Hrp mutant bacteria as biocontrol agents: Toward a sustainable approach in the fight against plant pathogenic bacteria

Abstract

Sustainable agriculture necessitates development of environmentally safe methods to protect plants against pathogens. Among these methods, application of biocontrol agents has been efficiently used to minimize disease development. Here we review current understanding of mechanisms involved in biocontrol of the main Gram-phytopathogenic bacteria-induced diseases by plant inoculation with strains mutated in hrp (hypersensitive response and pathogenicity) genes. These mutants are able to penetrate plant tissues and to stimulate basal resistance of plants. Novel protection mechanisms involving the phytohormone abscisic acid appear to play key roles in the biocontrol of wilt disease induced by Ralstonia solanacearum in Arabidopsis thaliana. Fully understanding these mechanisms and extending the studies to other pathosystems are still required to evaluate their importance in disease protection.

Seasonal variation in the biocontrol efficiency of bacterial wilt is driven by temperature‐mediated changes in bacterial competitive interactions

“…Competition between the pathogenic and non‐pathogenic bacteria is a major constraint for disease outbreaks in plant microbiomes (Hanemian et al . ; Wei et al . ; Raaijmakers & Mazzola ).…”

“…Competition between the pathogenic and non-pathogenic bacteria is a major constraint for disease outbreaks in plant microbiomes (Hanemian et al 2013;Wei et al 2013;Raaijmakers & Mazzola 2016). The outcome of microbial competition is however very variable and often depends on abiotic environmental conditions (Jiang et al 2011;Hanke et al 2015), the survival of the biocontrol agent (Hu et al 2016) and the interactive effects within microbiomes (Thomas & Sekhar 2016).…”

Type III Secretion–Dependent and –Independent Phenotypes Caused by Ralstonia solanacearum in Arabidopsis Roots

“…S3 shows that the capacity to colonize Arabidopsis plants of hrpB is significantly higher than of hrpG mutants. Thus, although hrp mutants had been already described to multiply in planta (Hanemian et al 2013), HrpG seems to be more essential than HrpB for the bacterium to colonize the plant xylem and reach the aerial tissues.…”

Modes of Action of Microbial Biocontrol in the Phyllosphere

“…Therefore, it is debatable whether competition for nutrients and space is an important mode of action. However, multiple studies show that inoculation with hrp mutants induces changes in gene expression which lead to a defense response and increased resistance against the pathogen (Faize et al, 2006;Feng et al, 2012;Hanemian et al, 2013).…”

“…Gram-negative bacteria use type III secretion systems to deliver effector molecules into the cytoplasm to suppress the immune system. When such a secretion system is inactivated (through mutations in hrp genes, which are required for a functional type III secretion system and elicitation of a hypersensitive response in plants) in pathogens, disease symptoms are reduced ( Hanemian et al, 2013 ). Such mutants are then unable to overcome pattern-triggered immunity and are unable to infect host tissue.…”