Cell Density-Dependent Gene Contributes to Efficient Seed Colonization by Pseudomonas putida KT2440

Abstract

We have characterized the expression pattern of a gene, ddcA, involved in initial colonization of corn seeds by Pseudomonas putida KT2440. The ddcA gene codes for a putative membrane polypeptide belonging to a family of conserved proteins of unknown function. Members of this family are widespread among prokaryotes and include the products of a Salmonella enterica serovar Typhimurium gene expressed during invasion of macrophages and psiE, an Escherichia coli phosphate starvation-inducible gene. Although its specific role is undetermined, the presence of ddcA in multicopy restored the seed adhesion capacity of a KT2440 ddcA mutant. Expression of ddcA is growth phase regulated, being maximal at the beginning of stationary phase. It is independent of RpoS, nutrient depletion, or phosphate starvation, and it is not the result of changes in the medium pH during growth. Expression of ddcA is directly dependent on cell density, being also stimulated by the addition of conditioned medium and of seed exudates. This is the first evidence suggesting the existence of a quorum-sensing system in P. putida KT2440. The potential implication of such a signaling process in seed adhesion and colonization by the bacterium is discussed.

Pumping iron to keep fit: modulation of siderophore secretion helps efficient aromatic utilization in Pseudomonas putida KT2440

“…Organisms belonging to this genus are widely found in association with plants, roots (rhizospheric zone) or as epiphytes or saprophytes (Espinosa-Urgel & Ramos, 2004). In addition, they are also one of the predominant groups of organisms found in environments contaminated with aromatics and other organic pollutants.…”

A Two-Component Regulatory System Integrates Redox State and Population Density Sensing in Pseudomonas putida

“…Although this microorganism is not known to produce any of the usual quorum-sensing signals and appears not to have the genetic potential to synthesize AHL or Pseudomonas quinolone signal, ddcA expression was shown to respond to a pHsensitive signal accumulated in the supernatant of KT2440 cultures during growth (11). In this paper we report the identification and characterization of a two-component regulatory system that modulates expression of ddcA and appears to be a key sensor/regulator of the redox state of P. putida cells.…”

“…In a previous work we had characterized a gene, ddcA, involved in colonization of plant seeds by P. putida KT2440, which shows a cell density-dependent expression profile (11). Although this microorganism is not known to produce any of the usual quorum-sensing signals and appears not to have the genetic potential to synthesize AHL or Pseudomonas quinolone signal, ddcA expression was shown to respond to a pHsensitive signal accumulated in the supernatant of KT2440 cultures during growth (11).…”

“…The P. putida KT2440 cosmid library used in this work has been described before (28). Plasmid pME510 carries a transcriptional fusion of ddcA to the reporter lacZ gene (11). Plasmid pRF1 was constructed by cloning an NsiI/BglII fragment containing roxS and roxR into plasmid pBBR1MCS-5 (16), as detailed in the Results section below.…”

“…Bacterial attachment was quantified after solubilization of the dye with ethanol and spectrophotometric measurement of absorbance at 550 nm. Attachment to corn seeds was tested as previously described (11,12). For rhizosphere colonization experiments, surface-sterilized seeds were allowed to germinate for 48 h before being inoculated with a 1:1 mixture of the mutant strain and a KT2440 derivative carrying an Sm resistance gene in single copy in the chromosome (KT2440-Sm), generated by site-specific insertion of mini-Tn7⍀Sm1 (15) at an extragenic location.…”