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a) Yes, the property of nitrogen fixation in a plant – leguminous or non-leguminous can be artificially induced by genetic engineering.It involves the introduction of a specific genes in the plant to produce nitrogenase enzyme which is involved in nitrogen fixation.

b) Mycorrihiza and Pine trees show symbiotic relationship.In this type of relationship both the organisms benefit from each other.

c) Yes, It is necessary for a microbe to be in close association with a plant to provide mineral nutrition.The most common association is in the form of nodules.The nodules has all the necessary biochemicals such as enzymes .For example the rod shaped Rhizobium has such relationship with several legumes like sweet pea, lentils, garden pea etc.