The ecological concept of costs of induced systemic resistance (ISR)

In: European Journal of Plant Pathology, Jg. 107 (2001), S. 137-146
ISSN: 0929-1873
Zeitschriftenaufsatz / Fach: Biologie
Plant defence is thought to provide benefits for the defended plants. Theoretical concepts must, therefore, explain
why there is variation in defensive traits, which naively might be assumed to be present constitutively in fixed
high amounts. Explanations are mainly based on the assumption of fitness costs. Investment in defence is thought
to reduce the fitness of plants in enemy-free environments. Fitness costs often result from allocation costs, i.e.
allocation of limited resources to defence, which then cannot be used for growth or other fitness-relevant processes.
This theoretical concept can provide a useful tool for the interpretation of induced plant responses against pathogens,
named induced systemic (or systemic acquired) resistance (ISR or SAR). Phenotypic plasticity, leading to induced
responses, might have evolved mainly to reduce costs, since investment in defence is restricted to situations actually
requiring defence. ISR can incur allocation costs and other, indirect costs, which ultimately may lead to fitness
costs. Evolution of any defensive trait depends on both what a plant ideally ‘should do’ and what it actually ‘is able
to do’. Costs of defence constrain its expression. This might have important influences on the evolution of plant
defensive traits, as well as on the exploitation of natural defences in agricultural crop protection.