Figure 1.

Maximal tension predictions for shell and multicellular-layered mechanical models of the shoot apical meristem (SAM) including organ emergence. (A) Depictions of a pressurized shell model of the SAM and an emerging lateral organ early (left) and later (right) during development. Internal pressure is depicted by the black arrows. Predicted maximum tension orientations along the surface are depicted by gray lines. Note that anisotropic circumferential orientations toward the SAM periphery compared to isotropic stresses toward the apex. Maximum tension orientations are circumferential around the primordium, where wall loosening has occurred. Note that these stress orientations match observed microtubule and cellulose orientations. Circumferential cellulose orientations around the organ primordium would be expected to promote proximodistal growth. (B) Depiction of maximal tension orientations for a multicellular-layered SAM and primordium. The degree to which tensions in this model match those of the shell model depends on the relative stiffness of the outer epidermal cell wall compared to inner cell walls. Maximal tensions along inner anticlinal walls are oriented anticlinally (perpendicular to the SAM surface) matching observations. (C) Depiction of maximal tension orientations (dark lines) in a cross section of multilayered mechanical model of a leaf. Light green marks internal tissues, blue line marks ad-ab boundary. Note that predicted internal tension orientations are aligned largely perpendicular to the ad-ab boundary (although not so much at the leaf margin) and match observed microtubule and cellulose orientations.