A spontaneous rifampicin-resistant mutant of the biocontrol agent Pseudomonas fluorescens CHAO was released as soil inoculant in large outdoor lysimeters and its ability to colonise the roots of winter wheat, spring wheat (grown after Phacelia) and maize at the later stages of plant development was investigated by colony counts. The inoculant (i.e. CHAO-Rif) colonised the rhizosphere and the interior of the roots of both wheat varieties but CFUs at ripening were about 2 log (g root)(-1) or lower. In contrast, the roots of maize were colonised poorly by the pseudomonad at flowering, but the latter was found at 3 or more log CFU (g root)(-1) on and inside the roots in late ripening stage. Furthermore, CHAO-Rif was recovered at more than 5 log CFU (g root)(-1) from the interior of several maize root samples. Whereas most cells of CHAO-Rif in soil were small and did not respond to Kogure's viability test, the pseudomonad was present as viable, unusually large (7 mm long) rods inside maize roots. In a microcosm experiment performed with similar sandy-loam soil, the CFUs of maize root-associated CHAO-Rif were higher where the shoots of the plant had been cut off, confirming that older and/or decaying maize roots represent a favourable niche for the inoculant. Overall, the results indicate that Pseudomonas inoculants have the potential to colonise the roots of certain crops (e.g. maize but not wheat for strain CHA0-Rif) at later stages of plant development.
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