MOLECULAR ANALYSIS OF ACTINORHIZAL SYMBIOTIC SYSTEMS - PROGRESS TO DATE

The application of molecular tools to questions related to the genetic s, ecology and evolution of actinorhizal symbiotic systems has been es pecially fruitful during the past two years. Host plant phylogenies ba sed on molecular data have revealed markedly different relationships a mong host plants than have previously been suspected and have contribu ted to the development of new hypotheses on the origin and evolution o f actinorhizal symbiotic systems. Molecular analyses of host plant gen e expression in developing nodules have confirmed the occurrence of no dulin proteins and in situ hybridization techniques have been successf ully adapted to permit the study of the spatial and temporal patterns of gene expression within actinorhizal nodules. The use of heterologou s probes in combination with nucleotide sequence analysis have allowed a number of nif genes to be mapped on the Frankia chromosome which wi ll ultimately contribute to the development of hypotheses related to n if gene regulation in Frankia. The use of both 16S and 23S rDNA nucleo tide sequences has allowed the construction of phylogenetic trees that can be tested for congruence with symbiotic characters. In addition t he development of Frankia-specific gene probes and amplification prime rs have contributed to studies on the genetic diversity and distributi on of Frankia in the soil.