Question

How do legume roots attract rhizobia? a. They secrete flavonoids. b. They secrete carotenoids. c. They secrete alkaloids. d. They secrete Nod factors. e. None of the above is a means of attracting rhizobia.

Short answer

The correct answer is option 'a': They secrete flavonoids.

Step-by-step solution

Interpret the question

Understand what is being asked - which substance is secreted by legume roots that attracts rhizobia.

Recall knowledge about legume-rhizobia interaction

Remember information about how legume plants and rhizobia bacteria interact. Rhizobia have a mutualistic relationship with legume plants where both benefit. The plant provides the bacteria with nutrients while the bacteria help the plant by fixing nitrogen.

Match the knowledge with the options

Now, match this information about the legume-rhizobia relationship with the options given in the exercise.

Determine the correct answer

The correct answer would be the substance that legume roots secrete to attract rhizobia which in this case is 'flavonoids'. Thus, option 'a' is correct.

Key concepts

Flavonoids

Legume roots secrete flavonoids as a key signal to attract rhizobia bacteria. Flavonoids are a group of naturally occurring compounds found in plants that serve various functions. In the context of legumes, these molecules act as chemical signals to initiate the legume-rhizobia interaction. Once released into the soil by the legume roots, flavonoids are recognized by the rhizobia. This recognition triggers a cascade of events leading to the production of Nod factors by the rhizobia.

These Nod factors are crucial because they signal the plant to begin forming root nodules, which are specialized structures where nitrogen fixation takes place. Together, flavonoids and Nod factors establish the critical communication bridge between plant and bacteria, ensuring successful nodulation and nitrogen fixation.

• Flavonoids are essential for plant-bacteria communication.
• These compounds are released from the root to attract rhizobia.
• They initiate the nodulation process by stimulating rhizobia.

Mutualistic Relationship

Legumes and rhizobia engage in a mutualistic relationship, meaning both parties benefit from their interaction. In this symbiotic association, legumes provide the rhizobia with carbohydrates and a niche environment to thrive, while the rhizobia contributes by fixing atmospheric nitrogen into a form usable by the plant. This exchange allows legumes to grow in nitrogen-poor soils without the need for synthetic fertilizers.

The mutualistic relationship between legumes and rhizobia is fundamental for sustainable agriculture. It reduces the dependency on chemical nitrogen fertilizers, which are costly and can be harmful to the environment. This natural fertilization mechanism promotes ecological balance and soil health.

• Both legumes and rhizobia benefit from the symbiotic relationship.
• Rhizobia convert atmospheric nitrogen, which is inaccessible to plants, into ammonia.
• This relationship contributes to the fertility of nutrient-poor soils.

Nitrogen Fixation

Nitrogen fixation is a critical process in the legume-rhizobia interaction. This natural procedure involves the conversion of inert atmospheric nitrogen ( N_2 ) into ammonia ( NH_3 ), which is a form that plants can readily uptake and utilize. Rhizobia bacteria are equipped with the enzyme nitrogenase, which facilitates this conversion process. The presence of this enzyme is essential since atmospheric nitrogen, although abundant, is not directly accessible to most living organisms.

Within the root nodules of legumes, rhizobia perform nitrogen fixation, providing the plant with a steady source of nitrogen. This is particularly beneficial in agricultural systems where synthetic nitrogen fertilizers are either too expensive or environmentally damaging. Hence, the biological nitrogen fixation carried out by rhizobia reduces the need for external nitrogen inputs and enhances the sustainability of agricultural practices.

• Nitrogen fixation transforms inert nitrogen gas into ammonia.
• This process is enabled by the enzyme nitrogenase in rhizobia.
• Legume root nodules are the site of nitrogen fixation.