Regulatory Mechanisms Underlying Biological Control Activity of Pseudomonas chlororaphis PA23.

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2010, 2012
Selin, Carrie Lynn
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FEMS Microbial Ecology
Biological control is an intriguing alternative to the use of chemical pesticides as it represents a safer, more environmentally friendly approach to managing plant pathogens. Pseudomonas chlororaphis strain PA23 was isolated from soybean root tips and it was found to be an excellent antagonist of sclerotinia stem rot. Our studies have shown that pyrrolnitrin (PRN) is the key metabolite required for S. sclerotiorum inhibition, while phenazine (PHZ) is important for biofilm establishment. For this reason, research efforts were directed towards elucidating the mechanisms governing PA23-mediated antibiotic production. To determine how these compounds were regulated, QS-deficient strains and an rpoS mutant were generated. The QS-deficient strains no longer inhibited the fungal pathogen S. sclerotiorum in vitro and exhibited reduced PRN, PHZ and protease production. Analysis of transcriptional fusions revealed that RpoS has a positive and negative effect on phzI and phzR, respectively. In a reciprocal manner, RpoS is positively regulated by QS. Characterization of a phzRrpoS double mutant showed reduced antifungal activity as well as PRN and PHZ production, similar to the QS-deficient strains. Furthermore, phzR but not rpoS was able to complement the phzRrpoS double mutant for the aforementioned traits, indicating that the Phz QS system is a central regulator of PA23-mediated antagonism. GacS/GacA, PsrA, RpoS and the PhzI/PhzR QS are members of a complex regulatory hierarchy that influence secondary metabolite production in PA23. An additional system, termed Rsm, was identified, adding yet another layer of complexity to the regulatory network. The Rsm system in PA23 appears to be comprised of a single small non-coding regulatory RNA termed RsmZ, and two RNA binding proteins RsmA and RsmE. We discovered that the expression of rsmZ, rsmA and rsmE all require GacA. In addition, both PsrA and QS were shown to positively regulate rsmZ transcription. For rsmE, GacA may indirectly regulate expression through PsrA, RpoS and QS, as all three regulators control rsmE transcription. Furthermore, we believe that the positive effects of PsrA and QS on rsmE transcription are likely mediated through RpoS as only RpoS show direct activation of rsmE in an E. coli background.
Biocontrol, Regulation, Pseudomonas chlororaphis, antibiotic production
Selin, C., Habibian, R., Poritsanos, N., Athukorala, S. N., Fernando, D. & de Kievit, T. R. (2010). Phenazines are not essential for Pseudomonas chlororaphis PA23 biocontrol of Sclerotinia sclerotiorum, but do play a role in biofilm formation. FEMS Microbiol Ecol 7, 73-83.
Selin, C., Fernando, W. G. D & deKievit, T. R. (2012). The PhzI/PhzR quorum-sensing system is required for pyrrolnitrin and phenazine production, and exhibits cross-regulation with RpoS in Pseudomonas chlororaphis PA23. Microbiology 158, 896-907.