| Peptide transport is mediated by specific membrane proteins in an energy-dependent manner and has been found to be involved in a number of biological processes such as nutrition, antibiotic and toxin absorption. Peptide transport systems have been well studied in a wide variety of organisms including bacteria, fungi, and animals, but there are only a handful of reports on their occurrance in plants. However, several lines of evidence have demonstrated that peptide transport not only exists in plants, but may also participate in important physiological processes in plants such as nutrition. The major goal of my project was to isolate and characterize the genes required for peptide transport in plants, and further elucidate the physiological role of these identified peptide transporters with regard to plant growth and development.; This document describes the isolation of two peptide transporter cDNAs, AtPTR2-A and AtPTR2-B, from an Arabidopsis thaliana cDNA library by functional complementation of a yeast peptide transport mutant deficient in ptr2, a yeast peptide transporter gene. Transfer of AtPTR2-A or AtPTR2-B to the yeast ptr2 mutant restored the ability to grow on di- and tripeptides but not peptides of four residues or longer. DNA sequence analysis showed that both AtPTR2-A and AtPTR2-B proteins are highly hydrophobic with twelve putative transmembrane domains. A search of the protein sequence data base reveals a high degree of similarity of AtPTR2-A and AtPTR2-B to the yeast peptide transporter Ptr2p, the A. thaliana nitrate transporter AtCHL1, the rabbit intestinal peptide transporter PepT1, the Candida albicans peptide transporter CaPtr2p, and Lactococcus lactis peptide transporter DtpT. Northern blot analysis showed that AtPTR2-B was highly expressed in roots, leaves, germinating seeds, stems, flowers, and siliques. AtPTR2-A mRNA could not be detected by Northern blot analysis, but was detectable in roots after reverse transcription and PCR amplification.; Transgenic Arabidopsis plants expressing antisense or sense AtPTR2-B showed that the endogenous AtPTR2-B mRNA levels was significantly reduced in transgenic leaves and flowers. Consistent with this reduction, all antisense lines and one sense transgenic line exhibited significant phenotypic changes including late flowering, arrested seed development, and increased lateral root and root hair formation. These phenotypic changes may reflect a nutritional deficiency due to reduced peptide transport mediated by AtPTR2-B, suggesting that AtPTR2-B may play a general role in plant nutrition. |
