Regulators and Regulation of Legume Root Nodule Development

Nitrogen is the nutrient plants require in the high- est amount, and in agriculture nitrogen availability has a major influence on both yield and product quality. In nature plants acquire nitrogen by assimi- lation of nitrate and ammonium or from dinitrogen through association with nitrogen-fixing bacteria. Symbiotic nitrogen fixation, where the plant supplies the carbon source for the energy-dependent reduc- tion of dinitrogen and protects the oxygen-sensitive nitrogenase enzyme, is among the most effective fix- ation systems. To establish a symbiosis, the bacterial microsymbionts gain access to single plant cells and install themselves in compartments surrounded by a plant membrane. In Gunnera sp. the cyanobacterium Nostoc sp. invades pre-existing stem glands and forms nitrogen-fixing heterocysts in infected cells. In most other symbiotic interactions, a specialized plant organ, the root nodule, is developed to provide op- timal conditions for the nitrogen-fixing bacteria. Among woody plant species belonging to eight dif- ferent families, an interaction with the gram-positive genus Frankia leads to the development of actinorhi-