Agriculture Reference
In-Depth Information
8.10 Role of expansin in regulating fruit softening
One potential regulator of pectin solubilization early in ripening is expansin. The mecha-
nisms by which expansins operate are still unclear, but they lack significant transglycosylase
or hydrolase activity (McQueen-Mason and Cosgrove, 1995). It has been hypothesized that
expansins modify cell wall structure by interfering with the ability of xyloglucans or other
hemicelluloses to hydrogen bond to cellulose microfibrils (Cosgrove, 1999). In fact, ex-
pansins have structural features similar to proteins that bind polysaccharides (Cosgrove,
1999). Expansin might diffuse down a cellulose chain, incrementally displacing hydrogen
bonds between cellulose and interacting glycan, allowing the conversion of existing wall
tension to polymer creep (Cosgrove, 2000).
Expansins are a class of proteins identified by their ability to induce the extension of
isolated plant cell walls. Expansins were originally identified as cell wall-loosening pro-
teins because of their unique ability to induce cell wall relaxation and extension in isolated
cell walls, and are now generally accepted to be key regulators of wall extension during
growth (Cosgrove, 2005). The expansins belong to a family of proteins that appear to be
involved in the disruption of noncovalent bonds between cellulose microfibrils and cross-
linking glycans. Expansin may also function to loosen the association between the structural
polysaccharide in the wall by disrupting hydrogen bonding the cellulose microfibrils and the
tightly associated xyloglucan polymers (McQueen-Mason and Cosgrove, 1995). Besides
playing an apparently key role in wall expansion, and hence in cell growth, expansins have
been implicated in an increasing number of processes during plant growth and develop-
ment such as fruit ripening, pollination, germination, and abscission (McQueen-Mason and
Rochange, 1999).
Expansins are encoded by an extensive multigene family and have been divided into
α
-
and
β
-expansins. Two further subfamilies
γ
- and
δ
-expansins, which are truncated versions
of
α
- and
β
-expansins, have been identified recently. Functional roles for
γ
- and
δ
-expansins
have yet to be defined, although recent data indicate a signaling role for
-expansins
(Li et al., 2003). The expression of an expansin specifically during the stages of fruit soft-
ening and wall breakdown suggests that expansins might function in cell wall disassembly
(Rose and Bennett, 1999). Brummell et al. (1999) reported that plants with overexpressed
LeEXP1 had softer fruits, whereas underexpressors had firmer fruit confirming the role of
expansins in tomato fruit softening. Powell et al. (2003) also demonstrated that concurrently
suppressing the expression of both LeEXPI and LePG increases fruit firmness more than
suppressed expression of either gene alone.
Depolymerization of polyuronides was markedly reduced in expansin-deficient plants as
they became red ripe or even overripe. However, no increased depolymerization of pectins
was observed in transgenic lines that overaccumulated recombinant expansin late in the
ripening. Although no direct effect of expansin on the action of PG or other pectinase
is reported, the confinement of obstructed pectin depolymerization to late-ripening stages
was consistent with the interpretation that expansin may have indirectly limited pectin
breakdown.
The expression of
γ
-expansin gene, MiExpA1 is correlated with softening in mango.
The expression of this gene is under dual control, being triggered by ethylene treatment
within 90 min followed by a ripening-associated peak in transcript accumulation on the
third day after ethylene treatment. At the protein level, expression of the expansin is
α
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