Figure A4.
Auxin is not suitable as a direct signal. In all three cases auxin is produced in a single epidermal cell (position is indicated with an arrow in the line graphs). For a notable increase of the auxin concentration in some parts of the segment, very high production rates are needed, even with slowed down transport. Moreover, in the inner cortical layers the auxin concentration increases a bit over a wide region and the maximum increase does not occur closest to the production side, but shootward of it (downstream considering the cortical flow direction). The bottom images show steady state auxin concentrations. The graphs on top show the concentrations in the indicated layers (thick lines: pericycle, 5× cortex, and epidermis). The resting state concentrations (i.e., before production started) are plotted in thin black (vascular) and gray (cortex) lines. (A,B) Default parameters (C): “slowed down” parameters from Figure 2B: all effective permeabilities are reduced by a factor 10. Production rates are much higher than in Figures 1E,H, as here only a single cell produces: 0.1 Cv μm–3 s−1 (A), 0.01 Cv μm–3 s−1 (B,C). Note that in (A) the producing epidermal cell produces the full amount of auxin present in a vascular cell (both have the same size) of the reference segment every 10 s. The total amount produced per second is slightly more than half of the total in Figure 2E and consequently from several cells rootward of the production site onward the auxin concentration is significantly increased in all cell files. We consider this value (A) absurdly high, but use it because hardly any change is seen with the already high production rate of (B). This is because the auxin is very efficiently transported away from the production site. Reducing the efficiency of this transport by reducing all effective permeabilities of the whole segment [“slowing it down” (C)], a less absurd production rate is enough to support an obvious accumulation, but the issues of ill confined auxin accumulation and a shift of the maximum in the inner cortical layers (C5–C3) remain.