Allocation of 15N from Nitrate to Nicotine: Production and Turnover of a Damage‐Induced Mobile Defense

Abstract

Nicotine is thought to be an excellent example of a "mobile" (sensu Coley et al. 1985) plant defense metabolite: it is synthesized in the roots, transported to leaves, and reported to be metabolically labile, with a half-life of <24 h. In a companion paper (Ohnmeiss and Baldwin 1994), we demonstrated that nondestructive damage dramatically increased the whole-plant allometric accumulation of nicotine in Nicotiana sylvestris (Solanaceae) and that damaged plants accumulated a larger proportion of their total nitrogen in nicotine pools than did undamaged plants. These damage-induced accumulations could arise from either increased nicotine production and/or decreased turnover with unaltered production. Here we ask how much of the change in whole-plant nicotine pools that is induced by damage is a result of increased nicotine production, with nitrogen derived from 15 N0 3 that was acquired, reduced, and assimilated after damage. In three experiments, we examine the rates of induced and constitutive nicotine production over 2 d, over 8 d under two nitrate supply rates, and over the lifetime of the plant. By examining 15 N-labeled nicotine pool sizes at different times after damage, we estimate the magnitudes of nicotine turnover and its importance in the induced changes in whole-plant nicotine pools.Two days after damage, damaged plants had accumulated 4.8 times the nicotine pool of undamaged plants, and 57% of the increase in the nicotine pool of damaged plants was synthesized with N from 15 N0 3 acquired after damage. Damage increased the rate of nicotine-N 15 production from 2.0 J.tg/h in undamaged plants to 6.3 J.tg/h in damaged plants.No evidence for nicotine turnover was found in either damaged or undamaged plants. The 8-d experiment conducted under two different nitrate supply rates confirmed these results. Furthermore, the 8-d experiment documented that N from 15 N0 3 acquired after damage was allocated to nicotine production in constant proportions within damaged and undamaged plants, independently of nitrate supply rate; 3.0-3.1 and 5. 7-6.1% of the total 15 N pool in undamaged and damaged plants, respectively, was used for nicotine production. Therefore, leaf damage doubles the allocation of recently acquired nitrogen to nicotine production in plants that do not differ in nitrate uptake or growth. In an experiment lasting 41 d after damage, we found that the damage-induced increase in total nicotine pools quantified after 5 d remained unchanged over the lifetime of the plants and that in undamaged plants the nicotine-' 5 N pool produced after 5 d was similarly unchanged at 41 d. In damaged plants the nicotine-15 N pool produced after 5 d decreased by 33.3% after 41 d. This decrease in the nicotine-15 N pools of damaged plants could be due to either metabolism or volatilization.We conclude that nicotine is not the metabolically labile secondary metabolite that it has been previously reported to be and that the changes in pool sizes induced by damage and nitrogen stress are principally due to changes in prod...

Tobacco nicotine uptake permease (NUP1) affects alkaloid metabolism

“…Nicotine is not appreciably degraded in planta (19), and whole-plant nicotine levels are held at MeJA-responsive allometric set points (20,21). Nicotine exhibits differential shoot accumulations over the course of plant development and seems to be mobilized by source-sink movement in the phloem (22,23).…”

Jasmonate-induced responses are costly but benefit plants under attack in native populations

“…In laboratory feeding trials, induced levels of nicotine protect plants against nicotinetolerant herbivores (16), but these herbivores may suffer lower rates of parasitism when feeding on plants with high nicotine concentrations (17), indicating that this induced defense may have both ecological benefits and costs. Moreover, because 6% of the total nitrogen content of an induced N. attenuata plant resides in this toxin alone (13,18), this nitrogen is unavailable for other activities such as seed production (18,19), suggesting that inducing nicotine production may incur large resource-based costs. Hence, it is reasonable to suppose that the fitness consequences of producing this toxin will vary greatly depending on a plant's habitat.…”

Diversity and variability of plant secondary metabolism: a mechanistic view

“…This result contrasts with studies reporting the half-life of nicotine as < ld (Robinson, 1974). But the high estimates of nicotine turnover are derived from studies in which labelled nicotine was introduced to the plant, rather than its biosynthetic precursors, and hence the high rates may reflect nicotine detoxification or other types of salvage metabolism, rather than normal nicotine turnover (Baldwin et al, 1994). These examples by no means categorically exclude the occurrences of turnover in secondary metabolism.…”

“…The only way to actually document turnover is to follow the fate of the endogenously synthesized secondary metabolites. Two of the most prominent and often quoted examples for 'turnover' later turned out to be artefacts: (1) monoterpenes synthesized with labelled 14CO2 in detached mint shoots exhibit pronounced turnover, but in the same experiment performed with rooted plants the labelled monoterpenes synthesized remain stable for at least 40 days (Mihaliak et al, 1991); (2) experiments with endogenously produced nicotine usinglSNO3 as biosynthetic precursor provided no evidence for nicotine turnover in Nicotiana sylvestris (Baldwin et al, 1994). This result contrasts with studies reporting the half-life of nicotine as < ld (Robinson, 1974).…”