Lichens mechanisms

Ei addition, clinical observations in EFN-a-treated hqiatitis C or melanoma patients have suggested a promoting effect of type I EFN in autoreactive skin disorders such as Lichen planus. This inflammatory skin disease was, however, not restricted to EFN-a treated patients but was found to be associated with expression by so far unknown mechanisms of the EFN-induced MxA... 

Polyols are present in desiccation tolerant lichens and liverworts, although not in mosses (Lewis, 1984). More generally starch hydrolysis and sugar accumulation occur in many plants experiencing severe water deficits (Hsiao, 1973). It is tempting to speculate that the accumulation of low molecular weight solutes in reponse to water stress represents a mechanism for the protection of membranes and proteins in the dry state. 

Numerous glucocorticosteroids for topical application are available. Essentially they all suppress the symptoms of inflammatory and hypersensitivity reactions and their mechanism of action is similar. Their indications include seborrhoeic and atopic dermatitis, phototoxic reactions, psoriasis, chronic discoid lupus, hypertrophic lichen planus and alopecia areata. However it has to be kept in mind that the use of corticosteroids for these conditions in most cases only gives symptomatic relieve and that the problem tends to recur on cessation of therapy. Traditionally topical corticosteroid formulations are grouped according to approximate relative efficacy. This efficacy is determined by both the potency of the agent and the concentration in which the corticosteroid is used. 

Topical doxepin hydrochloride 5% cream (Zonalon) may provide significant antipruritic activity when utilized in the treatment of pruritus associated with atopic dermatitis or lichen simplex chronicus. The precise mechanism of action is... 

Topical doxepin hydrochloride 5% cream (Zonalon) may provide significant antipruritic activity when utilized in the treatment of pruritus associated with atopic dermatitis or lichen simplex chronicus. The precise mechanism of action is unknown but may relate to the potent Hi and H2-receptor antagonist properties of dibenzoxepin tricyclic compounds. Percutaneous absorption is variable and may result in significant drowsiness in some patients. In view of the anticholinergic effect of doxepin, topical use is contraindicated in patients with untreated narrow-angle glaucoma or a tendency to urinary retention. 

The objectives of this paper are broad. Our first objective is to describe the primary mechanism of action of usnic acid on plants as ascertained by our laboratory.33 A second objective is to describe the phytotoxic activity of selected lichen anthraquinone analogues. In addition to the phytotoxic activity, we describe the effects of these secondary metabolites on phloem-feeding insects. Finally, we provide a hypothesis to explain the functional roles of these metabolites in the ecosystem. 

Sarret, G., Manceau, A., Cuny, D. et al. (1998). Mechanisms of lichen resistance to metallic pollution. Environmental Science and Technology, 32, 3325-30. 

Wessels, D. C. J. Schoeman, P. (1988). Mechanism and rate of weathering of Clarens sandstone by an endolithic lichen. South African Journal of Science, 84, 274-7. 

Fremy, P. (1945). Contributions a la physiologic des thallophytes marines perforantsetcariants les roches calcaires et les coquilles. Armales de ITnstitut Oceanographique, 22,107 14. Fry, E. I. (1927). The mechanical action of crustaceous lichens on substrata of shale, schist, gneiss, limestone and obsidian. Annals of Botany, 41, 437-60. 

Lichens are adapted to accumulate all the elements necessary for their life from the atmosphere. They have no root system and absorb very little from the substrate on which they grow. Atmospheric materials, including trace metals and radionuclides, can be concentrated by particulate entrapment, ion exchange, electrolytic sorption and processes mediated by metabolic energy (Crete et al., 1992). Passive particulate trapping is, however, thought to be the dominant uptake mechanism. This is also true for mosses which absorb nutrients directly through leaf and stem surfaces. 

Lichens have been used as good bioaccumulators of atmospherically transported heavy metals by different researchers (47-53). Lichens have neither roots, nor other adsorptive structures or a waxy cuticle and for mineral nutrition are largely dependent on atmospheric depositions. Lichens act as bioaccumulators via their capacity to adsorb metals from the environment. Three mechanisms have been proposed (a) intracellular uptake via an exchange process (b) intracellular accumulation and (c) trapping of metal-rich particles (54). In spite of that, there are still some gaps in the understanding of the overall process responsible for the metal uptake and accumulation in lichens. 

Klappa, C.F. (1979) Lichen stromatolites criterion for subaerial exposure and a mechanism for the formation of laminar calcrete (caliche). Journal of Sedimentary Petrology 49, 387-400. 

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