Casuarina glauca: A model tree for basic research in actinorhizal symbiosis.

Casuarina glauca is a fast-growing multipurpose tree belonging to the Casuarinaceae family and native to Australia. It requires limited use of chemical fertilizers due to the symbiotic association with the nitrogen-fixing actinomycete Frankia and with mycorrhizal fungi, which help improve phosphorous and water uptake by the root system. C. glauca can grow in difficult sites, colonize eroded lands and improve their fertility, thereby enabling the subsequent growth of more demanding plant species. As a result, this tree is increasingly used for reforestation and reclamation of degraded lands in tropical and subtropical areas such as China and Egypt. Many tools have been developed in recent years to explore the molecular basis of the interaction between Frankia and C. glauca. These tools include in vitro culture of the host and genetic transformation with Agrobacterium, genome sequencing of Frankia and related studies, isolation of plant symbiotic genes combined with functional analyses (including knock-down expression based on RNA interference), and transcriptome analyses of roots inoculated with Frankia or Rhizophagus irregularis. These efforts have been fruitful since recent results established that many common molecular mechanisms regulate the nodulation process in actinorhizal plants and legumes, thus providing new insights into the evolution of nitrogen-fixing symbioses.

Effect of salt stress on the physiology of Frankia sp strain CcI6.

Actinorhizal plants are able to overcome saline soils and reclaim land. Frankia sp strain CcI6 was isolated from nodules of Casuarina cunninghamiana found in Egypt. Phylogenetic analysis of Frankia sp. strain CcI6 revealed that the strain is closely related to Frankia sp. strain CcI3. The strain displays an elevated level of NaCl tolerance. Vesicle production and nitrogenase activity were also influenced by NaCl.