Fruit firmness

Fruit Firmness (Stewart Dalton, Spring 2013)

-http://hortsci.ashspublications.org/content/37/2/386.full.pdf

-Softening occurs with enzymatic digestion of cell wall components such as pectin, cellulose and hamicellulose.

-Pectin decreases as blueberries ripen from green to to blue- degradation of the cell wall and middle lamella is directly responsible for the loss of firmness in ripening fruits.

-Fruit firmness is an important economic trait in blueberries for many reasons.

-Expansin protein family- necessary for softening process. Expression of expansin mRNA and protein is correlated with growth and/or cell wall disassembly (Dotto et al, 2005)

-Expansin protein family website: http://genomebiology.com/2005/6/12/242

-pectate lyase gene expression causes softening in strawberries. It was seen that experimental transgenic strawberries had lower amounts of ionically bound pectins that control fruit. The pectate lyase gene is an excellent candidate for biotechnological improvement of fruit softening in the strawberry (and maybe the blueberry also!). Rapid postharvest softening is the primary reason for strawberry's short self life. Softening occurs via the degradation of the middle lammella of cortical parenchyma cells. Ripe fruits have thinner cell walls and show a loss of intercellular material. The underlying biochemical mechanism of strawberry softening is unclear. Percentage of water soluble pectins increases in ripe fruit but total quantity of polyuronide residues and polyuronide length are only slightly modified. Low poligalacuturonase activity found in ripe fruit. Degradation of the cellulose matrix is a major factor in loss of fruit firmness. Endoglucanase activity increases in ripening strawberries. The pectate lyase gene is a candidate for pectin degradation, which leads to loss of fruit firmness. Pectin lyase degrades pectins via a B-elimination reaction in contrast to the hydrolytic mechanism of polygalacturonases. (Jimenez-Bermudez et al, 2002)

-Pectins are broken down by pectinases. A list of different pectinases: pectin esterase (EC 3.1.1.11), polygalacturonase (EC 3.2.1.15), pectate lyase (EC 4.2.3.2) and pectin lyase (EC 1.4.2.10). [1]

-Alteration of the cell wall and middle lamella of fruits is responsible for textural changes during development and postharvest storage. Polysaccharides such as pectin, cellulose and hemicellulose are major components of the cell wall and undergo depolymerization in maturing fruit. This depolymerization contributes significantly to the softening of fruit tissue. Pectin is the predominant component of the middle lamella which is depolymerized during ripening. (Proctor and Peng, 1989)

-A list of blast results from arabidopsis for the enzyme family of pectinases: http://harvester.kit.edu/harvester/arabidopsis/IPI00522/IPI00522605/tabID_3.html