Role of Fungal Wall Components in Interactions Between Endomycorrhizal Symbionts

Abstract

Although endomycorrhizal symbioses represent the most widespread of plant-microbe interactions, the mechanisms leading to reciprocal functional compatibility are still unknown. In order to better understand why plant defense reactions are only weakly and locally induced in endomycorrhizas, it is interesting to determine whether endomycorrhizal fungi possess elicitors of plant defense reactions. In fact, walls of both ericoid and arbuseular mycorrhizal fungi contain chitin, β-1, 3 glucans and/or chitosan and glycoproteins which could potentially induce plant defense reactions. However, important wall modifications occur in these endomycorrhizal fungi during the root colonization process and therefore they could have strategies of self-camouflage so that they would not be recognized by the plant as non-self. Alternatively, plant defense reactions could be induced but then repressed. Evidence for the existence of such phenomena comes from studies of Myc-1 Nod- pea mutants which do not form arbuseular mycorrhizas nor nodules with rhizobial bacteria. Myc-1 mutants are characterized by the elicitation of strong plant defense reactions in epidermal and hypodermal cells in contact with fungal appressoria. Moreover, the fact that these mutants are resistant to both symbioses means that some plant genes are specifically involved in symbiosis establishment. Therefore, one role of symbiosis-related (SR) genes could be to repress a strong expression of plant defense reactions during endomycorrhization. Expression of these SR genes could somehow interact with fungal compatibility factors by mechanisms which are discussed.