Soil microbial communities
No plant in nature grows in a soil devoid of microorganisms. Plant roots are surrounded by a rich microbial community, which reaches greatest levels of abundance and diversity in the zone immediately surrounding the root, a micro-habitat known as the rhizosphere. It has been claimed that the rhizosphere is where most biodiversity on Earth exists (Hinsinger et al., 2009) and it is certainly one of the most dynamic and important ecosystems, through effects on plant growth and thus crop production and the structure and function of natural communities (Barrios, 2007).
The microbial community associated with plant roots contains a diverse array of bacteria, protozoa and fungi, some of which can be antagonistic to plant growth (pathogens), while others may appear to be benign or to have a range of beneficial effects. These latter effects include improved nutrient uptake by roots, chiefly through fixation and cycling of nitrogen, and mineralisation and uptake of phosphorus. Furthermore, soil microbes may increase plant growth by the synthesis of phytohormones (Costacurta & Vanderleyden, 1995), antagonism of deleterious soil bacteria and fungi by antibiotic production or depriving them of iron (Kloepper et al., 1980), alleviation of salt and drought stress (Evelin et al., 2009), enhancement of photosynthesis, and increasing resistance to foliar pathogens and insect predators (van der Ent et al., 2009). The fact that root-associated microorganisms can alter the resistance of foliar tissues to insect herbivores is a relatively recent discovery, and the aim of this review is to document these interactions and to explore their mechanisms.