University of Florida (UF) scientists achieved a major milestone in their quest to develop an HLB-resistant tree by sequencing the genome of a fruit plant that’s a close cousin to citrus trees. HLB is also known as greening disease.
UF Institute of Food and Agricultural Sciences researchers sequenced the genome from trifoliate orange, in collaboration with scientists from the University of California at Berkeley, the U.S. Department of Energy’s Joint Genome Institute and UF’s Interdisciplinary Center for Biotechnology Research. The work will help with breeding new citrus trees that will survive under today’s challenging conditions, including invasive pests, viruses and changing climates. See a summary of the research here.
“Very importantly, trifoliate orange and its hybrids have genes that can confer high tolerance to citrus greening and resistance to the Asian citrus psyllid, the insect that transmits greening to citrus,” said Zhanao Deng, a senior author of the new UF/IFAS-led study. “This genome can be used as a reference template to sequence widely used trifoliate orange hybrid rootstock varieties.”
“Most people – even citrus growers – rarely see trifoliate oranges. This is because they usually are the rootstock part of the tree, mostly underground,” said Fred Gmitter, a UF/IFAS professor of citrus breeding genetics and a co-author of the study.
Trifoliate oranges or their hybrids are grown at nurseries, and farmers use them as rootstock to grow the citrus that’s above ground. Trifoliate orange and its hybrids were used as the rootstock for more than three million citrus trees in Florida alone in 2018-2019, UF/IFAS researchers say.
Trifoliate orange and its hybrid rootstocks accounted for 82 percent of the top 20 rootstocks used in the 2018-2019 citrus propagation cycle in Florida.
“Our trifoliate orange genome will allow scientists to develop new tools that can more speedily transfer beneficial genes into sweet oranges, grapefruit and breeding of new scion cultivars, which grow above the ground,” Deng said.
“Releasing the first trifoliate orange genome can be valuable for our citrus gene-editing efforts,” Gmitter said. Scientists are using gene editing to produce canker-resistant and HLB-tolerant citrus.
“Because of our high-quality genome, re-sequencing of trifoliate orange hybrid rootstock varieties will be much easier, much quicker and much more cost-efficient,” said Deng. “Re-sequencing will enable development of new breeding tools, such as DNA marker-based selection, genomic selection of new rootstock varieties with resistance and tolerance to citrus greening, citrus tristeza virus and citrus nematodes. The new varieties might give higher yield and fruit quality.”
Citrus breeders want to introduce desirable genes from trifoliate orange into sweet orange, grapefruit and other varieties. It took decades to produce the first citrus scion variety (Sun Dragon) from crossing trifoliate orange and transferring some of its genes across multiple generations into sweet orange. With this new information from genome sequencing, that timeline can be dramatically reduced.
The research project was funded by grants from the Citrus Research and Development Foundation and the U.S. Department of Agriculture/National Institute of Food and Agriculture Citrus Disease Research and Extension program.
Source: University of Florida Institute of Food and Agricultural Sciences
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