The characteristic sour-sweet balance of citrus fruits is largely defined by citric acid accumulation in the vacuoles of juice sacs. Previous studies identified several proteins — such as CsPH8, CsAN1 and CsPH4 — that control acid storage and transport. However, little was known about the CsAN11 protein.
Many citrus cultivars exhibit wide variations in acidity, even among closely related varieties, suggesting the presence of additional genetic regulators. Due to these challenges, it is essential to investigate the mechanisms that modulate citric acid metabolism and storage in citrus fruits.
A research team from the College of Horticulture and Forestry Sciences at Huazhong Agricultural University in China has revealed how the CsAIL6 protein influences fruit acidity by repressing the CsAN11 protein. The study published in Horticulture Research provides fresh insights into the genetic control of citric acid accumulation in citrus. By integrating molecular, physiological and biochemical analyses, the authors demonstrated that CsAIL6 acts as a negative regulator of fruit acidity through its interaction with CsAN11.
The researchers first found that CsAIL6 expression was high in low-acid citrus cultivars but low in high-acid varieties, showing a strong negative correlation with titratable acid content. During fruit development, CsAIL6 levels increased as citric acid declined, suggesting a suppressive role in acid buildup. Transient overexpression of CsAIL6 in citrus and stable transformation in tomato both reduced fruit citric acid content, while silencing CsAIL6 had the opposite effect.
Further analysis of transgenic lines showed that overexpressing CsAIL6 specifically downregulated CsAN11, without significantly affecting CsAN1 or CsPH4. Assays confirmed a direct interaction between CsAIL6 and CsAN11. Overexpression of CsAN11 increased citric acid content. These findings establish that CsAIL6 functions as a key repressor of citric acid accumulation by targeting CsAN11.
“Our findings reveal an elegant regulatory mechanism where CsAIL6 acts like a molecular brake on fruit acidity,” said corresponding author Prof. Yong-Zhong Liu. “By binding to CsAN11, it prevents excessive citric acid buildup, providing a genetic explanation for why some citrus cultivars taste less sour.”
The discovery of CsAIL6 as a key repressor of citric acid accumulation offers valuable genetic resources for developing citrus varieties with tailored acidity and improved consumer appeal. Manipulating CsAIL6 expression could help breeders produce sweeter or more balanced citrus fruits without compromising other quality traits.
Source: Newswise
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