New insights in the molecular events underlying actinorhizal nodulation in the tropical tree Casuarina glauca

Background Actinorhizal root nodules result from the interaction between a nitrogen-fixing actinomycete called Frankia and roots of dicotyledonous trees and shrubs belonging to 8 plant families and 25 genera. Most actinorhizal plants are capable of high rates of nitrogen fixation comparable to those found in legumes. As a consequence, these trees are able to grow in poor and disturbed soils and are important elements in plant communities worldwide. This facility for adaptation has drawn great interest to actinorhizal plants, particularly to several species of Casuarinaceae such as Casuarina glauca, which can be used for fuelwood production, agroforestry, and land reclamation in the tropics and subtropics. The basic knowledge of the symbiotic association between Frankia and actinorhizal plants is still poorly understood, although it offers striking differences with the Rhizobium-legume symbiosis [1]. Recently, the development of genomics in some actinorhizal plants such as C. glauca[2], together with the possibility to obtain transgenic actinorhizal plants following Agrobacterium gene transfer [3], offer new approaches to understand the molecular basis of the actinorhizal process. We will highlight recent progress in the molecular knowledge of the early stages of the actinorhizal symbiosis. A comparative analysis of the symbiotic pathway in actinorhizal trees and legumes will be presented.