Molecular analysis of actinorhizal symbiotic systems: Progress to date

The application of molecular tools to questions related to the genetics, ecology and evolution of actinorhizal symbiotic systems has been especially fruitful during the past two years. Host plant phylogenies based on molecular data have revealed markedly different relationships among host plants than have previously been suspected and have contributed to the development of new hypotheses on the origin and evolution of actinorhizal symbiotic systems. Molecular analyses of host plant gene expression in developing nodules have confirmed the occurrence of nodulin proteins and in situ hybridization techniques have been successfully adapted to permit the study of the spatial and temporal patterns of gene expression within actinorhizal nodules. The use of heterologous probes in combination with nucleotide sequence analysis have allowed a number of nif genes to be mapped on the Frankia chromosome which will ultimately contribute to the development of hypotheses related to nif gene regulation in Frankia. The use of both 16S and 23S rDNA nucleotide sequences has allowed the construction of phylogenetic trees that can be tested for congruence with symbiotic characters. In addition the development of Frankia-specific gene probes and amplification primers have contributed to studies on the genetic diversity and distribution of Frankia in the soil.