Polar auxin transport

Polar auxin transport is the regulated transport of the plant hormone, auxin, in plants.

It is suggested that it involves the components of the cytoskeleton, plasma membrane, and cell wall. [http://www.plantphys.net/chapter.php?ch=19 Exploring the Cellular Basis of Polar Auxin Transport] ]

Chemiosmotic Model

The model is to explain the basipetal transport of auxin in the living plant cells.

Influx carrier protein, located on the top side of the cells, uptake auxin into the plant cell by both passive and active transport while the efflux carrier protein, located on the base, transport auxin out of the cell.

In the plant cells, the proton pump continuously pumps the hydrogen ion (H⁺) from the cytoplasm. The pH value of the cytoplasm is maintained at about 5; the IAA is deionized in the pH condition. As the cell membrane is not permeable to IAA, the transport of IAA required the efflux carrier protein.

Some experiments suggest that the asymmetrical development of efflux carrier protein requires the localized targeting of vesicles and the interaction with actin on cytoskeleton.

Polarity

Polarity is set up in the cell as the efflux carriers are only located on the base of the plant cells.

Pin-formed (PIN) auxin efflux facilitator is linked to the polarity of the transport. [ [http://www.plantcell.org/cgi/content/abstract/tpc.021501v1 Variation in Expression and Protein Localization of the PIN Family of Auxin Efflux Facilitator Proteins in Flavonoid Mutants with Altered Auxin Transport in Arabidopsis thaliana - Peer et al., 10.1105/tpc.021501 - THE PLANT CELL ] ]

The polarity change due to the environmental stimuli, for example, directional light source and perceived gravity vector .Gloria K Muday, Alison DeLong. (2001)"Polar auxin transport:controlling where and how much". "Trend in Plant Science" 6(11):535-542] .

Inhibitors of the transport

Quercetin (a flavonol) and Genistein are naturally occurring transport inhibitors.

NPA and 2,3,5-triiodobenzoic acid are efflux protein inhibitors.

Regulation

Phosphorylation

Polar auxin transport can be regulated by reversible protein phosphorylation; protein kinases and protein phosphatases mediate the phosphorylation and dephosphorylation respectively. A study suggests that phosphatase inhibition can alter the activities of acropetal and basipetal auxin transport. Gloria K Muday, Alison DeLong. (2001) "Polar auxin transport:controlling where and how much". "Trend in Plant Science" 6(11):535-542] .

Effects of auxin polar transport

Roots bend in response to gravity due to the regulated movement of the plant hormone auxin. In roots, an increase in the concentration of auxin will inhibit cell expansion, therefore, the redistribution of auxin in the root can initiate differential growth in the elongation zone resulting in root curvature.

References