Plant-Microorganism Interactions: Effects on the Tolerance of Plants to Biotic and Abiotic Stresses - Crop Improvement: New Approaches and Modern Techniques - page 217

complex biochemical, genetic and physiological interactions and relationships that
depend on the nature of the soil and the environment.
Although there is no evidence for host-specific AMF, there is evidence of
functional specificity when considering the effects of these fungi on host plants
(Pouyu-Rojas et al. 2006 ). As fungal isolates may vary depending on environ-
mental changes, it is important to evaluate occurrence and functional diversity as
critical factors in the structure of the plant community and ecosystem productivity
(O'Connor et al. 2002 ).
Knowing the AMF community structure of a certain environment or biome and
evaluating the functional diversity of these symbionts are critically important when
trying to exploit the potential of these fungi. The functional diversity of arbuscular
mycorrhizae (AMs) has often been defined in terms of responses in plant growth,
which can range from negative to positive, depending on the particular plant-fungus
combination and environmental conditions (Johnson et al. 1997 ). This functional
diversity can be measured using the colonisation rate, absorption of nutrients and
plant growth effects. Plants respond differently to different AMF, and these respons-
es are observed both among the AMF of different species and among isolates of the
same species (Munkvold et al. 2004 ; Smith et al. 2004 ).
Pouyu-Rojas et al. ( 2006 ) suggested the existence of selectivity and differen-
tiated symbiotic compatibility, with preferred combinations in the formation of
AMs and variable responses depending on the mycorrhizal genotype involved in
the fungus-plant relationship. Some studies have shown that a given AMF species
originates from the same soil and colonises different plant species with distinct
sporulation patterns (Eom et al. 2000 ). In some cases, AMF that promote the host
growth in one plant species could inhibit growth in another (Smith and Read 2008 ),
and this beneficial or parasitic relationship depends on the fungus-plant combina-
tion and environmental conditions (Johnson et al. 1997 ; Smith and Read 2008 ).
Additionally, individual species of AMF can vary greatly in their response to
the growth of different plant species, and variations can occur both among AMF
isolates belonging to different species and isolates of the same species (Munkvold
et al. 2004 ; Smith et al. 2004 ). Consequently, the presence or absence of certain
AMF species influences structural changes in the population (Klironomos et al.
2000 ). For example, increasing the diversity of these fungi in the soil (Rillig
2004 ) influences the diversity, structure and productivity of the plant community
(Heijden et al. 2004 ), in experimental studies performed in greenhouses and in natural
ecosystems.
Inoculation with different AMF species differentially alters the growth and co-
existence of different plant species (Heijden et al. 2003 ), and increasing the species
richness of these fungi increases the diversity and productivity of plants. Santos
( 2008 ) verified the influence of AMF species richness in the soil community on
the initial growth of tree species native to Brazil. The results of this study showed
that the benefits of increased AMF richness are greater when plants are grown in
complex communities with a considerable amount of competition. However, those
studies used a single individual as a representative of each AMF species, and each
culture was initiated from a single spore. Therefore, these results cannot indicate