Site of Monoterpene Biosynthesis in Majorana hortensis Leaves

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Previous studies have shown that the monoterpene ketone I-(G_3HJ menthone is reduced to the epimeric alcohols i-menthol and d-neomenthol in leaves of flowering peppermint (Menthapiperita L.), and that a portion of the menthol is converted to menthyl acetate while the bulk of the neomenthol is transformed to neomenthyl-,6-Dglucoside which is then transported to the rhizome (Croteau, Martinkus 1979 Plant Physiol 64: 169-175). Analysis of the disposition of I-(3Hjmenthone applied to midstem leaves of intact flowering plants allowed the kinetics of synthesis and transport of the monoterpenyl glucoside to be determined, and gave strong indication that the glucoside was subsequently metabolized in the rhizome. Studies with d4G-3Hjneomenthyl-,j-D-glucoside as substrate, using excised rhizomes or rhizome segments, confirmed the hydrolysis of the glucoside as an early step in metabolism at this site, and revealed that the terpenoid moiety was further converted to a series of ethersoluble, methanol-soluble, and water-soluble products. Studies with d-JG-3Hjneomenthol as the substrate, using excised rhizomes, showed the subsequent metabolic steps to involve oxidation of the alcohol back to menthone, followed by an unusual lactonization reaction in which oxygen is inserted between the carbonyl carbon and the carbon bearing the isopropyl group, to afford 3,4-menthone lactone. The conversion of menthone to the lactone, and of the lactone to more polar products, were confirmed in vivo using I4-3Hjmenthone and l4G.?Hj3,4-menthone lactone as substrates. Additional oxidation products were formed in vivo via the desaturation of labeled neomenthol and/or menthone, but none of these transformations appeared to lead to ring opening of thep-menthane skeleton. Each step in the main reaction sequence, from hydrolysis of neomenthyl glucoside to lactonization of menthone, was demonstrated in cell-free extracts from the rhizomes of flowering mint plants. The lactonization step is of particular significance in providing a means of cleaving the p-menthane ring to afford an acyclic carbon skeleton that can be further degraded by modifications of the well-known #-oxidation sequence.Rapid turnover of monoterpenes has been shown to occur in mature leaves of flowering peppermint plants (Mentha piperita L.) (4,17 (14,17) (Fig. 1). The high degree of selectivity in the metabolic disposition of the epimeric reduction products of the ketone was subsequently shown to result from compartmentation (27). Examination of the specificity and location of each enzyme of the sequence revealed the NADPH-dependent, menthol-specific dehydrogenase and the relatively nonspecific acetyl CoA-dependent acetyl transferase to reside primarily in the epidermis (presumably the epidermal oil glands), whereas the NADPH-dependent, neomenthol-specific dehydrogenase and associated UDP-glucose:monoterpenol glucosyl transferase were located almost exclusively in the mesophyll (14,18,23,27).With the pathway of menthone metabolism in leaves well established, and with tenta...

supporting

confidence: 80%

Metabolism of Monoterpenes

Croteau¹,

Sood²,

Renstrøm³

et al. 1984

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“…In contrast to the 14CO2 incorporation studies, these results suggest that monoterpenes may be synthesized in mature tissue only from endogenous stored substrates (9). Related studies have demonstrated that label from [U-_4CJsucrose is incorporated into monoterpenes most rapidly in Majorana hortensis leaf discs during the stage of leaf expansion (14). However, as previous studies have suggested that the number of oil glands is fixed at the time of leaf emergence (10,11,23,24,36), lower rates of incorporation by discs from fully expanded leaves may simply represent a lower density of oil glands and, thus, fewer biosynthetic sites per unit area or mass of tissue.…”

contrasting

confidence: 45%

“…As the oil glands are the primary site of monoterpene accumulation and are considered to be the major site of monoterpene biosynthesis (14,28,30), the number of oil glands present on the sage leaf surface was determined as a function of leaf expansion. This procedure was complicated by the fact that S. officinalis possesses five distinct types ofoil glands, including stalked capitate glands bearing a single terminal secretory cell, small sessile capitate glands containing one, two, or four secretory cells surmounting a basal and stem (stipe) cell, and large peltate glands containing eight secretory cells surmounting a basal and stem cell and entirely surrounded by an enlarged extracellular oil-filled cavity (35).…”

Section: Resultsmentioning

confidence: 99%

“…The oil glands are presumed to be the site of synthesis of monoterpenes (14,28,30), and microscopic examination of peppermint has indicated that the extracellular secretory space of the oil glands may fill with terpenes at a very early stage, well before the leaves have fully expanded (1,2,25). Experiments with peppermint have also indicated that there is little de novo synthesis of monoterpenes from "CO2 in mature leaves, but only in immature leaves that are still expanding (8,21,28).…”

mentioning

confidence: 99%

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Relationship of Camphor Biosynthesis to Leaf Development in Sage (Salvia officinalis)

Croteau

Felton²,

Karp³

et al. 1981

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129

The camphor content of sage (Saivia officinalis L.) leaves increases as the leaves expand, and the increase is roughly proportional to the number of filled peltate oil glands which appear on the leaf surface during the expansion process. "4CO2 is more rapidly incorporated into camphor and its direct progenitors in expanding leaves than in mature leaves, and direct in vitro measurement of the key enzymes involved in the conversion of geranyl pyrophosphate to camphor indicates that these enzymes, including the probable rate-limiting cyclization step, are at the highest levels during the period of maximum leaf expansion. These results clearly demonstrate that immature sage leaves synthesize and accumulate camphor most rapidly.

“…While a number of non-iridoid monoterpenyl glycosides have been reported in plants (2,5,12,19,26,28,30,32,33), including a recent report on the occurrence of l-menthyl-fB-Dglucoside in the rhizomes of Japanese peppermint (Mentha arvensis Mal. x M. piperita L.) (27), and the suggestion made that such glycosides are transport derivatives (1 1, 15), studies with peppermint were the first to implicate glycosylation of monoterpenols directly as a prelude to monoterpene metabolism at sites quite distant from the presumed site of synthesis, the epidermal oil glands (6).…”

mentioning

confidence: 99%

Metabolism of Monoterpenes

Martinkus¹,

Croteau²

1981

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Previous studies have shown that the monoterpene ketone I1G-'3H1-menthone is reduced to the epimeric alcohols i-menthol and d-neomenthol in leaf discs of flowering peppermint (Mentha piperita L.), and that a portion of the menthol is converted to menthyl acetate while the bulk of the neomenthol is transformed to neomenthyl-,B--glucoside (Croteau, Martinkus 1979 Plant Physiol 64: 169-175 2Although the systematic name for l-menthone is (5R,2S)-trans-5-methyl-2-(1-methylethyl)cyclohexanone, we have utilized here the more common nomenclature based on numbering of thep-menthane system (i.e. menthone = p-menthan-3-one) in which the methyl-substituted carbon is IR and the isopropyl-substituted carbon is 4S.'Author to whom inquiries should be made.variety of sources, including tracer studies and analyses of both short term and long term variation in monoterpene content (23, 24). Rapid and permanent turnover of monoterpenes has been shown to occur in flowering peppermint plants (Mentha piperita L.), and during this period of apparent catabolism, at least some of the otherwise undamaged leaf oil glands are emptied of their contents (3, 9). Coincident with the decrease in monoterpene content of peppermint leaves is the conversion of the major monoterpene constituent, l-menthone, to i-menthol and to lesser quantities of 1-menthyl acetate and d-neomenthol (4). Similar metabolic processes occur in other Mentha species (14). Detailed studies of the metabolism of l-[G-3H]menthone in peppermint leaf discs confirmed earlier observations that menthone was converted to menthol and menthyl acetate, and furthermore revealed that a significant proportion of the 1-menthone was transformed to dneomenthyl-,8-D-glucoside (7,9) (Fig. 1). Little neomenthyl acetate or menthyl-,f-D-glucoside was formed from [G-3HJmenthone, indicating a high degree of specificity in the metabolic disposition of the epimeric reduction products of the ketone (i.e. menthol and menthyl acetate accumulate in the volatile oil, whereas neomenthol is specifically converted to the water-soluble glucoside). Further analytical studies and in vivo tracer studies indicated that roughly half of the 1-menthone metabolized was converted to 1-menthol (of which approximately 20%o was acetylated), while the remaining half was transformed to d-neomenthol (of which nearly all was glucosylated) (9). When l-[G-3H]menthone was applied to leaves ofintact mint plants, the resulting [3Hlneomenthyl glucoside could be detected in the roots, and was shown to undergo subsequent conversion to unidentified polar products at this location (9,18). While a number of non-iridoid monoterpenyl glycosides have been reported in plants (2,5,12,19, 26,28,30,32,33), including a recent report on the occurrence of l-menthyl-fB-Dglucoside in the rhizomes of Japanese peppermint (Mentha arvensis Mal. x M. piperita L.) (27), and the suggestion made that such glycosides are transport derivatives (1 1, 15), studies with peppermint were the first to implicate glycosylation of monoterpenols directly a...