Nondefense functions of plant specialized metabolites: ecological phenomena, molecular mechanisms, and evolutionary implications

Frameworks for studying plant chemistry often assume that defense chemical evolution is shaped primarily by herbivores. However, there is mounting evidence that defense chemical variation also influences a number of nondefense functions and interactions. These correlated effects are widespread and can arise from a diversity of underlying molecular mechanisms, ranging from pleiotropy to shared regulation to correlated past natural selection. Different molecular mechanisms underpinning correlated function result in different levels of genetic constraint on the evolution of multifunctional metabolites. Understanding defense chemical evolution thus requires: quantifying dual- or multifunctional effects of plant specialized metabolites; elucidating the genetic basis of correlated function; and assessing the consequences of multiple functions for natural selection.