Colonization of plant roots by pseudomonads and AM fungi: A dynamic phenomenon, affecting plant growth and health

Because of their enormously large range of plant hosts and role in plant nutrition, arbuscular mycorrhizal (AM) fungi represent an extraordinarily fascinating field of study. Plant growth promotion effects by AM fungi were described as early as 1900 (Sthal 1900) and several data obtained in the second half of the last century support the idea that these microrganisms can act as biocontrol agents (BCA). The extent of root colonization is variable in different plants and under different environmental conditions (Giovannetti and Hepper 1985). Some effects of AM colonization on plants have been reported to be dependent on the degree of root colonization, while others have not. Root exudation and pH are modified by the presence of AM fungi (Bansal and Mukerji 1994; Bago et al. 1996), therefore AM fungi can affect the growth of rhizobacteria. Similarly, both root colonization by AM fungi and their effects on the plant can be affected by the presence of rhizobacteria, which can be plant growth-promoting, mycorrhiza helper or biocontrol agents.