Table 1.

Properties of some plant peptide transporters (PTRs)

Transporter	Species	Substrates	Expression pattern localization, activity	Mutant phenotype	Role
AtPTR1	Arabidopsis	Di- and tripeptides, phaseolotoxin	Plasma membrane in vascular tissues, seed germination, developing siliques, root tips		Seed development, reserve mobilization during germination, acquisition of soil nutrients
AtPTR2	Arabidopsis	Di- and tripeptides	Flower, leaf, stem	Delayed flowering, larger rosette leaves, reduced seed number, larger seeds	Seed development
HvPTR1	Barley	Di- and tripeptides	Scutellum of germinating grain, developing grain; activity controlled post-translationally		Redistribution of organic nitrogen from endosperm to embryo during germination, seed development?
VfPTR1	Faba bean	Dipeptides	Cotyledons during seed development and germination; axes, roots and root hairs of seedlings; dipeptides repress expression in roots		Redistribution of organic nitrogen in seed germination and development, nutrient uptake from soil
NaNTR1	Nepenthes	Not done	Phloem cells at base of pitchers, leaves, petioles		Transport of organic nitrogen from pitcher to sink tissues (phloem loading of peptides)

The known properties of five plant PTRs are compared. The substrates transported by these PTRs represent those identified in studies when the PTR proteins are produced in heterologous yeast or Xenopus oocyte expression systems. Consequently, these substrates represent only a minimal number of possible di- and tripeptide substrates that would be available to the PTR in planta. Expression patterns represent a summary of the tissues or developmental stages at which specific PTR transcripts have been detected in studies using either Northern analyses or in-situ hybridization approaches.