**3. Mechanism of mycorrhizal association**

Arbuscular Mycorrhizae fungi completely rely on host plants for their nutrition hence referred to as obligate biotrophs. The various stages involved in symbiosis are

Stage 1: This is a very vital stage of the colonization process in which fungi scout for host plant.

Stage 2: The second stage involves the invasion of host roots by fungi for colonization and development.

Few chemical substances known as bioactive agents are produced by special cells or tissues that persuade different organisms to function. Similarly, substances such as strigolactones produced by the roots enable fungi to recognize their host as well as induce enlargement and expansion. The fungi respond to these stimuli by releasing a series of factors referred to as Myc (Mycorrhizal Factors) which also have a critical part in the interaction between AM fungi and nitrogen-fixing bacteria. This association is facilitated further through the production of seven genes (SYM genes) [12]. After spore germination, few hyphae branches approach the root of host and enter into the cortical cell wall and finally invade the internal cortical cells resulting in the formation of a greatly branched structure called arbuscular, which acts as an platform for nutritional exchange [13]. The Myc impulses are recognized by Myc Factor Receptor(s) of host which leads to the release of cytosolic calcium in root cells. Another membrane-based protein (SYMPK) is stimulated, which encodes for a receptor-like kinase having the ability to identify fungal signals. SYMPK has the potential to convert these impulses from the cytoplasm to the nucleus by phosphorylation of unrevealed substances through its kinase domain [14]. The localization of all downstream elements present in the cytoplasm stimulates the fast impulse transmission between the cells and nuclei, this continuous to and fro moment of Ca2+ concentration is possible due to alternate action of Ca2+ channels and transporters. The calcium to and fro moments are coded by a Calmodulin-Dependent Protein Kinase (CCaMK). CCaMK phosphorylates the product of one of the SYM genes (CYCLOPS). This ultimately results in the control of other genes and consequently root colonization [13]. The mutualistic interaction of Arbuscular Mycorrhizal (AM) fungi and the roots of higher plants is of broad nature. Various studies have revealed that AM symbiosis is a principle component to overcome various stresses and in enhancing tolerance against various stresses by bringing modifications in phonology of host.
