Nicotine accumulation

Lindell G, Lunell E, Graffher H (1996) TransdermaUy administered nicotine accumulates in gastric juice, Eur J Clin Pharmacol 51(3 ) 315-318... 

Fig. 4 Current model for nicotine upregulation of a4p2 nAChRs. a Schematic of a ceU indicating major steps in the lifecycle of a nAChR. Nicotine accumulates within the cell. Within the endoplasmic reticulum, nicotine binds to nAChR subunits to facilitate assembly, or binds at the interface of an aP subunit pair to enhance maturation of a pentameric nAChR (Sallette et al. 2004, 2005). The strong influence of nicotine on maturation of the P2 subunit might also favour a change in nAChR stoichiometry, from (a4)3(P2)2 to (a4)2(P2)3 (Moroni et al. 2006). These actions could result in an increase in the membrane insertion of competent nAChRs. The possibflity of an additional action of nicotine to impede nAChR turnover or degradation is indicated by the dotted line, b Binding of nicotine to the extracellular domain of unassembled nAChR subunits facilitates assembly, c Binding of nicotine at an aP interface facilitates maturation of a pentameric nAChR. Items b and c adapted from Nashmi and Lester (2007), with permission from Elsevier...

Winz, R. A. and Baldwin, I. T. (2001). Molecular interactions between the specialist herbivore Manduca sexta (Lepidoptera, Sphingidae) and its natural host Nicotiana attenuata. IV. Insect-induced ethylene reduces jasmonate-induced nicotine accumulation by regulating putrescine A-methyltransferase transcripts. Plant Physiology 125 2189-2202. 

HAMILL, J.D., ROBINS, R.J., PARR, A.J., EVANS, D.M., FURZE, J.M., RHODES, M.J.C., Over-expression of a yeast ornithine decarboxylase gene in transgenic roots of Nicotiana rustica can leads to enhanced nicotine accumulation. Plant Mol. Biol, 1990,15, 27-38. 

Figure 9.1. Different defense strategies of Nicotiana plants that evolved in response to herbivore attack. Besides a specialized germination-behavior that efficiently reduces the over-all number of potential herbivores, Nicotiana plants evolved the inducible nicotine synthesis as a direct defense and the inducible emission of volatiles to attract parasitoids of the herbivore as an indirect defense. A specialized defense mechanism is triggered in response to attack by the herbivore Mctduca sexta, adapted to Nicotiana plants. Attack results in an ethylene burst, which down regulates nicotine accumulation and results in a fundamental transcriptional re-organization within the plant.

The correlation between nicotine accumulation and its defensive role in M sylvestris has been convincingly demonstrated. Increased alkaloid production may also be demonstrated by true herbivory. Tobacco plants subjected to leaf damage showed a fourfold increase in the alkaloid content of their undamaged leaves. This resulted from increased alkaloid synthesis and, as a result, a tenfold increase in alkaloids in the xylem. Experimental evidence has indicated that alkaloid induction maybe triggered by a phloem-translocated signal (Hartmann, 1991 and references therein). 

Once the supply of putrescine was enhanced, no larger increases in nicotine were found, presumably because other enzymes contributed, more than previously, to limiting nicotine accumulation. MPO is present at, t) ically, two-to fivefold higher levels than PMT, and therefore PMT may become limiting. Now that the pmt gene has been cloned (Hibi et al., 1994), this possibility can be tested directly. 

Nicotine biosynthesis also involves the incorporation of nicotinic acid (Fig. 2.2) (Robins et al., 1987), and the availability of this moiety can be as important in nicotine accumulation as that of the putrescine-derived portion. However, the enz)une responsible for the condensation of N-methylpyrrolinium with decarboxylated nicotinic acid, nicotine s)mthase (Friesen and Leete, 1990), was measured at only a very low level of activity, quite inadequate to account for the rates of nicotine accumulation observed in cultures. The molecular analysis of low-nicotine mutants of N. tabacum suggested the presence of regulatory genes (Me 1 and Me 2) governing the expression of nicotine bios)mthesis (Hibi et al, 1994). 

Some details of the stereochemistry of carotenoid cyclization have been elucidated. In the C40 series labelling with stable isotopes (deuterium) has been used for the first time in studies of carotenoid biosynthesis. A Flavobacterium species in the presence of nicotine accumulated the acyclic precursor lycopene (175). When the cells were washed free from the inhibitor and suspended in H20 cyclization of the lycopene proceeded, initiated by High-resolution n.m.r. 

Hamill JD, Robins RJ, Rhodes MJC (1989) Alkaloid production by transformed root cultures of Cinchona ledgeriana. Planta Med 55 354-357 Hamill JD, Robins RJ, Parr AJ, Evans DM, Furze JM, Rhodes MJC (1990) Overexpressing a yeast ornithine decarboxylase gene in transgenic roots of Nicotiana rustica can lead tq enhanced nicotine accumulation. Plant Mol Biol 15 27-38 Hamill JD, Rounsley S, Spencer A, Todd G, Rhodes MJC (1991) The use of the polymerase chain reaction in plant transformation studies. Plant Cell Rep 10 221-224 Hartmann T, Toppel G (1987) Senecionine N-oxide, the primary product of pyrrolizidine alkaloid biosynthesis in root cultures of Senecio vulgaris. Phytochemistry 26 1639-1643... 

Similar results have been published for other Solanaceae alkaloids, namely, those of the tropane group. Heine (1942) grafted Datura scions upon Nicotiana rustica and found that nicotine accumulated instead of tropane alkaloids. This finding was confirmed by Hills et al, (1946) with Nicotiana-Duboisia grafts. The tobacco scion on Duboisia root was found to accumulate tropane alkaloids. Surprisingly, the tobacco scion accumulated both hyoscine and the unesterified tropine. In all grafts between plants... 

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