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The Soconusco region of southeast Mexico has almost a quarter of the orchid species registered in Mexico and 37 threatened species (NOM-059-SEMARNAT-2001), many with severely reduced and non-viable populations. We chose two of the most threatened species, Rossioglossum grande (Lindl.) Garay and G.C. Kenn. and Cuitlauzina convallarioides (Schltr.) Dressler and N. H. Williams and a rare species recently discovered in the region, Rhynchostele bictoniensis (Bateman) Soto Arenas and Salazar, to study the mycorrhizal fungi associated with the roots, isolate them and use them to induce seed germination and promote development in asymbiotically produced protocorms, in the laboratory. We isolated ten strains of Rhizoctonia-like orchid mycorrhizal fungi from Rossioglossum grande and three from Cuitlauzina convallarioides. Using selected fungal strains from the same species, we tested for the promotion of further development of asymbiotically pre-germinated protocorms of R. grande and the promotion of seed germination of C. convallarioides. In the case of R. bictoniensis, we studied the effects on seed germination of nine strains of Rhizoctonia-like fungi isolated from other orchid species. For R. grande, after 10 months, one strain of Rhizoctonia promoted development of the pre-germinated protocorms, and almost 90% of the protocorms produced rhizoids. For C. convallarioides, after 3 months, one fungal strain promoted protocorm development to the stage where they produced green tissue under illumination, suggesting the onset of photosynthesis. For R. bictoniensis three of the fungal strains (from other orchid species) promoted germination and, after 4 months, autotrophic protocorms. Inglés

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