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RESEARCH PRODUCT

Colonization of Plant Roots by Pseudomonads and AM Fungi: A Dynamic Phenomenon, Affecting Plant Growth and Health

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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. Although several genera of microorganisms have been reported to behave as biocontrol agents (BCA) or plant growth-promoting rhizobacteria (PGPR), over these years attention has been focused mainly on fluorescent pseudomonads, because they are common inhabitants of rhizosphere and phyllosphere environments, synthesize a wide range of metabolites and enzymes (O’Sullivan and O’Gara 1992), are easily isolated from natural environments, utilize a large variety of substrates (Latour and Lemanceau 1997), and are easy to culture and manipulate genetically (Walsh et al. 2001). Altogether, these characters make them more reliable to scientific experimentation (Whipps 1997; Haas and Defago 2005). Due to their impressive capacity in producing different types of antibiotics such as 2,4-diacetylphloroglucinol, phenazines, oomycin, pyoluteorin, pyrrolnitrin,