Data from: Density and root exudates mediate Arabidopsis response to copper

Premise of the study: Plants grown at high densities show
increased tolerance to heavy metals for reasons that are not clear. A
potential explanation is the release of citrate by plant roots, which
binds metals and prevents uptake. Thus, pooled exudates at high plant
densities could increase tolerance. We tested this exclusion facilitation
hypothesis using mutants of Arabidopsis thaliana defective in citrate
exudation. Methods: Wild type Arabidopsis and two allelic mutants
for the Ferric Reductase Defective 3 (FRD3) gene were grown at four
densities and watered with copper sulfate at four concentrations. Plants
were harvested before bolting and dried. Shoot biomass was measured, and
shoot material and soil were digested in nitric acid. Copper contents were
determined by atomic absorption. Key results: In the highest
copper treatment, density-dependent reduction in toxicity was observed in
the wild type but not in FRD3 mutants. For both mutants, copper
concentrations per gram biomass were up to seven times higher thanwild
type plants, depending on density and copper treatment. In all genotypes,
total copper accumulation was greater at higher plant densities. Plant
size variation increased with density and copper treatment because of
heterogeneous distribution of copper throughout the soil.
Conclusions: These results support the hypothesis that citrate
exudation is responsible for density-dependent reductions in toxicity of
metals. Density-dependent copper uptake and growth in contaminated soils
underscores the importance of density in ecotoxicological testing. In
soils with a heterogenous distribution of contaminants, competition for
non-toxic soil regions may drive size hierarchies and determine
competitive outcomes.