Fig. 2

Tomato pericarp cell wall and some of the key enzymes involved in softening. Cellulose microfibrils are closely associated with pectin including homogalacturonan (HG) and rhamnogalacturonan I (RG-1I. HG chains can aggregate into highly organised structures and link to RG1 cores. The xyloglucans form “biomechanical hotspots” at limited locations in the CW, and tether cellulose microfibrils together. HG is present throughout the wall and especially in the middle lamella (ML) and the tricellular junctions (TCJ). Expansin, CEL2, alpha-xylosidase and β-1,4 endomannase are necessary for disrupting the associations between matrix polysaccharides and cellulose microfibrils. HG in TCJ and ML is disrupted by the combined activities of PL, PE and PG, allowing cell separation during tomato fruit softening. Based on evidence from tomato and other plant species including work reported by Paniagua et al. (2017), Pieczywek et al. (2020), Uluisik et al. (2016), Wang et al. (2019) and Shi et al. (2021)