Working model demonstrating that MdBT2 modulates malate accumulation and vacuolar acidification in response to nitrate. Bottom, Low nitrate or nitrate deficiency; MdCIbHLH1 enhances the activity of MdMYB73, which maintains the transcription of malate-associated genes at a high level, resulting in high malate accumulation and lower vacuolar pH. Top, MdBT2 ubiquitinates and degrades MdCIbHLH1 in response to high nitrate, which reduces the transcription of malate-associated genes by decreasing the activity of MYB73, leading to lower malate accumulation and high vacuolar pH. MBS, MYB-binding cis-elements.