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Interest in medicinal plants as a re-emerging health aid has been fueled by the rising costs of prescription drugs in the maintenance of personal health and well-being, and the bioprospecting of new plant-derived drugs. Natural products have served as an important source of drugs since ancient times and about 67% of the today's useful drugs are derived from natural sources (Newman, et. al., 2003). Search of chemical scaffolds for discovering new drugs is endless process. Recent technological progresses still enhance the significance of natural products in drug discovery arena because of their superiority over synthetic and combinatorial compounds in terms of numbers, diversity, design and biological relevance. It is now realized that these factors fundamentally decide the success rate in lead-finding process (Feher and Schmidt, 2003; Verpoorte, 1998). Such a huge contribution has come from merely 10 % of the world's total biodiversity that has ever been explored. Unusual life forms such as parasitic plants are worthy in this connection. Parasitic angiosperms are unique life forms that are parasitic to other higher plants. The proportion of drugs coming from natural resources is considerably high as in case of anti-platelet drugs (75%), anti-migraine (70%), anti-ulcer (47.6), and anti-inflammatory (32.5%).
India is endowed with a rich wealth of medicinal plants. Herbs have always been principal forms of medicine in India and presently they are becoming popular throughout developing countries, as people strine to stay healthy in face of chronic stress and pollution and to treat illness with medicines that works in concert with body's own defense (Prajapati, et. al., 2001).
Medicinal plants also play important role in lives of rural people, particularly in remote parts of developing countries like India with few health facilities. It is estimated that around 70,000 plant species, from lichens to towering trees have been used at one time or another for medicinal purposes. In Indian history, the uses of plants for therapeutic purposes have been mentioned in several books of Ayurveda. The other systems of medicine such as Homeopathy, Siddha, and Aromatherapy also use plants for therapeutic purposes. Now days it is a proven fact that the herbs provide starting materials for isolation or synthesis of conventional drugs. Medicinal plants have curative properties due to presence of various complex chemical substances of different composition, which are found as secondary plant metabolites in one or more parts of these plants. Such plant metabolites according to their composition are grouped in various classes such as alkaloids, glycosides, tannins, etc. All these classes are medicinally very important for example, alkaloids, such as morphine, codeine from opium poppy, strychnine and brucine from strychnus nuxvomica, quinine from cinchona, emetine from ipecac, vasicine from vasaka all these are alkaloids plays important role in medicines. Glycosides such as digoxine from digitalis, strophanthin from stropanthus, glycyrrhizin from glycyrrhiza, all are medicinally important glycosides. Some of essential oils from plant origin also possess medicinally potent activity such as peppermint.
Medicinal and aromatic plants are found in forest areas throughout South Asia, from plains to high Himalayas, with greatest concentration in tropical and subtropical belts and also in arid regions of Thar dessert. India recognizes more than 2,500 plant species which have medicinal values, however, large flora waiting for investigation for their medicinal properties.
Plant parasite has originated multiple times during angisperm evolution, and consequently, parasitic genera vary considerably in their habits and hosts ranges (Kuijt, 1969). Visaceae and Loranthaceae (mistletoe families) parasitize aerial parts of woody plants. Cuscutaceae (dodder families) are parasitic vines that entwine herbs and shrubs, and scrophulariaceae (figwort families) and related Orobanchaceae are subterranean parasites that invade roots of nearby plants. A more direct plant-plant interaction is between parasitic plants and their hosts (Press and Graves, 1995). Parasitian originated at least eight times independently in the evolution of higher plants and about 3000 species of angiosperms (approximately 1%) are parasites (Kuijt, 1969). Parasitic plants have different modes of invading host plants; some invade host root, where as others invade aerial parts of the plant .In all cases invasion of hosts tissues and extraction of host's resources is mediated by haustoria, specialized multifunctional organs that uniquely define the parasitic plant.
Loranthaceae consists of 77 genera and about 950 species, widely distributed from the tropics to temperate regions, particularly in the South. The aerial stem-parasites commonly known as mistletoes belong to several families, of which the major ones are Loranthaceae and Viscaceae. These two families were formerly regarded as subfamilies, but are now considered to have originated separately, the Loranthaceae related to Olacaceae, the Viscaceae to Santalaceae. They differ principally in that Loranthaceae have hermaphrodite flowers with a calyx and showy corolla, while Viscaceae have small inconspicuous flowers and only one perianth whorl.
1.1 Introduction: Parasitic plants
1.1.1 Nutritional modes of parasitic plants
The parasitic plants are evolved with complex nutritional mode which is quite different from 'normal' photosynthetic green plant. Probably this unusual nutritional mode of parasitic plants could be the origin of their mystic and ethnomedicinal use. Similarly this unusualness is also the subject of much curiosity to the scientific community since ancient times. On the basis of nutritional modes, flowering plants can be categorized as autotrophic and heterotrophic (Daniel, 2002). The majority of green plants (angiosperms) are autotrophic i.e. they produce all their own food via photosynthesis. While large number of plants, which have adopted a heterotrophic mode whereby all or some of their carbohydrates are obtained from another organism. The heterotrophs are further divided into two groups- mycotrophs and haustorial parasites (Furman and Trappe, 1971). Mycotrophs obtain carbohydrates and other nutrients from a mycorrhizal fungus (mycotrophs sometimes mistakenly referred to saprophytes. Only fungi are true saprophyte). Haustorial parasite that form modified roots called haustoria constituting the morphological and physiological connection with host plant. Haustorial parasitism appears to have evolved only in flowering plants (dicots).
Among the various unrelated families of parasitic plants, two basic types of parasitism exist: holoparasites and hemi-parasites. Holoparasites are totally achlorophyllus (or nearly so), non-photosynthetic, and obtain all their water and nutrient from host xylem and phloem. Most holoparasites occur on host roots; however, some species of Cuscuta are stem parasites that have lost thylakoids, chlorophyll and light-dependent CO2 fixation (Machado and Zetsche, 1990). Holoparasitism has evolved independently in at least seven lineages: Cynomorriaceae, Convolvulaceae Balanophoraceae, Scrophulariaceae, Rafflesiales, Lennoaceae and Hydnoraceae.
Hemi-parasites are chlorophyllus and photosynthetic (at least during some part of their life cycle) yet they obtain water and nutrient via haustorial connections to the host plant. Depending upon their degree of dependence on host, hemiparsites can be grouped into facultative and obligate. Facultative hemiparasites do not require a host to complete their life cycle but are photosynthetic and, when presented with host roots, invariably form haustorial connection. When attached to host roots, these parasites extract water and dissolved mineral via direct, cell-to-cell connections to the xylem. Facultative hemiparasites found in several root parasitic families such as Serophulariaceae, Oleaceae, Opiliaceae, Santalaceae, and Krameriaceae.
Obligate hemiparasites needs to attach to a host to complete their life cycle. These are further grouped as primitive and advanced type. The primitive type includes stem parasites of Loranthaceae, Misodendraceae and some Viscaceae. These parasites are photosynthetic xylem feeders, but being stem parasites, they cannot exist independent of host plant. The advanced obligate hemiparasites attach not only to host xylem but also obtain host carbon via phloem connections. Concomitant with this nutritional is the loss of photosynthetic function, at least to some degree or during some stage of life cycle. It includes most species of Phacellaria (Santalaceae), Cuscuta (Convolvulaceae), Arceuthobium (Viscaceae), Cassytha (Lauraceae) and Striga gesnerioides (Scrophulariaceae).
1.1.2 Distribution of parasitic plants
Although parasitic plants do not constitute dominant life form in an ecosystem, there exists approximately 3900 species of haustorial parasitic plants (about 1% of flowering plants). These are distributed in 278 genera of 18 families. About 700 species found in Scrophulariaceae alone while Cuscuta, Amyema, Viscum and Phoradendron are largest genera having 100 or more species. Thus about 30% of all parasitic plants are present in above seven genera.
Parasitic plants are reported from in nearly every habitat type found throughout world. Families such as Rafflesiaceae, Loranthaceae, Balanophoraceae, Mitrastemonaceae and Olaceae have significant numbers of genera and species in moist tropical habitats. Grassland and Savannah ecosystems that receive less participation also harbor diverse parasitic floras, particularly in families Scrophulariaceae and Loranthaceae. Even in xeric habitats such as deserts, parasitic plants from Cynomoriaceae, Hydnoraceae and Apodanthaceae can be found. Distribution is further affected by water potential of parasitic plants. Lower water potential than host plant is necessary requirement for parasitic plants in order to ensure the flow of water and nutrients through haustorial connections. For many parasites, this is accomplished by maintaining higher transpiration rates than that of their host plants. This may be reason that only few number of stem parasitic Loranthaceae and Viscaceae are found in dense (hence dark) tropical rainforest conditions. Savannas, with their high solar incidence and numerous host trees, provide ideal situation for these mistletoes and significant adaptive radiations in these families have been taken place in such dry ecosystems in Central and South America, Africa and Australia. In terms of overall numbers, the majority of parasitic plant species occur in ecosystem undisturbed by humans. The habitat is topographically and ecologically defined by the host tree. Host size and canopy characteristics determine where mistletoe can grow (Dawson, et. al., 1990).
1.1.3 Host relationships
The great diversity of host species which can be parasitized by hemi-parasites has been recorded. Further, the number and taxonomic diversity of host species for each mistletoe species vary considerably (Paul, 1998). Host plants are mostly dicotyledonous angiosperms, although there are a small number of gymnosperms. No host species were herbaceous (as expected of woody aerial parasites), although many of host genera contain herbaceous species. It was observed that several mistletoe species opportunistically parasitized exotic species as well native species, the significance of which is poorly understood. Several taxonomically close mistletoe species shared many common host genera however this could be a result of distribution of both host and mistletoe.
The term host-specificity (Barlow and Wiens, 1977) gives no indication of the number of host species host-specific mistletoe parasitizes. Additional terms like host range, host preference and host selection have also been applied (Musselman and Press, 1995). However our understanding is still inadequate in defining these terms in comparison to other plant symbiotic associations (Musselman and Press, 1995). The role of host species play in this parasitic interaction is rarely considered to be important. However, the host species may play a role in determining its parasitic constituents, through host resistance to haustorial penetration (Kuijt 1969). It was recorded that the nitrogen content of host species may be an important host selection factor (Dean, et. al., 1994). However along with the nitrogen, several secondary chemicals including phenolics are also transferred (Jadhav, et. al., 2005). Further studies, however, are necessary to understand the chemical ecology of host-parasite interaction involved in the host selection. It was also observed that despite a large host range, some dominant tree species are never infected by mistletoes.
1.1.4 Co-evolutionary relationships
One of the most co-evolutionary relationships that exist is the presence of one parasitic angiosperm upon another. Two forms of association can be distinguished, facultative and obligate. As suggested by Wiens and Calvin (1987), the term hyperparasitism should be used to describe facultative association between different parasite species. In contrast, the obligate situation called epiparasitism is known from mistletoes of Loranthaceae, Viscaceae and Santalaceae in both the Paleotropics and Neotropics. At least ten species of epiparasitic Viscum have been documented from Australia and Asia with Loranthaceae being the most frequent host. Here also epiparasite maintain water potential which is 1000 kPa less than its host mistletoe (Visser, 1982). However, the reason for this parasitic interaction are not clearly understood, one reason maybe attributable to the lack of other potential host species in particular area. Hyper-parasitism also occurs within species, the occurrence of which maybe actually much higher than is recorded. The ecological significance of hyper-parasitism and the pressures hyper-parasites place on their parasite hosts are poorly understood.
In addition to their relationships with their hosts, parasitic plants often develop complex associations with other organisms they encounter throughout their life. Among mistletoes, birds are considered the primary source of mistletoe dissemination (Docters, 1954; Kuijt, 1969). Indeed, for Loranthaceae mistletoes, birds (Dicacidae) have highly specialized digestive tracts and behaviors that aid in dispersing loranth seeds (Reid, 1990). In addition to mistletoe-birds, frugivorous birds are known to play significant role in mistletoe dissemination. Frugivorous birds are 'paid in advance' so there is no incentive to void seeds in an appropriate site for germination (Wheelwright and Orians, 1982). Thus, mistletoe dissemination by frugivores to a potential host is to some extent a random event. How this influences host range is not clearly understood, but some authors (e.g. Blakely, 1922) have suggested that the occurrence of mistletoe on 'unusual' hosts is attributed to minor disseminators.
A significant yet poorly known aspect of parasitic plant biology is their association with microorganisms (fungi and bacteria). Afsatt (1973) suggested that haustoria were first produced in response to microbial parasitism and subsequently modified for water and nutrient uptake. A number of fungi are associated with mistletoes, usually as facultative hyperparasites but obligate associations are also recorded (Gill and Hawksworth, 1961).