Quantification of growth–defense trade‐offs in a common currency: nitrogen required for phenolamide biosynthesis is not derived from ribulose‐1,5‐bisphosphate carboxylase/oxygenase turnover

Summary Induced defenses are thought to be economical: growth and fitness‐limiting resources are only invested into defenses when needed. To date, this putative growth–defense trade‐off has not been quantified in a common currency at the level of individual compounds. Here, a quantification method for 15 N–labeled proteins enabled a direct comparison of nitrogen (N) allocation to proteins, specifically, ribulose‐1,5‐bisposphate carboxylase/oxygenase (RuBis CO ), as proxy for growth, with that to small N‐containing defense metabolites (nicotine and phenolamides), as proxies for defense after herbivory. After repeated simulated herbivory, total N decreased in the shoots of wild‐type ( WT ) Nicotiana attenuata plants, but not in two transgenic lines impaired in jasmonate defense signaling (ir LOX 3 ) and phenolamide biosynthesis (ir MYB 8 ). N was reallocated among different compounds within elicited rosette leaves: in the WT , a strong decrease in total soluble protein ( TSP ) and RuBis CO was accompanied by an increase in defense metabolites, ir LOX 3 showed a similar, albeit attenuated, pattern, whereas ir MYB 8 rosette leaves were the least responsive to elicitation, with overall higher levels of RuBis CO . Induced defenses were higher in the older compared with the younger rosette leaves, supporting the hypothesis that tissue developmental stage influences defense investments. We propose that MYB 8, probably by regulating the production of phenolamides, indirectly mediates protein pool sizes after herbivory. Although the decrease in absolute N invested in TSP and RuBis CO elicited by simulated herbivory was much larger than the N‐requirements of nicotine and phenolamide biosynthesis, 15 N flux studies revealed that N for phenolamide synthesis originates from recently assimilated N, rather than from RuBis CO turnover.