ROLE OF MYCORRHIZAE

Due to rapid urbanization and increasing population, people are searching for underexploited areas to grow more food. Even to mitigate the climate change scenario and to maintain soil fertility, symbiotic microorganisms played a significant role in the agricultural production system. An interest in soil microbes and their active role and use in agriculture flared a few decades ago. Despite of that, most of the commercial products quickly flickered out, and laboratory successes had not translated to the field application to revive the microbial contribution to agricultural development. One of the few agricultural microbes that did catch hold was AMF for crop growth in almost all agricultural crops and the bacterium Rhizobium, which helps in nitrogen fixation through legumes in soils.

The major challenges of the new millennia are to grow food and other economic crops at adverse weather conditions to feed the billions of rural poor and to sustain food security. Under the climate change scenario, agricultural crops are exposed to the extent of biotic or abiotic stresses such as drought, flood attenuation, freezing stress, and salt loading that influence plant growth and development and thereby productivity. Water stress (WS) is one of the major abiotic stresses that directly influence plant productivity. Agriculture production also affected by drought and salinity at a major part of the world (Gueta-Dahan et al., 1997) as well as in India. Wang et al. (2003) stated that WS alone is responsible for crop loss worldwide, decimating average yields by 50%. Water deficit in plant tissue resulted in inhibition of photosynthesis, thus leading to a negative effect on yield. Some plants are drought tolerance, i.e., the ability to maintain the photosynthetic activity under WS. Comic (1994) explained that plants response to water deficit through affecting transpiration and rapid closure of stomata. Apart from photosynthesis, respiration, metabolism, and translocation of growth promoters and nutrient ions, the colony of AMF prevent to develop other fungi related to plant diseases in the rhizosphere (Alam, 1999; Jaleel et al., 2008). Smith and Read (2008) observed that the hyphae of the AMF not only uptake the water but also absorb nutrients like phosphorus (P), nitrogen (N), potassium (K), zinc (Zn) or copper (Cu). In another study, Brachmann and Pamiske (2006) also reported that the symbiotic association between plant roots and AMF was known to be one of the beautiful and widespread plant strategies to increase nutrient and water uptake to cope with adverse weather condition for mutual benefit. The soil fungi which had ultra-radical mycelium (IRM), which elongated in the root cortex, absorb the nutrient for the host plant. Extra-radical mycelium (ERM) of AMF spread in the soil around the root and provide the surface area by which the AM fungus absorbs nutritional elements such as for transport and transfer to the host. Zaidi et al. (2003) reported that the presence of nrycorrhizal fungi in the roots of chickpea or Bengal gram, which improves the growth and yield, especially in phosphorus- deficient soils. Even AMF imparts a positive role in various leguminous crops, particularly the enhancement of phosphate uptake and growth through their symbiotic association (Atimanav and Adholeya, 2002). In abiotic stress condition, drought reduces both nutrient and water uptake by the roots and decrease the transport of metabolites and ions from the roots to the shoots due to restriction of transpiration rates and impaired membrane permeability and thereby active transport. Mycorrhizal fungi generally increase fertilizer use efficiency. The research studies about mycorrhizal fungi in a wide range of conditions, plants, and soils were studied since 1981 to find out a viable symbiotic association for crop growth and soil health. The studies were also performed in diverse climatic conditions from field to greenhouse and even in nurseries of horticultural importance. The vesicular-arbuscular myconhizal fungi (AMF), i.e., Glomus deserticola, G. intraradices, G. fasciculatum, G. mosseae, and G. etunicatum have been identified as promising in arable agriculture world till now. Aguilera-Gomez et al. (1999) tried to demonstrate the importance of mycorrhizae for phosphate solubilization and more efficient utilization of P in a sustainable agricultural production system, particularly in the paper industry. The useful effect of myconhizal colonization increased leaf number, large leaf area, and greater shoot: root ratio and fnrit mass as compared to non-VAM plants through higher water absorption and nutrient uptake. At high P levels, the increment of reproductive growth was 4.5 folds in myconhizal plants. Due to greater uptake of P from the soil and greater extra radical lryphae of fungi, enable the plants to become more vigorous and even stay strong and stout at adverse weather conditions or soil moisture stress. This was supported by Al-Karaki (1998), who found that AMF inoculation on growth and water-use efficiency (grams dry matter produced per kilogram water evapo-transpired) while studied with two wheat genotypes grown under (drought-sensitive and drought- tolerant) under stress and normal condition. The result of this study revealed that biomass production was higher in well water plants than stress plants. The water-use efficiency of myconhizal plants was higher than that of non-myconhizal plants. So far, shoot dry matter is concerned; differences between non-myconhizal and myconhizal plants were due to the positive effect of arbuscular myconhizal fungal-root associations. Total P mobilization of AMF colonization increased by both genotypes, in spite of their' water-stress level. Myconhizal plants due to symbiosis had increased root growth, which paved the way for a greater absorption area for nutrient uptake. In addition to P nutrition, the absorption of water and other nutrients by myconhizal plants possessed better growth and development as compared to the non-myconhizal plant. The benefit/cost ratio was higher in even AMF, which increased host plant dry matter under WS condition in wheat than that of well-watered plants.

Absorption rates of AMF treated plant on growth and uptake of phosphorus (P) and other micronutrients like zinc (Zn), copper (Cu), and manganese (Mn) for barley plant grown with and without drought stress. The different rates of AMF inoculums were highly depended on soil mois- hire. Rate of root AMF colonization depended on inoculum rates resulted in even higher growth rates grown with WS than that of non-water str essed (NWS), unlike wheat. Plant biomass and nutrient content of P, Zn, Cu, and Mn increased due to an increase in inoculums rate up to 240 spores of Glomus mosseae per 100 g dry soil where soil moisture is an apparent character. Al-Karaki et al. (2004) believed that myconhizal colonization was the maximum in well-watered inoculated plants with AMF as compared to water-stressed plants. Biomass and grain yields were higher in AM myconhizal plots inespective of soil moisture condition. Plants inoculated with G. eutunicatum had higher biomass and grain yields than that of colonized by G. mosseae. Shoot P and Fe concentration were higher in myconhizal plants as compared to non-myconhizal (plants) where soil moisture effect was nil. VAM fungal inoculation act as a bio-enhancer in tenns of plant growth and yield by regulating uptake of P and Cu, especially under stress of moisture in the soil. Soil pore space is perfectly used by AMF and improves nutrient and water uptake in adverse conditions. In wheat crop, myconhizal application paves the way for a reduction in the effects of drought stress in semi-arid areas of the world.

Bethlenfalvay et al. (1988) found that the dry weights of AMF inoculated plants were greater at severe stress than that of the non-treated plant. G. mosseae was a very useful AMF, and its colonization at roots is insensitive to stress. The biomass of fimgi along with the length of the ERM, was also greater in stressed condition rather than in non-stressed soil conditions to plants. Growth enhancement by AMF treated plants with relation to P-fertilization w'as attributed to increased uptake of w-ater as well as several minerals along with efficient P absorption. The survival of AMF tr eated the plant to soil WS than that of non-VAM plants, even in lower wilting points are the beauty of the association of mycorrhizae. Brejeda et al. (1998) reported that seedlings inoculated with rhizosphere microflora showed positive improvement of root and shoot growth by 15-fold greater shoot and root yields, recovered up to 6-fold more N and 36-fold more P than seedlings inoculated with rhizosphere microflora. These responses were consistent for all four cultivars of switchgrass and were mainly due to AMF. Seedlings inoculated with rhizosphere populations from seeded switchgrass stands averaged 1.5-fold greater shoot and root yields than seedlings inoculated with rhizosphere populations from native prairies. AMF induced plant growth-promoting fimgi (PGPF) and other rhizosphere fimgi, due to symbiotic association, may be responsible for the substantial increase in shoot and root weights, P, and N uptake. This result was further confirmed by Fidelibus et al. (2001). They found that w'ater-use characteristics of AMF treated plants differed from those of non-AM plants regarding w-ell supplied with P. They used AM fungal isolates of different geographic origins. Inoculation of citrus seedlings with Glomus isolates from arid and semi-arid areas resulted in different patterns of water uptake and thereby plant growth and development. AMF induced plants and non-AMF plants had similar shoot size but different in root growth (diy weight and length). Leaf nutrient analysis of the AMF plant showed that P concentr ation was 12-56% higher as compared to non-AM plants. There was a positive correlation between enhanced root growth and leaf P concentration. They also found that a faster recovery from moisture stress by AM plants and AMF induced plants had lower leaf conductance than non-AM plants in case of gradual soil drying. Glomus mycorrhizae had a beneficial effect on Citrus growth and also in rootstock survival after grafting by providing greater P concentration to rootstock and minimizing stress effect to the grafted rootstock mitigating desiccation.

Similarly, Ruiz-Lozano et al. (1995) observed that seven Glomus species had effects on plant growth characters like mineral uptake of P and stornatal conductance. They also recorded additional characters like transpiration, the CO, exchange rate, water use efficiency, and pro line accumulation under drought and irrigated condition. Seven AM fungal species showed a wide array of host plant drought tolerance. They thought that the alleviating stress appeared to be based on physiological processes rather than nutrient uptake by AM responsive host plant. AMF association protected myconhizal plants against WS by the endophytes. Plant physiological processes like leaf conductance and transpiration and P and К uptake were enhanced by Glomus spp. Glomus deserticola, a potential AMF mainly responsible for the aggressive colonizer and effective species, winch mitigated drought enormously as compared to G. occultum, which w'as least in colonization and ultimately less water and nutrient absorption.

The study of Subramanian and Charest (1999) reflected the effect of myconhizal fungi on maize plants when the external hyphae of an arbuscular myconhizal helped in the uptake of nitrogen in fungus (Glomus intraradices). Rapid myconhizal colonization by G. intraradices augmented nutritional status and N uptake and its assimilation in maize plants under drought stress. N acquisition and increased capacity of its assimilation resulted to sustain the host plant in drought stress (moderate). The protein enzymatic activity like glutamine synthetase activity was increased by 30% under drought conditions, which might be due to the hyphal transport of N (N03 or NH4). P status under AM colonization rendered positive root growth at stress conditions. This finding was further confirmed with the study of Tobar et al. (1994b), which entailed us the importance of the external mycelium with regard to transport of N from

15N-labeled nitrate responsible for plant nutrition in case of well-irrigated or water-stressed inoculated plants. They confirmed that AMF provided transport of N from nitrate source though the hyphal network important for diy agricultural soils (predominant with N03). Similarly, Tobar et al. (1994a) stated that myconhizal activity directly related to N uptake under water-stressed conditions and had positive effects on plant growth. They grew Glomus mosseoe and G. fasciculatum in a neutral agricultural soil and found both the fungal species increased the 15N enrichment of plant tissues under water-stressed. This indicated in relatively diy soil, where a direct effect of AMF onN acquisition is essential. Both the fungi unproved biomass production of the host plant. It had been found that G. mosseae had more effect on N uptake than that of G. fasciculatum under water- stressed conditions in agricultural field. This result has also corroborated the findings of Tarafdar (1996), where AMF tested on the crop Vigua aconitifolia, Prosopis juliflora, and Cenchrus ciliaris in marginal land. In poor fertility sandy soil and indigenous myconhizal fungi, they found more N assimilation shoot biomass, and other nutrients uptake like, P, K, Fe, Zn, and Cu. In arid conditions, the more pronounced effect was observed in Prosopis juliflora with Glomus fasciculatum. The effect of mycorrhizae under semi-arid condition is very much conducive to the growth and development of host plants and thereby enhanced agriculture production in arable crops.

The horticultural crops like fruits and plantation crops, which are perennial in nature thrive best under arable and marginal land. The mycorrhizas play a vital role for yield and quality production of many fruits plantation crops and medicinal plants. The fruit crops like litchi, Л АМ fungi association is necessary for plant growth and development and, ultimately the yield.

Subramanian et al. (2006) established a positive relationship between tomato plant with ЛАМ fungi and recorded higher growth in terms of fruit yield, and quality attributes nutrient content, reproductive behavior, and water status under stress conditions. They found that drought tolerance was higher when inoculation with Glomus intraradices was done. It also showed enhanced nutritional status of the host plant, with regard to N and P. They further added that Vitamin C and reduced acidity from AM associated tomato fruit. Myconhizal colonization also lessened the deteriorating fruit quality caused by drought. In general, myconhizal fungi improved the nutritional status and w^ater content of the host plant to withstand in field conditions facing different intensities of drought. Runjin

(1989) determined the influence of AMF on nutrient and water uptake of apple plants. They gave special emphasis to establish a relationship of water uptake at the seedling stage with ЛАМ colonization. In sterilized soil medium, the apple plant inoculated with Glomus versiforme and G. macrocarpum enhanced drought tolerance, elements uptake, improved available water status, overall growth, and development of the plant. AM colonization increased the transpiration rate of the apple leaves and reduced the stomatal resistance. The plant is also experienced with the rate of recovery from the WS. External hyphae growth of AMF paved the way for enhancing the absorption and translocation of water. Zn and Cu absorption was strengthening by the roots and weakened the P-Cu and P-Zn interactions. Plant growth, mineral nutrition, especially nutrients uptake and water relations, are interlinked with the effects AMF. Isliii et al. (2005) reported that (AM) fungi had better fruit quality with larger trees when applied within the rhizosphere of Satsuma mandarin trees, than non-VAM control trees. Under low' concentrations of applied phosphorus (P), AMF showed higher growth and development of citrus tree (Shrestha et al., 1995). Reduction of P enhances the number of AM spores in the soil, thereby, AMF infection in the roots increased. The juice content w'as markedly increased with AMF, even sugar-acid ratio, and TSS (total soluble solids) w’ere increased, and chlorophyll b and the b value of peel color get affected by -P and -P+CH increased as compared to control. Carotenoids in the peel wrere affected by the reduction of P in soil. In particular, the reduction of P fertilizer positively affected the growth of AMF. It also increased the percentage of AM infection in citrus roots in the preliminary stage, which gave a boost to the yield of citrus trees. Thus, insoluble P fertilizer in the form of bone dust might be useful for the propagation and maintenance of AM inoculums. Waterer and Coltman (1989) stated that inoculation timing on the development and yields of AMF colonized bell peppers increased nutrient and water availability. They gave a treatment of Glomus aggregation under filed conditions and greenhouse. Tissue P concentrations when low’ (P soil inoculation), winch ultimately gave better plant weights and fruit yields than that of non-inoculated plants. Seedling inoculation wns better than transplanted later inoculation. AMF association with bell pepper plant gave better P utilization, which may be a substituted for a greater portion of phosphate fertilizer. Regarding plant biomass and diy matter, Yano-Melo et al. (1999) stated the effects of three native AM fungal species collected from the Brazilian semi-arid region on in vitro growth of banana plantains. A significant difference in plant growth characters like height, leaf area, the weight of shoot and root, and fresh and diy matter of shoot between inoculated and non-inoculated plants were observed after three months of acclimatization. Leaf area and height of inoculated plants were approximately 57% and 32% higher. Dry matter of shoots was increased by 45-64% in mycorrhizal plants. Plants inoculated with Glomus clarum showed an increment of around 45% in the fresh weights of shoots and roots. Inoculation with AMF increased the growth of micro-propagated banana plantlets during the acclimatization period. This might be due to higher rates of photosynthesis and nutrient transport from AM to plantain.

Further study by Jin et al. (2005) determined the nitrogen path in AMF developed by Glomus intraradices when carrot roots were grown in vitro. After providing 15N in NH, to the system, the fungal synthesized predominantly arginine (Arg). It indicated that NH4 was the most likely form of N transferred to host cells after Arg breakdown. Extrametrical mycelium, formed by AMF, played an important role in N nutrition to the carrot. While studied with cadmium (Cd) toxicity, Yu et al. (2005) found that the importance of earthworms and AMF on the phytoremediation of soils. Earthworms and mycorrhizal fungi survived to decompose Cd in ryegr ass shoot biomass. Inoculation of both earthworms and mycorrhizal fungi increased ryegrass shoot Cd uptake. The earthworms, mycorrhizal fungi, and then interaction paved the way for alleviating the process of eliminating the metal contamination in soil.

 
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