Agriculture Reference
In-Depth Information
respectively, was converted to Put, but the enzymes involved in this reaction have
not been identified so far.
3 Subcellular Localization and Transport
Polyamines are present in all cell compartments and may be specially detected in
actively growing tissues where cell division or elongation takes place. Cytochemi-
cal, immunochemical, autoradiographic and subcellular fractions studies suggest
that the largest PAs reservoirs in plants are the cell wall and the vacuole (Bagni
and Pistocchi
1991
; Mariani et al.
1989
; Slocum
1991
). In addition, PAs have been
found in the cytoplasm, nucleus, plasma membrane, mitochondria and chloroplasts.
In the latter compartment, PAs are associated with components of the electron
transport chain, by both electrostatic and covalent interactions (Bagni and Pistoc-
chi
1991
; Kotzabasis et al.
1993
; Torrigiani et al.
1986
; Votyakova et al.
1999
).
The information regarding subcellular localizations of enzymes involved in plant
polyamine metabolism is scarce. Immunocytochemical and bioinformatic studies
indicate that ADC is mainly present in the chloroplast and to a lesser extent in the
nucleus (Borrell et al.
1995
; Bortolotti et al.
2004
; Illingworth et al.
2000
). Inhibi-
tor binding and fractionation studies suggest that ODC is located in the cytoplasm
and nucleus (Slocum
1991
). In plants of
A. thaliana
, ODC activity was observed
in plastid membrane (Tassoni et al.
2003
). In contrast, Spd synthase and SAMDC
activities are generally located in the cytoplasm (Slocum
1991
), whereas there is no
information on Spm synthase. As mentioned above, Cu
+
2
- and flavin oxidases oc-
cur predominantly in the apoplast, although they have also been suggested to have
cytoplasmic and vacuolar localization (Cervelli et al.
2004
; Cona et al.
2003
).
Using cell cultures, protoplasts and petals as models, it has been shown that the
transport of aliphatic amines through the plasma membrane of plant cells is bidirec-
tional, saturable, energy dependent and under hormonal control, at least for auxins
and cytokinins (Bagni and Pistocchi
1991
). It has also been demonstrated that the
transport of Put in maize roots is non-competitively inhibited by inorganic cations
(Ca
+2
, Mg
+2
) and Spm. On the other hand, the existence of at least two transport
systems, one for diamines and another for PAs has been put forward (Di Tomaso
et al.
1992
; Hart et al.
1992
). It was also proposed that the interaction between
PAs and membranes would arbitrate important cellular events, such as receptor-
mediated signal transmission. In
E. coli
, several periplasmic proteins that bind PAs
are known such as POTD and PotF, which are part of two transmembrane transport
systems (pPT104 and pPT79) and bind Spd and Put, respectively (Sugiyama et al.
1996
; Vassylyev et al.
1998
). In vascular plants like zucchini (
Cucurbita pepo
) and
maize (
Zea mays
), plasma membrane proteins that specifically bind Spd have been
identified, purified and analyzed (Tassoni et al.
1996
,
2002
). Despite being slightly
mobile cations, due to their strong interaction with cell wall components, the distant
translocation of PAs through xylematic and phloematic conducts has been demon-
strated (Antognoni et al.
1998
; Bagni and Pistocchi
1991
; Caffaro et al.
1994
).
