Abstract
In root nodule symbiosis, coordinated regulation between rhizobial infection and nodule organogenesis is essential for development of fully effective nodules in which rhizobia reside and fix atmospheric nitrogen. Recent studies with non-nodulating mutants of Lotus japonicus have led to isolation of a number of symbiotic genes. However, involvement of these genes in infection thread formation in epidermis and/or nodule organogenesis in cortex remains obscure. To dissect symbiotic cellular responses occurred in different cell layers, epidermis/cortex specific expression systems which make it possible to express the symbiotic gene only in epidermis or cortex, are required. In Arabidopsis thaliana, the promoter of ExpansinA7 gene (AtEXPA7) is known to contain root hair-specific cis elements (RHEs), which confer root hair-specific expression to AtEXPA7. Using a promoter region of AtEXPA7 homologs in Lotus, we developed an epidermis specific expression system, epi-system, in Lotus roots. Based on the results of complementation tests of symbiotic mutants with causative genes expressed by epi-system, we will propose a model for regulating mechanism of epidermal infection process in Lotus.
