Laboratory and ®eld tracer experiments with 14 C-labelled senecionine N±oxide (SO) and distant biosynthetic precursors such as [ 14 C]putrescine revealed that pyrrolizidine alkaloid N-oxides (PAs) in Senecio vernalis Waldstr. & Kit. (Asteraceae) show no signi®cant turnover over periods of up to 29 d. However, PAs are spatially mobile, they are continuously allocated, and labelled PAs are even detectable in leaves and capitula developed weeks after tracer application. Chemical diversi®cation of SO, the common product of PA biosynthesis in roots, was studied in ®ve Senecio species (i.e. S. vernalis Waldstr. & Kit., S. vulgaris L, S. inaequidens DC, two chemotypes of S. jacobaea L. and S. erucifolius L.). Tracer experiments revealed that shoots are capable of transforming [ 14 C]SO into the unique species±speci®c PA patterns. Within a plant, the transformation eciency of SO can vary quantitatively and qualitatively between shoot organs (i.e. leaves, stems and in¯orescences). All transformations proceed position-speci®cally and stereoselectively. They comprise simple one-step or two-step reactions such as hydroxylations, epoxidations, dehydrogenations, and O-acetylations, as well as the more complex conversion of the retronecine into the otonecine base moiety (e.g. SO into senkirkine). Taking all the evidence together, the qualitative and quantitative composition of the Senecio PA pattern is a dynamic and sensitive equilibrium between a number of interacting processes: (i) constant rate of de-novo synthesis of SO in roots, (ii) continuous longdistance translocation of SO into shoots, (iii) eciency of SO transformations which may vary between plant organs, (iv) continuous allocation of PAs in the plant, and (v) eciency and tissue selectivity of vacuolar storage. We suggest that in constitutive plant defence, without signi®cant turnover of its components, such a highly plastic system provides a powerful strategy to successfully defend and possibly escape herbivory.
Uptake of abscisic acid by 5 mm long decapped root tips is a linear function of the external ABA concentration in the range of 2.9 × 10-8m to 10-4м and decreases dramatically with increasing pH. At pH 8.0 uptake rate is extremely low, even at high ABA concentrations. This indicated that nearly all of the ABA is taken up as the undissociated molecule ABAH. Uptake of ABA is influenced by agents modifying the pH gradients between the medium and the tissue such as salts of weak acids incubated at low external pH (inhibition of uptake and stimulation of ABA release by abolishing the pH gradients), protonophores such as CCCP (inhibition of uptake) and fusicoccin (stimulation of uptake by increasing the pH between medium and cytoplasm). It is concluded that ABA distributes between the compartments of the root cells according to the pH gradients with the undissociated molecule as the only penetrating species. Uptake and release occur without participation of a saturable component by diffusion. In contrast IAA permeates the plasmalemma as both IAAH and IAA-.
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