Synthesis Of Plant Metabolites In Specialized Structures Or Tissues

Plants do not always produce their products in every cell of the organism. Often plants have developed tissue-specific locations for synthesis of certain compounds which not only accentuates the compound's specific function but perhaps avoids the toxic effects that the compound may have on the plant itself. Indeed, this is the case for all plant products within each cell of every plant, but the plant as a whole must have a system for dealing with potentially hazardous substances. The following are a few examples.

2.5.1 Synthesis of Monoterpenes in Leaves of Peppermint (Mentha piperita)

It has been shown by Croteau and Winters14 at Washington State University that leaves can synthesize a variety of monoterpenes from geranyl pyrophosphate (GPP), as shown in Figure 2.22. GPP production in the terpenoid pathway is the universal precursor of all monoterpenes. Monoterpenes, as well as some sesquiterpenes, in general serve as antiherbivore agents that have significant insect toxicity while having negligible toxicity to mammals. Mixtures of these low molecular weight volatiles, called essential oils, are what give plants such as peppermint, lemon, basil, and sage their characteristic odors, and many are commercially important in flavoring foods and in making perfumes.

Of particular interest in peppermint, is the pathway of l-menthone metabolism illustrated in Figure 2.23. The branch of this pathway at the top of the figure shows the biosynthesis of l-menthol and l-menthyl acetate from l-menthone. These substrates and the enzymes that lead to their biosynthesis occur in the glandular hairs that arise from leaf epidermal tissue. The products are stored in a modified extracellular space between the cuticle and the cell wall. Well known to repel insects, menthol at the very surface of the leaves (in hairs) seems to deter herbivores before they even get a chance to take a trial bite. In contrast the branch in the pathway at the bottom of the figure that leads to the synthesis of d-neomenthol

FIGURE 2.22 Major pathways and cofactor requirements for monoterpene biosynthesis in peppermint (Mentha spicata). [M2+] is the divalent metal ion cofactor (either Mg2+ or Mn2+ required by monoterpene cyclases). (From McCaskill, D., Gershenzon, J., and Croteau, R., Planta, 187, 445-454, 1992. With permission.)

FIGURE 2.22 Major pathways and cofactor requirements for monoterpene biosynthesis in peppermint (Mentha spicata). [M2+] is the divalent metal ion cofactor (either Mg2+ or Mn2+ required by monoterpene cyclases). (From McCaskill, D., Gershenzon, J., and Croteau, R., Planta, 187, 445-454, 1992. With permission.)

and d-neomenthyl glucoside occurs not in the epidermal hairs, but rather, in the photosynthetic mesophyll tissue of the leaves that lies inside the epidermis. The ultimate product, d-neomenthyl glucoside, is then translocated from the leaf mesophyll tissue to the phloem in the leaf vascular bundles, and from there to the roots of the plant where it is stored. This difference in cell/tissue compartmentation for monoterpene biosynthesis in peppermint leaves is of particular interest to biochemists, physiologists, and cell biologists as a model for the control of gene

FIGURE 2.23 Pathways of l-menthone metabolism in peppermint (Mentha spicata). The percentages indicate the approximate distribution of the products derived from l-menthone in mature leaf-tissue. (From Croteau, R. and Winters, J. N., Plant Physiology, 69, 975-977, 1982. With permission.)

FIGURE 2.23 Pathways of l-menthone metabolism in peppermint (Mentha spicata). The percentages indicate the approximate distribution of the products derived from l-menthone in mature leaf-tissue. (From Croteau, R. and Winters, J. N., Plant Physiology, 69, 975-977, 1982. With permission.)

expression in different tissues and for the study of translocation of compounds within the plant. For the plant, it most likely evolved this bifurcation in the l-menthone biosynthetic pathway in response to predation pressures by insects and herbivores which prey on both the leaves/stems and roots of these plants. However, not all monoterpenes and sesquiterpenes are repellents. Sometimes their primary function is to attract.

2.5.2 Synthesis of Monoterpenes in the Flowers of Clarkia breweri and Other Species

Apart from the coloration factors discussed above and in Section 2.6.6, the flowers of many plant species attract pollinators by producing different complex mixtures of volatile compounds within the various floral organs (i.e., stigma, style, ovary, filaments, petals, or sepals). It is the combinations of the constituents of this scent mixture that give each flowering plant species a unique fra-grance.15,16 The fact that insects can distinguish between these different floral scent mixtures is the key to the reason that many specific plant species often have specific pollinator species. For example, plants that make flowers which produce linalool (a monoterpene) very often attract moth pollinators during the night, while species that may look very similar and live in the same area but do not produce linalool do not attract moths.17 They are pollinated by other insects, usually bees or butterflies during the daytime. Thus, the components of a floral scent have important implications for the pollination success of the plants that produce them.1819-21

Although floral scent production is crucial, it has been virtually ignored by the biochemical and molecular biology disciplines. Few of the biochemical pathways that produce the vast array of scent compounds have been elucidated, and although many of these compounds are monoterpenes, only one of the enzymes that directly produces a monoterpenoid floral scent compound has been identified at this time. This enzyme, linalool synthase (LIS), catalyzes the conversion of GPP directly to linalool (Figure 2.24). Linalool is a common acyclic monot-

UnflJotf OhX Lluniwrih

FIGURE 2.24 The linalool and linalool oxides pathway. (Courtesy of Eran Pichersky and Leland Cseke).

UnflJotf OhX Lluniwrih

FIGURE 2.24 The linalool and linalool oxides pathway. (Courtesy of Eran Pichersky and Leland Cseke).

erpenoid floral scent compound produced by the flowers of many plant spe-cies.1820-24 In Clarkia breweri plants (a small annual plant native to California and the only species where LIS activity is well characterized), it is produced predominantly by the epidermal cells of the petals which are responsible for the majority of linalool emission from the flower. Linalool also has its oxide forms that are produced through a suspected epoxide intermediate by an as-yet unidentified epoxidase (see Figure 2.24). These oxides are produced predominantly in the transmitting tissue of the stigma and style of each flower where pollen tubes grow during pollination. The oxides, however, are a minor component of the floral scent mixture. Both linalool and it's oxides are only produced when the flower is open, beginning as soon as the flower opens and ending just after the flower is pollinated. This timing has a distinct advantage for the plant since it avoids wasted energy by the production of compounds when they are not needed. Linalool is known to be toxic to some insects such as fleas. There is also some evidence through transgenic studies that linalool production is toxic to young plant tissue. Thus, producing linalool only when a more mature tissue, such as a flower, has developed may avoid other toxic effects within the plant. In any case, the primary activity of linalool itself seems to be to attract a specific moth pollinator (a hawkmoth) that lives in the same regions as C. breweri. The oxides may also play a part in this role, but it seems likely from their expression pattern that linalool oxides have potential roles (1) in directing the visiting insect specifically to the stigma where it is most advantageous for the plant to have pollen placed or (2) in the inhibition of pollen tube growth of other species or the stimulation of pollen tube growth from the same species. The true function of the oxides, however, is not known. Like other monoterpenes, linalool is also important in industry as a starting material in the production of perfumes and as a flavoring compound in food and drink.25 So, its study not only helps us understand how plants communicate with insects but may also benefit industry and agriculture — especially with the potential for the modification of scent production through transgenic plants of crop plants that are grown outside of their natural pollinator's living range and thus suffer from lower crop yields.

Another interesting part of the Clarkia project deals with the general question of how the ability to produce linalool changes over evolutionary time. As mentioned above, species that produce linalool are generally pollinated by moths, while species that do not produce linalool are pollinated predominantly by bees and butterflies. This part of the study focuses on the differences in the molecular genetics and biochemistry of scent production between Clarkia and Oenothera (evening primrose) species that determines the differences in primary pollinators. Oenothera and Clarkia are in the same family (Onagraceae) and are thus very closely related. Most Oenothera produce scent including linalool, yet only two species within the Clarkia genus, C. concinna and C. breweri, produce any linalool at all.22-24 Flowers of C. concinna, like those of all other Clarkia species, are odorless to the human nose. However, linalool and its pyranoid and furanoid oxides have been detected in C. concinna stigmas using gas chromatography/mass spectrometry (GC-MS), but at levels a thousand-fold less than in C. breweri. Additionally, chromosomal, morphological, and genetic data suggest that C. breweri has evolved relatively recently from C. concinna.20,24 These observations raise at least two questions: (a) What is the function of the linalool pathway in nonscented plants such as C. concinna; and (b) What is the mechanism of evolution that allows the scent trait to be switched off and on over evolutionary time? This evolution could occur through several mechanisms

— enzymatic, morphological, or genetic — but research so far has narrowed the possibilities for differential scent production between C. breweri and C. concinna to control at the level of transcription.26 The project aims to determine if the same type of mechanism is involved in the biosynthesis and emission of linalool in Oenothera species. It is generally accepted that Oenothera and Clar-kia species share a common ancestor, yet they show a surprising diversity in the ability to produce linalool. By characterizing the expression and regulation of the gene encoding linalool synthase, the project promises to uncover how scented species, represented by Oenothera, evolve into nonscented species, represented by most Clarkia, and yet retain the ability to evolve into scented species again

— represented by C. breweri.

2.5.3 Synthesis of Oleoresin Terpenes in Conifers

Oleoresin is a mixture of terpenoid compounds in the tissues of many species but is best characterized in conifers. Oleoresin from pine trees, also known as "pitch", is composed mainly of monoterpene olefins (turpentine) and diterpene resin acids (rosin).27 So-called constitutive oleoresin is synthesized in epithelial cells surrounding resin ducts in the needles and stem28 as well as in resin blisters on the bark of the tree trunk. In contrast, induced resin arises from nonspecial-ized cells located adjacent to site(s) of injury that are not normally associated with oleoresin biosynthesis.29 This resin is secreted in response to physical wounding and/or attack by fungal pathogens and insects such as bark beetles. Resins, however, are not all related to gum which may have the same function in other species. This defense reaction by conifers is adaptively important to the survival of conifers in natural habitats because the oleoresins are antifungal and toxic to bark beetles. Wounded areas in the bark of a tree trunk or branch physically become sealed by the solidification of the resin acids after the terpen-tine has evaporated.29 This may also serve to prevent loss of water.

The biosynthetic pathway for the synthesis of monoterpene olefins and abietic acid (the primary diterpenoid resin of grand fir, Abies grandis) is shown in Figure 2.25.27 Note that the starting substrate in the pathway is acetyl-CoA. From it, oleoresin biosynthesis proceeds stepwise via mevalonate, isopentenyl pyrophosphate, dimethylallyl pyrophosphate, farnesyl pyrophosphate, and ger-anylgeranyl pyrophosphate (GGPP) in the same biochemical processes that produce the precursors of menthol and linalool. The GPP leads directly to synthesis of monoterpene olefins such as a- and 0-pinene, 3-carene, 0-phellan-drene, and limonene catalyzed by monoterpene cyclases. The substrate, GGPP, leads to the synthesis of the diterpenoid resin, abietic acid, via four enzymatic steps involving a single cyclase, two hyroxylases, and a dehydrogenase. Each of these enzymes has been isolated and assayed for the production of respective products by liquid scintillation spectrometry, using [1(2)-14C] acetic acid as the starting substrate.27

2.5.4 Secretion of Sodium and Potassium Chloride from Salt Glands of Plants that Grow in Saline Environments (Halophytes)

Over evolutionary time, plants have developed mechanisms that allow them to survive in a given environment which has specific conditions. Some times these environmental conditions are quite harsh. A number of plants which are tolerant of and grow in saline environments actually secrete salts from their leaves using specialized salt glands. In fact, when one tastes the leaves, they taste salty because of these saline secretions. One such plant is salt grass, Distichlis spicata, which grows in such areas as the "playas" or salt flats near the Great Salt Lake and the Bonneville Salt Flats in Utah or in the saline soils of the Sacramento Valley of California. Scanning electron micrographs of the

FIGURE 2.25 Outline of the biosynthesis of monoterpene olefins and abietic acid, the principal diterpenoid resin of grand fir (Abies grandis) oleoresin. IPP, isopentenyl pyrophosphate; DMAPP, dimethylallyl pyrophosphate; GPP, geranyl pyrophosphate; FPP, farnesyl pyrophosphate; GGPP, geranyl-geranyl pyrophosphate; 1, monoterpene cyclases; 2, abietadiene cyclase; 3, abietadiene hydroxylase; 4, abietadienol hydroxylase; 5, abietadienal dehydrogenase. (From Funk, C. et al., Plant Physiology, 106, 999-1005, 1994. With permission.)

FIGURE 2.25 Outline of the biosynthesis of monoterpene olefins and abietic acid, the principal diterpenoid resin of grand fir (Abies grandis) oleoresin. IPP, isopentenyl pyrophosphate; DMAPP, dimethylallyl pyrophosphate; GPP, geranyl pyrophosphate; FPP, farnesyl pyrophosphate; GGPP, geranyl-geranyl pyrophosphate; 1, monoterpene cyclases; 2, abietadiene cyclase; 3, abietadiene hydroxylase; 4, abietadienol hydroxylase; 5, abietadienal dehydrogenase. (From Funk, C. et al., Plant Physiology, 106, 999-1005, 1994. With permission.)

surfaces of the leaves of saltgrass reveal glands and toothpaste-like secretions that emanate from these glands. If one makes X-ray analysis maps for sodium, potassium, and chlorine of the same area photographed with the scanning electron microscope, the images seen on the CRT (cathode ray tube) will reveal bright-dot images over each toothpaste secretion. This tells researchers which elements are present in the secretions. From this information, it was shown that these secretions are potassium chloride and sodium chloride, corresponding to the predominant salts in the soil in which these plants grow. So, to avoid possible damage due to the osmotic effects of salt, these plants simply secrete the salt that is taken up by the roots thus keeping it out of the plant's cells.

Was this article helpful?

0 0
Food Fanatic

Food Fanatic

Get All The Support And Guidance You Need To Be A Success At A Food Business. This Book Is One Of The Most Valuable Resources In The World When It Comes To Turning Your Love For Cooking Into A Money Maker.

Get My Free Ebook


Post a comment