References

11 AA transport in plant versus neutrotransmission 165
(Grallath et al. 2005; Rentsch et al. 1996). AtProT1isexpressedinthe
phloem and phloem parenchyma cells, whereas AtProT2 expression is
restricted to the epidermis and cortex cells of roots. AtProT3 finally is only
expressed in the aerial parts of the plant, in the epidermal cells of the leaves.
Induction of the expression of the AtProT upon salt stress implies a role
in stress adaptation or a function in organs of plants that desiccate like
pollen and seeds (Rentsch and Frommer 1996; Schwacke et al. 1999). In
mangroves, where salt concentrations are permanently high, the transport
of compatible solutes is thought to be an important factor for adaptation
to the environment (Waditee et al. 2002).
In plants nothing is known about the compartimentation of AA or the
transport steps into the vacuole. Some plant ATF1 members might fulfil
a function in import or export of solutes in vesicles and vacuoles. A similar
mechanism like the concentration of GABA in synaptic vesicles via VGAT
(Fig. 11.1) is possible for the related uncharacterized plant proteins in the
ATF family.
The plant members of the ATF family investigated so far all function as
H + -coupled systems, probably playing a role in accumulating AA within
the plant cell. Subcellular localization studies could help to identify the
transporters involved in the compartimentation of AA into the vacuole.
11.4
Conclusions and Future Prospects
Taken together, plant AA transporters can be grouped in three superfamilies
with homologs from the animal kingdom (Lalonde et al. 2004; Wipf
et al. 2002): (1) the ATF1 superfamily (SLC32, SLC36 and SLC38 and Arabidopsis
ATF1 members) (Fig. 11.3); (2) the APC superfamily (SLC7, AtCATs
and AtLATs) (Fig. 11.2); (3) AA transporters within the major facilitator
superfamily (MFS) (SLC17, homologs of unknown function in Arabidopsis
(Wipf et al. 2002).
As described, several export steps are required for AA distribution in
plants (Fig. 11.3): phloem-to-xylem transfer, release of AA into the apoplast
of the leaf mesophyll as the first step for phloem loading, unloading in sink
organs, supply of symplasmic isolated cells, e.g., growing embryo, guard
cells and pollen. Multiple possibilities exist, including vesicular transport
or carrier-mediated transport, but until now no exporter has been identified
in plants. Vesicular export is well established for AA and analogs in
mammalian nerve cells. Carriers for the active uptake of AA into secretory
vesicles have been identified (Fig. 11.1) and homologous plant genes exist
(Fig. 11.3). Furthermore, certain mammalian AA transporters are implicatedincellularexportattheplasmamembrane,especiallyfromintestinal