Quantitative estimation of fitness cost associated with glyphosate resistance in Echinochloa colona

Sou Sheng Goh

    Research output: ThesisDoctoral Thesis

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    A population of Echinochloa colona from the northern region of Western Australia
    evolved glyphosate resistance after 10 years of glyphosate selection and the expression
    of associated fitness costs in this population was studied. When assessing fitness costs
    associated with resistance, it is crucial to ensure that glyphosate-susceptible (S) and -
    resistant (R) individuals share a similar genetic background, except for the resistance
    alleles. A plant cloning technique followed by phenotypic identification and glyphosate
    selection was the experimental approach employed to identify S and R phenotypes
    within the segregating glyphosate-resistant E. colona population. For selection of R
    plants, seedlings were treated with 2,160 g ha-1 of glyphosate (Roundup 540 g L-1) at the
    two- to three-leaf stage and the survivors were allowed to produce seeds. For selection
    of S plants, 300 g ha-1 of glyphosate (Roundup 540 g L-1) was applied and the clones of
    the killed plants were used for seed production. Plants from these seeds served as the
    materials for whole-plant dose response and resistance mechanism studies as well as
    evaluation of fitness costs associated with glyphosate resistance in this E. colona
    population. Estimations of LD50 (dose causing 50% mortality) and GR50 (dose causing
    50% reduction in growth rate) showed an eightfold glyphosate resistance in this
    population. More shikimate accumulated in leaves of the S phenotype than in the R
    phenotype after glyphosate treatment. Glyphosate resistance was not found to be due to
    any resistance endowing target site EPSPS gene mutation. It is also unlikely that the
    resistance mechanism is due to amplified EPSPS gene expression or enhanced
    glyphosate metabolism as no differences were found between the S and R phenotypes.
    Similarly, there were no differences in leaf glyphosate uptake and translocation at the
    whole plant level between the S and R phenotypes. However, there is evidence
    suggesting a reduced cellular glyphosate uptake in the R phenotype.

    The glyphosate resistance mechanism in this R phenotype of E. colona remains to be elucidated and
    non-target-site based resistance mechanisms (such as vacuole sequestration and/or
    membrane uptake) should be explored. Seeds of the S and R phenotypes kept at warmer
    temperatures (after-ripening dry storage at 15-35C) or on the ground surface outdoors,
    germinated better than those seeds kept for the same period (14 days) at lower
    temperature (8C). Light and longer dry after-ripening times increased the rate of seed
    dormancy release and germination of both S and R phenotypes. Equal decline in
    seedling emergence with increasing soil burial depth was observed in both S and R
    phenotypes. Thus, the S and R phenotypes exhibited similar characteristics of seed
    dormancy release and germination, and seedling emergence. Under non-competitive
    conditions, plants from the S phenotype showed greater leaf area, vegetative
    aboveground biomass and root biomass as well as higher relative growth rate compared
    to plants of the R phenotype during the vegetative growth stage. At the reproductive
    stage, plants of the S phenotype also produced more reproductive biomass than plants
    from the R phenotype. A physiological fitness cost of 31 (seed mass) or 22 (seed
    number) was associated with the R phenotype. The results clearly suggest there is an
    evolutionary trade-off between glyphosate resistance and vegetative and reproductive
    growth in resistant E. colona plants when grown in glyphosate-free environments.
    Plants of the S phenotype were also stronger competitors with a rice crop than plants of
    the R phenotype, and allocated more resources to reproductive organs than the R
    phenotype. An ecological fitness cost of 60 was expressed in the R phenotype under
    interspecific (vs rice) competitive conditions. Similarly, a greater reduction in seed mass
    and number was exhibited by R plants when competing with S plants. In addition, a
    significant decline of 14% in the phenotypic frequency of glyphosate resistance was
    estimated after two generations in field conditions under no glyphosate selection.
    Overall, results from the present investigation demonstrate that there is a significant
    fitness cost associated with glyphosate resistance in the studied E. colona population. It
    is possible to speculate that under no further glyphosate selection, the frequency of
    glyphosate resistance is likely to remain low or even be eliminated in agroecosystems.

    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2016


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