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Fig. 1. Testing a classical ecological theory with a biochemical tool. The optimal defense theory posits that the distribution of chemical defenses within a plant is based on the relative fitness value of plant parts. In wild-type A. thaliana, glucosinolates are present in much higher concentration in young leaves (y) than in mature (m) or old (o) leaves, due to the transport of glucosinolates out of their sites of biosynthesis in the old and mature leaves to young leaves, where they accumulate (A). To test whether this pattern confers fitness benefits, Hunziker et al.(1) employ a double glucosinolate transporter mutant in which the concentration of glucosinolates is the same in all leaf age classes (B). On wild-type plants, caterpillars fed only on older leaves, with little effect on plant survival to reproduction. However, on the transporter mutant, feeding occurred only on younger leaves, leading to premature death, and showing the …