MYCORRHIZAE OF ANTINORHIZAL PLANTS

As pioneer species for land revegetation, actinorhizal plants are pred isposed to form with mycorrhizal fungi symbiotic associations that are as essential for plant growth as the associations they form with Fran kia. As expected, mycorrhizas stimulate the development and the N-2-fi xing activity of the actinorhizal symbiosis by improving mineral nutri tion of the host plant. In addition to mycorrhizas, actinorhizal plant s are also able to form an unique type of roots called ''proteoid root s'' or ''cluster roots'' in response to the detrimental effects of nut rient deficiencies in soil. As an alternative of mycorrhizas, cluster roots are specialized roots capable of assisting plant growth in margi nal, nutrient deficient soils. There is no competition between Frankia and mycorrhizal fungi for infection sites but extensive mycorrhizas c an affect Frankia growth and the subsequent development of young lobes of actinorhizal nodules. By contrast, the presence of Frankia in plan ta has no incidence on mycorrhizal infection processus. With reference to Alnus and Allocasuarina the possible implications of Frankia and m ycorrhizal fungi on the mechanisms of recognition and chemical signali sation between the host plant and these microsymbionts are discussed. Strikingly, ii should be noticed that Frankia and arbuscular endomycor rhizal fungi exhibit a number of similarities that could be taken into account for developing future research.