What Determines the Sweetness and Acidity of Citrus Fruits? Recent research findings from the team led by Associate Professor Lu Xiaopeng at Hunan Agricultural University, published in New Phytologist, revealed that prolonged drought periods from July to September (during fruit expansion phase) significantly impede fruit expansion in sugar oranges after 20 days of continuous drought. Moreover, after 30 days of continuous drought, there is a remarkable impact on fruit acid accumulation, especially with irreversible increases in lemon acid content. Even subsequent rehydration cannot reverse this effect.
During the summer drought period, citrus fruits. Photo by the interviewee.
Organic acids are crucial components of citrus fruit quality, directly impacting fruit flavor and consumer acceptance. Although Chinese citrus-producing regions generally receive ample rainfall, seasonal droughts during the summer and autumn are common. These periods coincide with fruit expansion and acid reduction, and prolonged drought significantly affects both fruit expansion and acid accumulation. Therefore, investigating the regulatory mechanisms of citrus fruit acidification mediated by seasonal drought during the summer and autumn is of great significance for improving fruit quality.
The team identified two key genes, CsAN1 and CsPH8, involved in drought-regulated citric acid accumulation through transcriptome data analysis of drought-stressed fruits. Subsequent experiments confirmed that both genes respond to drought/ABA signals and promote citric acid accumulation. Promoter element analysis combined with transgenic GUS staining revealed that ABRE is a core element in the promoters of CsAN1 and CsPH8 that respond to drought/ABA signals.
Mature citrus fruits. Image by contributor.
Inspired by the ABRE element, the team further explored the ABF family member CsABF3 in the ABA signaling pathway. Subsequently, it was discovered that CsABF3 responds to drought/ABA signals. Genetic transformation in citrus callus and tomato confirmed that CsABF3 positively regulates citric acid accumulation. Molecular biology evidence from ChIP and EMSA assays suggests that CsABF3 transcriptionally activates the expression of CsAN1 and CsPH8.
According to reports, this study reveals the molecular regulatory pathway of drought-induced citric acid accumulation in citrus fruits during the summer and autumn seasons. This provides an important theoretical basis for elucidating the mechanism of drought-induced acidity in citrus fruits and targeted regulation of fruit quality.
The research was supported by the National Citrus Industry Technology System, the National Key Research and Development Program, the National Natural Science Foundation of China, and the Natural Science Foundation of Hunan Province.
Related paper information: https://doi.org/10.1111/nph.19671
