Forging a symbiosis: transition metal delivery in symbiotic nitrogen fixation.

Symbiotic nitrogen fixation carried out by the interaction between legumes and rhizobia is the main source of nitrogen in natural ecosystems and in sustainable agriculture. For the symbiosis to be viable, nutrient exchange between the partners is essential. Transition metals are among the nutrients delivered to the nitrogen-fixing bacteria within the legume root nodule cells. These elements are used as cofactors for many of the enzymes controlling nodule development and function, including nitrogenase, the only known enzyme able to convert N 2 into NH 3 . In this review, we discuss the current knowledge on how iron, zinc, copper, and molybdenum reach the nodules, how they are delivered to nodule cells, and how they are transferred to nitrogen-fixing bacteria within.