ABSTRACT
Physiological aspects of symbionts are vital to the development and sustenance of mycorrhizal symbiosis. During the past decade, considerable knowledge has been accrued about the cytological and molecular changes related to regulation of mycorrhizal symbiosis. Yet there are innumerable physiological processes that require detailed investigation. The developmental physiology of AM symbiosis begins with dormant spores. Dormancy period is variable, depending on the availability stimuli and congenial environment. Extension of germ tubes and pre-symbiotic mycelium depend on the energy stored in spores. It is a crucial phase lasting 24 to 48 h, until the pre-symbiotic mycelium encounters a congenial host root. Formation of appresoria, infection and establishment of fungus inside the root cortex requires another 48 to 72 h. Hectic cytological and physiological changes occur inside fungal spores, mycelium and root cells. Franken et al. (2002) have argued that pre-symbiotic mycelium should possess a full complement of active genes required for growth and extension. Obviously, metabolic processes that sustain a limited growth fungal growth include uptake and utilization of carbohydrates. Several other metabolic processes such as cell cycle events, chromatin duplication, respiratory activity and molecular signals that occur in pre-symbiotic mycelium and during its transition to symbiotic phase are being intensively investigated. Molecular physiology of host and fungus during formation of appresoria, invasion into cortical tissue and establishment of AM fungus inside the roots has been delineated excellently using a series of host mutants (Marsh and Schultz, 2001).
