how UPL3 influenced seed weight, Miller looked for genes already known to be involved in seed development, whose activity was also affected in the mutant Arabidopsis. She found three genes, all regulated by the same protein, LEC2, a transcription factor that increases their expression, leading to larger and oilier seeds.

Previous research had shown that the UPL3 protein is part of a cellular cleanup crew, flagging other proteins for demolition by affixing them with ubiquitin molecules. Miller and her colleagues suspected that UPL3 typically flags LEC2 for breakdown, but this didn’t happen in plants with UPL3 disabled. They confirmed their suspicions, proving the gene’s mechanism of action, by demonstrating that plants mutated to overexpress UPL3 also had high levels of ubiquitin associated with their LEC2, flagging the protein for demolition.

While the genes and proteins in this study are not new to science, geneticist George Haughn of the University of British Columbia in Vancouver, Canada, says UPL3’s role in seed size and oil production is new. “They’ve identified another part of the process of the regulation of seed development,” he says. Sleuthing out the gene’s function is more additive than boundary breaking for the field, he adds. What makes this recent study truly noteworthy, in Haughn’s opinion, is the natural variation in the gene across rapeseed varieties, which breeders haven’t yet tapped to make plants with big, oily seeds.

Miller’s team contacted rapeseed breeders across the United Kingdom to ask which varieties they favor. Responses didn’t trend toward varieties with low UPL3 expression, suggesting breeders are not yet taking advantage of this added potential, she says. The Innes team patented the technology to reduce UPL3 expression several years ago.

A next question, says developmental biologist Raju Datla, is how long those bigger, oilier seeds will take to mature. From his vantage at the National Research Council Canada’s Aquatic and Crop Resource Development Research Center, in Saskatoon, Saskatchewan, Datla is thinking of the fields of canola that cover the prairies of his province, Canada’s main canola-growing region, and the farmers who already have a short, northern growing season. An oilier seed sounds good, it just has to grow fast enough, he says.

It will take more research to thoroughly probe Datla’s question and to determine if manipulating UPL3 expression has any unexpected effects on crop plants. But the UPL3 discovery could benefit other oil crops as well. Rapeseed’s UPL3 gene is very similar to that of other plants, including soybeans, Miller explains, so breeders may be able to tweak UPL3 expression across multiple species.

“The improvements in oil production in Brassica napus have stalled in the last few decades,” Haughn says. “This might give them hope they could get a better line out of breeding Brassica napus.”

Categories: Agriculture | Genetics | Plant Biology and tagged | | | | | |
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