A Novel Bacterial Luxr-Type Regulator Involved in Interkingdom Signaling Between Plant Growth Promoting Bacteria and Plants

Difference with transcriptional regulators in bacterial quorum sensing systems containing a paired LuxR transcriptional regulator and a LuxI cognate signal synthase, solo LuxRs are not associated with signal synthase and have been shown to bind to exogenous signals. LuxR solos are common in plant-associated bacteria and increasingly recognized as playing important roles in plant-microbe interkingdom signaling. I identified the plant LuxR solo, PcsR2, in the plant growth-promoting species Pseudomonas chlororaphis 30-84 that was highly upregulated when bacteria were grown on plant roots or in root macerates. The encoding gene pcsR2 is part of a chromosomal region containing 11 genes in three operons that is highly conserved among P. chlororaphis species and not other P.spp. Bioinformatic analysis suggests that the adjacent operons are involved in fatty acid desaturation, a function typically related to cell membrane fludity, and therefore, may be involved in bacterial adaptation to the plant niche. Chapter 2 demonstrated that expression of pcsR2 and genes encoding two fatty acid desaturases in adjacent operons were upregulated when this strain was grown on wheat roots and pcsR2 responded specifically to root-derived substrates as compared to leaf-derived substrates. Moreover, a pcsR2 mutant was impaired in the ability to utilize root nutrients and to colonize wheat roots and in bacterial phenazine production, a trait required for effective root colonization, biofilm production and biological control activity. I hypothesize that these changes are related to alterations in cell membrane fluidity, which may affect the activity of membrane bound transporters and regulators. Consistent with this hypothesis, the activity of a two-component system with a membrane-located signal-sensor controlling the phenazine regulatory network was reduced in the pcsR2 mutant, and expression of phenazine regulatory genes in trans restored phenazine production. Chapter 3 demonstrates that mutation of pcsR2 led to altered fatty acid production, and resulted in modified altered anisotropy consistent with alterations in cell membrane fluidity. Expression of two desaturases in trans partially demonstrated their role in these phenotypes. Mutation of pcsR2 also affected stress tolerance. Preliminary evidences in Appendix suggest broader effects of pcsR2 mutation on enzyme secretion, nutrient uptake and utilization, and plant immunity response, but requires further study