Amino Acid Transport

xylem in the root (Pate 1983; Cooper and Clarkson 1989; Schurr and Schulze 1995).
Thus, cycling amino acids between the two vascular systems in mature leaf and root
tissue (see Fig. 1) permits dynamic adjustments of carbon and nitrogen metabolism in
response to developmental queues or environmental change. For example, the metabolic
needs of the mature root system emphasizes catabolic reactions associated with maintenance
reactions versus the anabolic reactions that drive early growth. Therefore, excess
amino acids transported to the root under these conditions can be redirected to the shoot
for subsequent incorporation into developing seed. Significantly, excess amino acid nitrogen
arriving at the root may also represent a dynamic signal that triggers the downregulation
of nitrate assimilation (Cooper and Clarkson 1989; Crawford 1995; Padgett
and Leonard 1996).
In addition to its role in primary assimilation, amino acid transport is a key process
in leaf senescence and seed germination. In rice, for example, more than 60% of the
amino nitrogen delivered to developing leaves and ears is derived from amino acids
retrieved from senescing older leaves (Mae et al. 1983, 1985; Feller and Fischer 1994).
Likewise, amino acids released from storage proteins during seed ge~nation are the
principal nitrogen source in growing seedlings (Schobert and Komor 1989). Taken together,
it seems clear that amino acids are the currency of nitrogen exchange in the plant.
Amino acids are transported into plant cell by proton-coupled symporters that link translocation
across the plasma membrane to the proton-motive force generated by the €I?-
pumping ATPase (Fig. 2; Bush 1993a). These are widely expressed carriers that appear
to be the primary pathways for amino acid transport into plant cells. However, there is
some evidence for facilitated transporters that mediate passive amino acid transport, although
little biochemical and no molecular characterization of such porters has been
published.
The amino acid symporters have been investigated in recent years using purified
membrane vesicles and imposed proton electrochemical potential differences (Bush
1993a). Purified membrane vesicles are a very useful experimental system for studying
membrane transporters. Although membrane transport activity can be examined with
intact cells, there are many complications associated with metabolism and intracellular
comp~mentation that limit experimental interpretation. Additionally, it is difficult to
dissect the bioenergetics of a transport system in intact tissues because of complex interactions
among the primary pumps, ion channels, and other unrelated porters. Isolated
membrane vesicles, on the other hand, allow one to focus on specific transport processes
while minimizing problems associated with the living cell. The major attributes of the
membrane vesicle approach include (1) unequivocal identification of membrane location
using purified vesicles, (2) elimination of posttransport metabolism and compartmentation,
and (3) the ability to control both intra- and extravesiculq solution composition
(Bush 1992). The ability to control solution co~position on both sides of the membrane
is particularly useful for investigating the bioenergetics of the transport process. With
this approach, the basic transport properties of the plant amino acid symporters have
been described (Bush 1993a).
Plant amino acid symporters are electrogenic transporters. Electrogenicity was