CsMYB1-CwINV6 Module Involves in the Promotion of Soluble Sugar Accumulation in Citrus Fruits Under Drought Stress.

Drought can promote soluble sugar accumulation in fruits by increasing the fruit sink strength. Cell wall invertase (CwINV) plays a pivotal role in determining sink strength by regulating sucrose partitioning into the extracellular matrix. Research has demonstrated that drought stress significantly increases the transcript level of citrus CwINV6, but the transcriptional mechanisms governing its regulation under drought conditions remain elusive. In this study, we characterised the MYB transcription factor gene CsMYB1 from the citrus genome. CsMYB1 is localised in the cell nucleus, and CwINV6 is localised in the cell wall. Furthermore, the transcript levels of both CsMYB1 and CwINV6 significantly increased in 'Nanfeng' tangerine fruits (Citrus reticulata) in response to drought or ABA treatment. Transient overexpression of CsMYB1 or CwINV6 promoted the accumulation of glucose and fructose in 'Nanfeng' fruits. Conversely, transient VIGS of CsMYB1 or CwINV6 resulted in the opposite trend. Additionally, stable overexpression of CsMYB1 or CwINV6 significantly increased the soluble sugar content in the fruits of the 'Micro-Tom' tomato lines. Y1H and luciferase assays confirmed that CsMYB1 can bind to the CwINV6 promoter and positively regulate its expression. Taken together, our findings reveal that drought promotes soluble sugar distribution in citrus fruits by increasing sink strength via the CsMYB1-CwINV6 module.