The OPT Family Functions in Long-Distance Peptide and Metal Transport in Plants

The OPT family was first described six years ago, and much progress has been made in understanding the role these transporters play in their respective organisms. Plants are the only organisms in which both YS- and PT-type transporters have been characterized, and all of these OPTs appear to be plasma membrane-bound proteins, suggesting that they import substrates from the apoplasm or the environment. YS1 is the only OPT known to translocate substrates from the rhizosphere, whereas all the other OPTs seem to function in long-distance transport of peptides or metals. The sum of all the studies covered in this review suggest the model for OPT function in plants depicted in Figure 4. Peptides, metal-NA, and metal-MAs complexes (Strategy II plants only) are loaded into the xylem stream in the root for long-distance transport. OPTs unload the xylem by importing substrates into sink tissues such as leaves and by transloading the phloem. Peptides and metal-NA complexes exit the leaf symplasmically or by importation into the phloem from the apoplasm by OPTs. The filial tissues (endosperm and embryo) are apoplasmically separated from the maternal tissues, and OPTs may also function in loading the developing seed. Similarly, seedlings are symplasmically disconnected from the endosperm and OPTs may help move nutrients to the growing plant. Much progress has been made in the last two years toward understanding OPTs in plants, although several fundamental questions remain unanswered. Namely, what is the level of redundancy? Is there any substrate overlap between YS and PT OPTs? How crucial are their respective roles? Are there additional functions beyond peptide and metal transport? Given the recent pace of discovery, we may not have to wait long to find out the answers.