The Erwinia chrysanthem phoP-phoQ operon plays an important role in growth at low pH, virulence and bacterial survival in plant tissue

We have studied the role of acidic pH as a barrier for the colonization of the plant apoplast by Erwinia chrymsanthemi. A minitransposon containing a promoterless reporter gene, gus, was used for random mutagenesis of the bacterial genome. An acid-sensitive mutant, named BT1119, was isolated and had the following differential features with respect to the wildtype strain: (i) inability to grow at pH less than or equal to 5.5; (ii) decreased survival at acid pH and in plant tissues; (iii) increased susceptibility to antimicrobial peptides; (iv) decreased virulence in chicory leaves and pear fruits; (v) reduced polygalacturonase production; and (vi) reduced ability to alkalinize chicory tissues after infection. The sequence of the interrupted gene was highly similar to the phoQ gene, which is involved in environmental sensing in several bacteria, such as Yersinia pseudotuberculosis, Erwinia carotovora, Salmonella typhimurium and Escherichia coli and thus, this designation was used for the E. chrysenthemi system. This gene was induced at low Mg2+ concentrations and in planta. These results suggest that E. chrysanthemi PhoP-PhoQ system plays an important role in bacterial survival in plant tissues during the initial infection stages.