Induced Systemic Resistance as a Mechanism of Disease Suppression by Rhizobacteria

Abstract

Abstract:Plant growth-promoting rhizobacteria can suppress diseases through antagonism between the bacteria and soil-borne pathogens, as well as by inducing a systemic resistance in the plant against both root and foliar pathogens. The generally non-specific character of induced resistance constitutes an increase in the level of basal resistance to several pathogens simultaneously, which is of benefit under natural conditions where multiple pathogens may be present. Specific Pseudomonas strains induce systemic resistance in e.g. carnation, cucumber, radish, tobacco and Arabidopsis, as evidenced by an enhanced defensive capacity upon challenge inoculation. Although some bacterial strains are equally effective in inducing resistance in different plant species, others show specificity, indicating specific recognition between bacteria and plants at the root surface. In carnation, radish and Arabidopsis, the O-antigenic side chain of the bacterial outer membrane lipopolysaccharide acts as an inducing determinant, but other bacterial traits are also involved. Pseudobactin siderophores have been implicated in the induction of resistance in tobacco and Arabidopsis, and another siderophore, pseudomonine, may explain induction of resistance associated with salicylic acid (SA) in radish. Although SA induces phenotypically similar systemic acquired resistance (SAR), it is not necessary for the systemic resistance induced by most rhizobacterial strains. Instead, rhizobacteria-mediated induced systemic resistance (ISR) is dependent on jasmonic acid (JA) and ethylene signaling in the plant. Upon challenge inoculation of induced Arabidopsis plants with a pathogen, leaves expressing SAR exhibit a primed expression of SA-, but not JA/ethylene-responsive defense-related genes, whereas leaves expressing ISR are primed to express JA/ethylene-, but not SA-responsive genes. Combination of ISR and SAR can increase protection against pathogens that are resisted through both pathways, as well as extend protection to a broader spectrum of pathogens than ISR or SAR alone.

Plant responses to plant growth-promoting rhizobacteria

“…ISR confers on the plant an enhanced defensive capacity Van Loon and Bakker 2005). Upon infection with a challenging pathogen this enhanced defensive capacity is manifested as a reduction in the rate of disease development, resulting in fewer diseased plants or in lesser disease severity.…”

“…ISR elicited by almost all strains was found to be SA-independent, also by strains such as P. fluorescens CHA0 and Serratia marcescens 90-66, that can themselves produce SA as an additional siderophore (reviewed in Van Loon and Bakker 2005). Only the systemic resistance induced by Pseudomonas aeruginosa strain 7NSK2 was SA-dependent (De Meyer et al 1999).…”

“…Whereas generally rhizobacteria are not dainty in colonizing roots of different plant species, the perception by the plant of bacterial determinants that trigger ISR appears to be quite specific Meziane et al 2005;Van Loon and Bakker 2005). Apparently, one or more bacterial components need to be recognized by specific plant receptors.…”

MYB72 Is Required in Early Signaling Steps of Rhizobacteria-Induced Systemic Resistance in Arabidopsis

“…Although some rhizobacterial strains can activate the SA-dependent SAR pathway (De Meyer and Hö fte, 1997), the large majority of the reported resistance-inducing fluorescent Pseudomonas spp. strains have been shown to trigger ISR in a SA-independent manner (Van Loon and Bakker, 2005). WCS417r-mediated ISR functions independently of SA as well, as demonstrated by observations that Arabidopsis genotypes impaired in SA accumulation or biosynthesis (i.e.…”

Root Exudation: The Ecological Driver of Hydrocarbon Rhizoremediation

“…Phytoremediation of hydrocarbons depends primarily on rhizoremediation (Figure 1) which involves the breakdown of contaminants in soil as a result of microbial activity at the roots [66,68,69]. Microorganisms can colonize three distinct areas of the root zone of a plant [46,63]: (1) the endosphere, i.e., all the cells inside the roots [70]; (2) the rhizoplane which is the root surface [63,71], usually as biofilm (i.e., multiple layers of mature microcolonies covered by mucus [63]); and (3) the rhizosphere, i.e., the soil immediately adjacent to roots (a few millimeters thick) and influenced by plant roots [72,73]. Rhizosphere is a complex ecosystem characterized by a large amount of microniches-spatially close, but chemically heterogeneous-containing a high diversity of microorganisms [74].…”