DIVERSIFICATION OF PSEUDOMONAS CORRUGATA STRAIN 2140 IN WHEAT-SOIL MICROCOSMS IN RELATION TO BIOCONTROL OF TAKE-ALL DISEASE OF WHEAT
SJ BARNETT1,2, I SINGLETON3 and MH RYDER1
1CSIRO Land and Water, PMB 2 Glen Osmond 5064, South Australia; 2Department of Crop Protection, University of Adelaide, PMB 1 Glen Osmond 5064, South Australia; 3Department of Soil Science, University of Adelaide, PMB 1 Glen Osmond 5064, South Australia
Background and objectives
Bacteria are known to change phenotype and the production of variant phenotypes is a common laboratory phenomenon. It is not known to what extent phenotype plasticity occurs in vivo. If it does occur, it could have an impact on (1) the rhizosphere populations of disease antagonists introduced as biocontrol agents, or (2) disease control by the introduced bacteria. Strain 2140R was used as a model Pseudomonas disease antagonist to investigate changes in the population structure and control of take-all disease by an biocontrol agent introduced after culture on wheat roots.
Materials and methods
Single 2140R colonies with the wild type and variant colony morphologies were isolated, purified, and assayed for in vitro inhibition of Ggt, Rhizoctonia solani and Pythium irregulare, control of take-all disease (pot assay) and GC-FAME profiles (as used for taxonomic identification). Wild type and variant colony types were compared with the ancestral 2140R isolate stored in 15% glycerol at -70°C.
Results and conclusions
This work shows that a population of the disease antagonist, P. corrugata 2140R, started initially from a single isolate, can become a mixed population of more than one phenotype after as early as one 10-week cycle on wheat roots. Variant colony types can eventually dominate the population in some pot cultures. Wild-type colonies isolated after a number of cycles can diverge from the ancestral type as much as colony morphology variants. By evolutionary theory, the persistence and multiplication of new phenotypes depends on the selection pressure exerted on the population by biotic and abiotic conditions. It is hypothesized that other soil bacteria can also change phenotype in the root-soil environment. This may have implications for variable effectiveness of biocontrol agents. It may also have implications for the development of disease suppressive soils where selection for a disease-suppressive microbial community could be either from within the existing soil community or from new phenotypes which may arise.