FUTURE OF AMF AND SYMBIOTIC ASSOCIATION
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Researchers across the world, particularly from North America, Europe, and Africa, strongly believed that AMF is very usefirl by saving water, and improving nutrition to plant could help the stakeholder belongs to rural poor from arable farmland. A lot of field trials have been conducted using AMF inoculums, principally on many economically sound crops like apple, plum, and cherry in temperate regions. Due to different types of soil availed in these regions, the application of phosphate fertilizer was very effective (acid soils of mountain and terrace). In developing countries, phosphate fertilizer was relatively cheaper. Therefore, farmers in developing countries belong to tropical and subtropical climate have no great motivation to switch on to cormnercial mycorrhizal inoculums for yield maximization. The farmers and stakeholders are not interested in AMF due to the lack of awareness. This situation might be changing with increasing phosphate prices, which might influence the decision of the farmers to switch on myconiiizae application in then agricultural production system. The most of the tropical soils are phosphate deficient or where exceptionally low phosphate levels and phosphate retaining capacities. Use of AMF in future cases may be encouraged if degraded land brought under cultivation and prices of the phosphate fertilizer undergone high. Tropical region is where the judicious use of AMF mycorrhizal inoculums could potentially bring about a substantial amount of saving for the fanners by decreasing the amount of phosphate fertilizer and other fertilizers with phosphate combinations. Therefore, technology developed by highly facilitated laboratories of a developed country is looking for applications of AMF through biotechnological intervention to launch its products in tropical agriculture system of the developing county like Africa, Asia, and other sub-continents. It is a fact that enormous potential for mycor- rhizal application in the tropics could pave the way for further research and development on AMF and could establish its scientific importance in agriculture as well as in plant kingdom.
MYCORRHIZAE AND CARBON ASSIMILATION
In soil, plant, mycorrhiza interface there was significant carbon flow from host plant to mycorrhizal fungi (Ho and Trappe, 1973; Bevege et al.,
1975) and arguments from analogy with saprophytic and other symbiotic fungi (Lewis and Harley, 1965a, b; Smith et al., 1969) made sugars strong candidates (myconiiizae) for the forms of carbon transferred. Woolhouse (1975) postulated that the host root cortex may release sugars and other metabolites to the symbiotic association (plant to fungal interfaces) by passive efflux that could be stimulated by the presence of native as well as in symbionts fungus. It has been observed that no plant transporters are involved in such type of carbon efflux (Sauer et al., 1994). Further study is essential to have better identification of any carriers (Hairison, 1999). Shachar-Hill et al. (1995) studied with nuclear (atomic) magnetic resonance spectroscopy using isotopic labeling in AMF roots and intra-radical hyphae (Solaiman and Saito, 1997). Results revealed that fungal symbionts efficiently used hexose within the root cortex. On the contrary, there was no evidence to the uptake of Glycogen (Glc), fructose (Fru), mannitol, or succinate (Sue) by the ERM (Pfeffer et al., 1999). Intra-radical hyphae utilized the modest amount of Sue (Solaiman and Saito, 1997) and raised the possibility based on fractional enrichments to make less probable in vivo for carbon utilization (Shachar-Hill et al., 1995). In diverse biotro- phic symbiosis, including AM, host extracellular (acid) invertase imparts (Dehne, 1986; Farrar and Lewis, 1987; Snellgrove et al., 1987) a considerable amount of hexose that was dominantly taken up. For disease in powdery mildew/wheat association, recent results by Sutton et al. (1999) reveal that Sue is indeed hydrolyzed before uptake. In the case of ecto- mycorrhizal fungi (Amanita muscaria and Hebeloma crustuliniforme) Sue utilization depended on invertase activity in the root cortex of the host spruce (Salzer and Hager, 1991). Fungi might have active or passive carbon flow systems (Blumenthal, 1976; Lagunas, 1993), and the probable happening in AMF cannot be ruled out. The concentration gradient at the interface might be passive, which propels fungal carbon uptake and conversion (hexose to trehalose and glycogen) easy and rapid (Shachar- Hill et al., 1995). In another way, there may be active transport together with the active earner, which penetrates the cortical cell wall easily. In fungi, H+-hexose co-transport is a common phenomenon (Sanders, 1988).
METABOLISM IN THE MYCORRHIZAL ROOT
In the case of AMF, root carbohydrate pools are significantly obseived as compared to uncolonized plants (Douds et al., 2000). Expression of hexose transporters were enhanced at the level of gene expression, probably due to the involvement of uptake of sugar in cortical cells in intraradical hyphae (Harrison, 1996). Changes in the expression of invertase had also been reported by Blee and Anderson (2000) in arbuscules of AMF fungi. As the respiration rate in myconhizal roots was substantially higher than that of non-myconhizal fungi (Shachar-Hill et al., 1995; Douds et al., 2000; Graham, 2000), the transfer of carbohydrate to the fungus and with myconhizal roots showed stronger sink for photosynthates than another fungi (Douds et al., 2000). Martin et al. (1998), the acquired Glc had direct incorporation in cells of AMF, and again these converted into trehalose and glycogen via mannitol. Glycogen, trehalose, and tetrahalose were detected successfully in the intra and extra-radical (Shachar-Hill et al.,
1995) mycelium and genninated fungal spore (Bago et al., 1999; Pfeffer et al., 1999). Their presence might be due to cytoplasmic hexose, which may act as a buffer.
NUTRIENT UPTAKE AND TRANSPORT BY MYCORRHIZAE
The major function of mycorrhizae is the ability to uptake the nutrients such as inorganic phosphorus, mineral or organic nitrogen, and amino acids from the soil and transported to host plant and sometimes specialized transporters located on their membrane adsorb nutrient efficiently (Table 1.2). P and N transporters located in the root hairs and epidermis, uptake the nutrients directly fr om the soil-root interface due to high affinity is termed as direct pathways (DP) (Smith et al., 1969). On the other hand, the mycorrhizal pathway (MP) involves the nutrients absorption from the soil by the ERM and its translocation to the IRM thereby; finally, the nutrients are utilized by the host from the fungal-plant interface (Harrison et al., 2002). It is interesting to speculate that the AM fungus could use the downregulation of the DP to increase its C availability. A higher dependence on the MP for nutrient uptake has been shown to stimulate the C allocation to the root system (Nielson et al., 1998). The plant provides sucrose, which is broken down by invertase or sucrose synthase (plant- derived) into lrexoses, which the fungus takes up through high-affinity monosaccharide transporter (Helber et al., 2011). Induction of plant acid invertase in the mycorrhizal interface is essential for the AMF because it is not able to utilize sucrose as a C source. It is a fact that an increase in the C availability, the AM fungus stimulates the P transport in the AM symbiosis (Bucking et al., 2005). It has been found that C also acts as a trigger for fungal N uptake and transport. Stimulation in N transport is driven by changes in fungal gene expression (Fellbaum et al., 2012).
AMF FOR NODULATION AND DISEASE RESISTANCE
hi natural ecosystems, active symbiosis paves the way for the nitrogen and phosphate and potash economy, particularly on infertile soils or in marginal land (Wheeler and Miller, 1990). All bacteria and fungi present in soil did not help to the potential symbiotic association through mycorrhizae. Some bacteria help mycorrhizae for nutrient transfer and disease protection. Garbaye (1994) acknowledged about bacterial help for mycorrhizal establishment. Kosuta et al. (2003) reported that the AMF have been found to release myc (Mycorrhiza), responsible for accelerating the activities of the nodulation factor's inducible gene MtEnodll (Pour early nodulin 11 of Medicago truncatula). AMF also helped to the rhizobial bacteria through help in nodulation. Symbiotic association of AMF with the
economically important plant's changes in root exudation patterns induced by AM colonization (Soderberg et al., 2002; Marschner and Baumann,
2003), which also augment the influence for the composition of bacterial communities in the plant rhizosphere or mycorrhizosphere. With the help of PGPR, the manipulation of crop rhizosphere is unique and easy, and thereby bio-control of plant pathogens had remarkable results (Nelson, 2004; Ren et al., 2007). In soil, changes in the bacterial community were influenced by complex interactions between plant and fungal genotypes (Marschner and Baumann, 2003; Marschner and Timonen, 2005). In another study, Toro et al. (1997) demonstrated that there was a synergic effect between both Enterobacter sp. and Bacillus stibtilis and Glomus intraradices, which promoted the establishment of the myconhizal association and increased the host plant growth.
With regard to plant disease control, Grandmaison et al. (1993) stated that there was increasing deposition of phenolic compounds in AMF root and then bound to cell walls showed resistance of AMF root to pathogenic fungi. Several mechanisms of plant disease control by myconhizal fungi have been developed from time to tune. Myconhizae develop microbial networks at the rhizosphere of the host plants, which imparts the development against disease infection. Myconhizae act as a mechanical barrier for the pathogen infection and subsequent spread at the plant root system. During the growth process via lignifications, thickening of the cell wall happened, which in him prevent the entry of soil bom plant pathogens (Dehne and Schoenbeck, 1979). Host root also gathers or retains different metabolites (terpenes and phenols), by the signal of myconhizal root and propel defense system of the host plant (Knipa et al., 1973; Sampangi,