A Model for the Development of the Rhizobial and Arbuscular Mycorrhizal Symbioses in Legumes and Its Use to Understand the Roles of Ethylene in the Establishment of these two Symbioses

We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis–outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants

Arbuscular mycorrhizal community structure on co-existing tropical legume trees in French Guiana

Aims We aimed to characterise the arbuscular mycorrhizal fungal (AMF) community structure and potential edaphic determinants in the dominating, but poorly described, root-colonizing Paris-type AMF community on co-occurring Amazonian leguminous trees. Methods Three highly productive leguminous trees (Dicorynia guianensis, Eperua falcata and Tachigali melinonii were targeted) in species-rich forests on contrasting soil types at the Nouragues Research Station in central French Guiana. Abundant AMF SSU rRNA amplicons (NS31-AM1 & AML1-AML2 primers) from roots identified via trnL profiling were subjected to denaturing gradient gel electrophoresis (DGGE), clone library sequencing and phylogenetic analysis. Results Classical approaches targeting abundant SSU amplicons highlighted a diverse root-colonizing symbiotic AMF community dominated by members of the Glomeraceae. DGGE profiling indicated that, of the edaphic factors investigated, soil nitrogen was most important in influencing the AMF community and this was more important than any host tree species effect. Conclusions Dominating Paris-type mycorrhizal leguminous trees in Amazonian soils host diverse and novel taxa within the Glomeraceae that appear under edaphic selection in the investigated tropical forests. Linking symbiotic diversity of identified AMF taxa to ecological processes is the next challenge ahead

Colonization patterns in a mycorrhiza-defective mutant tomato vary with different arbuscular-mycorrhizal fungi

Interactions between a mycorrhiza-defective tomato (Lycopersicon esculentum) mutant, rmc, and different species of arbuscular mycorrhizal (AM) fungi were investigated and compared with those with the wild-type cv. 76R. Both cv. 76R and rmc were challenged with Glomus intraradices, G. mosseae, G. coronatum, G. versiforme, G. etunicatum, G. fasciculatum, Gigaspora margarita and Scutellospora calospora using a nurse pot inoculation system. Cv. 76R demonstrated normal colonization patterns for all fungal species. By contrast, the development of different fungal species with rmc was impaired at different steps. Development of G. intraradices, G. etunicatum and G. fasciculatum was arrested on the root surface. However, G. mosseae, G. coronatum, G. margarita and S. calospora frequently penetrated the root epidermis, but colonization of the cortex was rare. G. versiforme achieved relatively normal colonization in rmc compared with the other species. This is the first report on the variation of colonization patterns in a mycorrhiza-defective mutant by different species of AM fungi, and highlights the need for previously described mutants in legumes to be challenged by more than one fungus. © New Phytologist (2001)