The roles of auxins and cytokinins in mycorrhizal symbioses

Most land plant species that have been examined exist naturally with a high er fungus living in and around their roots in a symbiotic partnership calle d a mycorrhiza. Several types of mycorrhizal symbiosis exist, defined by th e host/partner combination and the morphology of the symbiotic structures. The arbuscular mycorrhiza (AM) is ancient and may have co-evolved with land plants. Emerging results from gene expression studies have suggested that subsets of AM genes were co-opted during the evolution of other biotrophic symbioses. Here we compare the roles of phytohormones in AM symbiosis and e ctomycorrhizas (EC), a more recent symbiosis. To date, there is little evid ence of physiologic overlap between the two symbioses with respect to phyto hormone involvement. Research on AM has shown that cytokinin (CK) accumulat ion is specifically enhanced by symbiosis throughout the plant. We propose a pathway of events linking enhanced CK to development of the AM. Additiona l and proposed involvement of ether phytohormones are also described. The r ole of auxin in EC symbiosis and recent research advances on the topic are reviewed. We have reflected the literature bias in reporting individual gro wth regulator effects. However, we consider that gradients and ratios of th ese molecules are more likely to be the causal agents of morphologic change s resulting from fungal associations. We expect that once the individual ro les of these compounds are explained, the subtleties of their function will be more clearly addressed.