CsABF3-activated CsSUT1 pathway is implicated in pre-harvest water deficit inducing sucrose accumulation in citrus fruit

Pre-harvest water deficit (PHWD) plays an important role in sugar accumulation of citrus fruit. However, the mechanism is not known well. Here, it was confirmed that PHWD promoted sucrose accumulation of citrus fruit, but had limited effect on fructose, glucose and total acid. A sucrose transporter, CsSUT1, which localizes to the plasma membrane, was demonstrated to function in sucrose transport induced by PHWD. Compared to wild-type, CsSUT1 overexpression in citrus calli stimulated sucrose, fructose and glucose accumulation, while its silencing in juice sacs reduced sucrose accumulation. Increased sugar accumulation in transgenic lines enhanced plant drought tolerance, and resulted in decreased electrolyte leakage, malondialdehyde and hydrogen peroxide contents, as well as increased superoxide dismutase activity and proline contents. An abscisic acid (ABA)-responsive transcription factor, CsABF3, was found with a same expression pattern with CsSUT1 under PHWD. Yeast one-hybrid, electrophoretic mobility shift assay and dual-luciferase assays all revealed that CsABF3 directly bound with the CsSUT1 promoter by ABA responsive elements. When CsABF3 was overexpressed in citrus calli, the sucrose, fructose and glucose concentration increased correspondingly. Further, transgenic studies demonstrated that CsABF3 could affect sucrose accumulation by regulating CsSUT1. Overall, this study revealed a regulation of CsABF3 promoting CsSUT1 expression and sucrose accumulation in response to PHWD. Our results provide a detail insight into the quality formation of citrus fruit.