Biochemistry of Flower Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 70

4.14 Cell wall changes during wilting

Cell walls are composed of a strong flexible network of cellulose surrounded by a hydrated
matrix of acidic polysaccharides (pectin), hemicelluloses, structural glycoproteins, and
enzymes (Cosgrove, 2000). Cell walls provide protection and structure for individual cells
and must accommodate rapid cell or tissue-specific expansion during petal opening. The
onset of senescence-associated wilting of floral organs has been temporally linked with
modifications of the cell wall in carnation (Devetten et al., 1991), and recent work on
Sandersonia also supports this view (O'Donoghue et al., 2005). Anatomical changes to
flower petal cells suggest that cell walls swell or break down as the internal mesophyll cells
become separated from each other and collapse during petal expansion and subsequent
senescence (O'Donoghue et al., 2002; Wagstaff et al., 2003). The loss of cell order that
occurs during petal senescence is often accompanied by an increase in activity of cell wall
hydrolases, depolymerization of hemicelluloses, and loss of neutral sugars, particularly
galactose and arabinose (O'Donoghue et al., 2002). Sandersonia , in particular, loses nearly
50% of cell wall galactose as flowers begin to wilt, accompanied by a marked increase in β -

galactosidase (O'Donoghue et al., 2002, 2005). A putative role for cell wall -galactosidases

in ripening fruit tissue is the degradation of pectin-associated galactan, which increases wall
porosity and enables access of other cell wall-modifying enzymes (Brummell and Harpster,
2001).

β

-Galactosidase gene expression, enzyme activity, and galactose loss from the cell
walls of Sandersonia petals may be associated with the onset of wilting (O'Donoghue et al.,
2005), but the theory of increased wall porosity may not apply in these flower petals. The
senescence-associated expression of Sandersonia

β

-galactosidase genes occurs in concert
with protease genes and is delayed after sugar-feeding treatment, which also delays the
onset of petal wilt (O'Donoghue et al., 2005). Sandersonia

β

-galactosidase genes are also
differentially expressed after water stress treatments. Expansins are noncatalytic enzymes
that are believed to disrupt the hydrogen bonding that links cellulose microfibrils with matrix
polysaccharides. Two roles for expansins in plants have been proposed: enabling cell walls
to expand in response to increased turgor pressure within the cell (Cosgrove, 2000) and cell
wall disassembly during tissue senescence (Brummell et al., 1999). The role of expansins
in petal senescence is less clear, one hypothesis is that expansins function to destroy cell
wall integrity by indiscriminate binding (Gookin et al., 2003). In petunia, downregulation
of

β

-expansin gene ( PhEXP1 ) produced plants with flowers of a smaller size (Zenoni et al.,
2004). The petals had a smaller epidermal cell area and altered cell wall morphology and
composition. The reduced cell wall thickness in transgenic plants was accompanied by a
reduction in crystalline cellulose (Zenoni et al., 2004). The expansins play a key role in
regulating cell expansion in petunia petals by coordinating cellulose synthesis, deposition,
and spatial organization in relation to other polymers and protein constituents of the cell
wall (Zenoni et al., 2004).

α

4.15 Changes in nucleic acids and proteins

While senescence is a degradative process, this degradation is not merely the result of
increased rates of protein turnover and decreases in the synthesis of proteins and RNA.
Although general decreases in total protein and RNA levels are observed in senescing floral
and foliar tissues, specific proteins and mRNAs have been found to increase (Borochov