Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE BREAKDOWN OF CELL WALL COMPONENTS 165
made of a (1,4)-α-D-xylan with occasional single glucuronic acid and arabinose side chain.
Other sugars attached to arabinoxylans include glucuronic acid and ferulic acid esters.
Glucomannan (alternating regions of 1,4-β-D-glucan and 1,4-β-D-mannan), one of the
other components of the primary cell wall, may have roles similar to xyloglucan and
arabinoxylan.
Hemicelluloses present in the ripening fruits include glucuronoarabinoxylans, xylans,
and glucomannans that are loosely associated with cellulose microfibrils that are long, rigid,
inextensible fibers.
Pectins represent a complex and heterogeneous group and can contain as many as
17 different monosaccharides (Vincken et al., 2003). Like hemicellulosic components,
pectins are also synthesized in the Golgi apparatus and deposited to the wall surface
via vesicles. Pectin polymers are broadly divided into several distinctive categories
that include galacturonan, rhamnogalacturonan I (RG-I), and arabinogalactan II (AG-
II), with each category having further subgroups. Galacturonan shares a backbone of
1,4-linked α-D-GalpA (galacturonic acid polymer) residues and includes homogalacturonan
(HG), rhamnogalacturonan II (RG-II), and xylogalacturonan (XGA). Homogalacturonans
are initially synthesized and secreted into plant cell wall with a high degree
of methylesterification (Carpita and McCann, 2000), which declines during development
due to the action of apoplastic pectin methylesterase (Willats et al., 2001a). Some
of the GalA residues of HG can be methylesterified at C-6 or carry acetyl moiety on
O-2 and O-3 position. RG-II contains a few rhamnose residues that are present only
in the side chains and not in the backbone. The branched galacturonan XGA contains
β-D-Xylp-(1→3) side chains with the degree of xylosylation varying between 25 and 75%
in watermelon and apple, respectively (Vincken et al., 2003). Some of the GalA residues
of XGA can be methylesterified. The methylesterification of HG plays significant role in
processing attributes of fruits, in particular (Thakur et al., 1996a, b), and the industrial
properties of pectins, in general (Thakur et al., 1997).
The RG-I backbone contains repeating disaccharide unit [→2)-α-L-Rhap-(1→4)-α-D-
GalpA-(1→] n where the n can be larger than 100. Like RG-II, some of the RG-I galacturonyl
residues can be acetylated at O-2 and O-3 and the rhamnosyl residues substituted
with neutral sugars at O-4. Although RG-I can exist as unbranched molecule, generally
20–80% of Rha are branched. RG-I may contain single (α-D-Galp-(1→4)) as well as
polymeric side chains such as arabinogalactan I (AG-I) and arabinan (50 glycosyls or
more residues). The AG-I backbone is composed of a 1,4-linked α-D-Galp and α-L-Ara f
residues. The backbone of arabinans is consist of a 1,5-linked α-L-Ara f with possible substitutions
with α-L-Ara f -(1→2)-, α-L-Ara f -(1→3)-, and/or α-L-Ara f -(1→3)-α-L-Ara f -
(1→3) side chains. The hairy pectins that vary with plant species include mostly complexes
of RG-I, AG-I, and arabinan. It is not yet established if the arabinogalactan II (AG-II) is
part of the pectic complex. AG-II is primarily associated with arabinogalactan proteins
(AGPs) and often coextracted with pectin suggesting covalent link between these moieties.
The backbone of AG-II contains 1,3-linked β-D-Galp with short side chains of α-L-
Ara f -(1→6)-[α-D-Galp-(1→6)] n with n = 1, 2, or 3. The galactosyl residues of the side
chains can be substituted with α-L-Ara f -(1→3) residues. Amount of structural proteins
is very low, and it ranges from 1 to 10% on dry weight basis. AGPs contain over 90%
polysaccharides and the protein moiety rich in Pro/Hyp, Ala, Ser, and Thr (Vincken et al.,
2003).