Amino acid transporters are localized to transfer cells of developing pea seeds

To determine the nature and cellular localization of amino acid transport i n pea seeds, two cDNA clones belonging to the AAP family of H+/amino acid c o-transporters (PsAAP1 and PsAAP2) were isolated from a cotyledon cDNA libr ary of pea (Pisum sativum L.). Functional expression in the yeast amino aci d uptake mutants 22 Delta 6AAL and 22 Delta 8AA showed that PsAAP1 mediates transport of neutral, acidic, and basic amino acids. RNA-blot analyses sho wed that PsAAP1 is expressed in seeds and vegetative organs, including amin o acid sinks and sources, whereas PsAAP2 could not be detected. For develop ing seeds, transcripts of PsAAP1 were detected in coats and cotyledons, wit h seed coats giving a weak signal. In cotyledons, expression was highest in epidermal-transfer-cell-enriched tissue. RNA in situ hybridization analysi s showed that PsAAP1 was predominantly present in epidermal transfer cells forming the outer surface of cotyledons, which abuts the seed coats. Overal l, our observations suggest that this transporter, which is localized in tr ansfer cells of cotyledons, might play a role in the uptake of the full spe ctrum of amino acids released from seed coats.