A cost-effective way of feeding the world
Hybrid strains of rice, which are produced by crossing two pure strains, can yield almost double the harvest, but producing them is expensive and requires farmers to purchase new seed each year. If the hybrid plants could reproduce asexually, farmers could save seed from one year to the next. How to engineer asexually reproducing rice has been a puzzle that scientists have been trying to solve for more than 30 years.
Sundaresan’s team previously showed that BBM1 is an essential trigger for plant embryo development, and that activating this gene in eggs can override the need for fertilization.
“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,” said Sundaresan. “One of the things we wondered was, maybe BBM1 is not enough; maybe it needs help.”
By examining which genes are turned on in fertilized plant eggs, the researchers identified a gene, WOX9A, for which only the sperm-carried copy of the gene is expressed. When they simultaneously activated both BBM1 and WOX9A in rice egg cells, it resulted in embryo formation 90% of the time, though activating WOX9A 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,” said Sundaresan. “So, then WOX9A comes in and clamps down on the switch so that it doesn’t flip back.”
Hybrid vigor without the need for hybrids
Because they arose from unfertilized eggs, the plants produced via this method are haploid, meaning they contain half the usual number of chromosomes. Though haploid rice plants do germinate and grow, they tend to be stunted compared to diploid plants that carry two copies of each gene.
“Haploids are valuable tools in plant breeding for producing pure lines, which enable uniform crop production,” said corresponding author Imtiyaz Khanday, an assistant professor of plant reproductive biology at UC Davis in the Department of Plant Sciences in the College of Agricultural and Environmental Sciences. “These findings also have significant implications for producing clonal seeds at high frequencies that retain the benefits of hybrid vigor.”
The next step, the researchers say, is to combine this method of activating both BBM1 and WOX9A with “synthetic apomixis,” a technique that they previously developed for asexually producing clonal seeds. This will mean that farmers can reap the benefits of hybrid vigor year after year by simply saving some of the harvest to plant the following year.
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