Food Biochemistry and Food Processing (2 edition)

BLBS102-c27 BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come

546 Part 5: Fruits, Vegetables, and Cereals

Ethylene

receptor

C 2 H 4

Gene

expression

Phospholipid

Outside

Inside

Outside

Inside

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

Ca

H

H

H

H

H

H H

H

Ca

Ca

PLD

PLD

PLD

PLD

Phospholipase D

Increased cytosolic

Ca2+, H+

Phosphatidic acid

Diacylglycerols

Phosphatidate

phosphatase

Free fatty acids

Fatty aldehydes

Calmodulin

Lipoxygenase

Alkanes

Peroxidized fatty

Free radicals acids

Gel phase

formation, reduced

membrane fluidity

Leakage

Damage to Ca2+

-H+ATPase

Lipolytic acyl

hydrolase

Ca^2

Autocatalytic

Figure 27.5.Diagrammatic representation of the autocatalytic pathway of phospholipid degradation that occur during fruit ripening/harvest

stress in horticultural produce.

PLD to the membrane where it can initiate membrane lipid

degradation. The precise relation between the stimulation of the

ethylene receptor and PLD activation is not fully understood,

but could involve the release of calcium and migration of PLD

to the membrane, formation of a metabolising enzyme complex

(metabolon) with other lipid degrading enzymes of the pathway

as well as calmodulin. PLD alpha appear to be the key enzyme

responsible for the initiation of membrane lipid degradation in

tomato fruits (Pinhero et al. 2003). Antisense inhibition of PLD

alpha in tomato fruits resulted in the reduction of PLD activity

and consequently, an improvement in the shelf life, firmness,

soluble solids and lycopene content of the ripe fruits (Whitaker

et al. 2001, Pinhero et al. 2003, Oke et al. 2003, Paliyath et al.

2008a). There are other phospholipid degrading enzymes such

as phospholipase C and phospholipase A 2. Several roles of these

enzymes in signal transduction processes have been extensively

reviewed (Wang 2001, Meijer and Munnik 2003).

LOX exists as both soluble and membranous forms in tomato

fruits (Todd et al. 1990). Very little information is available on

phosphatidate phosphatase and lipolytic acyl hydrolase in fruits.

Proteolysis and Structure Breakdown

in Chloroplasts

The major proteinaceous compartment in fruits is the chloro-

plast which is distributed in the epidermal and hypodermal lay-

ers of fruits. The chloroplasts are not very abundant in fruits.

During senescence, the chloroplast structure is gradually disas-

sembled with a decline in chlorophyll levels due to the degra-

dation and disorganisation of the grana lamellar stacks of the

chloroplast. With the disorganisation of the thylakoid, globular

structures termed as plastoglobuli accumulate within the chloro-

plast stroma, which are rich in degraded lipids. The degradation

of chloroplasts and chlorophyll result in the unmasking of other