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Plant-Microbe Associations
• production of various phytohormones, including auxins (e.g., indole3-acetic acid, cytokinins, and gibberellins) that affect growth;
• dissolution of minerals such as phosphorus which become readily
available to plant;
• production of compounds which can influence growth and development, for example, 1-aminocyclopropane-1-carboxylic acid.
In the presence of pathogenic microorganisms, rhizosphere bacteria can synthesize various biocontrol agents (antibiotics, enzymes, siderophores, etc.)
that suppress the growth of unwanted microbiota (Loper & Henkels, 1999;
Mitter et al., 2016). Being developed on roots, the rhizosphere microbiomes
affect plants by converting complex organic substances into accessible forms
which stimulate growth and affect the morphology and physiology of plants;
production of other specific metabolites, for example, ethylene, which causes
early flowering (Glick, 2006).
Plant growth-promoting rhizobacteria (PGPR) are useful free-living rhizosphere bacteria that stimulate the plant’s growth and are in association with
them; they are found inside and around plant roots (Kloepper & Schroth,
1981; Belimov et al., 2005, 2009). These microorganisms are involved in complex ecological interactions in the rhizosphere, where they can influence
plant health, growth, and stress response under unfavorable conditions,
since some of them have a high destructive potential for pollutants (Glick,
2003; Hayat et al., 2010; Mitter et al., 2016).
PGPR have been widely used for improving crop development as biologically friendly fertilizers (Adesemoye & Kloepper, 2009; Ortíz-Castro et al.,
2009; Simpson et al., 2011; Singh et al., 2011). Later the area of PGPRs application expanded, and they are considered for use in soil bioremediation, since
many rhizosphere bacteria belonging to the PGPR group are resistant to pollutants (Costa et al., 2014). PGPRs are involved in processes of metal migration
and have converted them to biologically available and soluble forms through
effects of siderophores, organic acids, biosurfactants, biomethylation, and
redox processes as part of the designed plant–microbial complexes (Ali et al.,
2013; Belimov et al., 2005; Oh et al., 2015; Zhu et al., 2015). Effectiveness of
phytoremediation depends on the activity of plant microbiome (Afzal et al.,
2014; Weyens et al., 2009).
Campanella and Paul (2000) suggested that the root system of Cucurbita pepo
secretes exudates of protein origin, which play an important role in the biodegradation of organochlorine compounds. Microbial degradation of pesticides mediated by enzyme systems is a promising approach for destruction
of these toxic substances (Pascal-Lorber & Laurent, 2011) and assisted in the
degradation of 4,4′-dichlorodiphenyl trichloroethane. Due to root exudates,
the plant ensures the stable functioning of microorganisms, and they, in
turn, contribute to the growth and development of the plant (Doornbos et al.,
2012). Synergistic effects caused by interaction of plants and microorganisms