plant surface microbiology.pdf

19 Functional Diversity of Arbuscular Mycorrhizal Fungi on Root Surfaces 341
duce glomalin, but the amount varies considerably for different species
(Wright and Upadhaya 1998). If AM fungal communities differ in their sensitivity
to disturbance, the capacity of the species present to form hyphae as
well as their ability to produce glomalin should influence the degree of soil
aggregation.
6 Environmental Influence on Functional Diversity of
Arbuscular Mycorrhizal Fungi
The soil environment includes many physical and chemical properties that
are continuously being modified by dynamic biological processes. Rhizosphere
soil is influenced by plant root exudates and microbial activity and the
AM fungi that live in this habitat have adapted to a wide range of environmental
conditions (Stahl and Christensen 1991; Giovannetti and Gianinazzi-
Pearson 1994). Environmental stresses on AM fungi could include: (1) high or
low levels of nutrients, (2) waterlog and drought, (3) soil acidity, (4) salinity,
(5) high levels of toxic metals, (6) biotic factors (e.g. fauna that feed on
hyphae), and (7) absence of suitable host plants for long periods. AM fungi
can adapt to both low and high levels of soil nutrients (Solaiman and Hirata
1997). As they are aerobic, waterlogging has a considerable impact on their
diversity and on their functioning. One of the most important soil factors
influencing the distribution of species of AM fungi is soil pH (Robson and
Abbott 1989). AM fungi show considerable diversity in their response to soil
pH and changes in soil pH can affect the relative abundance of species inside
roots (Sano et al. 2002). This has potential to influence the structure of communities
of AM fungi in soil. There is also evidence that agricultural practices
such as pesticide applications, cropping sequences and soil disturbance can
affect diversity of AM fungi in soil (Dodd and Jeffries 1989; Sieverding 1991;
Johnson et al. 1997).
7 Role of Plant Mutants in Studying the Interactions
Between Arbuscular Mycorrhizal Fungi and Roots
In symbiotic associations between AM fungi and plant roots, genetic control
imposed by each symbiont is poorly understood. Understanding of the
genetic and molecular basis of this symbiosis has been prevented by the
obligate nature of the fungal symbiont and by the lack of mycorrhiza formation
on the model plant Arabidopsis thaliana. Recently, Medicago truncatula
and Lotus japonicus have been chosen as model plants for research of plant –
microbe symbioses (Sagan et al. 1995; Jiang and Gresshoff 1997; Bonfante et
al. 2000; Senoo et al. 2000). The use of molecular genetic approaches in model
legumes will rapidly increase knowledge of host genetic determinants of