Abstract
Actinorhizal plants are able to establish a symbiotic relationship with Frankia bacteria leading to the formation of root nodules. The symbiotic interaction starts with the exchange of symbiotic signals in the soil between the plant and the bacteria. This molecular dialog involves signaling molecules that are responsible for the specific recognition of the plant host and its endosymbiont. Here we studied two factors potentially involved in signaling between Frankia casuarinae and its actinorhizal host Casuarina glauca: (1) the Root Hair Deforming Factor (CgRHDF) detected using a test based on the characteristic deformation of C. glauca root hairs inoculated with F. casuarinae and (2) a NIN activating factor (CgNINA) which is able to activate the expression of CgNIN, a symbiotic gene expressed during preinfection stages of root hair development. We showed that CgRHDF and CgNINA corresponded to small thermoresistant molecules. Both factors were also hydrophilic and resistant to a chitinase digestion indicating structural differences from rhizobial Nod factors (NFs) or mycorrhizal Myc-LCOs. We also investigated the presence of CgNINA and CgRHDF in 16 Frankia strains representative of Frankia diversity. High levels of root hair deformation (RHD) and activation of ProCgNIN were detected for Casuarina-infective strains from clade Ic and closely related strains from clade Ia unable to nodulate C. glauca. Lower levels were present for distantly related strains belonging to clade III. No CgRHDF or CgNINA could be detected for Frankia coriariae (Clade II) or for uninfective strains from clade IV.
Highlights
Legumes and actinorhizal plants form a N2-fixing root nodule symbiosis in association with rhizobia and Frankia bacteria, respectively (Vessey et al, 2005)
root hair deformation (RHD) is one of the earliest visible responses induced upon recognition of rhizobial Nod factors (NFs) by the host plant, and the development of a bioassay based on RHD was crucial to identify the chemical nature of NFs (Lerouge et al, 1990)
In actinorhizal plants infected intracellularly such as C. glauca and A. glutinosa, RHD is one of the first visible responses to Frankia inoculation and factors able to induce RHD in Alnus (AgRHDF) have been partially purified and characterized (Cérémonie et al, 1999)
Summary
Legumes and actinorhizal plants form a N2-fixing root nodule symbiosis in association with rhizobia and Frankia bacteria, respectively (Vessey et al, 2005). The establishment of these beneficial bacterial-plant relationships requires communication between the partners. Actinorhizal symbioses, which play an important ecological role (Dawson, 2008), have been less well studied and the molecular dialog between Frankia and their host plants is still poorly understood. Recent progress including the sequencing of several Frankia genomes (Normand et al, 2007; Tisa et al, 2016), transcriptomic studies (Alloisio et al, 2010; Benson et al, 2011), proteomic studies (Mastronunzio and Benson, 2010; Ktari et al, 2017) together with functional studies on several actinorhizal species (Svistoonoff et al, 2014) have opened new avenues for identifying components involved in the initial symbiotic dialog between the two partners
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