Involved in the biosynthesis of the nodulation enhancer compound rhizobitoxine. Catalyzes the final step of the pathway, the introduction of a carbon double bond into the C3 position of dihydrorhizobitoxine to produce rhizobitoxine. dihydrorhizobitoxine + 2 H(+) + O2 + 2 reduced [2Fe-2S]-[ferredoxin] = 2 H2O + 2 oxidized [2Fe-2S]-[ferredoxin] + rhizobitoxine Mutant can produce serinol and dihydrorhizobitoxine, but not rhizobitoxine (PubMed:11679318, PubMed:14711685). Mutant produces no symptoms of foliar chlorosis on G. max cv. Lee and does not accumulate rhizobitoxine in the nodules or upper shoots (PubMed:14711685). It shows significantly less nodulation competitiveness than the wild-type strain on M. atropurpureum (PubMed:14711685). Belongs to the fatty acid desaturase type 1 family.