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cg12 is an early actinorhizal nodulin gene from Casuarina glauca encoding a subtilisin-like serine protease. Using transgenic Casuarinaceae plants carrying cg12-gus and cg12-gfp fusions, we have studied the expression pattern conferred by the cg12 promoter region after inoculation with Frankia. cg12 was found to be expressed in root hairs and in root and nodule cortical cells containing Frankia infection threads. cg12 expression was also monitored after inoculation with ineffective Frankia strains, during my-corrhizae formation, and after diverse hormonal treatments. None of these treatments was able to induce its expression, therefore suggesting that cg12 expression is linked to plant cell infection by Frankia strains. Possible roles of cg12 in actinorhizal symbiosis are discussed.