Identification of Pseudomonas fluorescens Chemotaxis Sensory Proteins for Malate, Succinate, and Fumarate, and Their Involvement in Root Colonization

Pseudomonas fluorescens Pf0-1 exhibited chemotactic responses to L-malate, succinate, and fumarate. We constructed a plasmid library of 37 methyl-accepting chemotaxis protein (MCP) genes of P. fluorescens Pf0-1. To identify a MCP for L-malate, the plasmid library was screened using the PA2652 mutant of Pseudomonas aeruginosa PAO1, a mutant defective in chemotaxis to L-malate. The introduction of Pfl01_0728 and Pfl01_3768 genes restored the ability of the PA2652 mutant to respond to L-malate. The Pfl01_0728 and Pfl01_3768 double mutant of P. fluorescens Pf0-1 showed no response to L-malate or succinate, while the Pfl01_0728 single mutant did not respond to fumarate. These results indicated that Pfl01_0728 and Pfl01_3768 were the major MCPs for L-malate and succinate, and Pfl01_0728 was also a major MCP for fumarate. The Pfl01_0728 and Pfl01_3768 double mutant unexpectedly exhibited stronger responses toward the tomato root exudate and amino acids such as proline, asparagine, methionine, and phenylalanine than those of the wild-type strain. The ctaA, ctaB, ctaC (genes of the major MCPs for amino acids), Pfl01_0728, and Pfl01_3768 quintuple mutant of P.fluorescens Pf0-1 was less competitive than the ctaA ctaB ctaC triple mutant in competitive root colonization, suggesting that chemotaxis to L-malate, succinate, and/or fumarate was involved in tomato root colonization by P.fluorescens Pf0-1.