Orchid–fungus Fidelity: A Marriage Meant to Last?

Abstract

The characteristics of plant-mycorrhizae associations are known to vary in both time and space, but the ecological consequences of variation in the dynamics of plant-fungus interactions are poorly understood. For example, do plants associate with single fungi or multiple fungi simultaneously, and do the associations persist through a plant's lifetime or do plants support a succession of different fungi? We investigated these and other questions related to plant-fungus interactions in Goodyera pubescens, an evergreen terrestrial orchid of the eastern United States, that interacts with closely related fungi in the genus Tulasnella. Unlike the mycorrhizal associations of other plants, orchid-mycorrhizal associations only benefit the orchid, based on current evidence. Many terrestrial orchids have been found to associate with specific groups of fungi. This characteristic could potentially limit orchids to relatively narrow ranges of environmental conditions and may be a contributing factor in the decline of many orchids in the face of changing environmental conditions. We found that G. pubescens protocorms (developing embryos prior to leaf production) and adults associated with only one fungal individual at a time. The orchid-fungus association persists for years, but during a drought period that was associated with the death of many plants, surviving plants were able to switch to new fungal individuals. These results suggest that G. pubescens interacts with the same fungal partner during periods of modest environmental variation but is able to switch to a different fungal partner. We hypothesize that the ability to switch fungi allows G. pubescens to survive more extreme environmental perturbations. However, laboratory experiments suggest that switching fungi has potential costs, as it increases the risk of mortality, especially for smaller individuals. Our findings indicate that it is unlikely that switching fungi is a common way to improve tolerance of less severe environmental fluctuations and disturbances. These findings may have important implications for plant responses to severe climatic events or to more gradual environmental changes such as global warming.

Interactions with mycorrhizal fungi in two closely related hybridizing orchid species

“…Early molecular studies have revealed that terrestrial orchids interact with a wide range of mycorrhizal fungi. Some orchids are extreme specialists and are associated to a single fungal species (MCCORMICK et al 2004, TAYLOR et al 2004, DEARLANEY 2007, SHEFFERSON et al 2008, while others are generalists, hosting many different fungi (MCCORMICK et al 2006, BONNAR-DEAUX et al 2007, IRWIN et al 2007). …”

Orchidioid fungi of the form-genus Rhizoctonia associated with the roots of Chloraea cuneata Lindl. from Araucanía, Chile

“…It is possible that other fungal species can associate with C. cuneata in the field that can not be isolated by the methods used in this work. Also, the fungal partner may change with plant ontogeny and/or with environmental conditions (McCormick et al 2006). In our lab we are currently testing the effect of the isolated fungi on seed germination.…”

Mycorrhizal fungi isolated from native terrestrial orchids of pristine regions in Córdoba (Argentina)

“…Although in other species this behavior cannot be ruled out, these results must be confirmed with a larger number of isolates. McCormick, Whigham, and O'Neill, (2004), Otero et al (2005), McCormick Whigham, Sloan, O´Malley, andHodkinson (2006) and Dearnaley (2007) have established that there is a considerable variation in the colonization of terrestrial orchid roots (Dearnaley, 2007;McCormick et al, 2004;Otero et al, 2005, McCormick et al, 2006. Some species are associated with different fungi along their life cycles.…”