IMMUNOLOGICAL MEMORY IN THE INDUCED ACCUMULATION OF NICOTINE IN WILD TOBACCO

Nicotine, an inducible defense in Nicotiana sylvestris, is produced in the roots and transported throughout the plant after leaf damage. Roo t nicotine production is activated by an endogenously produced signal, jasmonic acid (JA), which is synthesized in response to leaf damage a nd either directly through transport or indirectly via another signal, increases JA pools in the roots. The addition of JA or its methyl est er (MJ) to the roots of hydroponically grown plants stimulates nicotin e production in the same way that leaf damage does. In this paper we u se MJ to stimulate the induced defense without damaging plants in orde r to examine the hypothesis that these plants have an immunological '' memory'' of prior inductions. Such a ''memory'' can be measured as a c hange in either the speed or the amount of induced nicotine production or accumulation that results from prior inductions. ''Memory'' is an important component of the induced defenses of animals and is predicte d to evolve in systems where an initial attack is a reliable predictor of future attacks. We induced 200 plants 1, 2, or 3 times during an 1 8-d period of rosette growth, allowed 6 d between inductions for the r elaxation of the response between inductions and quantified changes in biomass, whole-plant nicotine pools and de novo rates of nicotine pro duction from N-15-labeled nitrate acquired at the time of induction. I nduced changes in nicotine pools and the rates of nicotine production varied across the three induction periods: the highest rates and induc ed pools occurred during the second induction period, but the highest concentrations were found during the first induction period. The rates of de novo nicotine production from N-15-labeled nitrate were dramati cally increased by MJ stimulations during all three induction periods. We found no evidence for alterations in the rates of nicotine product ion in plants not currently exposed to MJ; 6 d after an induction, pla nts had relaxed their rates of nicotine production to levels that were not significantly different from those found in plants without a prio r induction history. Moreover, the maximum pools of N-15-labeled nicot ine were equivalent among all induced plants regardless of prior induc tion history. In summary, one or two prior inductions did not markedly affect nicotine production from recently acquired nitrate as measured in either induced or uninduced plants. However, the ecologically more relevant measure of the speed of induction is the change in whole-pla nt nicotine pools, and plants with one and two prior inductions increa sed their whole-plant nicotine pool significantly faster during the th ird induction period than did plants not previously induced. Plants wi th two prior inductions attained significant increases in their nicoti ne pools 2 d earlier than did plants with one or no prior inductions. Moreover, the effect was additive: plants with two prior inductions we re faster than plants with one. The effects of prior inductions on the speed of the induced response did not translate into increases in the magnitude of the response at the end of the 18-d experiment. The lack of change in the magnitude of the induced response may reflect the me thods used in our experiment more than the plant's capabilities. The w hole-plant induced response is limited by the amount of signal transpo rted to the roots, and since we used the same amount of MJ in all stim ulations, our experiment would not detect ''memory'' in the production of the signal in the damaged leaf or its transport to the roots. Thes e results demonstrate that plants, like animals, alter their induced d efense in response to their prior experiences.