Amino Acid Transport in Plants and Transport of Neurotransmitters in Animals: a CommonMechanism? - Communication in Plants: Neuronal Aspects of Plant Life - page 166

cells, and it has been speculated that the corresponding plant homologs
might fulfil similar functions (Wipf et al. 2002). Quite recently a mecha-
nism similar to the vesicular secretion of neurotransmitters has been shown
for yeasts. A membrane protein localized to internal membranes is respon-
sible for secretion of AA to the surrounding medium in yeasts (Velasco
et al. 2004). The recent analysis of an Arabidopsis mutant hypersecreting
glutaminefromhydathodesledtotheidentificationofanewclassofmem-
brane proteins possibly involved in this process (Pilot et al. 2004). These
proteins, which contain only a single transmembrane domain, might help
to identify the mechanism for AA export from plant cells. However, the
high number of proteins putatively involved in AA transport in plants (up
to 53; Wipf et al. 2002) as well as in animals, and the different mechanisms
prevented significant progress. To better understand the mechanisms un-
derlying the loading and unloading of AA from phloem and xylem and
the exchange between the two systems, it will be essential to analyse each
member individually.
Acknowledgements. We thank Felicity de Courcy for critical reading of
the manuscript. Our work has been funded by grants from the Deutsche
Forschungsgemeinsschaft.
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