Exploring the Pathogenicity of Pseudomonas brassicacearum Q8r1-96 and Other Strains of the Pseudomonas fluorescens Complex on Tomato

Abstract

Pseudomonas brassicacearum and related species of the P. fluorescens complex have long been studied as biocontrol and growth-promoting rhizobacteria involved in suppression of soilborne pathogens. We report here that P. brassicacearum Q8r1-96 and other 2,4-diacetylphloroglucinol (DAPG)-producing fluorescent pseudomonads involved in take-all decline of wheat in the Pacific Northwest of the United States can also be pathogenic to other plant hosts. Strain Q8r1-96 caused necrosis when injected into tomato stems and immature tomato fruits, either attached or removed from the plant, but lesion development was dose dependent, with a minimum of 10⁶ CFU ml^-1 required to cause visible tissue damage. We explored the relative contribution of several known plant-microbe interaction traits to the pathogenicity of strain Q8r1-96. Type III secretion system (T3SS) mutants of Q8r1-96, injected at a concentration of 10⁸ CFU ml^-1, were significantly less virulent, but not consistently, as compared with the wild-type strain. However, a DAPG-deficient phlD mutant of Q8r1-96 was significantly and consistently less virulent as compared with the wild type. Strain Q8r1-96acc, engineered to over express ACC deaminase, caused a similar amount of necrosis as the wild type. Cell-free culture filtrates of strain Q8r1-96 and pure DAPG also cause necrosis in tomato fruits. Our results suggest that DAPG plays a significant role in the ability of Q8r1-96 to cause necrosis of tomato tissue, but other factors also contribute to the pathogenic properties of this organism.

Keywords: 2,4-diacetylphloroglucinol; ACC deaminase; Pseudomonas; prokaryotes; tomato; vegetables.

Fig. 1.

Cross sections of cherry tomato fruits (cv. Sweet 100) injected with strain Q8r1-96, DAPG or T3SS deficient mutants, or a recombinant strain suspended in sterile water with 0.85% NaCl at a dose of 10⁸ CFU mL⁻¹. Treatments included wild type Q8r1-96, Q8r1-96ΔOPQR, Q8r1-96rspL, Q8r1-96rscV, Q8r1-96acc, Q8r1-96phlD and 0.85% NaCl (control). Each treatment had six replicates with each fruit serving as a replicate. Fruits were green when injected and ten days after infection they were cut in half and rated for disease on a 0–3 scale.

Fig. 2.

Necrosis in a tomato stem following injection of strain Q8r1-96 (A); necrosis in detached tomato fruits following four injections of DAPG (B); necrosis in detached tomato fruits following four injections of cell-free filtrates of strains Q8r1-96 and Q8r1-96phlD (C).