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Fig. 11.4. AA transporters in plants. Although a wide spectrum of proteogenic AA accumu-
late in phloem (100-200 mM) and xylem sap (about tenfold lower concentration than in
phloem), amines and AA predominate. The actual intracellular, apoplasmic, and phloem
composition of AA is thought to be similar, thus the relatively unspecific AA permeases
( AAP )mightberesponsibleforfunctionslikeuptakeofAAfromthexylemintothecelland
phloem loading. No organellar transporter has been identified to date. (Reprinted Wipf et
al. 2002, with permission from Elsevier)
showed that the members of the AAP family are associated with the vas-
cular tissue or the reproductive organs (Hirner et al. 1998; Okumoto et
al. 2002, 2004). The properties of AAP described thus match the activities
expected for transport proteins involved in moving a wide spectrum of
AA into the phloem sap and are consistent with the properties described
for uptake kinetics measured in plasma membrane vesicles of leaves (Li
and Bush 1990a, b). Antisense repression of a related AA transporter from
potato, St AAP1, expressed in source leaves led to reduced concentration of
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