Os07g0100600
Os07g0100600(OsPTR1) is a member of peptide transporters which enables the directed movement of peptides, compounds of two or more amino acids where the alpha carboxyl group of one is bound to the alpha amino group of another, into, out of, within or between cells.
Contents
Annotated Information
Function
Nitrogen (N) is the most important nutrient for plant growth and productivity. Plants have evolved diverse and complex transport systems to facilitate uptake and reallocation of nitrogenous compounds. Besides uptake of inorganic nitrogen molecules such as nitrate and ammonium by specific transporters, plants also take up organic nitrogen as a nitrogen source through their corresponding transporters in the form of amino acids, small peptides and proteins.
Organic nitrogen sources (amino acids and peptides) in the soil play important roles in the nitrogen economy of grasslands, particularly in nitrogen-limited terrestrial ecosystems. There is evidence that plants can use di-/tripeptides as a nitrogen source. Peptide transporters have important roles in organic nitrogen translocation for plants. Furthermore, nitrogen remobilization from older to younger leaves is also an important process for sustaining growth of developing organs. A significant contribution for nitrogen remobilized from older parts to growing young leaves and for filling seeds has been observed in rice, tobacco, and soybean. In plants, peptide transporters have been demonstrated to be involved in recycling of organic nitrogen and regulating germination of barley, wheat, rice and maize grains from the endosperm to the embryo by physiological tests.
Peptide transport in plants is accomplished by two distinct gene families: the OPTs(the oligopeptide transporters) and the PTRs (the peptide transporters). Higher plants contain a far greater number of genes for these transporters than do other eukaryotes. This may indicate the relative importance of small peptides and their transport to plant growth and metabolism. The OPT family transports tetra-and pentapeptides, while the peptide transporter (PTR) family transports di-and tripeptides. Stacey et al. consider that an important distinction between the two families is the much more selective nature of the OPT family for peptides when compared to the PTR family. However, substrates for the vast majority of OPTs have not been discovered, and for those few that have had their substrates identified, these molecules range from diverse peptides to modified tripeptides that are bound to metals. Nine members of the OPT family have been found in the model plants Arabidopsis and rice, and some rice OPT members (OsOPTs) have been demonstrated to transport ferrous iron.
For the PTR family, the best-characterized plant PTR is the carrier responsible for mobilization of peptides across the scutellum from the endosperm to the embryo during barley grain germination. The barley scutellar peptide transporter HvPTR1 has been functionally characterized by expression in Xenopus oocytes and shown to be localized to the plasma membrane of scutellar epithelial cells. Several PTR members have also been identified from Ara-bidopsis and exhibit various functions in the plant. AtPTR5 facilitates peptide transport into germinating and possibly maturing pollen, ovules, and seeds. In contrast, AtPTR1 has a role in uptake of peptides as the sole nitrogen source by roots.
Although barley, wheat, rice, and maize demonstrate rapid uptake of peptides into the scutellum in previous physiological experiments, some differences exist among these four cereals. A large gene family of small peptide transporters exists in the Arabidopsis and rice genomes. More than 50NRT1/PTR homologs have been found in Arabidopsis, and 80 are present in rice. However, physiological functions for these genes are unknown. Here, we report the primary functions and expression patterns for eight rice PTRs(OsPTRs).
Expression
Organ-specific expression of the eight rice OsPTR members was first examined when the plants were grown under normal conditions. Various organs, including flag leaves (F), nodes and internodes (S) and panicles (F) at the pollen meiosis stage, plus roots (R) and shoots (SH; including culms and leaves) from 3-week seedlings, were used to analyze OsPTR expression by a sq-RT-PCR assay (Fig. 1A). Transcripts of OsPTR4 and OsPTR8 were detected in all tested organs and indicate constitutive expression; the other six OsPTRs showed organ-specific expression.
The expression of rice OsPTRs was also analyzed in developing seeds, including grain-filling seeds and germinating seeds. In germinating seeds having imbibed for 24 h, seven of eight OsPTRs were expressed, the exception being OsPTR5. The relative order of transcript levels, from high to low, were OsPTR2, OsPTR8, OsPTR3, OsPTR1, OsPTR7, OsPTR4 and OsPTR6 (Fig. 1B). After pollination of rice spikelets, seeds begin grain-filling. All OsPTRs were expressed in grain-filling seeds (Fig. 1C). During the grain-filling stage, expression of OsPTR1 and OsPTR5 was stable, while expression of OsPTR2, OsPTR3 and OsPTR6 in developing seeds was higher at the early stage compared to the later stage. However, expression of OsPTR4, OsPTR7 and OsPTR8 in grain-filling seeds was lower at the early stage than the later stage.
Evolution
To search for genes homologous to peptide transporters in the rice genome, the protein database of RAP-DB(Rice Annotation Project database: http://rapdb.dna.affrc.go.jp/) was searched by BLASTP with AtPTR1(At3g54140) and ScPTR2(CAA82172) protein sequences, using a cutoff E-value of e−5. The DNA database of RAP-DB was again searched using “oligopeptide transporter” as a keyword. As a result, 32 homologs were found in the rice genome sequence database after removing all duplicate sequences, and they formed three distinct groups(Fig. 2) when deduced protein sequences were used to construct a phylogenetic tree. One clade is similar to the OPT family, another clade is similar to the PTR family and the third clade is similar to neither(Fig. 2). The OPT family in rice has nine members and has been primarily studied by Vasconcelos et al. , and some of these proteins are able to transport ferrous iron. Here we describe eight rice OsPTRs of the PTR family, including their functions and expression patterns under normal or stress conditions. They are OsPTR1 (AK100112), OsPTR2 (AK068351), OsPTR3 (AK101055), OsPTR4 (AK101099), OsPTR5 (AK070216) OsPTR6 (AK101480), OsPTR7 (AK070036) and OsPTR8 (AK072691). Their basic characteristics are listed in . They are highly similar to ScPTR2, AtPTR1 and AtNRT1.
The peptide transporter family (PTR) has also been considered similar to the plant low affinity nitrate transporters (NTR1), and their members are often combined into a clustered homology group called theNTR1/PTR family. OsPTRs were compared with plant low affinity nitrate transporters (NTR1) with known functions. An unrooted phylogenetic tree was constructed by comparing amino acid sequences from OsPTR1–8 with 14 additional NTR1/PTR members. The predicted amino acid sequences of OsPTR1-8 display significant similarity to NRT1/PTRs of rice, Arabidopsis and barley (Fig. 3). Phylogenetic analysis also reveals that OsPTR1, OsPTR7 and OsPTR8 are more closely related to PTR members(HvPTR1, AtPTR1, AtPTR2 and AtPTR5) with a known di-/tripeptide transport function. By contrast, OsPTR2, OsPTR4, OsPTR5 and OsPTR6 exhibit higher similarity to NTR1 members(OsNRT1.1, BnNRT1.2, AtNRT1.1, AtNRT1.2, AtNRT1.4, AtNRT1.5, AtNRT1.6 and AtNRT1.7) with a known nitrate transport function. The eight rice OsPTR orthologs display from 30.9% (OsPTR2 and AtNRT1.5) to 87.1% (OsPTR7 and HvPTR1) sequence similarity to other NRT1/PTR family members(data not shown).
To obtain more information on the genomic organization of rice OsPTR genes, the intronexon structures for the eight OsPTRs were determined(Fig4A). According to the annotation of the rice genome, OsPTR1 and OsPTR5 do not contain introns, OsPTR2 contains two exons interrupted by one intron, OsPTR3 and OsPTR7 contain two introns, OsPTR6 contains three introns and OsPTR4 and OsPTR8 contain four introns.
The eight OsPTR cDNAs are predicted to contain an ORF encoding highly hydrophobic polypeptides (grand average hydrophobicity 0.27 to 0.57) with molecular weights between 61.9 and 66.4 kDa. The aliphatic indices of OsPTR1–8 ranged from 93.8 to 103.0, whereas their pIs ranged from 4.96 to 9.17(Table 1). They were predicted to contain 11 or 12 transmembrane domains. Hydrophobicity plots of OsPTR1-8 were similar to that of AtPTR1(Fig4B). The sizes of OsPTR predicted proteins varied from 570 amino acids (i.e.OsPTR5) to 610 amino acids(i.e.OsPTR3; Fig. 4B). Like AtPTR1, all OsPTRs possess a long hydrophilic loop between TM5 and TM6(OsPTR1, OsPTR5, OsPTR7, OsPTR8) or between TM6 and TM7 (OsPTR2, OsPTR3, OsPTR4, OsPTR6; Fig. 4B). OsPTR1, OsPTR7 and OsPTR8 contain an FYXXINXG conserved motif, termed the FYING motif. However, a second motif GXGXXXXXXXXXGXDEQF, termed the G3DQF motif,has been identified in all PTRs from rice, Arabidopsis and barley, and the function of the G3DQF domain should be further tested.
Gene Family
The rest members of peptide transporters are OsPTR2 (Os12g0638200), OsPTR3 (Os10g0470700), OsPTR4 (Os07g0603800), OsPTR5 (Os04g0597600) OsPTR6 (Os04g0597800), OsPTR7 (Os01g0142800) and OsPTR8 (Os03g0719900)
Labs working on this gene
Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, Chinese Academy of Sciences
References
Jie Ouyang, Zhaoyan Cai.Identification and analysis of eight peptide transporter homologs in rice.Plant Science, 2010, 179(4): 374-382
