Mutualism and antagonism in the mycorrhizal symbiosis, with special reference to impacts on plant community structure

Abstract

Examination of the roots of land plants has revealed the occurrence of mycorrhiza in the majority of species, over 70% of which are hosts to zygomycetous fungi that form vesicular–arbuscular (VA) associations. On the basis of experiments with a small number of host species showing enhancement of growth following colonization, it is widely assumed that wherever mycorrhizas are observed, the symbiosis is of the mutualistic type. The value of definitions based on structural rather than functional attributes is here brought into question by experiments simulating the ecologically realistic circumstance in which seeds germinate in soil in the presence or absence of established VA mycelium. These reveal a spectrum of fungal impacts in which some species respond mutualistically, while others, putative hosts or nonhosts, are antagonised, showing reduction of yield and survivorship and, hence, a loss of fitness relative to plants grown without VA fungi. Antagonised species normally grow in disturbed, open habitats and fail to establish in closed communities. It is hypothesised that their turf incompatibility arises from a sensitivity to interference by VA fungi, which consigns them to ruderal habitats. Mycorrhizal fungi, thus, play a role in defining the ecological niches occupied by plants and in determining of plant community composition. Key words: mycorrhiza, vesicular–arbuscular, mutualism, symbiosis, antagonism, plant community.

Mycorrhizal fungi suppress aggressive agricultural weeds

“…Moreover, Chenopodium album and Echinocloa crus-galli belong to the top ten of the World's most aggressive weeds (Holm et al 1977). Three of the investigated weed species (Amaranthus retroflexus; Chenopodium album and Sinapis arvensis) are recognized as being nonmycorrhizal or poorly colonized by AMF (Harley and Harley 1987;Francis and Read 1995). Seeds of the weed species were obtained from the company Herbiseed (www.herbiseed.com).…”

“…However, AMF are not only beneficial, and interactions between plants and AMF can range from highly mutualistic to antagonistic where AMF reduce plant growth Read 1994, 1995;Johnson et al 1997;van der Heijden 2002;Klironomos 2003). For instance, studies performed with plants from natural communities show that AMF often have detrimental effects on non-hosts (Grime et al 1987;Allen et al 1989;van der Heijden et al 1998), on plants grown at high nutrient availability or on plant species characteristic of ruderal environments where there is considerable disturbance (Francis and Read 1995).…”

Diversity and classification of mycorrhizal associations

“…Damage to non-host roots by ECM fungi has most often been reported in sterile culture experiments using hosts and fungi that do not normally associate together (Molina & Trappe, 1982 b). During succession in many habitats, NM plants are outcompeted by mycorrhizal species (Brundrett, 1991;Francis & Read, 1995 ;Brundrett & Abbott, 2002). This probably occurs because the mycorrhizal species are more efficient at acquiring limiting soil nutrients such as phosphorus (Newman, 1988 ;Brundrett, 1991), but direct antagonism of non-host plants by mycorrhizal fungi may also occur in some cases as inoculum levels increase during succession (Allen et al, 1989).…”

Diversidade de fungos micorrízicos arbusculares em área de Caatinga, PE, Brasil

“…Os FMA promovem maior absorção de nutrientes, beneficiando o hospedeiro principalmente com a translocação do fósforo (He et al 2002),e também outros íons, proporcionam aumento da resistência vegetal contra patógenos (Liu et al 2007) e maior tolerância a estresses hídrico (Beltrano & Ronco 2008) e salino (Maia & Yano-Melo 2005), contribuindo ainda para a agregação do solo (Mergulhão et al 2010). Além disso, atuam na definição de nichos ecológicos ocupados pelos vegetais, determinando a composição das comunidades de plantas (Francis & Read 1995). Considerando que os FMA participam de processos importantes para a manutenção dos ambientes terrestres, estudos sobre esses fungos podem contribuir para esclarecer aspectos importantes da sua atuação e funcionalidade na Caatinga.…”