Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

170 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
modifications of cell wall polysaccharides (Alba et al., 2005; Handa et al., 2007). For example,
the expression of LePG in ripening tomato is accompanied by increases in other
proteins whose functions are to depolymerize or modify polysaccharides in cell wall, such
as pectin methylesterase (PME) (Harriman et al., 1991), endo-1,4,β-glucanase (Lashbrook
et al., 1994), xyloglucan endo transglycosylases (Arrowsmith and de Silva, 1995), β-
galactosidases (Smith et al., 1998; Carey et al., 2001), and pectate lyases (Marin-Rodriguez
et al., 2002).
Pectin modification provides an excellent example of the modification of polysaccharide
domains. A number of enzymes modify or degrade pectin including enzymes that act
on pectins such as exo- and endo-acting PGs, PMEs, β-galactosidases, and pectate lyases
(PLs) (Table 8.2). Most of these enzymes exist in multigene families with a subset of one
or more gene family members regulating the cell wall modification processes associated
with fruit ripening. Complex interactions between gene family members of each enzyme
class and between different enzymes may also occur. A well-characterized cooperative relationship
exists between PG and PME. PME plays critical roles in establishing structurally
and functionally distinct classes of pectin, important in many stages of plant growth and
development (Ridley et al., 2001; Willats et al., 2001a).
Pectin is the most abundant class of macromolecules within the cell wall matrix and in
the middle lamella between primary cell walls. During fruit softening, pectin typically undergoes
solubilization and depolymerization processes that are thought to contribute to wall
loosening and disintegration. PG, which catalyzes the hydrolytic cleavage of galacturonan
linkage of pectin, is the most abundant pectin-degrading enzyme whose action coincides
Table 8.2
Pectin-modifying and degrading enzymes in fruits
Substrate Enzymes Products
Pectin Pectin methyl esterase Pectic acid + methanol
Endopolymethyl galacturonase Methyl oligogalacturonides
Rhamnogalacturonase
α-(1,2)Linked L-Rha, α-(1,4)
linked D-Gal
Endopectin lyases
Unsaturated oligogalacturonides
Hairy pectin Rhamnogalacturonan acetylesterase Pectin + acetic acid
Pectin acetyl esterase
Pectin + acetic acid
Smooth pectins Lyases Oligogalacturonides
Protopectin Protopectinase Pectin
Pectic acid Endo-PG Oligogalacturonides
Exo-PG
Monogalacturonides
Endopectate lyases
Oligogalacturonides
Exopectate lyases
Unsaturated digalacturonides
Trigalacturonic acid Oligogalacturonide hydrolase Monogalacturonides
4:5 (galacturonide)n 4:5 unsaturated
oligogalacturonide hydrolase
Unsaturated monogalacturonide +
galacturonides (n−1)
Unsaturated digalacturonate Oligogalacturonide lyases Unsaturated monogalacturonides
Arabinans α-L-Arabinofuranosidase α-L-Arabinose
(1,5)-α-Arabinans Endoarabinanase Arabinose and higher
oligosaccharides
Galactans β-D-Galactanase β-D-Galactose