Abstract
Metal hyperaccumulation, in which plants store exceptional concentrations of metals in their shoots, is an unusual trait whose evolutionary and ecological significance has prompted extensive debate. Hyperaccumulator plants are usually found on metalliferous soils, and it has been proposed that hyperaccumulation provides a defense against herbivores and pathogens, an idea termed the ‘elemental defense’ hypothesis. We have investigated this hypothesis using the crucifer Thlaspi caerulescens, a hyperaccumulator of zinc, nickel, and cadmium, and the bacterial pathogen Pseudomonas syringae pv. maculicola (Psm). Using leaf inoculation assays, we have shown that hyperaccumulation of any of the three metals inhibits growth of Psm in planta. Metal concentrations in the bulk leaf and in the apoplast, through which the pathogen invades the leaf, were shown to be sufficient to account for the defensive effect by comparison with in vitro dose–response curves. Further, mutants of Psm with increased and decreased zinc tolerance created by transposon insertion had either enhanced or reduced ability, respectively, to grow in high-zinc plants, indicating that the metal affects the pathogen directly. Finally, we have shown that bacteria naturally colonizing T. caerulescens leaves at the site of a former lead–zinc mine have high zinc tolerance compared with bacteria isolated from non-accumulating plants, suggesting local adaptation to high metal. These results demonstrate that the disease resistance observed in metal-exposed T. caerulescens can be attributed to a direct effect of metal hyperaccumulation, which may thus be functionally analogous to the resistance conferred by antimicrobial metabolites in non-accumulating plants.
Highlights
Metal hyperaccumulation is described as the accumulation of exceptionally high concentrations of metallic elements in the aerial parts of a plant [1,2]
We have developed a model system to study the elemental defense hypothesis for plant metal hyperaccumulation using the bacterial pathogen Pseudomonas syringae pv. maculicola M4
We have shown that Psm displays T3SS-dependent pathogenesis in Thlaspi caerulescens plants grown in low metal concentrations and that metal hyperaccumulation by T. caerulescens at higher metal concentrations provides an effective defense against Psm
Summary
Metal hyperaccumulation is described as the accumulation of exceptionally high concentrations of metallic elements in the aerial parts of a plant [1,2]. The phenomenon has evolved in around 450 plant species, distributed across several families [2,3,4], and most hyperaccumulator species are endemic to metalliferous soils, either natural or anthropogenic in origin [5] This unusual characteristic has attracted considerable interest, and a number of hypotheses have been proposed to explain the evolution of the hyperaccumulation phenotype [6,7]. Some evidence has been provided that plants exposed to high metal concentrations have reduced susceptibility to various pathogens, no study has demonstrated that the metal itself was directly responsible for this effect. It is not possible, at present, to state that metal hyperaccumulation trait evolved as an antimicrobial defense
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