Metalheads! Patterns of extreme mineral accumulation in plants

Balanced and controlled maintenance of mineral levels in plant tissues is essential for the healthy growth of all plants. However, there is variation in the amount of minerals present in the tissues of different plant species, and in the variety of different minerals they accumulate. In extreme examples, some plant species are ‘hyperaccumulators’ meaning that they accumulate certain minerals to levels well above those normally found in other species. There is much interest in plant hyperaccumulators  both from the fundamental perspective of how the achieve and tolerate such high levels of mineral accumulation, as well as their potential use for removing contaminant minerals from land.

However, knowledge is lacking about the natural variety of plant hyperaccumulators, how these relate to particular soil types, and how accumulation of particular minerals in hyperaccumulator species impacts the levels of other minerals in these plants. Using plant species growing in the ultra-diverse Sabah region of Malaysia, van der Ent and colleagues in their recent Annals of Botany study investigate trends in patterns and levels of mineral accumulations to further understand the variety of plant hyperaccumulators in natural environments, what form these hyperaccumulators take, and how they may achieve this.

The Sabah region on which van der Ent and colleagues focus is known to contain hyperaccumlators of nickel, cobalt and manganese, as well as many plant species that are not hyperaccumulators. Hyperaccumulators of nickel and other minerals typically grow on ultramafic soils (soils low in essential potassium and phosphorus and high in minerals such as nickel, manganese and cobalt). The first question the authors ask is whether plants growing on ultramafic soils in the Sabah region have differences in leaf mineral concentrations compared to plants growing on non-ultramafic soils. Perhaps unsurprisingly, they find that the leaf concentrations of plants growing on ultramafic soils have a much greater concentration range for nickel, cobalt, manganese and some other minerals compared to on non-ultramafic soils. Interestingly however, plants on ultramafic soils maintain concentrations in the same range for minerals that are typically lower in ultramafic soils, such as potassium and phosphorus, as plants growing on non-ultramafic soils.

Related to this is investigating whether there are correlations between leaf elemental composition and the surrounding soil conditions. van der Ent and colleagues find that the majority of plant species exclude high-concentration minerals present in ultramafic soil, reinforcing the notion that even in ultramafic soils, hyperaccumulator species still make up only a small proportion of plant species. As already touched upon, minerals such as potassium and phosphorus that are generally low in ultramafic soils were generally accumulated to levels higher than in the surrounding soil, indicating that plants in these areas actively try to maintain concentrations of these minerals.

The authors then addressed whether mineral accumulation distribution in the surveyed plants follows a normal distribution or whether there are clear shifts away from this distribution? They find that on ultramafic soil, all the tested minerals follow a normal distribution in leaves except for nickel, which follows a clearly bimodal distribution, and for cobalt and chromium, which follow less stark bimodal distributions. The bimodal distributions indicate that for these minerals, the plants growing on ultramafic soils fall generally into either a non-accumulator or accumulator mode. This indicates a scenario that hyperaccumulators, particularly of nickel, are not in the upper region of a normal distribution but have a distinct trait that is separate from most other plants. van der Ent and colleagues point out that previous studies have indicated that such bimodality indicates that the particular trait depends on a small number of genes, whereas a normal distribution indicates more a complex genetic background.

van der Ent and colleagues therefore provide greater insight into the patterns of mineral accumulation of a variety of plants growing on ultramafic soils, as well as giving hints as to how the rare but striking examples of hyperaccumulators may undertake their unusually high accumulation of certain nutrients. This will likely be of interest to those wishing to harness hyperaccumulating plants for bioremediation purposes, as well as increasing our knowledge of how nutrient content of plants may vary in real environments rather than in often-studied lab conditions.