Handbook of Herbs and Spices - Volume 3

504 Handbook of herbs and spices

synthase, ocimene synthase and 4-S limonene synthase (4S-LS). The last stage of

monoterpenes biosynthesis includes several secondary transformations which start

with limonene and lead to a great diversity of final products. Perusal of reviews (Chand

et al., 2004) show that very little work has been reported on catabolism of monoterpenes

and regeneration of their synthesis. Gershenzon et al. (2000) are of the opinion that

loss of monoterpenes by catabolism and volatilization occurs at a very low rate.

It has been demonstrated that accumulation of monoterpenes varies during the

maturation of leaves. Brun et al. (1991) observed that the enzymes are developmentally

regulated at the level of gene expression (McConkey et al., 2000). Earlier elegant

reviews on the biosynthesis of different monoterpene families are available in Wise

and Croteau (1999) and Davies and Croteau (2000).

Recently a family of 40 terpenoid synthase genes was discovered in Arabidopsis

thaliana by genome sequence analysis (Aubourg et al. 2002) and over 30 cDNA

encoding plant terpenoid synthases involved in plant primary and secondary metabolism

in different plants have been cloned and characterized (Trapp and Croteau, 2001).

These terpene synthases were classified into six sub-families based on their sequence

homology and into three groups based on the numerical variation in introns. Particularly

in Mentha, about eight genes concerned with the biosynthesis of secondary metabolites

(terpenes) have been cloned (Diemer et al., 2001) and about 1400 nucleotide sequences

in the EMBL database, most of which are the EST (expressed sequence tags) sequence

from the trichomes. Chand et al. (2004) in their review emphasized that ESTs

would be of immense help in rapidly constructing the physical mapping of genes for

terpenoid biosynthesis and in determining the phylogenetic relationship between

species of Mentha.

31.3 Cultivation and production............................................................

The essential oil, a product obtained from the plant is located in the leaves of the

spearmint plant. The vegetative growth for the higher production of leaves can be

stimulated by the application of the following improved cultivation practices, enumerated

by several workers especially Husain et al. (1988), Ram (1999), and Khanuja et al.

(2004).

31.3.1 Soil and climate

Although spearmint thrives well in the cool climates of hills, it can be profitably

cultivated in tropical, sub-tropical plains and foothill areas having sub-tropical agro-

OPP HO O

OH

M. citrata

O—C—CH 3

O

M. spicata

M. cardiaca

M. viridis

linalyl acetate linalool geranyl

pyrophosphate

(–) limonene (–) trans carveol (–) carvone

Fig. 31.1 Hypothesized biosynthetic pathway for important monoterpenoids of three Mentha

species; (spicata, cardiaca and virids).