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

548 Part 5: Fruits, Vegetables, and Cereals

Figure 27.6.Isoprenoid biosynthetic pathway in plants.

HMG CoA reductase expression and activities in apple fruits

are hormonally regulated (Rupasinghe et al. 2001, 2003). There

are two genes for HMGR in apples designated ashmg1and

hmg2,which are differentially expressed during storage. The ex-

pression ofhmg1was constitutive, and the transcripts (mRNA)

were present throughout the storage period. In contrast, the ex-

pression ofhmg2increased during storage in parallel with the

accumulation of alpha-farnesene. Ethylene production also in-

creased during storage. Ethylene stimulates the biosynthesis

of alpha-farnesene as evident from the inhibition of alpha-

farnesene biosynthesis and the expression ofhmg2by the

ethylene action inhibitor 1-methylcyclopropene (MCP). Thus,

biosynthesis of isoprenoids is a highly controlled process.

Carotenoids, which are major isoprenoid components of

chloroplasts, are biosynthesised through the Rohmer pathway.

The precursors of this pathway are pyruvate and glyceraldehyde-

3-phosphate, and through a number of enzymatic steps, 1-

deoxy-D-xylulose-5-phosphate (DOXP), a key metabolite of the

pathway, is formed. NADPH-mediated reduction of DOXP leads

ultimately to the formation of IPP. Subsequent condensation of

IPP and DMAPP are similar as in the classical mevalonate path-

way. Carotenoids have a stabilising role in the photosynthetic re-

actions. By virtue of their structure, they can accept and stabilise

excess energy absorbed by the light-harvesting complex. During

the early stages of fruit development, the carotenoids have pri-

marily photosynthetic function. During fruit ripening, the com-

position of carotenoids changes to reveal the coloured xantho-

phylls pigments. In tomato, lycopene is the major carotenoid

pigment that accumulates during ripening. Lycopene is an inter-

mediate of the carotene biosynthetic pathway. In young fruits,