Casuarina Root Exudates Alter the Physiology, Surface Properties, and Plant Infectivity of Frankia sp. Strain CcI3

Abstract

ABSTRACTThe actinomycete genusFrankiaforms nitrogen-fixing symbioses with 8 different families of actinorhizal plants, representing more than 200 different species. Very little is known about the initial molecular interactions betweenFrankiaand host plants in the rhizosphere. Root exudates are important inRhizobium-legume symbiosis, especially for initiating Nod factor synthesis. We measured differences inFrankiaphysiology after exposure to host aqueous root exudates to assess their effects on actinorhizal symbioses.Casuarina cunninghamianaroot exudates were collected from plants under nitrogen-sufficient and -deficient conditions and tested onFrankiasp. strain CcI3. Root exudates increased the growth yield ofFrankiain the presence of a carbon source, butFrankiawas unable to use the root exudates as a sole carbon or energy source. Exposure to root exudates caused hyphal “curling” inFrankiacells, suggesting a chemotrophic response or surface property change. Exposure to root exudates altered Congo red dye binding, which indicated changes in the bacterial surface properties at the fatty acid level. Fourier transform infrared spectroscopy (FTIR) confirmed fatty acid changes and revealed further carbohydrate changes.Frankiacells preexposed toC. cunninghamianaroot exudates for 6 days formed nodules on the host plant significantly earlier than control cells. These data support the hypothesis of early chemical signaling between actinorhizal host plants andFrankiain the rhizosphere.