Arabidopsis response to copper is mediated by density and root exudates: Evidence that plant density and toxic soils can shape plant communities.

PREMISE: 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 might 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. 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 than for wild 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 heterogeneous distribution of contaminants, competition for nontoxic soil regions may drive size hierarchies and determine competitive outcomes.

Genetic analysis of pod dehiscence in pea (Pisum sativum L.).

The inheritance of the dehiscent pod character was investigated in two recombinant inbred populations using a simplified correlation analysis. The approach identified three regions on the pea genome that affect the expression of pod dehiscence. The region on linkage group III corresponded to the expected position of Dpo, a gene known to influence pod dehiscence. A locus on linkage group V appeared to have a slightly smaller effect on expression of the phenotype. The third region was observed only in one cross, had a greater effect than Dpo, and was postulated to be yellow pod allele at the Gp locus