Roles and regulation of phenazines in the biological control strain Pseudomonas chlororaphis 30-84

Abstract

Pseudomonads are well known for the production of diverse secondary metabolites, including phenazines that are essential for the control of plant diseases. Emerging evidence demonstrates that phenazines play multiple roles in the ecological fitness of the producing microbe. It is now understood that phenazines modify cellular redox state, act as electron shuttles altering electron flow patterns, contribute to biofilm formation and architecture, act as cell signals that regulate patterns of gene expression, and contribute to the survival of the producer. When associated with eukaryotic hosts, phenazines modify numerous host cellular responses. Of particular interest in defining their functional impact are the observations that bacterial species may produce different types of phenazines and many produce more than one phenazine derivative. The amount of each derivative produced may change during growth or in response to the intracellular or extracellular environment. Furthermore, environmental conditions influence the redox state of the derivatives, altering their functionality. The regulatory complexity governing phenazine production likely reflects the complexity of the roles they play for the producing microbe. Here, we highlight briefly some of the roles phenazines play in the biological control activity of pseudomonads belonging to the Pseudomonas fluorescens subgroup, with special emphasis on Pseudomonas chlororaphis 30-84. We also review the regulatory network governing phenazine production in P. chlororaphis 30-84, with special emphasis on how the different regulatory controls function in terms of the signals to which they respond and the suites of genes they control. Despite the recognized roles phenazines play in bacterial fitness and biological control, growing evidence suggests that phenazine non-producing phenotypic variants defective in the GacS/GacA two component regulatory system are common components of plant-associated bacterial communities. We provide additional discussion on why these phenotypic variants should not be overlooked.