Mycorrhizae

Mycorrhizae are the tremendous component of the earth ecosystem. Technically mycorrhiza is a symbiotic relationship between arbuscular fungus and roots of higher plants. The mycorrhizal fungi found in symbiosis with plant roots are an important connecting link between the plant and soil and are responsible for increased mineral uptake mainly by increasing the volume of soil available to the host through the production of extra-matrical hyphae (Miller et al.,1985). Convensionally there are  seven types of mycorrhiza present in the environment, viz., endo-, ecto-, ectendo-, arbutoid, monotropoid, ericoid and orchidaceous mycorrhiza (Amadou et al., 2014). Among  them endomycorrhiza or Vesicular Arbuscular Mycorrhiza (VAM) have gained much attention by the modern researchers. Taxonomically VAM belongs to the phylum Glomeromycota. Fungal component of VAM is often called as Arbuscular Mycorrhizal Fungi or AMF. As per the recent literature Arbuscular Mycorrhizal Fungi  comprise symbiotic relationship with more than 80% of land plants (Smith and Read, 1997). Paleobotanical evidences and nucleotide sequence data established that the symbiotic relationship between AMF and plants is almost of 400 years (Mohammadi et al., 2011).

 

Taxonomically AMF belongs to Glomeromycota which has three classes (Glomeromycetes, Archaeosporomycetes and Paraglomeromycetes). The class Glomeromycetes is further devided in to five orders namely, Glomerales, Diversisporales, Gigasporales, Paraglomerales, Archaeosporales. Collectively it includes 14 families, 26 genera and 150 specis (Bagyaraj, 2014). Glomeromycetous fungi are obligate symbionts and produce relatively large, thick walled and multinucleate spores  ranging from 40-800 µm. Some of them contain several hundreds to thousands of nuclei which remain viable even in adverse soil or other hazardous environment. Under favourable condition spores germinate and forms appressoria on surface of fine roots of the host plant. Soon after appressoria enter into the root and ramify in the root cortex and established a new mycorrhizal symbiosis. After the entry longitudinal intercellular hyphae appear like arbuscules and vesicles in the cortical cells. Arbuscules are branched hyphal structures and vesicles are thin walled structures containing oil droplets that functions as storage organs. Arbuscular mycorrhizal fungi improves crop productivity in several ways. Now a days it gains much attention as it can increase tolerance level of  host plants against several abiotic stresses like drought stress, radiation stress, salinity, alkalinity, sodicity, thermal stress etc (Ref). They mitigate the effects such stresses, enhance plant growth and bio mass, prevent nutrient deficiency and ion toxicity. Many findings concluded that existence of AMF in plant can adjust the osmotic imbalance, detoxify Reactive Oxygen  Species level (ROS), prevent membrane damage, mitigate drought stress, enhance growth and nutrient uptake, enhance soil fertility (Kapoor et al., 2013 ). Further investigations revealed the molecular mechanism of AMF induced resistance of plant againt environmental stresses (Latef et al.,2016). In present communication we have focused upon the current scenario of germination and further development of AMF under some critical situations and stress conditions.

The growth of AM fungal germ-tubes is influenced by various exogenous factors, which affect overall length and/or branching. Several studies reported  that  based on exogenous factors AM fungi appear as different morphological form  such “fan-like” structures (Powel,1976) of predominantly septate hyphae, branching of germ-tubes (Graham 1982), elicited hyphae (Giovannetti et al. 1993), or hyphal elongation (Elias and Safir 1987; Bécard and Piché 1989; Bécard et al. 1992; Chabot et al. 1992b; Morandi et al. 1992; Poulin et al. 1993), following the application of root exudates, flavonoids and/or CO₂ to germinated spores (Vierheilig et al. 1998). Mycorrhizal fungi apparently have a limited range of tolerance against environmental conditions (Stahl et al., 1988) and possess specific adaptations to the habitat in which they occur.