Spatially distinct expression of two new cytochrome P450s in leaves of Nepeta racemosa: identification of a trichome-specific isoform

Morphology of glandular trichomes of Japanese catnip (Schizonepeta tenuifolia Briquet) and developmental dynamics of their secretory activity

Schizonepeta tenuifolia Briquet, commonly known as Japanese catnip, is used for the treatment of colds, headaches, fevers, and skin rashes in traditional Asian medicine (China, Japan and Korea). The volatile oil and its constituents have various demonstrated biological activities, but there is currently limited information regarding the site of biosynthesis. Light microscopy and scanning electron microscopy indicated the presence of three distinct glandular trichome types which, based on their morphological features, are referred to as peltate, capitate and digitiform glandular trichomes. Laser scanning microscopy and 3D reconstruction demonstrated that terpenoid-producing peltate glandular trichomes contain a disk of twelve secretory cells. The oil of peltate glandular trichomes, collected by laser microdissection or using custom-made micropipettes, was demonstrated to contain (-)-pulegone, (+)-menthone and (+)-limonene as major constituents. Digitiform and capitate glandular trichomes did not contain appreciable levels of terpenoid volatiles. The yield of distilled oil from spikes was significantly (44%) higher than that from leaves, while the composition of oils was very similar. Oils collected directly from leaf peltate glandular trichomes over the course of a growing season contained primarily (-)-pulegone (>80% at 32 days after germination) in young plants, while (+)-menthone began to accumulate later (>75% at 80 days after germination), at the expense of (-)-pulegone (the levels of (+)-limonene remained fairly stable at 3-5%). The current study establishes the morphological and chemical characteristics of glandular trichome types of S. tenuifolia, and also provides the basis for unraveling the biosynthesis of essential oil in this popular medicinal plant.

Research progress relating to the role of cytochrome P450 in the biosynthesis of terpenoids in medicinal plants

Terpenoids are an extensive and diverse group of plant secondary metabolites. To date, they have been applied in many fields including industry, medicine and health. The wide variety of terpenoid compounds cannot arise solely from simple cyclisations of a precursor molecule or from a single-step reaction; their structural diversity depends on the modification of many specific chemical groups, rearrangements of their skeletal structures and on the post-modification reactions. Most of the post-modification enzymes that catalyse these reactions are cytochrome P450 monooxygenases. Therefore, the discovery and identification of plant P450 genes plays a vital role in the exploration of terpenoid biosynthesis pathways. This review summarises recent research progress relating to the function of plant cytochrome P450 enzymes, describes P450 genes that have been cloned from full-length cDNA and identifies the function of P450 enzymes in the terpenoid biosynthesis pathways of several medicinal plants.

Glandular trichomes: what comes after expressed sequence tags?

Glandular trichomes cover the surface of many plant species. They exhibit tremendous diversity, be it in their shape or the compounds they secrete. This diversity is expressed between species but also within species or even individual plants. The industrial uses of some trichome secretions and their potential as a defense barrier, for example against arthropod pests, has spurred research into the biosynthesis pathways that lead to these specialized metabolites. Because complete biosynthesis pathways take place in the secretory cells, the establishment of trichome-specific expressed sequence tag libraries has greatly accelerated their elucidation. Glandular trichomes also have an important metabolic capacity and may be considered as true cell factories. To fully exploit the potential of glandular trichomes as breeding or engineering objects, several research areas will have to be further investigated, such as development, patterning, metabolic fluxes and transcription regulation. The purpose of this review is to provide an update on the methods and technologies which have been used to investigate glandular trichomes and to propose new avenues of research to deepen our understanding of these specialized structures.

A candidate cDNA clone for (-)-limonene-7-hydroxylase from Perilla frutescens

Cytochrome P450 mono-oxygenases from peppermint, spearmint and perilla (all members of the family Lamiaceae) mediate the regiospecific hydroxylation of the parent olefin (-)-limonene to produce essential oil components oxygenated at C3, C6 and C7, respectively. Cloning, expression and mutagenesis of cDNAs encoding the peppermint limonene-3-hydroxylase and the spearmint limonene-6-hydroxylase have allowed the identification of a single amino acid residue which determines the regiospecificity of oxygenation by these two enzymes. A hybridization strategy provided a cytochrome P450 limonene hydroxylase cDNA from perilla with which to further evaluate the structural determinants of regiospecificity for oxygenation of the common substrate (-)-limonene. The perilla cDNA was a partial clone of 1550bp (lacking the N-terminal membrane insertion domain), and shared 66% identity with the peppermint 3-hydroxylase and spearmint 6-hydroxylase at the amino acid level. The perilla cytochrome P450 was expressed in Escherichia coli as a chimeric protein fused with the N-terminal membrane insertion domain of the limonene-3-hydroxylase. The kinetically competent recombinant protein was characterized and shown to produce a mixture of C3-, C6- and C7-hydroxylated limonene derivatives with a distribution of 33%, 14% and 53%, respectively.

Geraniol 10-hydroxylase, a cytochrome P450 enzyme involved in terpenoid indole alkaloid biosynthesis

Geraniol 10-hydroxylase (G10H) is a cytochrome P450 monooxygenase involved in the biosynthesis of iridoid monoterpenoids and several classes of monoterpenoid alkaloids found in a diverse range of plant species. Catharanthus roseus (Madagascar periwinkle) contains monoterpenoid indole alkaloids, several of which are pharmaceutically important. Vinblastine and vincristine, for example, find widespread use as anti-cancer drugs. G10H is thought to play a key regulatory role in terpenoid indole alkaloid biosynthesis. We purified G10H from C. roseus cells. Using degenerate PCR primers based on amino acid sequence information we cloned the corresponding cDNA. The encoded CYP76B6 protein has G10H activity when expressed in C. roseus and yeast cells. The stress hormone methyljasmonate strongly induced G10h gene expression coordinately with other terpenoid indole alkaloid biosynthesis genes in a C. roseus cell culture.

A cytochrome P450 gene is differentially expressed in compatible and incompatible interactions between pepper (Capsicum annuum) and the anthracnose fungus, Colletotrichum gloeosporioides

The anthracnose fungus, Colletotrichum gloeosporioides, was previously shown to have an incompatible interaction with ripe-red fruit of pepper (Capsicum annuum). However, the fungus had a compatible interaction with unripe-mature-green fruit. Using mRNA differential display, we isolated and characterized a PepCYP gene expressed in the incompatible interaction. The PepCYP gene encodes a protein homologous to cytochrome P450 proteins containing a heme-binding domain. The expression level of PepCYP is higher in the incompatible interaction than in the compatible interaction, and then remains elevated in the incompatible interaction. In the compatible interaction, the expression of PepCYP is transient. The induction of PepCYP gene is up-regulated by wounding or jasmonic acid treatment during ripening. Analysis of PepCYP expression by in situ hybridization shows that the accumulation of PepCYP mRNA is localized in the epidermal cell layers, but not in the cortical cell layers. An examination of transverse sections of the fruits inoculated with the fungus shows that the fungus invades and colonizes the epidermal cell layers of the unripe fruit at 24 and 72 h after inoculation, respectively, but not those of the ripe fruit. These results suggest that the PepCYP gene product plays a role in the defense mechanism when the fungus invades and colonizes the epidermal cells of fruits in the incompatible interaction during the early fungal infection process.

MOLECULAR-GENETIC ANALYSIS OF PLANT CYTOCHROME P450-DEPENDENT MONOOXYGENASES

Cytochrome P450-dependent monooxygenases are a large group of heme-containing enzymes, most of which catalyze NADPH- and O2-dependent hydroxylation reactions. The cloning of plant P450s has been hampered because these membrane-localized proteins are typically present in low abundance and are often unstable to purification. Since the cloning of the first plant P450 gene in 1990, there has been an explosion in the rate at which genes encoding plant P450s have been identified. These successes have largely been the result of advances in purification techniques, as well as the application of alternative methods such as mutant- and PCR-based cloning strategies. The availability of these cloned genes has made possible the analysis of P450 gene regulation and may soon reveal aspects of the evolution of P450s in plants. This new knowledge will significantly improve our understanding of many metabolic pathways and may permit their manipulation in the near future.