Abstract
Bacteria that colonize plant roots and promote plant growth are referred to as plant growth-promoting rhizobacteria (PGPR). PGPR are highly diverse and in this review we focus on rhizobacteria as biocontrol agents. Their effects can occur via local antagonism to soil-borne pathogens or by induction of systemic resistance against pathogens throughout the entire plant. Several substances produced by antagonistic rhizobacteria have been related to pathogen control and indirect promotion of growth in many plants, such as siderophores and antibiotics. Induced systemic resistance (ISR) in plants resembles pathogen-induced systemic acquired resistance (SAR) under conditions where the inducing bacteria and the challenging pathogen remain spatially separated. Both types of induced resistance render uninfected plant parts more resistant to pathogens in several plant species. Rhizobacteria induce resistance through the salicylic acid-dependent SAR pathway, or require jasmonic acid and ethylene perception from the plant for ISR. Rhizobacteria belonging to the genera Pseudomonas and Bacillus are well known for their antagonistic effects and their ability to trigger ISR. Resistance-inducing and antagonistic rhizobacteria might be useful in formulating new inoculants with combinations of different mechanisms of action, leading to a more efficient use for biocontrol strategies to improve cropping systems.
Highlights
The rhizosphere is the narrow zone of soil influenced by the root system (Dobbelaere et al, 2003)
The following rhizospheric environment and bacterial antagonistic activities can be highlighted: (1) synthesis of hydrolytic enzymes, such as chitinases, glucanases, proteases, and lipases, that can lyse pathogenic fungal cells (Neeraja et al 2010; Maksimov et al 2011), (2) competition for nutrients and suitable colonization of niches at the root surface (Stephens et al, 1993; Kamilova et al 2005; Validov S, 2007, PhD thesis, Leiden University, The Netherlands), (3) regulation of plant ethylene levels through the ACC-deaminase enzyme, which can act to modulate the level of ethylene in a plant in response to stress imposed by the infection (Glick and Bashan, 1997; Van Loon, 2007), and (4) production of siderophores and antibiotics
Vleesschauwer and Höfte (2009) proposed the terminology Induced systemic resistance (ISR) to depict induced systemic resistance promoted by non-pathogenic rhizobacteria or plant growth-promoting rhizobacteria (PGPR), irrespective of the signaling pathway involved in this process, while the term systemic acquired resistance (SAR) is used to describe salicylic acid-dependent induced resistance triggered by a localized infection
Summary
The rhizosphere is the narrow zone of soil influenced by the root system (Dobbelaere et al, 2003). The following rhizospheric environment and bacterial antagonistic activities can be highlighted: (1) synthesis of hydrolytic enzymes, such as chitinases, glucanases, proteases, and lipases, that can lyse pathogenic fungal cells (Neeraja et al 2010; Maksimov et al 2011), (2) competition for nutrients and suitable colonization of niches at the root surface (Stephens et al, 1993; Kamilova et al 2005; Validov S, 2007, PhD thesis, Leiden University, The Netherlands), (3) regulation of plant ethylene levels through the ACC-deaminase enzyme, which can act to modulate the level of ethylene in a plant in response to stress imposed by the infection (Glick and Bashan, 1997; Van Loon, 2007), and (4) production of siderophores and antibiotics.
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