ABSTRACT
Recent progress in identifying, sequencing and characterizing genes relevant to AM and ECM symbiosis has been spectacular. The 'Symbiotic Regulatory gene' is a very useful concept. First, it helps us in focusing our investigation to only those genes whose expression is significantly altered due to symbiosis. Thus far, we have known that SR genes play a crucial role in establishment, maintenance and senescence of AM/ECM symbiosis. No doubt, understanding the molecular regulatory aspects of symbiosis in time and space may need more systematic and elaborate effort. Considering the large number of host-mycorrhizal fungal combinations that are being investigated and their inherent variation, it is time that we concentrated on a few selected specific symbiotic partners. Even then, by analyzing a large number of SR genes, we are actually attempting to decipher a complicated system at molecular level. The situation gets proportionately more complicated if a tripartite relationship of plant, mycorrhizal fungus and nodule bacteria or pathogen is involved. A single aspect such as colonization itself may involve a wide range of SR genes whose expression fluctuates due to a variety of factors related to host, fungus and environment. Such investigations invariably require a great deal of patience and perseverance. It might be worthwhile to systematically identify and classify the SR genes with regard to their origin (host or fungus), stage of symbiosis affected, cellular functions influenced. Figure 3.7 depicts a chart that helps in classifying SR genes suitably based on stages or functional aspects. Practically, in future, we will encounter a maze of gene induction/repression patterns and corresponding SR gene activity. Perhaps, we can focus further and confine analysis to a few important SR genes with the ability to influence symbiotic aspects drastically. Careful manipulation of SR genes may lead us to important applications in agriculture/forestry. They may hold the key for enhanced P uptake, translocation and growth stimulation of host species.
