Trade‐offs between metal hyperaccumulation and induced disease resistance in metal hyperaccumulator plants

Metal hyperaccumulation is an unusual trait involving the uptake and storage of high concentrations of metals in the aerial tissues of plants. A number of hypotheses have been proposed to explain the evolution of the metal hyperaccumulation trait, of which the hypothesis that accumulated metal provides a defence against herbivores or pathogens has received most attention and support. Metal hyperaccumulation requires a range of physiological adaptations that enable plants to take up, transport, sequester and tolerate high concentrations of metal. Such adaptations may confer a fitness cost, and it has been suggested that metal hyperaccumulator plants may compensate for this cost by reducing investment in other traits such as induced disease resistance. This is supported by recent work that shows that metal hyperaccumulators such as N occaea spp. show reduced or altered production of key components of induced disease resistance. However, an alternative explanation exists, which is that the physiological adaptations involved in metal hyperaccumulation require alterations to, or compromise, functional aspects of plant defence mechanisms. Here, the evidence for trade‐offs between metal hyperaccumulation and disease resistance mechanisms is reviewed, and the nature of the physiological adaptations involved in metal hyperaccumulation and their potential to impact other forms of plant defence is discussed. It is suggested that defensive trade‐offs may have been key to the evolution of the metal hyperaccumulation trait, resulting in increased dependence upon the protection conferred by metals.