hcl mutants are Nod-minus, Hac-minus (no marked root hair curling with a refractile spot), but root hair deformations and cortical cell divisions are induced in the presence of Rhizobium. In response to Nod factors, hcl mutants respond by wild type epidermal nodulin gene expression, root hair branching and calcium spiking. Studies of nuclear position and of the microtubule cytoskeleton network of hcl mutants show that root hair as well as cortical cells are activated in response to S. meliloti. However, the asymmetric microtubule network typical of curled root hairs does not form in the mutants, and activated cortical cells do not become polarised and do not exhibit the microtubular cytoplasmic bridges characteristic of the pre-infection threads induced by rhizobia in M. truncatula. These data suggest that hcl mutations alter the formation of signalling centers that normally provide positional information for the reorganisation of the microtubular cytoskeleton in epidermal and cortical cells.