Figure 1.

Mechanical and biochemical cues in computer models of plant development. (A) The direction of maximal expansion in plant cells depends on the orientation of cellulose microfibrils. Disordered microfibrils cause the isotropic all-around growth. A globally ordered structure of cellular microfibrils determines the anisotropic elongation, such that the growth direction is biased. (B) The acid-growth hypothesis; auxin promotes the proton pump activity which then acidifies the cell wall through the extrusion of H⁺ into the apoplast. Acidification promotes expansins and other cell wall-related enzymes, leading to the relaxation of the cell wall material, and successive cell growth.