Fig. 7: Working model showing that the BTB-BACK-TAZ domain protein MdBT2 modulates malate accumulation and vacuolar pH.

Left, with environmental stimuli (such as nitrate signaling), MdBT2 ubiquitinates MdCIbHLH1 and MdMYB73 for degradation, which reduces the transcription of malate-associated genes by MdMYB73, leading to lower malate accumulation and higher vacuolar pH. Right, with no environmental stimulus, MdCIbHLH1 enhances the activity of MdMYB73, promoting the transcription of malate-associated genes, thus resulting in higher malate accumulation and lower vacuolar pH