Exploring Lower Limits of Plant Elemental Defense by Cobalt, Copper, Nickel, and Zinc

Abstract

Elemental defense is a relatively newly recognized phenomenon in which plants use elements present in their tissue to reduce damage by herbivores or pathogens. In the present study, neonates of the generalist herbivore, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), were fed artificial diets amended with varying concentrations of Co, Cu, Ni, and Zn that are hyperaccumulated by plants to determine minimum lethal concentrations (MLC) and minimum sublethal concentrations (MSC) for each metal. MLC values (dry mass) for Co (45 μg/g), Ni (230 μg/g), and Zn (280 μg/g) were below published minimum hyperaccumulator levels. MSC levels (dry mass) for Co (15 μg/g), Ni (140 μg/g), and Zn (200 μg/g) were at concentrations lower than published minimum accumulator levels. Furthermore, both MLC and MSC values for Zn were within normal tissue concentrations. These results indicate that elemental defense for Co, Ni, and Zn may be effective at concentrations lower than hyperaccumulator levels and so may be more widespread than previously believed.

Ecology of nickel hyperaccumulator plants from ultramafic soils in Sabah (Malaysia)

“…Stepwise evolution of the Ni hyperaccumulator trait, starting with relatively low foliar metal concentrations and associated competitive advantages over other plants with normal foliar concentrations has been hypothesised to lead to hyperaccumulation (Boyd 2012;Cheruiyot et al 2013). The question arising from this hypothesis is why some Ni hyperaccumulators reach extremely high Ni concentrations (for example, 23,300 lg g -1 in Phyllanthus cf.…”

“…The question arising from this hypothesis is why some Ni hyperaccumulators reach extremely high Ni concentrations (for example, 23,300 lg g -1 in Phyllanthus cf. securinegoides or [100-fold higher than MLC for the generalist insect herbivore in Cheruiyot et al 2013), which presumably comes at an energetic cost. The answer could lie in a biogeochemical 'arms race' with Ni-tolerant insects (Boyd 2004(Boyd , 2009.…”

“…In the context of 'elemental herbivory defense', foliar Ni accumulation has the distinct benefit of requiring limited energetic resources (although the uptake and transport physiology requires Ni complexing ligands such as citrate) because Ni is not produced but rather translocated from the soil and, contrary to organic molecules, Ni cannot be broken down or metabolised to avoid toxicity (Martens and Boyd, 1994;Boyd and Martens 1998). However, whereas the synthesis of organic toxic molecules by plants is relatively flexible in evolutionary terms, Ni accumulation is not (Cheruiyot et al 2013).…”

Local adaptation is associated with zinc tolerance inPseudomonasendophytes of the metal-hyperaccumulator plantNoccaea caerulescens

“…Metal-hyperaccumulating plants maintain exceptionally high concentrations of metals in their aerial tissues [1][2][3][4], often exceeding 1% of tissue dry biomass for elements such as zinc, cadmium, nickel and manganese, a phenotype that is thought to protect them from herbivory [5][6][7][8][9][10][11][12] and disease [13][14][15]. There is evidence that the use of metals in defence has led to an evolutionary trade-off between metal hyperaccumulation and some pathogen-induced defences [16][17][18].…”

Molecular basis of natural variation and environmental control of trichome patterning

“…Some heavy metal tolerant plants accumulate heavy metals in trichomes serving for detoxification purposes (Choi et al, 2001, 2004; Marmiroli et al, 2004; Freeman et al, 2006; Harada and Choi, 2008; Sarret et al, 2009; Quinn et al, 2010). Heavy metal loaded trichomes might contribute to elemental defense strategies (Boyd, 2012; Cheruiyot et al, 2013; Kazemi-Dinan et al, 2014). …”