The current model of this process involves the lateral transport of the hormone auxin through the first layer of cells in the stem apical meristem via the auxin efflux carrier protein PIN1. Computer models composed of all these components are capable of producing many of the observed patterns of phyllotaxy.
Introduction
- Qualitative Observations
- Mechanism of Cell Plate Positioning
- Stress and Strain as Controlling Factors in Cell Division
- Modeling
For example, the actual growth direction is rarely aligned with the main geometrical axis of the cell, and therefore division cannot simultaneously satisfy the requirements of shortest length and perpendicularity. So, in most cases, the cell plate is set up with the organization of the pre-prophase band.
Methods
- Plants
- Time-Series Images
- Static Images
- Image Processing
- Projection to Local Euclidean Plane
- Alignment of Principal Axes
Manual seeding of each cell in the image was then performed followed by this Mesh.Segment_Mesh(20000) Python code. After translation to the local coordinate system, each cell is represented by a matrix of data points.
Results
Cell Expansion and Division in Space and Time
As shown in Figure 1.2b, the cells in the center of the meristem expand very little and in some cases shrink between time points. This phenomenon is observed at all time points and does not fluctuate with the expansion waves observed elsewhere in the meristem.
Analysis of Traditional Heuristics of Cell Division
Errera did not specify the position of the new cell wall (the junction of the new wall with the cortex). The ratio of the area after cell division (Aaf ter =A1+A2, where A1 and A2 are the areas of the sibling pair) to the area before cell division (Abef ore) is illustrated in figure 1.11.
Model
The ratio of the area after cell division to the area before cell division is shown. The cell division potential was minimized and the fitness function was calculated for each of the six methods described in the shoulder.
Fitness Functions
A mean value (corresponding to the fitness function averaged over all 207 cell divisions) was assigned to each value of the weight vectors. In posterior projections, the outer boundary of the daughter cell pair was used to represent the pre-division cell wall.
Cellzilla Growth Model
The location of the new cell wall can be assigned either by the modern interpretation of Errera's rule or a potential model as described elsewhere in the paper. There was very little difference as either of the first two components (area and length) are varied.
Model Predictions
In this analysis we applied the previous authors' concept of a thermodynamic model to our data; we applied their model to our data to see how their potential function ΔV = lp compares to ours (4.5) in a thermodynamic analysis. Simulation results for both the modern interpretation of Errera's model and the optimized potential model are illustrated in Figure 1.15 after 1000 cell divisions have occurred (see also movies in Supplementary Material 4 and 5).
Comparison to the Besson-Dumais Model
This difference in results for the potential function VBD can be explained by the different ways we and Besson-Dumais use the same potential function. Thus, in the direct comparisons that have been made, we find a significantly lower effective temperature 1/β and thus significantly less randomness using our proposed potential function rather than Besson-Dumais.
Discussion
Gamma-tubulin is essential for microtubule organization and development in Arabidopsis.” In: The Plant cell18.6 (June 2006), pp. Localization of the microtubule end-binding protein EB1 reveals alternative pathways of spindle development in Arabidopsis suspension cells. In: The plant cell 17.6 (June 2005), pp.
![Figure 1.17: Comparison of our model with that of Besson and Dumais [9]. Histograms show log|∆V |, the difference between the predicted and actual potential used by the cell, for each model.](https://thumb-ap.123doks.com/thumbv2/123dok/10416427.0/43.918.183.760.265.1018/figure-comparison-besson-dumais-histograms-difference-predicted-potential.webp)
Introduction
- Synthesis
- Transport
- Auxin Receptors and Signal Propagation
- Auxin Maxima
- Meristem Patterning
- Root Patterning
- Auxin-Driven Patterning in other Tissues
- Modeling
The most studied member of the auxin family is Indole-3-acetic acid (IAA), which is found in concentrations of up to 250 pg·mg-1 in mature leaves, cotyledons, and shoot and root meristem tissues[3, 4 .]. Furthermore, recent work has shown that in brassinosteriod mutants, changes in cell size in the vascular ring lead to changes in the number of vascular bundles[89]. Leaf primordia arise from founder cells at the periphery of the vegetative SAM at sites of high auxin concentration.
Methods
Plants and Growth Conditions
The sterilized seeds were dried on sterile filter paper and then applied to the MS agar plates. The pots were arranged in trays of 12 pots and stored at 4°C for three days before being transferred to a growth chamber with 24 hours of lighting.
Fluorescent Lines
Roots with high expression of GFP in the roots were marked and later transferred to soil. Lines showing visible GFP expression in the meristem (in addition to expression in the roots) were selected for further experiments.
Meristem Experiments
Root Experiments
Vascular Experiments
Fixing
Embedding
The samples were removed from the 1xPBS solution and dehydrated with the following series of ethanol solutions. Adding eosin to the samples made it easier to find them later once they were embedded in the semi-transparent paraffin. The samples in the weighing boat were transferred to the petri dish with a hot metal spatula.
Sectioning
The histoclear/paraffin mixture was then decanted from the weigh boat, leaving the samples with a minimal amount of histoclear/paraffin. The next day, the melted paraffin was replaced with fresh paraffin every four hours, three times. This solution was added to a glass slide jar and the slides were placed in the solution for 30 minutes.
Imaging and Analysis
These liquids were placed in slide holders so that the slides could easily stand up in the liquid and hold part of the slide above the liquid for easy handling. Slides were then air-dried for 30 min and then either stored in a slide box at 4°C or imaged immediately.
Leaf Experiments
After removing the slides from the refrigerator, the drop of FM4-64 solution was blotted with a Kimwipe. The sample was excited using a single 488 nm laser through a 488 nm dichroic mirror which can excite both FM4-64 and GFP. A 500–525 nm bandpass filter was used to capture light from GFP and a 650 nm long-pass filter was used to capture light from FM4-64 simultaneously.
MorphoGraphX Image Processing
In order for MorphoGraphX to correctly interpret the image file, the first image in the set must be the top of the meristem. Hold down Ctrl and Alt and use the mouse to select the entire bottom of the grid. The following procedure was used to detect cell edges in the grid:
Results
- Phyllotaxis Model
- Modeling Auxin Movement
- Shoot Apical Meristem
- Shoots
- Roots
- Leaves
The transport function is only applied to elements on cell boundaries. Neglecting intracellular diffusion, the spacing of auxin peaks depends on cell size. In addition, some areas of the meristem divide faster than others, as shown in the first chapter.
Discussion
History of Phyllotaxis
He speculated that the same phyllotaxis mechanism was used throughout the plant's lifespan. When they made cuts at the top of the meristem, the wounds were pulled open, indicating that the tissue was under tension. When they made cuts around the circumference of the meristem, the wounds remained closed, indicating that the tissue was under circumferential compression.
Measurement of Phyllotaxis
In this technique, a mold is made of the meristem using the same fast-setting polymers used in dental molds. Advances in light microscopy allow high-resolution fluorescence imaging of the meristem in three dimensions. A third approach to this problem is to use tomographic techniques to build a 3D model of the entire plant.
Tomography
This can be done with multiple stacks of parallel slices, as is the case with confocal microscopes or when imaging the surface of opaque objects. An image taken from one side of the object only captures information about the object visible on that side. By combining images from different angles, a more complete model of the object can be built.
Methods
- Plant Growth Conditions
- Microscopy
- Meristem Image Processing
- Divergometer
- Tomography Photography
- Tomography Software
The following Bash script (OSX, Linux, or Cygwin environments only) was used to reverse the numerical order of the TIF filenames when necessary. Click Start and the stack will appear in the main window in a semi-transparent state, as shown in Figure 2.2A. The same general procedure for building a mesh from a z-stack was used as described in section 2.2.11.5. The point of negative curvature was determined to be the apex of the SAM.
Results
- Meristem Size Via Fluorescent Markers
- Meristem Size Via Topology
- Comparison of Meristem Measurement Methods
- Measuring Phyllotaxis with 3D Tomography
- Mutant Phyllotaxis Comparison
The tip of the meristem is determined by the maximum closest to the center of the figure. The rest of the meristem size measurements in this study were made using the geometric method. If this is true, then changing meristem size should result in changes in divergence angles.
Discussion
Requirement for the Auxin Polar Transport System in Early Stages of Arabidopsis Flower Bud Formation." In:The Plant Cell 3.7 (July 1991), pp. Changes in the pattern of cell arrangement at the surface of the shoot apical meristem in Hedera helix L. Meristem morphology is dependent on cell divisions and the placement of new walls.
Cell Size
In addition to the meristem, I also investigated the effect of cell size on patterning in the roots, leaf margins and shoot vessels. I counted vascular bundles and the number of cells around the ring in the same cell size mutants as before. The data show a linear relationship between the number of cells in the ring and the number of bundles.
Meristem Size
In that genotype, the meristem has about half the radius of wild type and shows a much larger divergence angle. These non-canonical divergence angles show that the phyllotaxis is affected by the changes to the meristem, even though the mean remains the same. The hypothesis is therefore partially true in that changing the size of the meristem changes the divergence angles.
New Model
The distribution of divergence angles of individual genotypes shows more variability than just looking at the means. Although, it is possible to obtain more pronounced phenotypes as seen in the model by significantly shrinking the meristem. Apparently there is also an upper threshold where phyllotaxy will change significantly when the meristem becomes large enough.
