Molecular engineering of monoterpene production

Genetic Manipulation of Biosynthetic Pathways in Mint

In recent years, the study of aromatic plants has seen an increase, with great interest from industrial, academic, and pharmaceutical industries. Among plants attracting increased attention are the Mentha spp. (mint), members of the Lamiaceae family. Mint essential oils comprise a diverse class of molecules known as terpenoids/isoprenoids, organic chemicals that are among the most diverse class of naturally plant derived compounds. The terpenoid profile of several Mentha spp. is dominated by menthol, a cyclic monoterpene with some remarkable biological properties that make it useful in the pharmaceutical, medical, cosmetic, and cleaning product industries. As the global market for Mentha essential oils increases, the desire to improve oil composition and yield follows. The monoterpenoid biosynthesis pathway is well characterised so metabolic engineering attempts have been made to facilitate this improvement. This review focuses on the Mentha spp. and attempts at altering the carbon flux through the biosynthetic pathways to increase the yield and enhance the composition of the essential oil. This includes manipulation of endogenous and heterologous biosynthetic enzymes through overexpression and RNAi suppression. Genes involved in the MEP pathway, the menthol and carvone biosynthetic pathways and transcription factors known to affect secondary metabolism will be discussed along with non-metabolic engineering approaches including environmental factors and the use of plant growth regulators.

Metabolic engineering of monoterpene biosynthesis: two-step production of (+)-trans-isopiperitenol by tobacco

Monoterpenoid biosynthesis in tobacco was modified by introducing two subsequent enzymatic activities targeted to different cell compartments. A limonene-3-hydroxylase (lim3h) cDNA was isolated from Mentha spicata L. 'Crispa'. This cDNA was used to re-transform a transgenic Nicotiana tabacum'Petit Havana' SR1 (tobacco) line expressing three Citrus limon L. Burm. f. (lemon) monoterpene synthases producing (+)-limonene, gamma-terpinene and (-)-beta-pinene as their main products. The targeting sequences of these synthases indicate that they are probably localized in the plastids, whereas the sequence information of the P450 hydroxylase indicates targeting to the endoplasmatic reticulum. Despite the different location of the enzymes, the introduced P450 hydroxylase proved to be functional in the transgenic plants as it hydroxylated (+)-limonene, resulting in the emission of (+)-trans-isopiperitenol. Some further modifications of the (+)-trans-isopiperitenol were also detected, resulting in the additional emission of 1,3,8-p-menthatriene, 1,5,8-p-menthatriene, p-cymene and isopiperitenone.

Enhanced levels of the aroma and flavor compound S-linalool by metabolic engineering of the terpenoid pathway in tomato fruits

The aromas of fruits, vegetables, and flowers are mixtures of volatile metabolites, often present in parts per billion levels or less. We show here that tomato (Lycopersicon esculentum Mill.) plants transgenic for a heterologous Clarkia breweri S-linalool synthase (LIS) gene, under the control of the tomato late-ripening-specific E8 promoter, synthesize and accumulate S-linalool and 8-hydroxylinalool in ripening fruits. Apart from the difference in volatiles, no other phenotypic alterations were noted, including the levels of other terpenoids such as gamma- and alpha-tocopherols, lycopene, beta-carotene, and lutein. Our studies indicate that it is possible to enhance the levels of monoterpenes in ripening fruits by metabolic engineering.

Expression of Clarkia S-linalool synthase in transgenic petunia plants results in the accumulation of S-linalyl-beta-D-glucopyranoside

Petunia hybrida W115 was transformed with a Clarkia breweri S-linalool synthase cDNA (lis). Lis was expressed in all tissues analysed, and linalool was detected in leaves, sepals, corolla, stem and ovary, but not in nectaries, roots, pollen and style. However, the S-linalool produced by the plant in the various tissues is not present as free linalool, but was efficiently converted to non-volatile S-linalyl-beta-D-glucopyranoside by the action of endogenous glucosyltransferase. The results presented demonstrate that monoterpene production can be altered by genetic modification, and that the compounds produced can be converted by endogenous enzymatic activity.

Developmental regulation of monoterpene biosynthesis in the glandular trichomes of peppermint

Monoterpene production in peppermint (Mentha x piperita L.) glandular trichomes is determined by the rate of biosynthesis, as determined by (14)CO(2) incorporation, and is restricted to leaves 12 to 20 d of age. Using oil glands isolated from peppermint leaves of different ages, in vitro assay of the eight sequential enzymes responsible for the biosynthesis of the principal monoterpene (-)-menthol indicated that all but one biosynthetic enzyme had a very similar developmental profile. Activities were highest in leaves 12 to 20 d of age, with a sharp peak centered at 15 d. The exception, (-)-menthone reductase, the last enzyme of the pathway, exhibited a later peak of activity, which was centered at approximately 21 d. The correlation between in vitro enzyme activity and the rate of biosynthesis measured in vivo suggests that monoterpene formation is controlled mainly by the coordinately regulated activity of the relevant biosynthetic enzymes. Developmental immunoblotting of limonene synthase, which catalyzes the committed step of the pathway, demonstrated a direct correlation between enzyme activity and enzyme protein, suggesting that the dynamic time course for the remaining pathway enzyme activities also reflects the corresponding protein levels. RNA-blot analyses indicated that the genes encoding enzymes of the early pathway steps are transcriptionally activated in a coordinated fashion, with a time course superimpossible with activity measurements and immunoblot data. These results demonstrating coincidental temporal changes in enzyme activities, enzyme protein level, and steady-state transcript abundances indicate that most of the monoterpene biosynthetic enzymes in peppermint are developmentally regulated at the level of gene expression.