Polar auxin transport is the regulated transport of the plant hormone auxin in plants. It is an active process, the hormone is transported in cell-to-cell manner and one of the main features of the transport is its asymmetry and directionality (polarity). The polar auxin transport functions to coordinate plant development; the following spatial auxin distribution underpins most of plant growth responses to its environment and plant growth and developmental changes in general. In other words, the flow and relative concentrations of auxin informs each plant cell where it is located and therefore what it should do or become.
Chemiosmotic model
Polar auxin transport (PAT) is directional and active flow of auxin molecules through the plant tissues. The flow of auxin molecules through the neighboring cells is driven by carriers (type of membrane transport protein) in the cell-to-cell fashion (from one cell to other cell and then to the next one) and the direction of the flow is determined by the localization of the carriers on the plasma membrane in the concerned cells. The transport from cell to the neighboring one is achieved through relatively complex combination of several sub-processes. To explain the mechanism behind unique character of auxin transport through living cell files of the plant, the so-called chemiosmotic model was formulated. The mechanism was first proposed in the 1970s by Ruberry and Sheldrake and this visionary prediction was finally proven in the 21st century. The mechanism below describes the process in which auxin is trapped in the cell by the so-called acid trap and how it can then leave the cell only by activity of specific carriers, which control the directionality of the flow from cells and generally the direction of auxin transport through the whole plant body.
Acid trap
As weak acids, the protonation state of auxins is dictated by the pH of the environment; a strongly acidic environment inhibits the forward reaction (dissociation), whereas an alkaline environment strongly favors it (see Henderson-Hasselbalch equation): The export of auxins from cells is termed auxin efflux and the entry of auxin in to cells is called auxin influx. The first step in polar transport is auxin influx. Auxin enters plant cells by two methods, first by passive diffusion as non-ionized protonated indole-3-acetic acid (IAAH) across the phospholipid bilayer, or second by active co-transport in the anionic form IAA−. As IAAH is lipophilic, it can easily cross the lipid bilayer.
IAAH ⇌ IAA− + H+, where IAAH = indole-3-acetic acid; IAA− = its conjugate base The inside of cells (pH ~ 7) is less acidic than the outside (the apoplast; pH ~ 5.5). So outside the cell a significant portion (17%) of the IAA molecules remain un-dissociated (proton-associated). This portion of auxin molecules is charge-neutral and therefore it is able to diffuse through the lipophilic lipid bilayer (lipid bilayer being constituent of cell membrane) into the cells. Once through the bilayer in the cell, the molecules are exposed to the more basic pH of the cell interior, and there they dissociate almost completely, producing anionic IAA−. These chemically polar ions are unable to passively diffuse across the cell membrane and remain trapped inside the cell.
Polarity of auxin export Once inside the cell, auxin cannot leave the cell on its own by crossing the lipid bilayer. Hence the export of auxin from the cell requires an active transport component in the plasma membrane - i.e. some membrane transport protein. Two protein families: The PIN proteins and ABCB (PGP proteins) transporters function as "auxin efflux carriers" and transport the anionic form of auxin out of the cell. While the PGP auxin efflux carriers are evenly distributed, the PIN proteins normally maintain polar (i.e. asymmetric) localisation on the plasma membrane. That is to say they are most concentrated on one side of the cell. Furthermore, the asymmetrical localisation of the PIN proteins is coordinated between neighbouring cells. As a result, the PIN proteins generate a directional flow of auxin at the tissue and organ scale. This PIN-generated flow is called auxin polar transport. For example, the cells located in the vasculature (at the center) of the root all show PIN1 proteins on their basal membrane only (i.e. on their lower side). As a result, in the root vasculature, auxin is transported directionally from the shoot to the root tip (i.e. downwards).
Role in plant development
Self-organisation of polar auxin transport See also "Uneven distribution of auxin" and "Organization of the plant" in the main Auxin article Auxin plays a central role in PIN protein polarity establishment. The regulation of PIN localisation by auxin creates a feedback loop where PIN proteins control the directionality of auxin fluxes, and auxin in turn controls PIN proteins localisation. These interactions between auxin and its own transporters confer to the system self-organizing properties, which explains for instance phyllotaxis (the regular and geometrical arrangements of lateral organ along the stem), the formation of leaf serrations, and the formation of vascular strands. This positive feedback regulation auxin on its own transport also plays an essential role in vascular development, which process is called canalization. PIN proteins are so named because mutant plants lacking the founding member of this family, PIN1, cannot develop flowers. The formation of flowers is triggered by regularly spaced local auxin accumulation at the surface of the shoot apical meristem and, for this PIN1 is required. As a result, the pin1 mutant plants produce a "pin-like" inflorescence consisting only of a naked stem. This highlights the importance of polar auxin transport in plant development.
… excerpt ends here. Continue reading the full article.
