Figure 1.
Sugar metabolism and accumulation in apple fruit [1], [14].
Both sorbitol (Sor) and sucrose (Suc) are unloaded to the cell wall space between sieve element-companion cell complex (SE-CC) and parenchyma cells in fruit [14]. Sor is taken up into parenchyma cells via sorbitol transporter (SOT). Suc is directly transported into parenchyma cells by plasma membrane-bound sucrose transporter (SUT), or converted to fructose (Fru) and glucose (Glc) in the cell wall space by cell wall invertase (CWINV), and then transported into the parenchyma cells by hexose transporter (HT). In the cytosol, Sor is converted to Fru by sorbitol dehydrogenase (SDH), while Suc can be converted to Fru and Glc by neutral invertase (NINV) or to Fru and UDP-glucose by sucrose synthase (SUSY). The resulting Glc and Fru can be phosphorylated to glucose 6-phsophate (G6P) and fructose 6-phosphate (F6P) by hexokinase (HK) and fructokinase (FK, specific for Fru). The conversions between F6P, G6P, G1P, and UDPG are catalyzed by phosphoglucoisomerase (PGI), phosphoglucomutase (PGM), and UDPG-pyrophosphorylase (UGP) in readily reversible reactions. The F6P produced in sugar metabolism enters glycolysis/TCA cycle to generate energy and intermediates for other processes. G1P is used for starch synthesis. UDPG can be used for cellulose synthesis or combined with F6P for re-synthesis of Suc via sucrose phosphate synthase (SPS) and sucrose-phosphatase (SPP). Most of the Fru, Glc and Suc that have not been metabolized are transported by special tonoplast transporters into vacuole for storage. Inside the vacuole, Suc can be also converted to Glc and Fru by vacuolar acid invertase (vAINV).
Figure 2.
Maximum likelihood phylogeny of Malus genes encoding enzymes and transporters involved in sugar metabolism and accumulation with those from Arabidopsis or Lycopersicon esculentum.
The tree was produced using MUSCLE and PhyML with the JTT amino acid substitution model, a discrete gamma model with 4 categories and an estimated shape parameter of 1.0. Bootstrapping was performed with 100 replicates. A, cell wall invertase (CWINV); B, neutral invertases (NINV), α and β type NINV according to Nonis et al. [37]; C, vacuolar acid invertase (vAINV); D; sucrose synthase (SUSY), different types according to Bieniawska et al. [38]; E, fructokinase (FK), cytosolic and plastid fructokinases in tomato according to Granot [19]; F, hexokinase (HK), different groups according to Karve et al. [39]; G, Sucrose phosphate synthases (SPS), Arabidopsis types according to Lutfiyya et al. [40]; H, Sucrose transporter (SUT), different groups according to Braun & Slewinski [41]; I, tonoplast monosaccharide transporter (TMT); J, vacuolar glucose transporter (vGT).
Table 1.
Comparison of relative mRNA expression for genes encoding enzymes involved in sugar metabolism (including MdSDHs, MdCWINVs, MdNINVs, MdvAINVs, MdSUSYs, MdFKs, MdHKs, and MdSPSs) among mature leaves, shoot tips, young fruit (40 DAB) and mature fruit (135 DAB) of apple.
Table 2.
Comparison of relative mRNA expression for genes encoding sorbitol transporter (SOT), sucrose transporter (SUT), tonoplast monosaccharide transporter (TMT), and vacuolar glucose transporter (vGT) among mature leaves, shoot tips, young fruit (40 DAB) and mature fruit (135 DAB) of apple.
Figure 3.
Relative mRNA expression for genes encoding enzymes involved in sugar metabolism (including MdSDHs, MdCWINVs, MdNINVs, MdvAINVs, MdSUSYs, MdFKs, MdHKs, MdSPSs) during apple fruit development.
Quantitative RT-PCR was performed with gene-specific primers, except for MdSDH2-9 where a pair of universal primer was designed from the conserved cDNA region of MdSDH2 to MdSDH9. For each sample, transcript levels were normalized with those of Actin, and the relative expression levels of each gene were obtained using the ddCT method while expression in 40-DAB-fruit was designated as ‘10’. Values are means of three replicates of the reverse transcribed RNA sample pooled from 5 biological replicates ± SD.
Figure 4.
Relative mRNA expression for genes encoding sugar transporters (including MdSOTs, MdSUTs, MdTMTs and MdvGTs) during apple fruit development.
Quantitative RT-PCR was performed with gene-specific primers. For each sample, transcript levels were normalized with those of Actin, and the relative expression levels of each gene were obtained using the ddCT method while expression in 40-DAB-fruit was designated as ‘10’. Values are means of three technical replicates of the reverse transcribed RNA sample pooled from 5 biological replicates ± SD.
Figure 5.
Activities of key enzymes involved in sugar metabolism during apple fruit development.
SDH: Sorbitol dehydrogenase; CWINV: Cell wall invertase; NINV: Neutral invertase; vAINV: Vacuolar acid invertase; SUSY: Sucrose synthase; FK: Fructokinase; HK: Hexokinase; SPS: Sucrose-phosphate synthase. Values are means of five replicates ± SD.
Figure 6.
Concentrations of sorbitol (Sor), sucrose (Suc), fructose (Fru), glucose (Glc), fructose 6-phosphate (F6P) and glucose 6-phosphate (G6P) during apple fruit development.
Values are means of five replicates ± SD.
Figure 7.
Starch concentrations during apple fruit development.
Values are means of five replicates ± SD.