Contribution of the cell wall biophysical factors in <i>Ectocarpus</i> and pollen-tube tip growth.

(A) For each cell type, the global stress σe was computed using measured values of turgor, curvature, and cell wall thickness (equation S2 in S1 Text). Knowing normal velocity Vn at each point, the expected strain rate was computed according to equation S10 in S1 Text. Note the different scales between Ectocarpus and the pollen tube. (B) Relationship between stress and expected strain rate in Ectocarpus apical cells (left) and in the tobacco pollen tube (right): instead of plotting each value against s, these values were plotted against each other to show how the stress results in strain. In Ectocarpus, but not in the pollen tube, behaves according to the Lockhart equation otherwise, with constant values for Φ and σy (compare with Fig 3B). (C) Robustness of this result was tested using a bootstrap analysis with 3,000 replicates. For each sample, the linearity of the part of the curve (where σe > σy) was estimated by linear regression. The distribution of the values of r2 shows that linearity is well supported (see also S4 Fig). Data are available as S4 Data. (D) Relationship between the three biophysical features of the cell wall: plastic yield threshold (σy, x-axis), thickness (δ, y-axis), and plastic extensibility (Φ, z-axis). In Ectocarpus, only variation of δ accounts for tip growth (brown line), whereas in pollen tubes, both σy and Φ vary while the wall thickness remains constant (green line).