Posttranslational mechanisms that can regulate the fruit ripening process. A, Different histone modifications differ functionally in how they influence fruit ripening. Histone acetylation can either accelerate or decelerate the ripening of fruit. Histone H3K4me3 promotes fruit ripening, whereas histone H3K27me3 inhibits fruit ripening. B, Important posttranslational events involved in fruit ripening. The membrane protein SlREM1 directly binds to SlSAM1, SlACS2, and SlACO1 to increase ethylene production. Meanwhile, SlREM1 undergoes ubiquitin-mediated degradation. In cytosol, SlACS2 is regulated by phosphorylation and ubiquitination. SlACS2 is stabilized via phosphorylation by a protein kinase, while dephosphorylated SlACS2 is unstable and then degraded by the ubiquitin–26S proteasome pathway. E3 ubiquitin ligase SlPPSR1 mediates the degradation of SlPSY1 precursor protein via ubiquitination, to maintain a steady-state level of mature SlPSY1 protein in the chromoplast of tomato. FveMAPK3 negatively regulates anthocyanin accumulation of strawberry and acts downstream of FveMKK4 and FveSnRK2.6. FveMAPK3 can phosphorylate both FveCHS1 and FveMYB10 to inactivate their involvement in anthocyanin accumulation of strawberry. In the nucleus, MdHXK1 can phosphorylate MdbHLH3 to activate its transcriptional activity and promote anthocyanin biosynthesis in apple. Ubiquitination-driven degradation of transcription factors is a common way by which the ripening and quality of fruit is regulated. The E3 ubiquitin ligases SlSP1 and SlSPL2 locate to chloroplast outer envelope membranes where they participate in tomato fruit ripening. Furthermore, SlSP1 is responsible for the ubiquitin-mediated degradation of the TOC complex to promote the chloroplast-to-chromoplast transition. In the vacuole, SlVPE3 cleaves SlKTI4 to regulate the resistance of fruit to pathogens. A vacuolar invertase inhibitor can abolish vacuolar invertase activity due to protein–protein interactions, thus affecting sucrose metabolism during fruit ripening. Sl: Solanum lycopersicum; Fve: F. vesca; Md: M. domestica.
