In a significant advancement in agricultural biotechnology, an international team of researchers, led by scientists from the University of California, Davis, have harnessed the power of the CRISPR-Cas genome-editing tool to create a variety of rice resistant to disease while maintaining high yields. The findings of this groundbreaking study were recently published in Nature.

The novel rice variant, created via the genome editing of a newly identified gene, demonstrated robust yield and resistance to the fungus that causes the devastating disease known as rice blast. These findings, gleaned from small-scale field trials conducted in China, carry immense significance considering rice's role as a staple food for half the global population.

The initial discovery of a mutant, known as a lesion mimic mutant, was made by Guotian Li while he was a postdoctoral scholar in the lab of Distinguished Professor Pamela Ronald at UC Davis, both of whom are co-lead authors of this study. Ronald is also affiliated with the Department of Plant Pathology and the Genome Center.

"This innovative improvement to the gene has the potential to be a game-changer for farmers, signifying its crucial importance," said Ronald.

The breakthrough began in Ronald's lab, where a staggering 3,200 distinct rice strains, each possessing a unique set of mutations, were created and sequenced. Among these strains, Li identified a strain marked by dark patches on its leaves.

"Li discovered that this strain was also resistant to bacterial infection. However, it was markedly small and low-yielding," Ronald stated. "Such 'lesion mimic' mutants have been previously discovered, but their utility to farmers has been limited due to their low yield."

“We demonstrated the power of genome editing for engineering broad-spectrum disease resistance in rice (Oryza sativa). We first isolated a lesion mimic mutant (LMM) from a mutagenized rice population,” the research authors wrote. “We then demonstrated that a 29-base-pair deletion in a gene we named RESISTANCE TO BLAST1 (RBL1) caused broad-spectrum disease resistance and showed that this mutation caused an approximately 20-fold reduction in yield. RBL1 encodes a cytidine diphosphate diacylglycerol synthase that is required for phospholipid biosynthesis.”

The authors continued, writing that the “mutation of RBL1 results in reduced levels of phosphatidylinositol and its derivative phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). In rice, PtdIns(4,5)P2 is enriched in cellular structures that are specifically associated with effector secretion and fungal infection, suggesting that it has a role as a disease-susceptibility factor. By using targeted genome editing, we obtained an allele of RBL1, named RBL1^Δ12, which confers broad-spectrum disease resistance but does not decrease yield in a model rice variety, as assessed in small-scale field trials.”

Continuing his groundbreaking work at Huazhong Agricultural University in Wuhan, China, Li employed the CRISPR-Cas9 genome editing technology to isolate the gene associated with the mutation. He then successfully recreated this resistance trait, identifying a strain with a robust yield that was resistant to multiple pathogens, including the fungus responsible for rice blast.

In disease-intensive fields, the innovative rice plants yielded five times more than the control rice, which was compromised by the fungus, Ronald noted.

"Rice blast is a globally pervasive disease, affecting virtually all rice-growing regions. This is especially critical given the sheer scale of rice as a crop," she added.

Going forward, the researchers plan to introduce this mutation into commonly cultivated rice varieties. Presently, the gene has only been optimized in a model variety, "Kitaake," which is not extensively grown. The team is also exploring the application of this gene in wheat to develop disease-resistant wheat varieties.

"Many lesion mimic mutants have been discovered and subsequently sidelined due to their low yield. We're hopeful that this will motivate researchers to revisit these mutants, using genome editing to strike a balance between resistance and high yield," Ronald concluded.

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