CIRCADIAN CLOCK ASSOCIATED 1 modulates drought-induced acid and sugar accumulation in citrus fruit.

Mar 1, 2026The Plant journal : for cell and molecular biology

The circadian clock gene CIRCADIAN CLOCK ASSOCIATED 1 influences acid and sugar buildup in citrus fruit during drought

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Abstract

A citrus circadian clock gene, CsCCA1, influences citrate and sugar levels in fruit under drought stress.

CsCCA1 exhibits circadian rhythmicity and affects citrate accumulation during drought stress in citrus fruit.
Overexpression of CsCCA1 is linked to increased concentrations of citrate, sucrose, fructose, and glucose.
Silencing CsCCA1 leads to decreased levels of citrate and sugars in citrus and tomato fruit.
Higher citrate and sugar concentrations may enhance drought tolerance in citrus callus.
CsCCA1 positively regulates the transcription factor ANTHOCYANIN 1 (AN1) and sucrose transporter 1 (SUT1) involved in citrate transport and sugar entry into cells.

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Citrus fruit accumulates acid at the early stage of fruit enlargement and sugar at the fruit maturity stage. Drought stress during these stages accelerates their accumulation. Here, we reported a citrus circadian clock gene, CIRCADIAN CLOCK ASSOCIATED 1 (CsCCA1), which exhibits circadian rhythmicity and impacts citrate accumulation in citrus fruit under drought stress, even naturally working in cultivars with high or low fruit citrate levels. Further, it was found to participate in fruit sugar accumulation. Overexpression of CsCCA1 in citrus juice sacs, calli, leaves, and tomato fruits was associated with an increase in the concentration of citrate, sucrose, fructose, and glucose, whereas the silenced lines showed the opposite effect. The higher concentrations of citrate and sugar enhanced the drought tolerance of citrus callus. We also found that CsCCA1 positively regulates the bHLH transcription factor ANTHOCYANIN 1 (AN1), which plays a role in citrate transport to the vacuole, and sucrose transporter 1 (SUT1), which facilitates the entry of sucrose into cells. This regulation occurs through the binding of CsCCA1 to the evening element sequences in their promoters. This indicates the circadian clock's role in coordinating drought signal with the accumulation of citrate and sugar, providing novel insights into the formation of citrus fruit quality.

CIRCADIAN CLOCK ASSOCIATED 1 modulates drought-induced acid and sugar accumulation in citrus fruit.

Abstract

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