Abstract
Nitrogen-fixing nodules induced by Frankia in the actinorhizal plant Discaria trinervis result from a primitive intercellular root invasion pathway that does not involve root hair deformation and infection threads. Here, we analyzed the role of auxin in this intercellular infection pathway at the molecular level and compared it with our previous work in the intracellular infected actinorhizal plant Casuarina glauca. Immunolocalisation experiments showed that auxin accumulated in Frankia-infected cells in both systems. We then characterized the expression of auxin transporters in D. trinervis nodules. No activation of the heterologous CgAUX1 promoter was detected in infected cells in D. trinervis. These results were confirmed with the endogenous D. trinervis gene, DtAUX1. However, DtAUX1 was expressed in the nodule meristem. Consistently, transgenic D. trinervis plants containing the auxin response marker DR5:VENUS showed expression of the reporter gene in the meristem. Immunolocalisation experiments using an antibody against the auxin efflux carrier PIN1, revealed the presence of this transporter in the plasma membrane of infected cells. Finally, we used in silico cellular models to analyse auxin fluxes in D. trinervis nodules. Our results point to the existence of divergent roles of auxin in intercellularly- and intracellularly-infected actinorhizal plants, an ancestral infection pathways leading to root nodule symbioses.
Highlights
Actinorhizal symbiosis comprise more than 200 species from 8 families distributed within the orders Fagales, Cucurbitales and Rosales, which form nitrogen fixing nodules with the soil actinomycete Frankia
These results indicate that inhibition of auxin influx activity perturbs nodule formation in D. trinervis to what was observed in C. glauca
As auxin has been involved in the intracellular infection of the actinorhizal plant C. glauca, here we analyzed the role of this phytohormone in the intercellularly infected D. trinervis
Summary
Actinorhizal symbiosis comprise more than 200 species from 8 families distributed within the orders Fagales, Cucurbitales and Rosales, which form nitrogen fixing nodules with the soil actinomycete Frankia. It has been suggested that rhizobia inhibit auxin transport at the early stages of the symbiotic interaction in legumes forming indeterminate type of nodules (Mathesius et al, 1998; De Billy et al, 2001), but not in L. japonicus that forms determinate type of nodules (Pacios-Bras et al, 2003) In both cases an accumulation of auxin occurs at the site of nodule initiation and auxin is supposed to stimulate cell divisions in the cortex and pericycle that lead to the formation of nodule primordia (Roudier et al, 2003). Using computer simulations we showed that the pattern of transporter distribution leads to auxin accumulation in infected cells, where auxins possibly induce changes in gene expression, cell metabolism, or in the cell wall properties (Perrine-Walker et al, 2010, 2011)
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