From sour to sweet: the genetic pathway of jujube domestication

Jujube (Ziziphus jujuba Mill.) is one of the oldest domesticated fruit trees, with its wild relative, Z. jujuba var. spinosa, known for its high organic acid content—primarily citric acid and malic acid—which gives it a distinct sour taste. Over the course of domestication, the acidity of cultivated jujube fruits has significantly decreased, resulting in their characteristic sweetness. However, the precise genetic basis for this transformation remained unclear, highlighting the need for in-depth research into the molecular mechanisms governing citric acid metabolism.

On January 3, 2024, researchers from Shandong Agricultural University and the Shandong Institute of Pomology published a study (DOI: 10.1093/hr/uhae003) in Horticulture Research that identified a key genetic mutation influencing citric acid degradation in jujube fruit. Their findings pinpointed a single nucleotide polymorphism (SNP) in the ZjACO3 promoter that is strongly correlated with fruit acidity levels.

The study revealed that a mutation at position −484 (A>G) in the ZjACO3 promoter altered the sequence from CAAGTG in wild jujube to CAGGTG in cultivated varieties. This seemingly small genetic change had a profound impact on acidity regulation. In cultivated jujube, the mutated promoter (CAGGTG) enhances the binding affinity of the transcription factor ZjbHLH113, leading to higher expression of ZjACO3. This, in turn, promotes citric acid degradation via the GABA pathway, reducing fruit acidity. In wild jujube, the original sequence (CAAGTG) results in weaker ZjbHLH113 binding, leading to lower ZjACO3 expression and higher citric acid accumulation, which contributes to its sour taste.

To validate these findings, the researchers conducted yeast one-hybrid screening, transient dual-luciferase assays, and overexpression analyses. They also developed a KASP marker based on the ZjACO3 promoter mutation, which can accurately predict citric acid content in jujube fruit. This marker provides a valuable genetic tool for breeding programs, allowing for the early selection of desired acidity levels in new cultivars.

“This study provides a clear genetic explanation for the variation in citric acid content between wild and cultivated jujube,” said Dr. Chunmei Zhang, a lead researcher on the project. “The identification of the ZjACO3 mutation and its regulatory mechanism represents a significant step forward in understanding the domestication process of this important fruit tree.”

The discovery has far-reaching implications for jujube breeding. With the ZjACO3-based KASP marker, breeders can now select for precise acidity levels in new jujube cultivars, improving fruit flavor to meet consumer preferences. Beyond jujube, this research contributes to the broader field of fruit tree genetics, offering insights into molecular domestication mechanisms that could inform genetic improvements in other fruit species. By unraveling the genetic control of fruit acidity, scientists are paving the way for a new era of precision breeding and enhanced fruit quality.

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References

DOI

10.1093/hr/uhae003

Original Source URL

https://doi.org/10.1093/hr/uhae003

Funding information

This work was supported by the National Natural Science Foundation of Shandong Province (ZR2019BC029), the China Postdoctoral Science Foundation (Grant No. 2019 M662416), Key R&D Program of Shandong Province, China (2023LZGC016), and the Introduction and Training Plan of Young Creative Talents at Universities in Shandong Province: Research Group of Forest Tree Biotechnology.

About Horticulture Research

Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2022. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.