ACTINORHIZAL SYMBIOSES - RECENT ADVANCES IN PLANT MOLECULAR AND GENETIC-TRANSFORMATION STUDIES

Infection of actinorhizal plants roots by the actinomycete Frankia lea ds to the formation of a nitrogen-fixing root nodule (actinorhiza) con sisting of multiple lobes, each of which is a modified lateral root. A ctinorhiza development involves several specific steps, for example, r oot hair infection, prenodule formation, and initiation of lobe primor dia from root pericycle. This article summarizes the latest developmen t in the isolation and characterization of nodule-specific and -enhanc ed transcripts isolated from actinorhiza. The amino acid sequence deri ved from the nucleotide sequence of the cDNAs, in combination with loc alization data, showed that gene products are involved in nitrogen, ca rbon, and oxygen metabolism. Furthermore, some transcripts represented encoded gene products that might be part of infection and senescence mechanisms in actinorhiza. The article also reviews experiments design ed to establish genetic transformation systems for actinorhizal plants . This research has led to the obtainment of transgenic plants of the Casuarinaceae family by using A. rhizogenes and A. tumefaciens. These new findings are discussed in view of future studies on actinorhizal s ymbiosis. As molecular and cellular studies on Casuarinaceae and Betul aceae are more advanced than on the other six actinorhizal plant famil ies, we concentrate primarily on species within these two families.