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Q13 - A dme Mutant and Nitrogen Fixation in Soybean Root Nodules

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Mineral Nutrition & Nutrient Uptake

The bacterium Bradyrhizobium japonicum can infect soybean (Glycine max) roots and form nodules. The nitrogen fixation catalyzed by nitrogenase occurs in the nodules and the nitrogenase activity can be measured easily by acetylene reduction instead of nitrogen reduction. Scientists generated a defective mutation of NAD+-dependent malic enzyme (dme mutant), the enzyme that generates pyruvate and NADH, and infected soybean seedling roots with wildtype and mutant bacteria. The seedlings were grown in nitrogen-free media. After 14 and 28 days of inoculation, the number and weight of nodules in the seedlings and acetylene reduction activity were recorded.

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. Nitrogen fixation activity in nodules of the same treatment at 28 days after inoculation is higher than that at 14 days after inoculation.
B. Both number and size of nodules increase with time from 14 to 28 days after inoculation with B. japonicum.
C. The reduction in nitrogen-fixing activity of nodules infected by the mutant at 28 days after inoculation compared to those at 14 days after inoculation is due to the reduction of nitrogenase activity and nodule formation.
D. Nitrogen fixation in B. japonicum-induced nodule is down-regulated by NAD+-dependent malic enzyme.
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Question reproduced from IBO 2016, Theoretical Paper A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)