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The role of auxin transport in plant patterning mechanisms.

Smith RS - PLoS Biol. (2008)

View Article: PubMed Central - PubMed

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A slightly increased concentration of the activator in a cell due to random variation can lead to a small local increase in production of both the activator and the inhibitor in the cell... If the inhibitor diffuses to neighbor cells much more quickly than the activator, it will reduce the inhibitor's negative effect on local activator self-enhancement, and suppress the activation of cells nearby... Based on these results, Reinhardt et al. proposed a model for organ initiation in which the local activation of cells is not caused by local self-enhanced production, as is the case in reaction–diffusion models, but rather by the directed transport of auxin to organ initiation sites (Figure 3)... Longer-range inhibition does not require a second substance; it is due to the removal of auxin from surrounding tissue... But this in itself does not provide a complete mechanism for patterning, as it does not answer the question as to what controls the orientation of the PIN1 transport proteins... A possible answer to this question came from computer simulation studies in which it was hypothesized that PIN1 proteins are able to react to the concentration of auxin in neighbor cells, and orient preferentially toward cells with higher concentration... Similar in concept to reaction–diffusion, if one cell has a slightly higher auxin concentration, then this causes the PIN1 proteins in neighboring cells to orient preferentially toward it, causing a further increase in concentration... In a tissue of cells, this can result in a spacing mechanism similar to Meinhardt and Gierer's activator–inhibitor system... As with shoots, the root must also periodically create lateral organs to extend its structure, and the experimental data suggest that this process is also triggered by elevated auxin levels... In the root, however, lateral root primordium initiation does not appear to be accompanied by dramatic changes in PIN relocation, as is the case with the patterning in leaves and shoots... Instead, an article by Laskowski et al. in this issue of PLoS Biology suggests that it is the auxin import protein AUX1, combined with the geometry of the cells themselves, that is the crucial player in the patterning mechanism behind lateral root initiation... They are essential for pattern formation, since organ initiation in the shoot is severely impaired when these proteins are knocked out... However, they do not polarize to one side of the cell as PINs do, nor are they preferentially expressed at organ initiation sites... To start the process, Laskowski et al. suggest that it is not simply noise, or distance from previous lateral root primordia, that gives select cells that initial slight advantage, but that it is the result of the geometry of the cells themselves... The article by Laskowski et al. demonstrates the utility of a combined approach involving both experimental and computer simulation methods.

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Activator–Inhibitor SystemThe activator is shown in blue, and the inhibitor is shown in red. (A) The activator enhances its own production as well as the production of the inhibitor. The inhibitor inhibits production of the activator. (B and C) A line of cells, with the height of the blue and red bars indicating activator and inhibitor concentration. A slight increase in the concentration of the activator in one cell causes an increase in production of both the activator and the inhibitor in that cell (B). The inhibitor diffuses away more quickly than the activator, allowing local activation to escalate, while simultaneously suppressing neighbor cells (C).
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pbio-0060323-g001: Activator–Inhibitor SystemThe activator is shown in blue, and the inhibitor is shown in red. (A) The activator enhances its own production as well as the production of the inhibitor. The inhibitor inhibits production of the activator. (B and C) A line of cells, with the height of the blue and red bars indicating activator and inhibitor concentration. A slight increase in the concentration of the activator in one cell causes an increase in production of both the activator and the inhibitor in that cell (B). The inhibitor diffuses away more quickly than the activator, allowing local activation to escalate, while simultaneously suppressing neighbor cells (C).

Mentions: There are two substances, an activator and an inhibitor. The activator enhances its own production as well as the production of the inhibitor. The inhibitor, in turn, inhibits production of the activator. Such a system is easy to envision as a feedback loop in a genetic regulatory network (Figure 1). A slightly increased concentration of the activator in a cell due to random variation can lead to a small local increase in production of both the activator and the inhibitor in the cell. If the inhibitor diffuses to neighbor cells much more quickly than the activator, it will reduce the inhibitor's negative effect on local activator self-enhancement, and suppress the activation of cells nearby. In a system of identical cells, each operating with these same rules, a small amount of noise can lead to a periodic pattern of peaks high in activator concentration. This then triggers differentiation, leading to visible pattern formation. Meinhardt proposed and simulated many variants on this basic idea, and was able to reproduce a wide variety of the patterns observed in nature [2,5]. Now that molecular data have become available, these types of models are increasingly being cast in terms of real substances, belonging to real genetic regulatory networks [6–8].


The role of auxin transport in plant patterning mechanisms.

Smith RS - PLoS Biol. (2008)

Activator–Inhibitor SystemThe activator is shown in blue, and the inhibitor is shown in red. (A) The activator enhances its own production as well as the production of the inhibitor. The inhibitor inhibits production of the activator. (B and C) A line of cells, with the height of the blue and red bars indicating activator and inhibitor concentration. A slight increase in the concentration of the activator in one cell causes an increase in production of both the activator and the inhibitor in that cell (B). The inhibitor diffuses away more quickly than the activator, allowing local activation to escalate, while simultaneously suppressing neighbor cells (C).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2602727&req=5

pbio-0060323-g001: Activator–Inhibitor SystemThe activator is shown in blue, and the inhibitor is shown in red. (A) The activator enhances its own production as well as the production of the inhibitor. The inhibitor inhibits production of the activator. (B and C) A line of cells, with the height of the blue and red bars indicating activator and inhibitor concentration. A slight increase in the concentration of the activator in one cell causes an increase in production of both the activator and the inhibitor in that cell (B). The inhibitor diffuses away more quickly than the activator, allowing local activation to escalate, while simultaneously suppressing neighbor cells (C).
Mentions: There are two substances, an activator and an inhibitor. The activator enhances its own production as well as the production of the inhibitor. The inhibitor, in turn, inhibits production of the activator. Such a system is easy to envision as a feedback loop in a genetic regulatory network (Figure 1). A slightly increased concentration of the activator in a cell due to random variation can lead to a small local increase in production of both the activator and the inhibitor in the cell. If the inhibitor diffuses to neighbor cells much more quickly than the activator, it will reduce the inhibitor's negative effect on local activator self-enhancement, and suppress the activation of cells nearby. In a system of identical cells, each operating with these same rules, a small amount of noise can lead to a periodic pattern of peaks high in activator concentration. This then triggers differentiation, leading to visible pattern formation. Meinhardt proposed and simulated many variants on this basic idea, and was able to reproduce a wide variety of the patterns observed in nature [2,5]. Now that molecular data have become available, these types of models are increasingly being cast in terms of real substances, belonging to real genetic regulatory networks [6–8].

View Article: PubMed Central - PubMed

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

A slightly increased concentration of the activator in a cell due to random variation can lead to a small local increase in production of both the activator and the inhibitor in the cell... If the inhibitor diffuses to neighbor cells much more quickly than the activator, it will reduce the inhibitor's negative effect on local activator self-enhancement, and suppress the activation of cells nearby... Based on these results, Reinhardt et al. proposed a model for organ initiation in which the local activation of cells is not caused by local self-enhanced production, as is the case in reaction–diffusion models, but rather by the directed transport of auxin to organ initiation sites (Figure 3)... Longer-range inhibition does not require a second substance; it is due to the removal of auxin from surrounding tissue... But this in itself does not provide a complete mechanism for patterning, as it does not answer the question as to what controls the orientation of the PIN1 transport proteins... A possible answer to this question came from computer simulation studies in which it was hypothesized that PIN1 proteins are able to react to the concentration of auxin in neighbor cells, and orient preferentially toward cells with higher concentration... Similar in concept to reaction–diffusion, if one cell has a slightly higher auxin concentration, then this causes the PIN1 proteins in neighboring cells to orient preferentially toward it, causing a further increase in concentration... In a tissue of cells, this can result in a spacing mechanism similar to Meinhardt and Gierer's activator–inhibitor system... As with shoots, the root must also periodically create lateral organs to extend its structure, and the experimental data suggest that this process is also triggered by elevated auxin levels... In the root, however, lateral root primordium initiation does not appear to be accompanied by dramatic changes in PIN relocation, as is the case with the patterning in leaves and shoots... Instead, an article by Laskowski et al. in this issue of PLoS Biology suggests that it is the auxin import protein AUX1, combined with the geometry of the cells themselves, that is the crucial player in the patterning mechanism behind lateral root initiation... They are essential for pattern formation, since organ initiation in the shoot is severely impaired when these proteins are knocked out... However, they do not polarize to one side of the cell as PINs do, nor are they preferentially expressed at organ initiation sites... To start the process, Laskowski et al. suggest that it is not simply noise, or distance from previous lateral root primordia, that gives select cells that initial slight advantage, but that it is the result of the geometry of the cells themselves... The article by Laskowski et al. demonstrates the utility of a combined approach involving both experimental and computer simulation methods.

Show MeSH