For a selected host plant-beneficial microorganism association, describe the morphological and physiological aspects of the relationship and discuss the factors affecting the interaction. Outline the induced effects of the relationship which results in reduced disease and/of improved health of the plant.
Mycorrhizaenext term and their potential use in the agricultural and forestry industries
Plant roots with mycorrihizal fungi associations amplify the absorption of nutrients, predominantly phosphorus, and hence boost the development of crop plants and trees. Approximately 80-90% of all vascular plants including most of the significant agricultural species have shown their relationship with vesicular-arbuscular mycorrhizae (VAM). Ectomycorrhizae can be seen in most of the economically relevent tree species of the temperate regions and in tropical trees. Such type of symbiotic relations is, so important in crop and biomass production. When we consider all of these factors, they are receiving substantial interest in agriculture and forestry. At present, VAM are utilized in fumigated soils, greenhouse crops, and in the retrieval of disturbed sites. Ectomycorrhizae can be employed in the administration of trees in nurseries, in afforestation programs, and in containerized seedlings productions.
Production of VAM and ectomycorrhiza inoculum for large scale projects is now feasible but many basic questions related to persistence of these fungi in field situations, competition with other microorganisms, and particularly the most efficient fungi to use for particular hosts remain largely unanswered.
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Symbiotic relationship between plant roots and particular fungi is concerned as mycorrhiza. Mycorrhizal fungi make available a better absorptive surface than root hairs and consequently help in the absorption of the immobile ions such as phosphate, copper and zinc present in the soil. Moreover, mycorrhizal plants have superior tolerance to toxic metals, to root pathogenic microorganisms, to drought, to elevated soil temperature, to unfavourable soil pH and to transplant shock. The two chief kinds of mycorrhiza in tropical ecosystems are ectomycorrhiza and arbuscular mycorrhiza (AM). Ectomycorrhizae mainly take place in tropical pines, Caesalpiniaceae, Dipterocarpaceae and Myrtaceae. Arbuscular mycorrhiza is able to form in most of the agricultural and horticultural crops and numerous tropical tree species. It has been shown that tropical plant cultivation systems are conventionalized on areas formerly occupied by dense species-rich forest ecosystems. This begets an extreme alteration mycorrhizal fungi biodiversity. Modem, high-input agricultural practices usually are harmful to mycorrhizal fungi, whilst the low-input sustainable agriculture approaches boost the mycorrhizal fungi population. Further researches and studies are necessary to understand the mycorrhizal functions and their role in soil aggregation. Also, a non-destructive approach has to be developed for studying mycorrhizal biodiversity in natural ecosystems.
Plant roots afford an ecological niche for numerous soil microorganisms. The term "mycorrhiza" was first coined by Frank in 1885 to illustrate the symbiotic alliance of plant roots and fungi. Mycorrhiza, precisely means 'fungus root'. It has well recognized now that mycorrhizal fungi progress growth of plant species that are relevant in agriculture, horticulture and forestry. For example, in tropical soils, phosphorus availability may be very low. Mycorrhizae make up well-organized root extension organs involved in uptake and translocation of phosphate molecules and other diffusion-limited nutrients. Consequently mycorrhizae have an important role in the rapid growth of plants in the tropic region.
The existence of mycorrhizae in natural ecosystems
The presence of high fungal species variety characterizes the natural tropical ectomycorrhizal ecosystems (Smits et al 1993). Usually natural forests are transformed into agriculture lands and industrialization purposes. This approach may affect the production of sporocarps of ectomycorrhizal fungi and sometimes ceases the production itself. Among the known mycorrhizae species in tropical ecosystems AM fungi dominate in their species diversty. It has been shown that the number of spores of AM fungi is very low in undisturbed forests [unaltered natural forests]. Spore production increase with stumpy to moderate degrees of interruption, species diversity, as measured out on the basis of spore morphology, does not typically peak in natural ecosystems. Nevertheless, intensive input agriculture decrease the number of AM and EM propagules and species richness (Sieverding, 1991).
Ectomycorrhizae in artificial plantations
New Zealand forests rich with enormous plant species such as ...............................................Radiata pine was first established to New Zealand in early 1850s for a trial purpose. These trial have been succeeded and the plant species could adapt with the micro -macro climates of the hostile environment. The trial based on widespread planting of pine as a forestry purpose could achieve a positive and warm attention in the new Zealand wood industry. This is because of the enriched pine development and Its excellent growth rates in the climate.By the first forestry planting boom in the 1920s and 1930s, it had been adopted as the species of choice. There are several factors have been shown with these peculiar adaptabilities of the plant species with its companion, ectomycorrhizae. It showed a versatile growth capabilities throughout New Zealand on a range of soil types, together with coastal sands, heavy clays, gravels and volcanic ash deposits.
In the 1950s, a genetic research programme was started to improve the quality of radiata pine. Before introducing these plant species to forest cultivation there are several studies carried out. Plants of better-quality growth and form were chose and propagated by grafting technics and also experimented its cultivation with artificially introduced ectomycorrhiza counterparts in the growing region like seedling orchards to prevent pollen contamination. The earliest enhanced radiata pines were planted in forests in 1970.
Improvement programmes have been continued with selection criteria, through controlled cross-breeding, hybridisation and sophisticated plant propagation techniques. Scientists have established more adaptive and resistant breeds with particular climatic zones and soil types. They have been shown resistance to foliage disease like dothistroma needle blight.
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Reduced familiarity of the silviculture of native trees (knowledge of which remains lacking) has been one of the causes of establishing ectomycorrhizal trees like pines and eucalyptus in many tropical regions to congregate the rising demand for wood products by an ever mounting population and budding wood industries. Open forests encompass been vacant and replaced by single-species feigned plantations, for the most part pines. Mainly of the plantations botched pending the part of ectomycorrhizae for tree development and survival was appreciated. Learn of ectomycorrhizae in next of kin to afforestation or agroforestry in the company of home-grown hardwoods is static in its infancy.
Diversity and function
Daft (1983) suggests that various colonization with mixed AM inocula consisting fungi exhibiting diverse ecological strategies is possibly far more useful to a plant than colonization by a single endophyte which may not be able to endure certain ecological alterations, especially in the case of tree species. Sieverding (1991), states that high variety of VAM fungal species may have advantages than low diversity species. He states that it is particularly improbable that plant production would be maximized with a highly diverse VAM species composition. It is more important in using high diversity in inocula in unpredictable and stressed environments in the tropics. The relationship between species diversity and plant primary production has an important role in the development of forest trees (Gupta and Germida, 1988).
Tolerance and adaptivness of ectomicorrhizae
In ectomycorrhizae, Munyanziza (1994) states that induced drought can lead to fluctuations in the composition of species colonizing miombo seedlings. Schoeneberger and Perry (1982) observed that the ectomycorrhizal fungus Cenococcum geophilum is more harshly exaggerated by burning than other fungi. Moreover ectomycorrhizal fungi with solid rhizomorphs, like Pisolithus tinctorius have been revealed to be very effective in water transport under dry weather conditions (Duddridge et al., 1980).
Any system with higher fungal varieties with diverse fungi supplying plant biomass increment, plant endurance and soil amelioration by aggregate structure is seems more stable and buffered aligned with environmental and artificial [man-made] disturbances than low-diversity systems. Such type fungal diversity may be enhanced by different range of habitats, diversity in host age and species inside an ecosystem. Anyway, a single effective endophyte will impart more benefits than would mixed inocula under less fluctuating conditions,
Boreal forest tree species consisting Scots pine (Pinus sylvestris L.) survive in symbiotic association with ectomycorrhizal (ECM) fungi. In such type of interaction the fungi gain photosynthates from the host plant [pine] and in turn, the fungi get better absorbance of nutrients and water in addition to protect the host plant against environmental strains. The symbiotic association can be characterized by alterations in mycelium and root morphology ensuing in development of hyphal mantle in the region of lateral roots and a hyphal structure, named a Hartig net among epidermal and cortical cells of roots of the plants. In the case of pine roots, the mycorrhizal relations may persuade dichotomous branching of the lateral roots.
ECM fungi have been exhibited to encourage the metabolism and growth-related gene expressions and result the better growth development much prior to the generation of physical ECM structures, and its so-called pre-mycorrhizal phase. For instance, in semi-aseptic cultures inoculation with Piloderma croceum has been exhibited in a progress in root length and leaf surface area in the host plant called Quercus robur throughout the long-standing pre-mycorrhizal phase. In vitro experiments with Scots pine seedlings and Suillus variegatus, the fungus-promoted lateral root configuration and also primary needle elongation taking place earlier than mycorrhiza development. These studies reveal that polyamine (PA) concentrations present in Scots pine seedlings fluctuate as the communication or interaction proceeds.
Free putrescine (Put) accumulated transiently in the needles and stems resulting in improved growth.
In the roots, spermidine (Spd) and spermine (Spm) accumulated as mycorrhiza formation started. A substance called PAs have been shown their importance in root growth and there by its elongation throughout a rooting process. The PAs function in root expansion is underlined by the study of ................................. with Scots pine embryos, in which arginine decarboxylase (ADC) mRNA and ADC protein, the enzyme generating Put [putrescine], and it is restricted in dividing cells of the meristem. The target of their research was to explore whether changes in the concentrations of PAs in Scots pine seedlings in the existence of the ECM fungus are took part in enhanced growth in the presence of ECM symbiosis.
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Soluble carbohydrates have numerous functions in roots. Root carbohydrates are also the most essential carbon resource for mycorrhizal fungi. Even though a few ectomycorrhizal fungi acquire restricted saprotrophic capabilities (Norkrans 1950), the carbon element they need is abounding by their host plants as simple carbohydrate (Lindeberg et al 1986). The soluble root carbohydrates concentrations may be positively concurrent with infection by both vesicular arbuscular (Thomson et al 1990) and ectomycorrhizal (Rudawska 1986) fungi. Sucrose is the main plentiful carbohydrate transported in the phloem of plants and once hydrolysed to glucose and fructose by plant enzymes [invertase], supplies the main carbohydrate source to mycorrhizal fungi (Schaeffer et al 1999). Several ectomycorrhizal fungi may lacking invertase (Salzer & Hager,1993) and consequently depend on the host [plant] to hydrolyse sucrose (Schaeffer et al. 1995). But In some situations, ectomycorrhizal fungi may cause acid hydrolysis of sucrose (Hampp & Schaeffer 1999). Winter-hardy plant species like Pinus resinosa (red pine) are acknowledged to gather sucrose in roots in response to decreased day length, ensuing in improved tolerance of freezing (Sakai et al 1987). Root carbohydrate concentrations hence differ seasonally (Ritchie et al 1986), resulting in seasonal discrepancies in carbon contribution to ectomycorrhizal fungi (Pankow et al. 1989). Mycorrhizal fungi may sturdily supply to the deposition of carbon into the soil (Smith & Read 1997). Trehalose and mannitol (usually well thought-out as fungal-specific carbohydrates), sucrose (considered as a hostspecific carbohydrate) and glucose, fructose, and myo-inositol [may take place in both plants and fungi] all these sugars are essential for fungal growth. Ectomycorrhizal fungal carbohydrate concentrations may be influenced straight by temperature, host carbohydrate concentration ( Rothe et al 1998).
Certain species of fungi are present somewhat constantly on the root surfaces or in the cells of the roots of many plant species, in order that dual organs of regular morphological and histological patterns are produced. They referred as mycorrhizae and these fungal species physiologically, ecologically and reproductively co-exist with the host plant for long time in a mutualistically active symbiotic state. Mycorrhizal fungi may affect the contribution of nutrients to the roots or to protect other organisms from pathogenesis, as other micro-organisms do the same in root provinces.
Contact of ectomycorrhizas with the substrate
Many ectomycorrhizas have a significant hyphal connection with the soil as simple branching hyphae or as hyphal strands which ramify in the substrate. According to Nye , any poorly mobile nutrient concentration in the soil would fall in the locality of an absorbing surface. The rate of absorption and diffusion occur simultaneously towards the root surface area and will go on to fall till the rate of uptake is same by its rate of replacement. At last a 'zero sink' [called as deficiency zone] may origin about the absorbing area, and followed by no physiological property of the living system can boost the uptake rate, which is completely reliant on the rate of the substance movement through the soil. Here the hyphae of mycorrhizal fungi provide to make use of a wider volume of soil outside the zero or deficiency zone and locate the absorbed nutrients to the root.
There are several researches have been done to study the mechanisms involved in the translocation of inorganic substances and carbohydrates in mycorrhizal hyphae. These studies have given an idea about hyphae as a vehicle and its role in the transporation and translocation of different kinds of substances [nutrients] into and from the mycorrhizal roots (Harley and Smith, 1983).
In some kind of soil, phosphate may in mild concentrations with low mobility. But in the case of ammonium, it is ten times more mobile but essential in as a minimum ten times greater quantity. Aluminium and phosphate ions are expected to be facilitating mycorrhizal infection. The ion potassium has also similarity with ammonium in their properties [here mobility and uptake rate] but its requirement is not relevant. Calcium is generally present in excess quantity in many soil types and nitrate, while require in large quantity if it is the chief resource of nitrogen, is very mobile and present in good amount, especially in eutrophic soil. But it is swapped by ammonium in acid soil regions with elevated phenolic content where bacterial mechanisms are low. In contrast, trace elements like copper and zinc have been shown their uptake by mycorrhizal plants.
The mycorrhizas, typically ectomycorrhizas, with moderately smooth surfaces (Smith et al, 1983) are established in unstable soils, like the surface litter layers of forests. In such regions mobility of solids may under the action of soil fauna, wind and rain. Zero sinks or deficient zones may not form the regions where drainage and movement of solution over mycorrhizal and root surfaces. That means extensive hyphal formations are not so important in such soil regions.
Researches about ectomycorrhizas has revealed that majority of phosphate absorbed from low concentrations, a few 70-80 %, accumulates in the fungus but a constant 20-30% passes on into the host. It is inorganic orthophosphate that translocates into the host plant but it has not conceded through the main accumulations in ectomicorrhiza, but by a further straight route (Harley and Loughman, 1963).
The polyphosphates can be observed in the fungus both as granules and as soluble polyphosphate (Martin et al 1983). During phosphate absorption, granules form at a rate approximately corresponding to total phosphate absorption (Chilvers and Harley, 1980). Around 30-40% of phosphate absorbed from high orthophosphate concentration can be converted to polyphosphates by ectomycorrhiza fungi. The significance of the polyphosphates uptake in fungal development indicate the relevance of phosphate instability and ready solubilisation due to strong phosphate bond presence (Harley and McCready,1981).
Absorption of nitrogen compounds
It has been observed that in eutrophic soil regions [usually consists nitrate], mycorrhizal plants do not take nitrogen up readily than non-mycorrhizal plants. In such places vesicular-arbuscular plants dominates in the number than others. Nitrate is usually deficient in the habitats of Ericaceae and in coniferous forests. On the other hand, ammonium and organic compounds may be present. A few ectomycorrhizas and ericoid mycorrhizas are incapable certainly to reduce and use nitrate but steadily use ammonium and organic compounds. Read and his colleagues have exhibited that ericoid and ectomycorrhizas can not only use ammonium [amino acids] but also hydrolyse, and use insoluble organic nitrogen compounds of humus. The nitrogen compounds created in the fungal tissues has been shown to be readily translocated in the hyphae to the host.
Glutamine [favourable for plant growth] is an essential amino acid released into the host tissues by its partner ectomycorrhiza (Lewis, 1976). The uptake mechanism of rapid accumulation of glutamine in the fungal hyphae involves not only the' exploitation of organic acids in the cell but also the dark CO2 fixation. Glutamate dehydrogenase (GDH) and glutamine synthesase (GS) pathways have been discovered in mycorrhizas (France and Reid, 1983), while it is feasible that the glutamine synthase pathway with lesser Km for ammonium is more essential.
Ectomycorrhizas are well-established to phosphates and ammonium compounds absorption from soils of elevated phenolic content where nitrates are scarce. The large fungal sheath tissue and fruit bodies' result increased carbon demand; therefore large prevailing plants [pine like plants] can sustain them best. The capability of the fungal sheath for nutrient storage and carbohydrates in times of heaps fits mycorrhiza to flourish in different seasonal climates like cold and warm, or dry and moist conditions; that is to seasonal development and requirement for nutrients by the host plant and fungus [symbionts]. The fungi also protect the host from toxic metals which are very soluble in acid soil solutions.
Reduction in mycorrhizal fungal types and pathological fungal infections has been shown that their relevance in the coniferous plants survival in Europe. Researches about the reason for declining coniferous plants in Central Europe has been reveal that SO 2 exposure and simulated acid rain have caused a considerable destruction of micorrhiza in that region due to the presence of industrialization. The result is an imperfect root branching and reduced mycorrhizal infection in conifer seedlings. These studies suggest that conifer mycorrhizas can be subjected to acidification of the soil in addition to indirectly through the alterations in host physiology due to "gaseous pollutants''.
The studied rootlet types have been showed that ectomycorrhizas have typical structural changes that occur in different environmental conditions. The most significant ultra structural alterations in the industrial environment are the atypical accumulation of tannin in the cortical and pericycle cells of both non-mycorrhizal and mycorrhizal rootlets. This symptom is most obvious in the monopodal rootlets and thin sheathed dichotomous mycorrhizas. Trees which have slight crown injuries show or indicate tannin accumulation in their roots. The high quantity of polyphenolic compounds in the root cortical and pericycle cells is considered as an indication of stunded root growth. This phenomenon may have suppressive effects on mycorrhizal infection and Hartig net development. Intracellular hyphae can be observed in the cortical cells. Intracellular invasions are very common and take place in reproducing cortical cells enclosed by degenerating Hartig nets. But in some cases living cortical cells are also penetrated and some straight connections to existing Hartig nets can be found out. The intracellular hyphae look like by their diameter and ultra structural facts the major mycorrhizal element of each rootlet. These observations expose that usually ectomycorrhizal fungi may develop intracellular invasions under hectic environmental circumstances.
Isolation trials and electrophoretic isoenzyme experiments are helpful for to identify the intracellular fungi. In the light of these studies it could identify the electron dense vacuolar accumulations in the rootlet types of industrial environment. In some experiments such structures have observed to rise in number in mycorrhizas when exposed to nitrogen fertilizers. The chemical composition and function of such structures are still mysterious one. The observed ecological and ultra structural alterations pointed out that harsh disturbances in the function of pine mycorrhizas and its composition take place in such type of industrial environments.
Trees and herbaceous plants growing on soil heaps of old metal mines have been shown their extensive tolerance towards metals. In recent times it has been observed that tolerance may alter with mycorrhizal status. Field studies in controlled environmental conditions with seedlings of birch, spruce and pine have been shown that mycorrhizal relationship with species of Amanita, Pisolithus, Rhizopogon and Scleroderma diminish the toxicity of zinc, copper, nickel and aluminium. In some cases the mycorrhiza fungal presence reduces accumulation of metals in shoots.
The significance of mycorrhizal fungi in tree sustenance is well recognized, and the variety of their identified effects is repeatedly being comprehended (Harley and Smith, 1983). Previous studies exhibited that phosphate uptake was repeatedly involved, and later it was established that nitrogen and water could also be channelled to the tree via fungus, leading to improved uptake in scarcity conditions. Mycorrhizas boost the uptake of macronutrients and also enhance the absorption of trace nutrients and conceivably even of non-nutrient metals.
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A more detailed understanding of the effects ofmycorrhizal fungi on metal tolerance is restricted by our considerable ignorance in general terms of mechanisms of metal tolerance in plants. The latter seem unable to generate metallothioneins on a large scale, although there have been reports of their induction by copper (Rauser, 1984), and there is no substantial evidence of intrinsically insensitive enzymes. It has been widely supposed that tolerant plants must sequester the metal in a harmless form, as there is no evidence of exclusion at the tissue level, but there is little agreement as to where, or in what form, this might be. Once again the various metals may well be different.
The significance of the soil microbial populations for plant mineral nutrition and nutrient cycling has long been documented.
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One of the vital interactions is the symbiotic relations [symbiosis] of plants with mycorrhizas. On the contrary, the soil microfauna and their effects on plant performance have hitherto inward slight awareness; while soil protozoa in particular, have been revealed to constructively affect plant growth. Scientists have been investigated through a laboratory experiment the impact of mycorrhiza and protozoa and their communication on performance in plants. Experiments in Spruce seedlings show that the ectomycorrhizal fungus Paxillus involutus (Batsch) Fr. can grow in defaunated forest soil with naked amoebae (Acanthamoeba sp.). The protozoan presence can result the generation of a more composite root system by increasing root length. The effects of protozoa can be well-defined in the absence of mycorrhiza. On the contrary to protozoa, the occurrence of mycorrhiza may result a less complex root system, that means improved root length, development of fine roots and generation of root tips may not be facilitated by mycorrhiza alone. Shoot height, and stem, shoot and needle mass were at a highest in combined treatment experiments with both mycorrhiza and protozoa. The occurrence of mycorrhiza and protozoa also exaggerated the concentrations of the plant nutrients. In treatments with protozoa shoots of spruce seedlings enclosed fewer nitrogen content, foremost, e.g. to an improved C/N ratio in the needles. On the other hand, in trials with mycorrhiza, phosphorus concentrations in needles were augmented. Rhizosphere microorganisms were affected by mycorrhiza and protozoan presence. Microbial biomass was declined in the presence of mycorrhiza, mainly due to a diminution in bacterial figures. Conversely, in the occurrence of protozoa the hyphal length in the rhizosphere was also restricted.
Effects of mycorrhiza and protozoa on plantnutrient acquisition and growth
A few mycorrhiza-forming species of fungi are highly dedicated, developing as they do mycorrhiza only inside a certain genus of trees. Other kinds are less expert and can generate mycorrhiza in different genera. The most specialized Boletus elegan,s which forms only on larch. To the some extent less dedicated belongs, for example, Lactarius deliciosus, which occurs mycorrhiza with pine and spruce trees. To the smallest amount specialized belong, between others, Amanita muscaria (exposed to be symbiotic with pine, spruce, larch etc.).
The studies of Hatch and Bjorkman  about the conditions for the development of mycorrhiza in seedlings of pine reveal that mycorrhiza is produced if the soil and therefore also the roots are lacking in one or more of the compounds nitrogen, phosphorus, potassium, or calcium etc. Bjorkman (1942) says, there are generally three factors that have a significant influence on the development of mycorrhiza of trees. ie; light and the amount of nitrogen and phosphorus present in the soil. In order to prove this Bjorkman planted pine and spruce seedlings under diverse intensities of light in different humus types. He could not observe a rapid mycorrhizal development In the dark or in weak light up to around 10% of full daylight. But when he increased the intensity of the illumination up to 25% there was a powerful increase in mycorrhiza formation. This experiment shows the importance of light intensities in the better development of mycorhizae.
Bjorkman also established that the leaves and needles which are drop during the autumn season in the Swedish forests consisted water-soluble, thermostable compounds that implement a strong antibiotic effect on tree-mycorrhiza fungi. This peculiarity can see in the case with leaf litter of maple, birch, beech, oak and pine like trees.
In order to prove this, he extracted the ground-up leaves with water and the extracts subjected to sterilization by autoclave or allowed it passed through a Seitz filter. In mild concentration, he noticed the stimulating effect of these extracts and also observed the cessation of mycorrhizal growth in high concentration. The occurrence of inhibitory substances is the reason that the mycorrhizal fungi do not flourish and grow in forest litter. However, he could assume that these antibiotic substances disappear with the aid of washing-out or decomposition [rain and flooding like factors] process in course of time.
The impacts of peat, mineral fertilizer and sewage waste on tree development and mycorrhizal status have also monitored after cultivating replicate enclosures consisting oil sands tailings with container-developed jack pine (Pinus banksiana Lamb.) and subalpine coal mine spoil with white spruce (Picea glauca Voss). The result is the normal enhanced development of mycorrhizal plants on coal spoil than oil sands tailings. High quantity of sewage sludge can greatly enhance the growth of jack pine and white spruce trees.
Mine spoils and tailings can be usually regarded as as unfavourable environments for plant development. They are infertile soils, deficient in organic matter, hold little nutrient reserves, have stumpy moisture-holding capabilities, are subject to leaching, erosion processes and elevated soil temperatures and, relying on the specific spoil, may have adverse chemical features (Jurgensen, 1978). As a result, plant life on spoils may be subjected to deferent stresses that might be as a minimum somewhat alleviated by both the quantity and quality of mycorrhizal relations, together with a raise in the effectiveness of water and nutrient uptake and preservation of the nutrient pool (Danielson, 1985). Mycorrhizas may facilitate preserve nutrients obliquely by stimulating plant growth development and the following incorporation of nutrients into biomass directly by promoting more competent utilization of soil. Due to these reasons, mycorrhizal symbioses are essential elements in the biology of mine spoils and tailings.
Acid precipitation and gaseous pollutants have been shown their impacts on mycorrhizal occurrence. These factors are tending to be reducing the root growth and mycorrhizal development (Reich et al, 1988). Pollutants have indirect effects in reduction of photosynthesis, and consequently carbon allocation to the root system, may also slow down mycorrhizal development. For example, a decline in edomycorrhizal fungi occured in the Netherlands coupled with pollution from S02 and NH3 (Termorshuizen and Schaffers,1987).
The rhizosphere and mycorrhizosphere of ectomycorrhizal (EM) pine seedlings vary noticeably from one another in plant and soil attributes. Mycorrhizal plants usually have larger shoots and smaller root systems paralleled to non-mycorrhizal ones.
Ectomycorrhizas form in definite families of woody gymnosperms (e.g., Pinaceae) and angiosperms (e.g., Dipterocarpaceae, Betulaceae) and are very essential in several temperate and boreal forests. The fungal partners in ectomycorrhizal (EM) relations account for an approximately 25-30 % of the microbial biomass in forest soils (Hogberg 2002). Ectomycorrhizas are a various group of minimum 6000 species of basidiomycetes, ascomycetes, and zygomycetes (Smith and Read 1997).
The presence of a enveloped fungal mantle with host roots and surrounded root epidermal or cortical cells with Hartig nets, all these structural characteristics [ectomycorrhiza] gives a large surface area for resource exchange. Hormonal communications among plant and fungus lead to severely distorted root design together with the suppression of root hairs. The outer component of EM links consists of hyphae with cross walls that separate cellular components. In many occasions hyphae coalesce into macroscopic structures named rhizomorphs that join the mycelium to sporocarps or can be morphologically analogous to xylem and serve in the uptake process of water (Smith and Read 1997). The peripheral mycelium of EM fungi may be more widespread than that of arbascular mycorrhizae fungi with as much as 200 m of hyphae per gram of arid soil (Read and Boyd 1986).
Ectomycorrhizal fungi are normally classified by means of the morphology of colonized roots and its sporocarps, such as well-known mushrooms and truffles. In the case of orchid and monotropoid associations there is no exchange of any micro-macro nutrients between the partners. But mycorrhizas involve plant exchange of photosynthates in return for fungal exchange of mineral nutrients in other cases. The union of so many distinct forms of mycorrhizas is a evidence for the mutual benefits of these trading associationships. To realize the dynamics of resource swap in mycorrhizas, we should study the mechanisms by which resources are attained by these symbionts. Mycorrhizal fungi amplify the nutrient uptake for plants, partially, by exploring or utilizing the soil more capably than plant roots.
Mycorrhizal fungal hyphae live in large volumes of soil, spreading far out ahead of the nutrient diminution zone that forms around roots. From the studies of Simard et al. , it is clear that the external hyphae of EM fungi generate a 50-60-fold raise in surface area. The small diameter sized hyphae extract nutrients from soil pore spaces too small for plant roots to utilize (van Breemen et al. 2000). Most mycorrhizal fungi rely heavily on plant photosynthate to gather their energy needs; arbascular mycorrhiza [AM] fungi are obligate biotrophs while ectomycorrhiza [EM] and cricoid fungi are biotrophs with various saprotrophic capabilities. The carbon cost of mycorrhizas is hard to exactly estimate, however field and laboratory researches imply that plants distribute 15-20 % of net primary production to fungal partners (Smith and Read 1997). Root colonization by mycorrhizal fungi frequently doubled the rates of host plant photosynthesis. Such type of effect has been endorsed to mycorrhizal development of plant nutritional status in some systems (Black et al. 2000). Mycorrhizal fungi especially ectomycorrhiza fungi are a considerable carbon sink for their host plants. Mycorrhizal biomass has been exposed to both raise and decline with increasing availability of soil nitrogen (Johnson et al. 2003).
Available data suggest that ectomycorrhizal plants have developed mechanisms to actively balance photosynthate costs with mineral nutrient profits. 1st, environmental factors that decrease photosynthetic rates, for example low light intensities can lead to declines in mycorrhizal growth (Gehring 2003). Second one is the host allocation to root structures is receptive to mycorrhizal benefits. This can see within ecotypes of the similar plant species. Plant taxa with low surface area [coarse root] are commonly more reliant upon mycorrhizas than those with high surface area [fibrous root]. This put forward that for extreme mycotrophic plant taxa, it is further modify to offer a fungal partner with photosynthates than to sustain fibrous root systems (Newsham et al. 1995). Mycotrophic plants have advanced to a certain degree of plasticity in their distribution to roots in response to their mycorrhizal systems. Mycorrhizal plants regularly have compact root- shoot ratios in comparison with non-mycorrhizal plants of the identical species grown under the same conditions (Colpaert et al 1996).
Populations and communities of rhizosphere bacteria can be affected by the quantity and quality of root exudates produced by the mycorrhizal fungi in that region (Linderman 1988). Modern data suggest that diverse combinations of plant-fungal pairs produce distinct effects on bacterial populations. For instance, Soderberg et al (2002) noticed that the outcome of AM population by Glomus intraradices on bacterial colonies diverse between plant species. Whereas some findings suggest that mycorrhizal fungi and soil bacteria may struggle for carbon in the rhizosphere, other observations pointed out that plant growth and its elevation by mycorrhizal fungi may neutralize these effects and in fact encouraging the rhizosphere bacterial activities (Soderberg et al 2002). For example, a few bacteria like fluorescent pseudomonads, are identified to function as mycorrhization helper bacteria (MHB) because of their capability to constantly improve mycorrhizal expansion (Garbaye 1994). This is supported by the experiments of Dunstan et al in 1998, in ectomycorrhiza formation on Eucalyptus diversicolor in plant nurseries shoed that fungal expansion could achieve up to 300% by MHBs. The discharge of volatile compounds by MHBs has been revealed to stimulate fungal growth and the mechanisms behind these effects are still mysterious. He suggests that such types of mechanisms and interactions are very complex. For instance, isolated MHBs from the Pseudotsuga menziesi-Laccaria laccata symbiosis are fungus-specific and not a plant-specific one. Usually MHBs bacteria elevate EM formation of the fungus L. laccata but introverted the configuration of ectomycorrhizas by other fungal species.
In the rhizosphere, mycorrhizal fungi co-inhabit with several saprotrophic and pathogenic fungal organisms. Saprotrophic interactions may be visible or intensified with EM and ericoid fungal associations due to their significant abilities to degrade organic matter. A trenching research by Gadgil et al in 1971, recommend that, EM fungi may trim down decomposition rates and take part in competition with saprotrophic fungi for nutrient availability. Ectomycorrhizal fungi may surpass saprotrophic fungi for rhizosphere inhabitation (Lindahl et al. 2001) by discharging organic acids into the rhizosphere region that may inhibit the growth of saprotrophs directly or indirectly diminish litter decomposition by saprotrophs by extracting water from the soil (Koide and Wu 2003). Fungal pathogens and mycorrhizal fungi also interact with one another. Many studies demonstrate significant protection from pathogens by mycorrhizal
fungi. In a meta-analysis of studies of interactions among AM fungi and fungal pathogens, Borowicz (2001) showed that plants generally grow better when they are mycorrhizal and this is especially true when plants are challenged by pathogens. Newsham et al. (1995) suggest that pathogen suppression may be more important than other benefits of AM symbioses in natural ecosystems. The mechanisms of pathogen suppression are highly varied and include improved nutrition of the host plant, changes in the chemical composition of plant tissues, and changes in rhizosphere bacterial communities (Linderman 2000).
Traditional models of nutrient cycling consider soil microbes as carbonlimited saprotroph that provide plants with inorganic nutrients in the soil. This view misses key aspects of nutrient cycling in ecosystems dominated by cricoids and EM mycorrhizas, and several authors have argued for a more mycocentric view of nutrient cycling. Lindahl et ah (2002) suggest that mineralization and the inorganic nutrients that result from it are relatively unimportant to nutrient cycling in many coniferous forests. Instead, organic sources of nutrients predominate in these ecosystems, and plant access to these nutrients depends upon the outcome of interactions between decomposers and EM fungi. In this view, EM and cricoid mycorrhizal fungi acquire some nutrients from the soil directly, and also capture organic nutrients from plant litter and, perhaps more importantly, from other soil biota including mycorrhizal and saprotrophic fungi, bacteria, protozoa, and soil microfauna (Klironomos and Hart 2001). Furthermore, EM fungi can sequester large quantities of nitrogen in their external mycelia. These nitrogen stores can be mobilized and utilized by host plants when nitrogen demands are high, for example, during bud break in the spring.
Rillig et al (2002) concluded that temperature increases in the ranges predicted by climate models may promote mycorrhizal fungi directly through temperature-dependent increases in fungal metabolism and indirectly through increases in plant growth and nutrient mineralization in the soil. These changes were expected to be most dramatic in regions near the poles where
potential increases in nutrient mineralization could favor AM fungi in areas formerly dominated by cricoid or EM associations. When combined with the changes predicted for nitrogen enrichment, these patterns further suggest a world of increasing AM dominance as human impacts increase. More variable precipitation regimes are also likely to affect mycorrhizal fungi. Root colonization by mycorrhizal fungi can respond significantly to soil moisture content (e.g., Swaty et al. 1998) and the relationship may be nonlinear and variable depending upon the taxa of plants and fungi involved. Mycorrhizal fungi have been shown to affect plant-water relations and drought tolerance in a number of AM and EM hosts (e.g., Boyle and Hellenbrand 1991; Auge 2001). Species and even isolates of mycorrhizal fungi vary in their ability to tolerate dry conditions and to assist their hosts in doing so (Stahl and Smith 1984). Simulated drought stress of beech (Fagus sylvaticus) resulted in significant shifts in EM community composition and increases in the production of sugar alcohols that are thought to play a role in compensation for drought stress (Shi et al. 2002). Querejeta et al. (2003) demonstrated that host plants may sustain mycorrhizal fungal hyphae during times of drought through hydraulic lift of moisture from deeper soil layers occupied by roots but not fungal hyphae. The maintenance of a functional mycorrhizal mycelium will not only provide benefits to the host plant and fungus but also to a variety of other rhizosphere organisms.
The drought tolerance of certain taxa of plants and fungi will be exceeded as climates continue to change. Plants are expected to be less tolerant of water stress than fungi because fungi can access smaller soil pore spaces and survive remarkably low water potentials. Some fungi are among the most xerotolerant organisms known (Kendrick 2000), though the drought tolerance of mycorrhizal fungi has not been broadly tested. Because of their dependence on host plant carbon, when drought is extreme, mycorrhizal fungi may share the same fate as their host plants regardless of their individual drought tolerance. For example, recent droughts in southwestern North America have resulted in substantial mortaUty of pinyon pines (Pinus edulis). Surviving pinyons in highmortality areas supported a less abundant, less diverse, and compositionally different EM community than neighboring pinyons growing in low-mortality sites on similar soils (Swaty et ah 2004). Furthermore, survival rates of pinyon seedlings were 50 percent lower in high-mortality sites that were depauperate in EM fungi compared to sites with high populations of EM fungi. Because pinyons are the only hosts for EM fungi in many of these habitats, their loss from the system may also mean loss of EM fungi from large tracts of woodland. Interestingly, AM fungi predominate in most water-limited desert environments.