The physiology and molecular biology of peptide transport in seeds

Peptide transport plays a major role in the nitrogen nutrition of the cereal embryo during germination. During germination, enzymatic hydrolysis of storage proteins in the cereal grain endosperm forms a reservoir of small peptides and amino acids which are translocated across the scutellum to supply organic nitrogen to the growing embryo. Uptake of these solutes by the scutellum, a modified cotyledon which functions in nutrient transport, is mediated by carriers localized to the plasma membrane of the scutellar epithelium. To date the peptide transporter HvPTR1 is by far the best characterized example of a nutrient transporter involved in reserve mobilization during germination. Peptide transport in the barley scutellum has been relatively well-characterized biochemically, and in recent years the barley scutellar peptide transporter HvPTR1 has been cloned and characterized at the molecular level. Here, we review the physiological role and importance of peptide transport in germination, focusing on recent characterization of the barley peptide transporter HvPTR1. In barley, the uptake of small peptides by the scutellum appears to be mediated by a proton-coupled peptide transport system capable of handling peptides 2-4 amino acid residues in length.