Protection of apple against fire blight induced by an hrpL mutant of Erwinia amylovora

Abstract

A regulatory hrpL non-virulent mutant of Erwinia amylovora is effective in controlling fire blight disease when inoculated on apple seedlings simultaneously with the pathogenic parental strain. Mechanisms involved in this protective effect were investigated. The use of two marker genes, uidA and lacZ, expressed in the hrpL mutant and the pathogenic strain, respectively, allowed to localize simultaneously the two inoculated strains in plant tissue. An anti-β-glucuronidase antibody was also used to detect the hrpL mutant. Both techniques indicated that the two strains localized mainly in separate areas of the leaf tissue. In addition, leaves infiltrated with the hrpL mutant exhibited a significant increase in peroxidase activity in contrast to a hrp secretion mutant known to be less effective in the protection. It is suggested that protection obtained with the hrpL mutant relies on the physical separation between the mutant and the parental strain after co-inoculation and the rapid and sustained activation of plant defense mechanisms in reactive tissue, i.e. not invaded by the virulent strain.Additional key words: apple defense response, histochemical assays, Malus × domestica.

The alternative σ factor HrpL negatively modulates the flagellar system in the phytopathogenic bacteriumErwinia amylovoraunderhrp-inducing conditions

“…The actual eliciting ability of flagellin of E. amylovora is not known. However, our present results showing that Ea hrpL maintains high motility in plant-mimicking conditions, together with our recent demonstration (Faize et al, 2006) that Ea hrpL elicits apple defenses and protects plant tissues against subsequent fire blight infection (unlike the TTSS mutant Ea hrpsec), are consistent with the hypothesis that elicitation of defense (and then protection from infection) is because of a larger amount of flagellins exported by Ea hrpL, and therefore that flagellins of E. amylovora could indeed act as general elicitors in apple. However, induction of apple defense responses by purified flagellin remains to be demonstrated.…”

Genetic characterization of the HrpL regulon of the fire blight pathogenErwinia amylovorareveals novel virulence factors

“…The remaining candidate hrp promoters are not oriented to transcribe mRNA related to type III secretion (Table 2) and may be involved in the regulation of the expression of other virulence factors, consistent with observations in E. amylovora that hrpL mutants exhibit increased peroxidase activity relative to type III secretion null strains and that hrpL mutants are also hyperflagellated and more motile than WT E. amylovora (Cesbron et al ., 2006; Faize et al ., 2006).…”

“…Although known to play a role in the regulation of the T3SS (Chatterjee et al ., 2002a; Innes et al ., 1993; Shen and Keen, 1993; Wei and Beer, 1995), previous examinations of HrpL function have also demonstrated that the HrpL regulon includes genes whose activity is not directly related to type III secretion (Cesbron et al ., 2006; Faize et al ., 2006; Sreedharan et al ., 2006; Yap et al ., 2005). To better understand the diversity of phenotypes observed in Δ hrpL strains, mutants in our selected HrpL‐regulated genes were assayed for alterations in their biofilm formation and motility phenotypes.…”

“…In E. amylovora , the hrp PAI encodes putative phaseolotoxin‐like biosynthetic proteins required for systemic fire blight development (Oh et al ., 2005). Additional virulence roles for HrpL are also suggested by type III secretion‐independent hrpL mutant phenotypes, including increased peroxidase activity and hypermotility (Cesbron et al ., 2006; Faize et al ., 2006).…”

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

“…38 (Similar observations had been made in apple seedlings inoculated simultaneously with a hrp mutant and a wild type strain of E. amylovora). 14 On the other hand, a subsequent inoculation with the virulent strain allowed a high protection rate associated with a decrease in the multiplication of the virulent strain. The delay required between hrp mutant and wild-type strain inoculations suggested that some plant signaling pathways had to be established before inoculation of virulent bacteria.…”