Microbial Antagonism at the Root Level Is Involved in the Suppression of Fusarium Wilt by the Combination of Nonpathogenic Fusarium oxysporum Fo47 and Pseudomonas putida WCS358
Two biological control agents, nonpathogenic Fusarium oxysporum Fo47
and Pseudomonas putida WCS358, were evaluated for suppression of
Fusarium wilt of flax grown in nutrient solution and for suppression of
the population density and metabolic activity of the causal organism F.
oxysporum f. sp. lini strain Foln3GUS on root surfaces. Due to the presence
of an introduced gusA reporter gene construct in Foln3GUS, the pathogen
expressed b-glucuronidase activity that was related to its carbon metabolism.
At a Fo47 to Foln3GUS inoculum ratio of 100:1, both the population
density of the pathogen and the b-glucuronidase activity on and in flax
roots were reduced by the nonpathogenic strain, and Fusarium wilt was
suppressed. At a Fo47 to Foln3GUS inoculum ratio of 10:1, Fo47 decreased
the severity of Fusarium wilt to a smaller extent and it also reduced
b-glucuronidase activity without reducing the density of Foln3GUS
on flax roots. At a nonpathogenic to pathogenic Fusarium strains ratio of
10:1, the addition of P. putida WCS358 further suppressed Fusarium wilt
and the density of the pathogen at the root level, whereas a mutant of
WCS358 deficient in pseudobactin production had no significant effect.
Iron availability to WCS358 on flax roots, assessed by ice-nucleation
activity conferred from a transcriptional fusion (pvd-inaZ) of an ice-nucleation
reporter gene to an iron-regulated promoter, was sufficiently low to
allow pseudobactin production. P. putida WCS358 did not reduce the severity
of Fusarium wilt of flax when inoculated without Fo47, and it did
not improve disease suppression achieved by high inoculum doses of Fo47
(a Fo47 to Foln3GUS ratio of 100:1). Together, these data provide evidence
that (i) suppression of Fusarium wilt of flax by Fo47 is related to reductions
in the population density and metabolic activity of the pathogen on
the root surface; (ii) WCS358 can enhance the biological control activity
of Fo47, but this enhancement depends on the population of Fo47 relative
to the pathogen; and (iii) pseudobactin contributes to suppression of Fusarium
wilt by the combination of Fo47 and WCS358 on roots in which
conditions are conducive to pseudobactin production by the bacterium