Infection of actinorhizal plants roots by the actinomycete Fmnkia leads to the formation of a nitrogen-fixing root nodule (actinorhiza) consisting of multiple lobes, each of which is a modified lateral root. Actinorhiza development involves several specific steps, for example, root hair infection, prenodule formation, and initiation of lobe primordia from root pericycle. This article summarizes the latest development in the isolation and characterization of nodule-specific and -enhanced transcripts isolated from actinorhiza. The amino acid sequence derived from the nucleotide sequence of the cDNAs, in combination with localization data, showed that gene products are involved in nitrogen, carbon, 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 designed to establish genetic transformation systems for actinorhizal plants. This research has led to the obtain-ment 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 symbiosis. As molecular and cellular studies on Casuarinaceae and Betulaceae are more advanced than on the other six actinorhizal plant families, we concentrate primarily on species within these two families.
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Franche et al. (1998) studied this question.