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44. | Elucidation of the relative contribution of the two alternative pathways for polyamine biogenesis in plants suggests a key role for the putrescine pool in controlling flux to the higher polyamines |
TERESA CAPELL, OLIVIA LEPRI, LUDOVIC BASSIE, PHAM TRUNG-NGHIA, PHAM THU-HANG, GEHAN
SAFWAT and PAUL CHRISTOU Molecular Biotechnology Unit, John Innes Centre, Norwich Research Park, Colney Lane, Norwich, NR4 7UH, United Kingdom |
Polyamines (PAs) are a group of low molecular weight polycationic compounds
that are ubiquitous in nature and important for many physiological, biochemical
and developmental processes (Smith 1985). Current views about the plasticity
of the PA pathway in plants are conflicting, with one school of thought
supporting the notion that the pathway is rigidly controlled and as a
result it is unlikely that any changes in end product accumulation can
be achieved by modulating levels of enzyme activity in the pathway (Burtin
and Michael 1997). An alternative view which is increasingly supported
by more in depth molecular and biochemical studies using transgenic plants
suggests that the pathway can indeed exhibit plasticity, however this
depends on spacial control of expression consistent with a threshold model
in terms of levels of accumulation of the first PA, Put, in specific tissues
(Bassie et al. 2000). Most profound changes appear to take place
in storage tissues such as seeds, followed by roots with the least effect
seen in vegetative tissues (Thrun- Nghia et al. unpublished). The
simplest PA, putrescine (Put), is derived from ornithine by ornithine
decarboxylase (ODC). Putrescine may then be converted into the longer
aliphatic PAs spermidine (Spd) and spermine (Spm) by spermidine and spermine
synthases respectively. Plants provide an interesting eukaryotic system
for investigating the physiological and biochemical role of PAs because
an alternative route to Put via arginine decarboxylase (ADC) is present.
Ornithine decarboxylase converts ornithine to Put directly whereas ADC
uses arginine as substrate to also generate Put via agmatine and Ncarbamoylputrescine
(Smith 1985). increase in Spd and Spm accumulation in leaves (Fig. 4A). We postulate
that the increases in Spd and Spm are consistent with the notion that
the transgenic Put pool in leaf tissue needs to reach a minimal level
before excess Put may be channeled into the biosynthesis of the higher
PAs. However, no linear correlation could be demonstrated between increases
in ODC activity in leaf or root tissue and end product accumulation in
seeds.
orientation) and odc, we conclude that ODC is the
main enzyme responsible for Put biosynthesis in plants.
References |
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