References

164 T. Müller, W. Koch, D. Wipf
AAinpotatotubers.Thisreducedtransporttosinkorganssupportsthehypothesis
of a role in phloem loading for some of the AAP (Koch et al. 2003).
Besides long-distance transport the delivery of reduced nitrogen to the
reproductive organs is a key factor for seed development. The import of AA
is an essential prerequisite for seed development since the accumulation of
storage proteins must be preceded by AA import. It could be shown that the
expression of the AA transporter in seeds indeed precedes the expression
of the storage protein AtS1 in Arabidopsis (Hirner et al. 1998). A similar increase
in the expression of a legume AA transporter has been shown in Vicia
faba (Miranda et al. 2001). In pea, it has been demonstrated that PsAAP1 is
expressed in the transfer cell layer of cotyledons and might be responsible
for taking up AA released from the seed coat (Tegeder et al. 2000). Ectopic
overexpression of an AAP in pea seeds increases AA uptake and increases
storageproteincontentby40–50%.Theresultssuggestthatnitrogensupply
limits protein synthesis and storage in the seed (Rolletschek et al. 2005).
From the second plant-specific branch of the ATF family, the LHT
(Fig. 11.3), two members have been functionally characterized so far.
AtLHT1 has been described as a transporter specific for lysine and histidine,
but other AA are also recognized (Chen and Bush 1997). LHT1
is present in all tissues, with expression being strongest in young leaves,
flowers and siliques. Recent characterization of a related protein, LHT2,
shows that this transporter is more likely to be a general AAP with a high
affinity for proline (Km ∼ 10 µM) and aspartate (Km ∼ 72 µM). Inhibition
studies indicate potentially similar affinities towards most AA except the
basic AA histidine, lysine and arginine (Lee and Tegeder 2004). Expression
of the gene in the tapetum tissue of the flower suggests a role in the transfer
of organic nitrogen to the tapetum tissue and in supplying the pollen with
nutrients. The affinity of LHT2 towards proline is a factor of 10 higher
than the affinity of AAP6 and is, together with the high affinity of CAT5
for arginine, among the highest determined for AA transporters in plants.
AA concentrations in phloem, xylem, and supposedly in mesophyll cells,
are highly variable depending on carbon supply, inorganic nitrogen and
light. Thus, it makes sense that the relatively immobile plants can respond
to variations in AA concentrations with transporters that have different Km
values and transportation speed.
ANT1, the only member characterized of a more distantly related branch
(Fig. 11.3) to the AAP and LHT mediates the transport of aromatic and
neutral AA (Chen et al. 2001). The substrate specificity found in the third
plant-specific branch of the ATF-family, the proline transporters (ProT;
Fig. 11.3) is different. In contrast to the plant carriers just described, ProT
preferentially transport the compatible solutes proline (Km values of 0.427,
0.500 and 1 mM for AtProT1–3, respectively), betaine (Km values of 0.115,
0.267 and 0.290 mM for AtProT1–3) and GABA (4.5, 4.01 and 5.12 mM)