All of the following statements concerning the actinomycetous filamentous soil bacterium Frankia are correct except that Frankia

Options

A. a. Forms specialized vesicles in which the nitrogenase is protected from oxygen by a chemical barrier involving triterpene hopanoids
B. b. Can induce root nodules on many plant species
C. c. Like Rhizobium, it usually infects its host plant through root hair deformation and stimulates cell proliferation in the host's cortex
D. d. Cannot fix nitrogen in the free-living state

Answer

To solve the question regarding the actinomycetous filamentous soil bacterium Frankia, we need to analyze each statement provided in the question carefully.

### Step-by-Step Solution:

1. **Understanding Frankia**: Frankia is a genus of actinobacteria that is known for its ability to fix nitrogen. It forms a symbiotic relationship with certain plants, particularly non-leguminous plants.

2. **Analyzing Statement A**:
- "Frankia forms specialized vesicles in which the nitrogenous is protected from oxygen by a chemical barrier involving triterpene hapenoids."
- This statement is correct. Frankia does form vesicles that protect nitrogenase (the enzyme responsible for nitrogen fixation) from oxygen.

3. **Analyzing Statement B**:
- "Frankia can induce root nodule on many plant species."
- This statement is correct. Frankia is known to induce root nodules in various non-leguminous plants, similar to how Rhizobium works with legumes.

4. **Analyzing Statement C**:
- "Like Rhizobium, it usually infects its host plant through root hair deformation and stimulates cell proliferation in the host's cortex."
- This statement is also correct. Frankia infects host plants through root hair deformation, leading to the formation of nodules.

5. **Analyzing Statement D**:
- "Frankia cannot fix nitrogen in the free living state."
- This statement is incorrect. Frankia can fix nitrogen in both symbiotic and free-living states, although it is more effective in symbiosis.

6. **Conclusion**:
- The only incorrect statement among the options provided is D: "Frankia cannot fix nitrogen in the free living state." Therefore, this is the answer to the question.

### Final Answer:
The statement that is incorrect concerning Frankia is: **D: cannot fix nitrogen in the free living state.**