Hubei Researchers Identify Key Gene Module to Shorten Citrus Breeding Cycle

On March 31, reporters learned from the Hubei Academy of Agricultural Sciences (HAAS) that the Institute of Fruit and Tea of HAAS, in conjunction with the College of Horticulture and Forestry Sciences of Huazhong Agricultural University, has identified a key gene module that can simultaneously regulate sucrose and citric acid metabolism in citrus. With this module, it is expected that the breeding cycle of new citrus varieties can be shortened from 10 years to less than 6 years in the future through gene editing technology, which can directionally design the sugar-acid ratio of citrus.

Relevant research results have been published online in The Plant Cell, a top international botany journal, providing a new theoretical basis and core target for the directional genetic improvement of citrus flavor quality. Sucrose and citric acid are the core components of citrus flavor quality. During citrus ripening, sucrose content continues to rise while citric acid gradually degrades, and the dynamic balance between the two directly determines the taste of citrus.

In the past, the academic community generally believed that the sugar and acid traits of citrus were independently regulated by different genes, and there were few studies on the molecular mechanism of coordinated regulation of sugar and acid. Researchers in Hubei selected 6 representative citrus varieties, systematically integrated multi-omics data around key time points of fruit development, and drew the first transcriptional and epigenetic co-regulation map of citrus fruit during ripening.

They noticed a pair of functionally coordinated genes in the map — CrHSFA6B and CrbZIP5b. Functional verification showed that activating the CrbZIP5b gene can promote sucrose accumulation, and activating the CrHSFA6B gene can promote citric acid degradation. Under the action of ABA, a key hormone affecting fruit ripening, the CrbZIP5b gene is not only activated itself, but also activates the CrHSFA6B gene simultaneously, enabling the two processes of sugar increase and acid reduction to proceed synchronously, and ultimately realizing the flavor transformation of citrus from sour to sweet during ripening.

Wu Liming, a researcher at the Institute of Fruit and Tea, Hubei Academy of Agricultural Sciences, explained that in the past, to breed citrus varieties with high sweetness, breeders had to sequentially improve the two target traits of high sugar and low acid — first screening out parts with qualified sugar content from a large number of breeding materials, then continuing to screen out parts with qualified acid content from them.

"This step-by-step improvement method has led to a breeding cycle of often as long as ten years or even longer for a single citrus variety," he said. "Now, it is like we have found a 'switch' in the citrus. By precisely regulating one gene, we can control the process of 'acid reduction and sugar increase' and accurately adjust the sugar-acid ratio of citrus fruits."

When the CrbZIP5b gene is activated, citrus can quickly "reduce acid and increase sugar"; when it is silenced, citrus will "reduce sugar and increase acid accordingly. This will greatly shorten the breeding cycle of new citrus varieties and make it possible to customarily cultivate citrus varieties with different sweetness levels.

The study also found that the module has high functional conservation in the genus Citrus and can be widely applied to the quality improvement of different citrus groups such as ponkan, sweet orange, pomelo and kumquat. The discovery of this key gene module marks a new breakthrough in citrus flavor improvement research, injecting strong impetus into the high-quality development of the citrus industry.