Figure 1.
Basic layout of the shoot apical meristem and our model.
A) Schematic representation of the SAM. The central zone (CZ) is located in the center of the shoot apex and contains the stem cells (SCs). CLV3 expression marks the SC domain. Directly below the SC domain is the organizing center (OC), which is defined by the expression of WUS. Lateral to the CZ is the peripheral zone (PZ), which consists of rapidly proliferating cells. STM expression in the three outer cell layers partially correlates with the PZ. The surface width of the STM expression domain (dashed line) as well as the surface distance between opposing primordia (solid line) is indicated. B) Schematic representation of the stem cell pool model. Cells of the SC pool (S) and the OC (O) proliferate with rate α1. SCs differentiate into cells of the OC with rate λ1 and into cells of the proliferation zone P with rate λ2. Cells of the proliferation zone can re-specify into SCs with a rate ρ1. This rate depends on signals from the OC as indicated by the dotted line. Cells of the OC terminally differentiate with a rate λ3, which depends on the SC pool.
Figure 2.
Representative slides of in situ hybridizations using CLV3, WUS, STM and HISTONE H4 RNA probes on tissue grown under three different environmental conditions. First row: vegetative meristems form short days, 23°C. Second row: transition meristems from long days, 16°C. Third row: inflorescence meristems from long days, 23°C.
Figure 3.
Quantification of SAM markers.
Box-plot representation of data derived by image analysis from SAM measurements and in situ hybridizations with CLV3, WUS, STM and HISTONE H4 RNA probes. A) Area of the CLV3 and WUS expression domain. B) Width of the STM expression domain and distance between opposing primordia both measured along the apex surface (see Figure 1A). C) Mitotic index (MI) of the CZ and the PZ. Mean and standard deviation of the data is given in Table 1. VM: vegetative meristem from short days, 23°C; TM: transition meristem from long days, 16°C; IM: inflorescence meristem from long days, 23°.
Table 1.
Mean and standard deviation of the expression area of CLV3 and WUS and the width of the STM expression domain.
Table 2.
Mean and standard deviation of the re-specification rate ρ1 and the cell proliferation rate of the CZ (α1) and of the PZ (α2) in the three investigated conditions.
Table 3.
Median and 95% confidence interval for differentiation rates of the three alternative models.
Figure 4.
Phase-plane diagrams of alternative models for the SC and OC pool sizes.
A) Basic model Equations (1–2). B) SC-based feedback model Equations (5–6), C) OC-based feedback model Equations (9–10). All graphs show the and
null clines for two different values of the cell proliferation rate α1. Solid curves: α1 = 0.0042, as in CZ of vegetative meristems (VM). Dashed curves: α1 = 0.0217, as in the CZ of transition meristems (TM). The intersection of each pair of null clines corresponds with a steady state (S ˙,O ˙) and is indicated by a black circle and the corresponding meristem type. Note that the increase in the steady state size of the SC pool (S*) due to an increase in α1 is smaller in both feedback models (B–C) compared to the basic model in A).
Figure 5.
Dependence of the CZ cell output rate and SC pool size.
The output rate is defined by the fraction of SCs that differentiate into PZ cells per unit time. Note that while the basic model Equations (1–2) show a linear increase in SZ size with, both models including a feedback on the SZ differentiation rate exhibit a reduced (Equations (5–6)) or almost absent (Equations (9–10)) change in the SC pool size while delivering the same increase in cell output rate. Circles: vegetative meristem from short days, 23°C. Diamonds: transition meristem from long days, 16°C. Squares: Inflorescence meristem from long days, 23°C.
Figure 6.
Cell pool sizes in response to changes in CLV3 or WUS activity.
The responses to elevated CLV3 signaling are indicated by solid lines, the effect of ectopic WUS expression are denoted by dashed lines. All domain sizes are given relative to the WT pool sizes denoted by S0 (SC) and O0 (OC), respectively. Note that only the basic model Equations (5–6) can recover the experimental phenotypes. Here, enhanced CLV3 signaling, simulated by increasing λ3, leads to a reduced OC and SC pool size. Ectopic WUS over-expression, simulated by including a constant SC production rate, enlarges the SC domain and leads to a decrease of the endogenous WUS activity. Black: basic model Equations (5–6). Red: SC-based feedback model Equations (9–10). Blue: OC-based feedback model Equations (9–10).