In this study, two previously analysed infection-related promoters were examined for their expression pattern in nodules formed on transgenic hairy roots of the actinorhizal species Datisca glomerata. The infected cell-specific expression pattern of a leghemoglobin promoter from the indeterminate nodule-forming legume Vicia faba was conserved in the determinate nodule-forming legume Lotus japonicus, the actinorhizal species Casuarina glauca (Fagales) and also in D. glomerata (Cucurbitales). Hence, the conservation of the infected cell-specific transcription factors responsible for the activation of leghemoglobin genes in legumes, and the symbiotic haemoglobin gene in C. glauca, extends to actinorhizal Cucurbitales. However, the infection thread containing cell-specific expression pattern of the C. glauca Cg12 promoter which had been found to be conserved in legumes (Svistoonoff et al. 2004), was not conserved in D. glomerata. In order to find out whether this result, in combination with the long-known fact that infection thread growth in D. glomerata does not involve the formation of pre-infection thread structures like in legumes and actinorhizal Fagales, points at an alternative infection thread growth mechanism in actinorhizal Cucurbitales, a cytological analysis of infection thread growth in D. glomerata vs. Alnus rubra (Fagales) was performed. In D. glomerata, infection threads crossed the cell wall through the center of pit fields, while in A. rubra no correlation between infection thread crossing points and thin cell wall regions could be found. Moreover, extensive ramification of infection threads in the cell periphery of D. glomerata infected cells leads to multiple pit field penetrations in a given host cell and a large degree of inter-host cell communication by the microsymbiont, in contrast to only a few wall penetration sites, aligned axially, in A. rubra.