Ecology and diversity of metal HYPERaccumulating Species: integrating studies of gene expression, Plasticity of Accumulation, individual fitness, functional syndromes and Complementary Effects – HYPERSPACE

The ability of some plants to hyperaccumulate toxic metal(oid)s in their leaves is fascinating. Physiological studies revealed the basis of this counterintuitive trait: it requires active absorption, translocation, detoxification and storage of metals. Clearly, for this trait to evolve, the fitness benefits have to outweigh direct (toxicity) and indirect costs (associated machinery). Several hypotheses exist to explain its ecological significance, including increased metal-tolerance, defense against enemies and competitors, or increased drought-tolerance. Unfortunately, physiologists and ecologists have mostly worked in isolation and on single species, limiting our ultimate understanding of ‘WHY’ hyperaccumulation has evolved. Here, we want to fill this gap by integrating studies on gene expression, metal allocation, individual fitness, functional trade-offs and community-level consequences of hyperaccumulation.
We first ask 1) ‘HOW’ plants hyperaccumulate metals using transcriptomic for several European hyperaccumulators and different metals, including, for the first time, Thallium (Tl). Gene expression will also be studied for genes involved in functions presumably related to hyperaccumulation, e.g. oxidative stress tolerance, enemy defense, photosynthetic capacity, and drought tolerance.
We then ask 2) whether hyperaccumulation increases WHEN specific stimuli occur (herbivory, competition, drought), suggesting a key role of these stimuli in selection. Plastic response to the stimuli will be tested by measuring metal allocation to leaves and roots in greenhouse experiments with three species (hyperaccumulating Ni, Zn, Cd, Tl). We then look at whether hyperaccumulation indeed deters herbivores or creates allelopathic effects.
We then ask 3) WHERE (soil types), for WHOM (populations with different hyperaccumulation ability) and under which circumstances (herbivory) benefits outweigh costs. In greenhouse experiments, we simultaneously study hyperaccumulation and metal tolerance, and its net effects on reproduction. We also evaluate how the balance between fitness benefits and costs changes under herbivory.
In search for generality, we 4) use multi-species studies to compare functional traits of hyperaccumulators with those of coexisting, non-accumulating species. I.e., we ask ‘WHO’ can meet the challenge of hyperaccumulation? E.g., acquisitive species, with high photosynthetic and metabolic activity, may better sustain this costly adaptation. Ecological strategies will be studied through functional trait measurements of 80 hyperaccumulating species (~ 10% of the world’s hyperaccumulators) and coexisting species across biomes and metal(oid)s (Ni, Zn, Cd, Tl, Se).
Finally, we will 5) scale up our findings to a community-level. We test for complementarity effects when hyperaccumulators grow WITH each other. We use rhizotron experiments (fine root interactions) and mesocosms (other interactions) to understand both community-level dynamics and ways for efficient for phytoremediation of contaminated soils.