Book Description

Since the first postulation of auxin function by the Darwins, many other plant hormones have been identified and most of them have been found to be synthesized at different sites from their places of action. Hormone transport and thus the responsible hormone transporters are therefore essential for a precise regulation of plant hormone action, which has been repeatedly supported by severe developmental and physiological phenotypes reported for hormone transporter loss-of-function mutants. Plant transporters have been shown to be involved in short and long-distance transport of hormones. Short-distance transport between cells seems to be sufficient for a local hormone action in some tissues (such as seeds), which seem to require exporter and importer proteins in adjacent cells as shown for example for abscisic acid. During long distance transport with the transpiration stream or in the phloem, demonstrated for many (but not all) plant hormones (including auxins, abscisic acid, cytokinins, gibberelins, strigolactones, and salicylic acid), transporters are thought to function in loading and unloading processes. Similarly, in cases where long-distance transport is achieved by cell-to-cell transport (such as for auxins), the highly coordinated action of import and export transporters at the contact surfaces of neighboring cells is apparently needed, however, all these processes are far from being understood on the molecular level. Currently, it appears that many hormones are transported by members of distinct transporter classes, ranging from primary active pumps (that couple hormone translocation to direct ATP hydrolysis), antiporters and symporters (that use the proton motive force to create hormone concentration gradients), and to facilitators. Among those, the ATP-binding cassette (ABC) family and the Nitrate transport1/ Peptide transporter family (NPF) seem to be dominant but currently it is unclear how individual transporters cooperate to achieve a systemic level of transport. Furthermore, in most cases several pairs of importers and exporters are required but how these are correct allocated in order to guarantee the function of a complex hormonal network is unknown. While remarkable progress has been made on hormone transporter regulation on the transcription and post-transcriptional level for transporters involved in long-distance transport (such as auxin), regulation of transporter trafficking, stability and activity is less understood for other hormones.