Cell wall modifications lead to progressive degradation of
cell wall polymers and loss of integrity of the middle lamella,
which is rich in pectins that control cell-to-cell adhesion, thus
influencing fruit texture (Brummell and Harpster, 2001). In
general, pectins are the first cell wall components to be modified
during fruit ripening. PE (pectin esterase) is the main
pectin de-esterification enzyme, and PG (polygalacturonase)
is the main enzyme promoting pectin solubilization. PEs may
act synergistically with PGs, with PE action resulting in the
generation of homogalacturonan (HG) substrates that are
more susceptible to PG-mediated hydrolysis (Seymour et al.,
2013). Depolymerization of hemicelluloses is also a common
feature of ripening fruit; endo-transglycosylase hydrolase
(XTH) plays an important role in this process by modifying
xyloglucan, the predominant hemicellulose of cell walls
(Brummell, 2006). In tomato, both ethylene biosynthesis
and signal transduction play important roles in regulating
fruit ripening-related textural changes (Li et al., 2010).