Figures (2)  Tables (1)

    • Figure 1. 

      A simplified model for malate metabolism, transport and accumulation in fruit. Major enzymes involved in malate metabolism in fruit: 1, phosphoenolpyruvate carboxylase (PEPC); 2, phosphoenolpyruvate carboxykinase (PEPCK); 3, NAD-cyMDH (NAD-cytoplasmic malate dehydrogenase); 4, NADP-cytoplasmic malate enzyme (NADP-cyME); 5, pyruvate kinase (PK); 6, NAD-mitochondrial malate dehydrogenase (NAD-mtMDH); 7, mitochondrial citrate synthase (mtCS); 8, NAD-mitochondrial malate enzyme (NAD-mtME); 9, pyruvate dehydrogenase (PDH); 10, mitochondrial aconitase (mtACO); 11, isocitrate dehydrogenase (ICDH); 12, α-oxoglutarate dehydrogenase (α-OGDH); 13, succinyl-coa synthase (SCS); 14, succinate dehydrogenase (SuDH); 15, fumarase; 16, cytoplasmic citrate synthase (cyCS); 17, glyoxylate aconitase (glyACO); 18, isocitrate lyase (ICL); 19, malate synthase (MS).

    • Figure 2. 

      Upstream regulators of major gene Ma1 regulating apple fruit acidity. The arrows represent positive regulation and the rest represent negative regulation.

    • Gene family Gene name Protein name Activation (+)/
      inhibition (−)      		 Module control Species
      MYB transcription factor MdMa1 MdMYB73 + MdBT2-MdCIbHLH1
      -MdMYB73      		 Malus domestica
      MdMa1/MdMa11 MdMYB123 + — Malus domestica
      MdMa1 MdMYB44 − MdbHLH49-MdMYB44 Malus domestica
      MdMa1 MdMYB21 − — Malus domestica
      MdVHA-A3/D2 Ma10 MdMYB44 − WD40-MdbHLH49
      -MdMYB44      		 Malus domestica
      MdVHA-B1/E MdVHP1
      MdtDT      		 MYB1/10 + MdTTG1-MdbHLH3
      -MdMYB1/10      		 Malus domestica
      MdVHA-A MdVHP1 MdMYB73 + MdBT2-
      WD40-MdbHLH1
      -MdMYB73      		 Malus domestica
      CitPH5 CitPH4 + CitTRL-CitPH4 Citrus reticulata
      WRKY transcription factor SlALMT9 SlWRKY42 − — Solanum lycopersicum
      PpALMT9 PpWRKY44 + PpABF3-PpWRKY44 Pyrus spp.
      ZjALMT4 ZjWRKY7 + — Ziziphus jujuba
      MdMa1 MdWRKY31 + — Malus domestica
      MdMDH1 MdWRKY126 + — Malus domestica
      bHLH transcription factor MdMDH1 MdbHLH3 + — Malus domestica
      NAC transcription factor AcALMT1 AcNAC1 + — Actinidia spp.
      ERF transcription factor MdMa1 MdERF72 − MdWRKY31-MdERF72 Malus domestica
      CitVHA-C4 CiERF13 + — Citrus reticulata
      PP2C family MdVHA-A3/B2/D2 Ma10 MdPP2CH − SAUR37-MdPP2CH Malus domestica

      Table 1. 

      The crucial genes and their upstream regulatory factors of fruit acidity regulation.