plant surface microbiology.pdf

20 Mycorrhizal Fungi and Plant Growth Promoting
Rhizobacteria
José-Miguel Barea, Rosario Azcón
and Concepción Azcón-Aguilar
1 Introduction
Soil microbial communities are crucial in maintaining a biological balance in
soil, a key issue for the sustainability of either natural ecosystems or agroecosystems
(Kennedy and Smith 1995). When provided with available carbon
substrates, soil microorganisms are able to develop a range of activities in the
microhabitats where they flourish and some of these activities are of great relevance
for plant growth and health and for soil quality (Bowen and Rovira
1999). Soil-borne microbes are found bound to the surface of soil particles or
in the soil aggregates, while others interact specifically with plant roots (Glick
1995). Particularly important from the point of view of plant surface microbiology
are the interactions at the root – soil interface where microorganisms,
plant roots and soil constituents interact (Lynch 1990; Azcón-Aguilar and
Barea 1992; Linderman 1992; Kennedy 1998; Bowen and Rovira 1999; Barea
2000; Gryndler 2000) to develop a dynamic environment what is known as the
rhizosphere (Hiltner 1904). The rhizosphere, therefore, is the zone of influence
of plant roots on the soil microbiota; a microcosm with physical, chemical
and biological properties different from those of the root-free bulk soil
(Bowen and Rovira 1999; Gryndler 2000; Barea 2000). A characteristic of the
rhizosphere is that microbial diversity is altered and that the activity and
number of microorganisms is increased (Kennedy 1998).
The supply of photosynthates and decaying plant material to the root-associated
microbiota is a key issue for rhizosphere formation and functioning.
The release of organic material is known to occur mainly as root exudates,
acting as either signals or growth substrates (Werner 1998). However, once
the microbial population is established, rhizosphere developments are
affected by microbially induced changes on rooting patterns and by the supply
of available nutrients to plants, which in turn modify the quality and
quantity of root exudates (Bowen and Rovira 1999; Barea 2000; Gryndler
2000). Microbial interactions in the rhizosphere are known to markedly influence
plant fitness and soil quality (Lynch 1990; Bethlenfalvay and Schüepp
Plant Surface Microbiology
A.Varma, L. Abbott, D. Werner, R. Hampp (Eds.)
© Springer-Verlag Berlin Heidelberg 2004