Polar auxin transport
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
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Polar auxin transport (PAT) is directional and
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.[1][2][3][4] The mechanism was first proposed in the 1970s by Ruberry and Sheldrake[1][5] and this visionary[5] 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
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
IAAH
⇌IAA− + H+
, whereIAAH
= 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%)[4] 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.[4] 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,[4] producing anionic IAA−. These chemically polar ions are unable to passively diffuse across the cell membrane and remain trapped inside the cell.[4]
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
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
PIN proteins are so named because
Tropisms
Other external and internal signals (e.g. blue light, mechanical stress, gravity or
For instance, the regulation of polar auxin transport is central in a process such as
Similar mechanisms occur in other tropic responses, such as
Generation of morphogenetic gradients
Polar auxin transport is required for the generation of auxin gradients throughout the plant body.
Regulation
Although the detailed molecular mechanism of PIN proteins polarity establishment remains to be elucidated, many endogenous and exogenous regulators of PIN proteins localisation have been characterised.
Auxin
Most importantly, PIN proteins localisation on the plasma membrane is controlled by auxin. Several mathematical models making different assumptions on the way auxin influences PIN localisation explain different observations. Some models assume PIN proteins polarize towards the neighbouring cell containing the highest cytosolic auxin concentration. These models are called "up-the-gradient" models and explain for instance phyllotaxis. Other models assume that PIN proteins localise on the side of the cell where the efflux of auxin is the highest. These models are called "with-the-flux" models and explain the formation of vascular strands in leaves.
The molecular mechanism responsible for these different behaviours of the system (with-the-flux and up-the-gradient) is not yet fully understood. Noticeably, an auxin receptor protein called ABP1 is thought to play a potentially significant role in the control of PIN proteins polarity by auxin.
Mechanical stress
Mechanical signals have been proposed to regulate PIN polarity.
Vesicle Trafficking
The
Inhibitors of the transport
In research, 1-N-Naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA) are used as specific inhibitors of the auxin efflux.[9]
9-Hydroxyfluorene-9-carboxylic acid (HFCA), TIBA, and trans-cinnamic acid (TCA) are also example of Polar Auxin Transport Inhibitors. They prevent the development of the bilateral growth of the plant embryo during the globular stage. All 3 inhibitors induce the formation of fused cotyledons in globular but not heart-shaped embryo.[citation needed]
Phosphorylation
Polar auxin transport can be regulated by reversible protein
References
- ^ S2CID 10724269.
- .
- .
- ^ PMID 20300209.
- ^ PMID 20739413.
- ^ S2CID 4348635.
- ISBN 978-0470650530.
- ^ PMID 12495745.
- ^ a b p.435 Plant Physiology Third Edition Taiz and Zeiger (2002)
- ^ Gloria K Muday, Alison DeLong. (2001)Polar auxin transport:controlling where and how much. Trends in Plant Science 6(11):535-542
- S2CID 218593545.