ABSTRACT
The increase in fertilizer use, especially nitrogen, has played a major role in recent productivity gains. However, increasing the amount of nitrogen applied decreases its use efficiency, and a large amount of it is left behind, causing environmental problems. Nitrogen, even though it is abundant in the atmosphere, is the most limiting nutrient for crop yields in many soils, while phosphorus is the most limiting nutrient under natural conditions in the tropics and the second most limiting plant nutrient for farmers. Biological nitrogen fixation by free-living microorganisms in the soil or those associated with plants has a great potential to contribute to food production. Biological nitrogen fixation uses energy derived from photosynthesis and does not accumulate excess nitrogen to cause pollution. Rhizobium, a genus of gram-negative soil bacteria, is capable of establishing nitrogen-fixing symbiosis within roots of plants belonging to the Leguminosae family. Legume nodules export symbiotically fixed nitrogen as amino acids, amides, or ureides apoplastically via xylem into the shoot of the host plant. Assimilates from photosynthesis are imported into nodules via phloem in the form of sucrose. Export of nitrogen compounds from the infected cells and import of energy-rich assimilates are essential requirements for efficient biological N2 fixation. The legume host supplies carbon and energy to the nitrogen-fixing bacteria, which in turn reduce nitrogen into ammonia and export it to the plants (Bergerson and Turner, 1967; Miller et al., 1988). The mutual symbiotic association between a plant and a fungus, mycorrhizae, is also playing major role in plant growth. A diverse group of soil fungi spanning the Zygomycetes, Ascomycetes, and Basidiomycetes (Harley and Smith, 1983) form mycorrhizae. Arbuscular mycorrhizae are formed between members of the order Glomales of Zygomycetes and the species of
angiosperms which also includes the most important agricultural crops. The arbuscular mycorrhizal fungi (AMF) are ubiquitous and are found in diverse habitats ranging from the arctic to the tropics, arid to aquatic environments, and stable plant communities to highly disturbed ecosystems. AMF are vital for uptake and accumulation of ions from soil and translocation to hosts because of their high metabolic rate and strategically diffuse distribution in upper soil layers. In fact, the fungus serves as a highly efficient extension of the host root system. The fungi derive most of their needed organic matter from their symbiotic niche in the roots and, in turn, help their host plants achieve better growth through enhanced phytochrome levels (Raman and Mahadevan, 1996) in absorption of phosphorous and other mobile elements from soil, impart tolerance to heavy metals, and afford protection against disease, salinity, drought, and temperature (Raman and Sambandan, 1998).
