the heart stage of embryogenesis. The promoter^ GUS analyses showed that AAP1 was expressed in the developing endosperm and cotyledon, whereas AAP2 was restricted to the vascular strand of sili- ques. Based on the timing and localization of its ex- pression pattern, AAP1 was proposed to function in transporting amino acids into the developing endo- sperm and embryo, whereas AAP2 is particularly abundant in the vascular tissue of the stem and sili- que, suggesting a role in amino acid retrieval [20].
Environmental regulation of amino acid transport activity and gene expression also plays an important role in diกerentiating the function of these essential transport proteins. For example, although ProT1 and ProT2 are widely expressed proline-speciขc amino acid transporters in Arabidopsis, they respond differentially to changes in water and salt stress [16].
ProT2 expression was strongly induced under stress conditions whereas ProT1 expression was relatively unchanged and several AAPs were repressed. Increased proline transport capacity is consistent with the role proline plays as a compatible solute under
water stress conditions. In addition to this environmental response, the expression of the tomato LeProT1 orthologue is restricted to the pollen and it appears to play a role in pollen maturation and germination. Signiขcantly, 70% of the amino nitrogen in
tomato pollen is proline, suggesting it plays an initial
role as a compatible solute and later as energy source for tube elongation [22]. A pollen-speciขc amino acid transporter has also been identiขed in Nicotiana syl- vestris [23].
Recent studies have shown that many important aspects of carbon and nitrogen metabolism are regu- lated by dynamic changes in C/N ratios where de- creases in C or N resources up-regulate genes in- volved in their acquisition while abundance of these resources induces genes associated with use and stor- age [24^26]. Nitrogen assimilation in Arabidopsis, for example, is regulated by changes in metabolic status. Light and sugars, which both increase C/N balance, up-regulate the expression of genes involved in am- monia assimilation into glutamine and glutamate by chloroplastic GS (GLU1) and Fd-GOGAT (GLN2), while they repress AS (ASN1) and GDH expression [25]. In dark-adapted plants, however, carbon skele- tons are less abundant (low C/N balance) and ASN1 gene expression is induced and there is a concomitant increase in asparagine levels observed in the phloem exudate [24]. Amino acid transporter gene expression also appears to be linked to the metabolic status of the plant C/N balance.
AAP1 (also known as NAT2) gene expression is regulated by light and carbon status. AAP1 tran- script abundance in leaf tissue increases within 6 h in dark-adapted plants exposed to light. Likewise,