Current Bioactive Compounds - Volume 8, Issue 1, 2012

Current Bioactive Compounds is now entering its 8th year of publication. Over the last seven years it has established itself as a very respectable journal in its field. The journal publishes authoritative reviews, original research and hot topic issues that are written by experts in the field on all the most important advances in bioactive compounds. The fields covered include new bioactive compounds of natural or synthetic origin with proven activities in various biological screenings and pharmacological models with a special emphasis on stereoeselective synthesis. The continued growth and success of Current Bioactive Compounds are due to the constant support from the Associate Editors, Guest Editors and Editorial Board Members as well as the eminent authors who have chosen to publish their work in this journal. I am most grateful to all of them. I also wish to thanks the editorial staff of Bentham Science Publishers including Ms Humaira Hashmi, Ms Sumera Jalees and Mr. Ansar Ahmed for their cooperation and hard work.

This issue “Monoterpenoids in plant-plant interactions” highlights the ecological role of these compounds in nature, peculiarly in natural and modified ecosystems, updating available literature. Terpenoids are a group of secondary metabolites with a variety of functions, playing important roles in plant-plant, plantinsect and plant-pathogens interactions. The production and accumulation of secondary metabolites, which inhibit and/or stimulate germination and development of other plants (allolochemicals), are important for understanding the mechanism of the ecological interactions. Among allelochemicals released by plants, volatile essential oils and monoterpenes have received much attention. Many studies have documented that volatile oils or their constituents inhibit and delay seed germination and inhibit the seedling growth of many weeds and cultivated crops. The phytotoxic effects of essential oil have increased the interest in exploring volatile oils from aromatic plants for potential weed management. The mechanisms by which monoterpenes released by certain species affect the germination and growth of other plants remain to be elucidated. The information available suggests that they interefere with some physiological processes, including respiration, mitosis, the action of phytohormones, antioxidant defence systems and membrane properties. Recently, in literature, the mechanism of action for α-pinene has been reported: such compound acts on mitochondria as an inhibitor of electron flow through cytochrome-oxidase pathway. This action likely leads to an increase in mitochondrial reactive oxygen species (ROS) production and consequently, to membrane lipoperoxidation. A perturabation on mitochondrial functions may thus be a primary mechanism of the phytotoxic action of some monoterpenes, particularly during seed germination and initial seedling growth when ATP production depends mainly on mitochondrial metabolism.

The essential oil composition of Juniperus phoniceae, Pistacia vera and Pistacia terebinthus was analyzed by GC and GC–MS analysis and their bioherbicidal activity was evaluated against four weeds species, Sinapis arvensis, Trifolium campestre (dicots), Lolium rigidium and Phalaris canariensis (monocots). Qualitative and quantitative differences between oils were observed. All oils were rich in monoterpene hydrocarbons, the major constituents of J. phoniceae were α-pinene (49%) and α-terpinene (8%) and the major components in P. vera and P. terebinthus were α-pinene (16-19%), α- terpinene (32-41%) and limonene (4-25%). Tested oils strongly inhibited the germination and seedling growth of all weeds, in a dose dependent manner with dicots weeds being significantly more sensitive than monocots, indeed, at the dose of 4µl/ml, germination of S. arvensis and T. campestre was totally inhibited, while for the same dose, germination and seedling growth of L. rigidium and P. canariensis were partially reduced. Essential oils increased the level of proline and caused a severe electrolyte leakage from the roots of all tested weeds indicating membrane disruption and loss of integrity. Our results show that essential oils of forestry trees have an important phytotoxic effect on seed germination and seedling growth of weeds. The phytotoxic activity of the essential oils may be attributed to their relatively high content of monoterpene hydrocarbons. It can be suggested that these essential oils have the potential to be used as a bioherbicide.

The Mediterranean flora is characterized by the abundance of aromatic plants. The feature differentiating these plants from all others, in spite of the fact that they belong to many different families, is the production of chemically related secondary compounds, the low molecular weight and volatile isoprenoids. This remarkable presence of aromatic species is important in determining the phytotoxic potential within this ecosystem. Such plants make a significant contribution to phryganic Mediterranean ecosystems both in terms of species number and biomass. Thus, the essential oils play an important role in this ecological context. Mediating various processes in the frame of an ecosystem, they become indirectly beneficial to the plants, considering their involvement in processes of adaptative character in Mediterranean ecosystem. For this reason, our research group carried out a series of studies on the possible phytotoxic properties of aromatic plants that, being rich in active principles, are considered a primary source of potential allelochemicals.The focus of this overview is direct to have an overall idea about the chemistry and antigerminative activity of essential oils of some Mediterranean aromatic plants and their main constituents.

The mechanisms by which monoterpenes released by certain species affect the germination and growth of other plants remain to be elucidated. The information that is currently available suggests that they interfere with some physiological processes, including respiration, mitosis, the action of phytohormones, antioxidant defence systems and membrane properties. The data obtained by our group and other data from the literature have enabled us to propose a mechanism of action for α-pinene, the most active monoterpene that has been identified in our works. α-Pinene acts on mitochondria as an inhibitor of electron flow through cytochrome-oxidase pathway. This action likely leads to an increase in mitochondrial reactive oxygen species (ROS) production and consequently, to membrane lipoperoxidation. These effects are manifested in intact tissues, as indicated by the inhibition of KCN-sensitive respiration (cytochrome-oxidase pathway), stimulation of KCN-insensitive respiration (mitochondrial alternative oxidase and extramitochondrial oxidases), activation of lipoxygenase and antioxidant enzymes, and increased concentrations of lipoperoxide products. A comparison of the effects of α- pinene on different plant species has revealed that the potential of α-pinene to induce cellular oxidative stress depends on the balance between the generation of ROS and the capacity of the antioxidant defence system. Seeds with high concentrations of lipoxygenase seem to be more susceptible to α-pinene-induced oxidative damage. Many of the cellular activities that have been reported to be altered by monoterpenes require the metabolic energy of ATP and/or membrane integrity. A perturbation on mitochondrial functions may thus be a primary mechanism of the phytotoxic action of some monoterpenes, particularly during seed germination and initial seedling growth when ATP production depends mainly on mitochondrial metabolism.

Monoterpene production by plants is not only dependent on the plant ability to produce secretory tissues where these molecules are accumulated. All plants are able to emit monoterpenes upon stress of both biotic and abiotic origin. Biotic stress induced by herbivory triggers the activation of signaling pathways eventually leading to the expression of genes coding for enzymes responsible for monoterpene emission. The emitted monoterpenes are used to defend the plant directly (in the case of terpenoid storing plants) and indirectly (for most of all other plants). Herbivore-induced monoterpene emission is perceived as a signal by neighboring plants. This activates specific defense responses in primed plants. This minireview describes herbivore-induced monoterpene production and the effect of emitted monoterpenes on plants with particular reference to calcium signaling.

Terpenoid production (emission and storage) within foliage plays direct and indirect defensive and protective functions for the plant, mediates complex trophic relationships and controls the oxidation capacity of the atmosphere. Both biotic and abiotic conditions alter terpenoid production, with herbivory, light and temperature effects being reasonably well understood. In this manuscript, the state of the science about nutrient effect on terpenoid production is reviewed. The focus is on isoprene emissions and mono- and sesquiterpenoid emissions and concentrations according to fertilizing treatments and their potential interaction with other environmental factors. Ecological, physiological, biochemical and biophysical hypothesis formulated over research investigations are exposed and several points are highlighted as future research perspectives which could help to elucidate the apparent contrasting results.

The abuse of synthetic pesticides in weed management causes serious environmental problems: disruption of ecological balance, harmful effects on human health and higher incidence of herbicide-resistant weeds. Due to this, efforts are being directed to develop new eco-friendly compounds with non-toxic effects and new modes of action. Some plant secondary metabolites are an important source to formulate new bioherbicides. Monoterpenes are one of the largest and most important groups of secondary metabolites, some of which have an already reported phytotoxic activity that makes them interesting as potential bioherbicides. This paper summarizes the proved effects of individual monoterpenes and their mode of action on plant-plant interactions.

The “Salvia phenomenon” is one of the most famous examples of allelopathic interaction between higher plants. The Salvia thickets are surrounded by zones of bare soil (“bare zone”, 1-3 m in width), which merge into areas of inhibited grassland (“zone of inhibition”) and finally undisturbed grassland at a distance of 3-9 m. This characteristic vegetation pattern was attributed to monoterpenes, especially 1,8-cineole and camphor, which volatilized from S. leucophylla leaves, got adsorbed in the soil around the Salvia thickets, and inhibited germination and seedling growth of annual herbs. Initially, continuity of hydrophobic environment (clay soil particles – cuticular waxes on the seed/seedling surfaces – plasmodesmata - plasma membrane) was regarded to be important for the lipophilic compounds to enter the target cells. However, monoterpenes can reach the target cells via aqueous route as well. Because monoterpenes produced by S. leucophylla all induce similar symptoms in the seedlings of target plants, their mode of action appears to be essentially common. They exert various deteriorating effects on the cells of target plants, which might be totally explained if the primary point of action resides in mitochondrial function (respiratory ATP synthesis) and/or generation of reactive oxygen species. In contrast to the previous belief that cuticular waxes act as the pathway of lipophilic monoterpene to enter the site of action or reservoir of the inhibitors, they may act as “adsorptive barrier” to prevent the entering of monoterpenes inside the cell wall.

The Genus Eucalyptus (Myrtaceae) consisting of over 700 tall evergreen trees is widely distributed and extensively planted for wood, pulp, medicines and aesthetic values. The foliage of the tree is rich in essential oils that are used extensively in perfumery, pharmaceuticals and food industries, and are held responsible for a wide spectrum of biological activity of the tree. Additionally, the terpenoid rich essential oil also plays a significant role in regulating floor vegetation under the trees due to its allelopathic properties. Of late, there have been attempts to utilize this property of the essential oils for the management of weeds, fungi, microbes and nematodes. The present paper attempts to discuss this broadspectrum potential of eucalyptus oil and constituent monoterpenes.