Figure 12.

Proposed model for alternative splicing‐mediated Ma1 (ALMT9) function in transporting malate across the tonoplast in apple. Alternative splicing of Ma1 generates two transcripts encoding two isoform proteins, Ma1α and Ma1β. Ma1β is 68 amino acids shorter with much lower expression than the full‐length protein Ma1α. They interact to form Ma1α‐Ma1β heterodimers as well as Ma1α homodimers, with the heterodimers having significantly higher malate transport activity than the homodimers. Ma1β does not transport malate itself, but a threshold level of Ma1β (Ma1β ≥1/8 of Ma1α) is required for enough Ma1α‐Ma1β heterodimers to form, generating synergy for malate transport. Ma1 operates at the threshold in wild‐type “Royal Gala” apples, maximizing its malate transport activity for vacuolar malic acid accumulation (shown at the center). Overexpression (OE) of Ma1α or Ma1β triggers feedback inhibition on the native Ma1 expression via transcription factor MYB73, decreasing either the Ma1β level well below the threshold (Upper right) or the Ma1α level to a low value that limits the formation of the heterodimers and the homodimers (Lower right), both of which diminish Ma1's malate transport activity for malic acid accumulation.