Factors influencing the effect of deleterious Pseudomonas putida rhizobacteria on initial infection of pea roots by Rhizobium leguminosarum bv. viceae

Strains of Pseudomonas putida identified as plant deleterious rhizobacteria, were used to investigate factors of rhizo-bacterial origin that may affect the in vitro growth of Rhizobium leguminosarum bv. viceae. Growth inhibitory effects on R. leguminosarum were found when it was grown in contact with the rhizobacteria and also in the presence of their extracellular metabolites. Thus, these strains of P. putida do not require direct contact with living bacterial cells to induce inhibitory effects on rhizobial growth. The negative impact caused by the rhizobacteria was not consistent at high pH levels and varied with nutrient status, thus environmental factors are important for mediating growth suppression in rhizobia. Interestingly, it was also found that the inhibitory effect of P. putida was strongly reduced in the presence of ferric iron, implying that its inhibitory substance(s) is to some extent under iron regulation. In addition, chemotaxis experiments showed that the inhibitory substance(s) produced by P putida could have a negative impact on R. leguminosarum chemotaxis, indicating that the initial infection process by rhizobia could be disrupted. Thus: certain rhizobacteria seem ro have a capacity to interact with the early infection processes of R. leguminosarum with potentially important agronomic implications. (C) 2001 Elsevier Science B.V. All rights reserved.