Erwinia chrysanthemi requires a second iron transport route dependent of the siderophore achromobactin for extracellular growth and plant infection

Abstract

SummaryFull virulence of the pectinolytic enterobacterium Erwinia chrysanthemi strain 3937 depends on the production in planta of the catechol-type siderophore chrysobactin. Under iron-limited conditions, E. chrysanthemi synthesizes a second siderophore called achromobactin belonging to the hydroxy/carboxylate class of siderophore. In this study, we cloned and functionally characterized a 13 kb long operon comprising seven genes required for the biosynthesis ( acs ) and extracellular release ( yhcA ) of achromobactin, as well as the gene encoding the specific outer membrane receptor for its ferric complex ( acr ). The promoter of this operon was negatively regulated by iron. In a fur null mutant, transcriptional fusions to the acsD and acsA genes were constitutively expressed. Band shift assays showed that the purified E. chrysanthemi Fur repressor protein specifically binds in vitro to the promoter region of the acsF gene confirming that the metalloregulation of the achromobactin operon is achieved directly by Fur. The temporal production of achromobactin in iron-depleted bacterial cultures was determined: achromobactin is produced before chrysobactin and its production decreases as that of chrysobactin increases. Pathogenicity tests performed on African violets showed that achromobactin production contributes to the virulence of E. chrysanthemi. Thus, during infection, synthesis of these two different siderophores allows E. chrysanthemi cells to cope with the fluctuations of iron availability encountered within plant tissues. Interestingly, iron transport mediated by achromobactin or a closely related siderophore probably exists in other phytopathogenic bacterial species such as Pseudomonas syringae .

Multiple biosynthetic and uptake systems mediate siderophore-dependent iron acquisition in Streptomyces coelicolor A3(2) and Streptomyces ambofaciens ATCC 23877

“…Multiple siderophore biosynthetic and uptake systems have been reported for other bacteria, including Bacillus anthracis (anthrachelin and anthrabactin; Cendrowski et al, 2004), Erwinia chrysanthemi (achromobactin and chrysobactin; Franza et al, 2005) and Pseudomonas aeruginosa (pyochelin and pyoverdin; Poole & McKay, 2003). These reports hint towards functional duplication conferring an advantage for the bacterium as it colonizes different ecological niches.…”

“…These reports hint towards functional duplication conferring an advantage for the bacterium as it colonizes different ecological niches. They also suggest that in these pathogenic strains only one siderophore is important during certain stages of infection (Cendrowski et al, 2004;Franza et al, 2005). Desferrioxamine production seems to be conserved among Streptomyces spp., yet several soil-dwelling non-actinomycetes can utilize the ferric complexes of these hydroxamate metabolites (Meyer & Abdallah, 1980;Berner et al, 1988;Kachadourian et al, 1996).…”

Microbial Siderophores Exert a Subtle Role in Arabidopsis during Infection by Manipulating the Immune Response and the Iron Status

“…This result confirms that CB can repress the expression of PDF1.2. Previous studies using bacterial mutants unable to produce CB or achromobactin have shown that these siderophores promote the infection process in host plants (Enard et al, 1988;Dellagi et al, 2005;Franza et al, 2005). We thus asked whether infiltration of CB onto Arabidopsis leaves prior to E. chrysanthemi inoculation could affect the bacterial growth.…”

“…As E. chrysanthemi triggers a set of defenses in Arabidopsis during infection, including the SAmediated signaling pathway (Fagard et al, 2007), we wondered if the activation of this pathway was reduced after inoculation of a siderophore-deficient mutant compared with the wild-type strain. Thus, we used the E. chrysanthemi CB-deficient mutant affected in the cbsE gene (Franza et al, 2005). Expression of the PR1 gene was monitored 3, 7, and 24 h after infiltration of Arabidopsis leaves with wild-type cells or the siderophore-deficient mutant.…”

“…Under iron deficiency, E. chrysanthemi releases two siderophores: the hydroxycarboxylate achromobactin, which is produced when iron becomes limiting (Mü nzinger et al, 2000), and the catecholate chrysobactin (CB; Persmark et al, 1989), which prevails under severe iron deficiency. CB and achromobactin production are required for the systemic progression of maceration symptoms on the hosts (Enard et al, 1988;Dellagi et al, 2005;Franza et al, 2005). Neema et al (1993) showed that the production of CB enables bacterial cells to compete with plant cells for iron, preventing sequestration of this metal by the plant ferritins.…”