ABSTRACT
References ................................................................................................... 232
Plants in soil offer a highly specific environment to naturally-occurring soil
microbial communities. Hence, over evolutionary time, soil microorganisms
have developed a range of strategies that have enabled them to interact with
plants. The composition of plant-associated microbial communities is highly
important for the performance of the plant, as the interaction with the plant
may be beneficial, neutral, or harmful. Different soil bacteria interact at
different distances and with varying degrees of intimacy with plants. They
may
1. Live in the soil influenced by the roots (i.e. the rhizosphere)
2. Colonize the root surface (i.e. the rhizoplane)
3. Colonize the intercellular spaces or vascular tissues inside plants
(endoplant habitat)
The rhizosphere has been defined as the compartment of soil which is
influenced by plant roots and by the compounds these release. Root deposits
and exudates represent important sources of substrates available to both
rhizosphere and rhizoplane microorganisms, and thus exert a great influence
on the structures of the microbial communities that are present. Furthermore, it
has been shown that the composition of these substrates can differ substan-
tially between plant species as well as between cultivars, and also depends on
the developmental stage of the plant. In addition, the composition of root
exudates may be affected by soil parameters, including pH, soil texture,
nutrient availability or limitation, and by exposure to plant pathogens. In
addition to the rather straightforward effects of root exudates, there are other
effects on microbes in the rhizosphere which result from spatial and temporal
changes in the roots and their physiology, caused by factors such as diurnal
variations, root aging, lateral root emergence leading to temporal wounds, and
plain root wounding.
