Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

164 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
Table 8.1
Common polymers of primary cell wall and their structures
Polymer
Cellulose
Xyloglucan
Glucomannan
Glucuronoarabinoxylan
Homogalacturonan
Rhamnogalacturonan I (RG-I)
Rhamnogalacturonan II (RG-II)
Structural proteins
Molecular structure
(1→4)β-D-glucan chains held together with hydrogen bonding, forming
very long crystalline microfibrils
Cross section contains 36 glucan chains
Backbone similar to cellulose, i.e., (1→4)β-D-glucan
Regular substitution on three out of four consecutive glucose residue with
α-D-xylose
Xylose occasionally extended with β-D-galactosyl-α-L-fucose or
α-L-arabinose in some species
The reducing end of unsubstituted glucose residues is susceptible to
cleavage by Trichoderma endo-(1→4)β-D-glucanases (EGases)
producing similar amounts of heptasaccharide (Glc4.Xyl3) and
nonasaccharide (Glc4.Xyl3.Gal.Fuc) xyloglucan subunit
oligosaccharides
Backbone contains regions of (1→4)β-D-glucan and (1→4)β-D-mannan
in nearly similar amounts galactomannan
Occasionally terminal has a side chains of single unit of α-D-galactose
Backbone of (1→4)β-D-xylan
Side chains of single unit of nonreducing terminal α-L-arabinose and
α-D-glucuronic acid
Made of long chains of (1→4)α-D-galacturonic acid
Initially highly methyl-esterified
Made of alternating α-D-rhamnose and α-d-galacturonic acid residues;
long side chains of either unbranched (1→4)β-D-galactan or branched
α-L-arabinans or type I arabinogalactans attached to the rhamnose
residues
Backbone made of (1→4)α-D-galacturonic acidlike homogalacturonan;
complex side chains of different types of neutral sugar. A minor cell
wall component. RG-II monomers can dimerize together as boron
diesters and may affect the cell wall porosity
Four different types including expansin; some are heavily glycosylated
cellulose. These models have provided useful starting points to understand ripeningassociated
changes in cell wall, that lead to change in fruit texture (Vincken et al., 2003;
Brummell, 2006; Vicente et al., 2007).
Cellulose microfibrils are composed of unbranched (1,4)-linked α-D-glucan and synthesized
in the plasma membrane by large hexameric complexes. Long crystalline ribbons
of about 3–5 nm width of cellulose microfiber, formed by extensive hydrogen bonding
among several parallel glucans chains, provide cell wall mechanical strength and resistance
to enzymatic attack. The matrix glycans (hemicelluloses) are neutral or weakly acidic,
composed mainly of neutral sugars, and do not contain galacturonic acid (GalA). The basic
structure of hemicellulose is similar to cellulose, but does not form microfibrils due to
its branching and other sugar modifications. Hemicelluloses are synthesized in the Golgi
apparatus and deposited to the wall surface by transport through vesicles. Xyloglucan and
arabinoxylan predominate among the various components of hemicellulose. The xyloglucan
backbone is similar to that of cellulose but has numerous regularly spaced xylose
side chains, some extended with either Gal-Fuc or Ara. The arabinoxylan backbone is