Question

We find that Rhizobium forms nodules on the roots of leguminous plants. Also, Frankia, another microbe, forms nitrogen-fixing nodules on the roots of non-leguminous plant Alnus.

a. Can we artificially induce the property of nitrogen fixation in a plant– leguminous or non-leguminous?

b. What kind of relationship is observed between mycorrhiza and Pine trees?

c. Is it necessary for a microbe to be in close association with a plant to provide mineral nutrition? Explain with the help of one example.

Answer 1

a) Induction of nodule formation:

Manipulation of genes by man is called genetic engineering. Through genetic engineering, foreign genes can be introduced into a plant, hence, it is possible to induce the property of nitrogen fixation in a plant. It involves the introduction of nif genes that cause the synthesis of nitrogenase enzymes by some vector in the desired plant.

b. Benefits to fungi​:

The fungi (which is achlorophyllous) has direct access to photosynthates from the Pine trees. Thus the plant is a source of food and energy for the fungal component.

Benefits to plant:

The fungal hyphae helps in the mobilisation of nutrients from the soil for the Pine.

Nature of relationship:

As both the members of the association are benefited, the association between mycorrhiza and Pine trees is mutualistic.


Close association:

The microbe must be in close association with the plant for providing nutrients. This is primarily because:-

● The microbe has to have morphological connections to the plant part for transport of the nutrients.

● The microbe also gains benefit from the plant which happens only if the microbe stays inside or on the plant.

Example:

An appropriate example, to explain that a close association is required, is the nitrogen fixation carried out by Rhizobium in the roots of leguminous plants. The following steps take place:-

● Leguminous plants secrete chemicals which attract Rhizobium which then begin to multiply.

● Rhizobium induces the root hair to curl and enters through the infection thread into the cortex of the roots.

● The infected cortical cells get differentiated into nitrogen-fixing cells. These sites of infection are called nodules.

● Nodules are formed at these sites and they establish vascular connections.

The bacteria that inhabit the nodules perform nitrogen fixation and allow these nitrogen derivatives to get discharged into the vascular system and get transported throughout the plant through xylem. As seen above, specific morphological connections are required for the symbiotic association to function.


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