Figure 9.

Mechanical stress and genetics can enhance stochasticity to promote heterogeneous growth. (A,B) Mechanical feedback loops can amplify variability in cellular growth rates to direct morphogenesis. (A) Microtubule (green) orientation is initially random when little mechanical force is exerted on cells. If some cells grow more rapidly than their neighbors (top and bottom cells in the left panel), they exert a force on the slower growing cells (in the middle). Once force surpasses a particular threshold (red arrows), microtubules will re-orient in a parallel manner to induce anisotropic growth that resists the mechanical stress. This further amplifies differential growth between neighboring cells. (B) On the organ scale, mechanical stress can amplify differences in growth rates to promote tissue morphogenesis. The shoot apical meristem (blue) endures increased mechanical stress and anisotropic growth in the peripheral zone, where the organ primordium is emerging (orange to red). Cortical microtubules (green) orient parallel to the emerging organ. Reprinted from Uyttewaal et al. (2012), Copyright, with permission from Elsevier. (C) JAGGED promotes variability by uncoupling cell size from the cell cycle in the organ primordia. In the floral meristem, cells grow isotropically and must reach a certain size to divide. This figure was reproduced from Schiessl et al. (2012), Copyright Elsevier. (D) JAGGED represses expression of the cyclin-dependent kinase inhibitors KRP2 and KRP4 to promote variability. Double and triple mutants with jag-1, krp4-1, and krp2-3 partially rescue the jag-1 phenotype, suggesting that JAG initiates cellular variability partially through repression of KRPs. Reprinted from Schiessl et al. (2014).