Metals as toxins in plants

Accumulating toxic substances from the environment and incorporating them into body tissues is a familiar strategy for preventing predation in insects such as the Monarch butterfly. This review explores the possibility that certain plant species, known as hyperaccumulators, employ the same strategy when they accumulate high levels of metals from their environments. Accumulation of toxic levels of selenium, nickel, copper, manganese, zinc, chromium and other metals in plant tissues, either as unutilizable metabolites, as chelates or isolated in subcellular structures, has been well documented. What is lacking is clear evidence that the practice of hyperaccumulating metals succeeds in reducing predation (herbivory). This may be due to the fact that, with the possible exception of selenium accumulators, these plants have failed to evolve unequivocal mechanisms for signalling the presence of their toxic contents.