Les mycorhizes des plantes actinorhiziennes

Abstract

Summary As pioneer species for land revegetation, actinorhizal plants are predisposed to form with mycorrhizal fungi symbiotic associations that are as essential for plant growth as the associations they form with Frankia. As expected, mycorrhizas stimulate the development and the N2-fixing activity of the actinorhizal symbiosis by improving mineral nutrition of the host plant. In addition to mycorrhizas, actinorhizal plants are also able to form an unique type of roots called « proteoid roots » or « cluster roots » in response to the detrimental effects of nutrient deficiencies in soil. As an alternative of mycorrhizas, cluster roots are specialized roots capable of assisting plant growth in marginal, nutrient deficient soils. There is no competition between Frankia and mycorrhizal fungi for infection sites but extensive mycorrhizas can affect Frankia growth and the subsequent development of young lobes of actinorhizal nodules. By contrast, the presence of Frankia in planta has no incidence on mycorrhizal infection processus. With reference to Alnus and Allocasuarina the possible implications of Frankia and mycorrhizal fungi on the mechanisms of recognition and chemical signalisation between the host plant and these microsymbionts are discussed. Strikingly, it should be noticed that Frankia and arbuscular endomy- corrhizal fungi exhibit a number of similarities that could be taken into account for developing future research.