Higher Plant Terpenoids

Langenheim J H (1994), Higher plant terpenoids-aphytocentric overview of their ecological roles , J Chem Ecol, 20, 1223-1280. 

This section briefly describes some results we have obtained from working with liverworts, primitive plants classified along with mosses (Musci) as Bryophyta. Liverworts are usually intolerant of drought and many thrive in the climate of the west of Scotland. The leaves of most liverworts contain oil-bodies, and the terpenoids found in them are often enantiomeric to those occurring in higher plants. Terpenoids with unusual carbon skeletons have been identified, e.g. tamariscol, 13, for which we used 2D INADEQUATE to trace out the carbon connectivity. Novel aromatic compounds have also been isolated, including 1, scapaniapyrone A, which was mentioned in section 1. 

Antitumor compounds, among them cyclic peptides, terpenoids, and alkaloids isolated from higher plants 99YZ529. 

The terpenoid precursor isopentenyl diphosphate, formerly called isopentenyl pyrophosphate and abbreviated IPP, is biosynthesized by two different pathways depending on the organism and the structure of the final product. In animals and higher plants, sesquiterpenoids and triterpenoids arise primarily from the mevalonate pathway, whereas monoterpenoids, diterpenoids, and tetraterpenoids are biosynthesized by the 1-deoxyxylulose 5-phosphate (DXP) pathway. In bacteria,... 

Many phytotoxic compounds produced by higher plants are phenolic compounds. Several of these have been implicated in allelopathy. Based on the biosynthetic pathway from which they are derived, phenolic compounds produced by higher plants fall into two general categories 1) terpenoid phenolic compounds derived from five... 

ARIGONI, D., EISENREICH, W, LATZEL, C., SAGNER, S., RADYKEWICZ, T, ZENK, M.H., BACHER, A., Dimethylallyl pyrophosphate is not the committed precursor of isopentenyl pyrophosphate during terpenoid biosynthesis from 1-deoxyxylulose in higher plants, Proc. Natl. Acad. Sci. USA, 1999, 96,1309-1314. 

Triterpenoids (C30 compounds) are the most ubiquitous of the terpenoids and are found in both terrestrial and marine flora and fauna (Mahato et al., 1992). Diterpenoids and triterpenoids rarely occur together in the same tissue. In higher plants, triterpenoid resins are found in numerous genera of broad-leaved trees, predominantly but not exclusively tropical (Mills and White, 1994 105). They show considerable diversity in the carbon skeleton (both tetracyclic and pentacyclic structures are found) which occur in nature either in the free state or as glycosides, although many have either a keto or a hydroxyl group at C-3, with possible further functional groups and/or double bonds in the side-chains. 

Carotenoids Organic compounds that are 40-carbon terpenoids. They serve as photosynthetic pigments in algae, photosynthetic bacteria, and higher plants. They are also present in some nonphotosynthetic bacteria. 

Higher plants produce a great variety of terpenoids (78) but only a few of these have been implicated in allelopathy. The monoterpenoids are the major components of essential oils of plants and they are the predominant terpenoid inhibitors that have been identified from higher plants. Many fungi (82) and algae (83) produce terpenoid allelochemicals also. 

Terpenoids are found in all parts of higher plants and occur in mosses, liverworts, algae and lichens. Terpenoids of insect and microbial origins have also been found. 

The terpenoids are secondary metabolites that are found in essential oils, resins, tissues of higher plants and micro-organisms, whilst recently some have also been located in liverworts [5,6]. The terpenoids are formed from linear arrangements of isoprene units, Fig. (1), which are derived from acetate metabolism through mevalonic acid (MVA). This pathway was found to be common to the whole range of natural terpenoid derivatives... 

Various studies have also been reported on sesquiterpenoid lactones in Xanthium strumarium and Vemonia species,452,453 diterpenoids of Isodon species,454 and triterpenoids in Lycopodium species.455 Analyses of the carotenoids of Medicago species and of berries from a range of sources reinforce previously held views that the distribution of carotenoids in these sources has no taxonomic significance.456,457 Although most higher plants that have been investigated do not retain the capacity to biosynthesize the normal pattern of terpenoids when in tissue culture, it has been reported that Ruta graveolens did retain this ability.458... 

Like chlorophyll, plastoquinone A has a nonpolar terpenoid or isoprenoid tail, which can stabilize the molecule at the proper location in the lamellar membranes of chloroplasts via hydrophobic reactions with other membrane components. When donating or accepting electrons, plastoquinones have characteristic absorption changes in the UV near 250 to 260, 290, and 320 nm that can be monitored to study their electron transfer reactions. (Plastoquinone refers to a quinone found in a plastid such as a chloroplast these quinones have various numbers of isoprenoid residues, such as nine for plastoquinone A, the most common plastoquinone in higher plants see above.) The plastoquinones involved in photosynthetic electron transport are divided into two categories (1) the two plastoquinones that rapidly receive single electrons from Peso (Qa and Qb) and (2) a mobile group or pool of about 10 plastoquinones that subsequently receives two electrons (plus two H+ s) from QB (all of these quinones occur in the lamellar membranes see Table 5-3). From the plastoquinone pool, electrons move to the cytochrome b f complex. 

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