Researchers at UC Davis and Berkeley Find Way to Trigger Rice Egg Cell Development Without Fertilization

Most farmers don’t grow high-yielding varieties of rice despite it being a staple food crop for more than half the world’s population as the seeds are too expensive. However, a team of researchers from the University of California’s Davis and Berkeley campuses found a way to activate two genes in rice egg cells that trigger their development into embryos without the need for fertilization which would efficiently create high-yielding clonal strains of rice and other crops.

Led by Venkatesan Sundaresan, a Distinguished Professor in the Department of Plant Biology and Plant Sciences at UC Davis, the team previously showed that a gene called BBM1 in rice egg cells could switch on the ability of a fertilized egg to form an embryo. The method only worked about 30% of the time, though.

In a recent collaboration with researchers from UC Berkeley’s Innovative Genomics Institute, the team has showcased that simultaneously activating a second gene, WOX9A, increases the success rate to around 90%. The finding is published in the journal Nature Plants.

“It’s remarkable that after 20 years of unsuccessful efforts in clonal hybrids, there has been so much recent progress—from showing that it is actually possible back in 2019, to showing now that it can work efficiently in 2024. I’m very optimistic now that hybrids will no longer be the barrier to achieving sustainable agriculture with high yields all over the world,” stated Sundaresan.

Hybrid strains of rice can yield almost double the harvest however, producing them is expensive and requires farmers to purchase new seed each year. Farmers could save seed from one year to the next if the hybrid plants could reproduce asexually. However, for more than 30 years, scientists have been trying to solve the puzzle of how to engineer asexually reproducing rice.

Previously, Sundaresan’s team showed that BBM1 is a vital trigger for plant embryo development and that activating this gene in eggs can override the need for fertilization.

Sundaresan stated, “Switching on BBM1 artificially in the egg cell is enough to start embryogenesis and make a new plant, but this process only worked about a third of the time. One of the things we wondered was whether maybe BBM1 is not enough; maybe it needs help.”

Examining which genes are turned on in fertilized plant eggs allowed the researchers to identify a gene, WOX94, for which only the sperm-carried copy of the gene is expressed. Simultaneously activating both BBM1 and WOX94 in rice egg cells resulted in embryo formation 90% of the time, however, activating WOX94 alone did not result in embryo initiation.

“We think BBM1 is flipping a switch that primes the egg cell to transition into an embryo, but the switch is not fixed. So, then WOX9A comes in and clamps down on the switch so that it doesn’t flip back,” explained Sundaresan.

Due to them being created from unfertilized eggs, plants produced via this method are haploid, meaning they contain half the usual number of chromosomes. While haploid rice plants do germinate and grow, they also tend to be stunted compared to diploid plants which are plants that carry two copies of each gene.

Corresponding author Imtiyaz Khanday, assistant professor in the Department of Plant Sciences of the UC Davis College of Agricultural and Environmental Sciences, said, “Haploids are valuable tools in plant breeding for producing pure lines, which enable uniform crop production. These findings also have significant implications for producing clonal seeds at high frequencies that retain the benefits of hybrid vigor.”

According to the researchers, the next step is to combine this method of activating both BBM1 and WOX94 with “synthetic apomixis,” a technique that they previously developed for asexually producing clonal seeds. This means that farmers can reap the benefits of hybrid vigor year after year by simply saving some of the harvest to plant the following year.

“If we combine this trick of making an egg cell turn into an embryo without fertilization, along with another technique that knocks out meiosis, we can efficiently produce high-yielding hybrid seeds. In a world where resources are increasingly limited, it provides a path forward for sustainable agriculture for rice farmers, and in the future, for other crops as well,” said Sundaresan.

Additional authors of the study are Hui Ren and Kyle Shankle, UC Davis, and Myeong-Je Cho and Michelle Tjahjadi, UC Berkeley.