Much depend on the situation you are in, the facilities you have available, but a above all: what are you interested in. A growing area of interest is the communication between plant and mycorrhizal fungus in response to its environment. That you observe in the field is a result of this communication. However, keep in mind that it is a dynamic situation, changing over time and variable in spatial scale. Get a thorough overview over existing literature, and perform various simple observations in your own situation. Out from that, you formulate new hypotheses you are going to test.

Thank you for your answer and sugestion Theo. I agree, a growing area of interest in te interactions between plant, micorrhizal fungus and, in my opinion, others soil microorganisms.

It depends on the objectives of your study..

Are u interested in mass production of AM fungi ?

or are u interested in taxonomy, molecular studies, ecology and interaction of AM fungi in the rhizosphere with other microrganism????????

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I am more interested in ecology and interactions of AM fungi with other rhizosphere microorganisms.

I will suggest Signaling in AM and plant symbiosis....

and how does it affect other rhizosphere organism.....????

root systems of different plant tree ??????

and how.all this affect....Carbon economy.......?????

if u want than you can select aquatic ecosystem...where....there is also influx of saline water....??????

Becard and Fortin, tested a medium with transformed roots of carrot in MMB or MM (artificial cultures) this methodology is excellent for certain mycorrhizal fungi (AMF), and species sporulate easily,to obtain pure cultures AMF for molecular identification ... however the production of AMF in living plants (host) is for me, the best production of competent species in field .

Identifying the molecular basis/ cell to cell signaling of the Interaction between mycorrhizal fungi and mycorrhizae helper bacteria seems to be a very exciting topic to me.