Induced resistance and allocation costs: What is the impact of pathogen challenge?

Expression of plant defences is often assumed to be costly, requiring diversion of resources away from plant growth and development. However, very little information exists on the costs of induced resistance to pathogens. It is argued that since such costs are likely to be outweighed by the benefits of induced resistance, quantification of costs can only be undertaken in an environment in which the beneficial effects cannot affect fitness. For studies of induced resistance to pathogen infection, this would require plants to receive the inducing treatment, but not the subsequent pathogen challenge. However, a common feature of induced resistance to pathogens is priming. Here, although defences may be triggered by the inducing treatment, there is further enhancement of those defences, or even triggering of other defences, following pathogen challenge. It is argued that to omit pathogen challenge from attempts to quantify the costs of induced resistance will lead to an underestimation of such costs.