| Claim/Annotation element | Evidence summary | System studied | Publication (authors, year) | URL/DOI | Context citation ID(s) |
|---|---|---|---|---|---|
| Enzyme reaction / substrate class | Tomato polygalacturonase (PG) is described as a pectin-degrading enzyme. In processing-focused work, PG2A/PG2B are the heat-labile catalytic forms collectively termed PG2, while PG1 contains PG2 plus an accessory subunit; this supports annotation of PG2 as the catalytic pectin-hydrolyzing component acting on cell-wall pectins. | Tomato fruit PG isoenzymes from ripening/processed fruit | Peeters et al., 2004 | https://doi.org/10.1002/bit.20134 | (pqac-00000011) |
| Role in ripening-associated pectin depolymerization | Antisense suppression of tomato PG reduced total PG activity by ~50–95% (up to ~99% in higher-copy lines) and decreased pectin depolymerization without preventing ripening. Authors concluded PG1 may contribute to pectin solubilization, whereas PG2, which was nearly abolished, may be mainly responsible for pectin depolymerization. | Transgenic antisense tomato fruit during ripening | Smith et al., 1990 | https://doi.org/10.1007/BF00028773 | (pqac-00000004) |
| Quantitative effect on pectin molecular mass | In normal fruit, soluble pectin molecular mass dropped to ~80 kDa by 14 days after breaker, whereas antisense lines remained at ~95 kDa and ~135 kDa, consistent with reduced PG2-linked depolymerization. Total EDTA-soluble uronic acid was not strongly changed, separating solubilization from depolymerization. | Transgenic antisense tomato fruit | Smith et al., 1990 | https://doi.org/10.1007/BF00028773 | (pqac-00000004) |
| Isoforms PG2A and PG2B | PG2 exists as two isoforms, PG2A and PG2B, with approximate apparent molecular masses of 45 kDa and 43 kDa, respectively; differences are attributed mainly to glycosylation rather than distinct catalytic identities. | Ripening tomato fruit polygalacturonase preparations | Pogson & Brady, 1993; Peeters et al., 2004 | https://doi.org/10.1007/BF00240897; https://doi.org/10.1002/bit.20134 | (pqac-00000005, pqac-00000011) |
| PG1 complex formation with beta/h-subunit | PG1 is formed from catalytic PG2 plus a glycoprotein beta/h-subunit (converter). In vitro, purified PG2 added to mature-green pericarp formed a PG1-like complex containing PG2 and the ~39 kDa beta-subunit, with PG1-like mobility and antigenicity. | Tomato fruit pericarp extracts and purified isoenzymes | Pogson & Brady, 1993 | https://doi.org/10.1007/BF00240897 | (pqac-00000005) |
| Properties of beta/h-subunit interaction | PG2 binding to the beta-subunit was robust across salt, pH, and reducing conditions and could occur even with denatured beta-subunit, indicating a strong non-covalent association. The complex forms under low ionic strength and pH 4–5.5, conditions relevant to the fruit apoplast/cell-wall space. | In vitro reconstitution using tomato PG fractions | Pogson & Brady, 1993 | https://doi.org/10.1007/BF00240897 | (pqac-00000005) |
| Thermal stability distinction between PG1 and PG2 | PG2A/PG2B are heat-labile catalytic polypeptides, while PG1 is relatively thermostable because it contains the h-subunit. The h-subunit can convert PG2 to PG1 in vitro, and the resulting PG1 resembles native PG1 in physicochemical behavior. | Tomato juice / purified isoenzymes under heat and pressure treatment | Peeters et al., 2004 | https://doi.org/10.1002/bit.20134 | (pqac-00000003, pqac-00000011) |
| In vivo relevance of PG1 | Processing work notes uncertainty over whether PG1 exists pre-formed in vivo or forms during extraction/handling, so the most secure annotation for P05117 is the catalytic PG2 polypeptide rather than the PG1 complex. | Tomato fruit PG isoenzymes | Peeters et al., 2004 | https://doi.org/10.1002/bit.20134 | (pqac-00000011) |
| Contribution to fruit softening | Reduced PG/SlPG2 expression is associated with firmer fruit, while higher PG activity accelerates softening. In SlGSNOR-silenced tomato fruit, firmness was significantly higher from 6–12 days postharvest, accompanied by reduced PG activity and lower SlPG2 transcript abundance. | Postharvest tomato fruit with SlGSNOR silencing | Liu et al., 2024 | https://doi.org/10.3390/ijms25052729 | (pqac-00000009, pqac-00000013) |
| Regulatory factor: NO/GSNOR axis | SlGSNOR positively promotes ripening-associated softening; when SlGSNOR is silenced or inhibited, SlPG2 transcript abundance decreases together with PG, pectate lyase, and cellulase activities. This implicates NO/GSNOR signaling upstream of SlPG2 expression during postharvest softening. | Postharvest tomato fruit treated with N6022/GSNO or infected with TRV-SlGSNOR | Liu et al., 2024 | https://doi.org/10.3390/ijms25052729 | (pqac-00000009, pqac-00000013) |
| Broader functional inference from modern tomato transgenics | In tomato, heterologous overexpression of raspberry RiPG2 increased PG activity, decreased firmness, increased water-soluble and ion-bound pectins, and decreased protopectin/CSP/cellulose/hemicellulose, supporting the conserved interpretation that PG2-class enzymes drive pectin solubilization/depolymerization and softening. | Transgenic tomato overexpressing RiPG2 | Li et al., 2024 | https://doi.org/10.3390/agronomy14010160 | (pqac-00000006, pqac-00000007, pqac-00000012) |
| Ripening-stage expression association | In the RiPG2 transgenic tomato study, PG activity remained higher than controls throughout development, and firmness decreased accordingly; the authors interpret PG activity as promoting cell-wall hydrolysis during ripening. This is consistent with historical tomato PG2 evidence even though the transgene is from raspberry. | Tomato fruits at 35, 40, and 45 days after flowering | Li et al., 2024 | https://doi.org/10.3390/agronomy14010160 | (pqac-00000012) |
| Identity/disambiguation note for tomato PG2 | Multiple tomato studies consistently use PG2/PG2A/PG2B to denote the catalytic fruit endopolygalacturonase distinct from PG1, which is the PG2-containing complex with the beta/h-subunit. This literature usage aligns with the UniProt description of P05117 as tomato Polygalacturonase-2. | Tomato fruit polygalacturonase literature | Smith et al., 1990; Pogson & Brady, 1993; Peeters et al., 2004 | https://doi.org/10.1007/BF00028773; https://doi.org/10.1007/BF00240897; https://doi.org/10.1002/bit.20134 | (pqac-00000004, pqac-00000005, pqac-00000011) |


*Table: This table summarizes key experimental evidence relevant to functional annotation of Solanum lycopersicum PG2/P05117, emphasizing catalytic role, isoform relationships, complex formation with the beta/h-subunit, and regulation during ripening. It is useful for tracing each major claim to specific publications and workspace citation IDs.*