Two papers in this issue of Development investigate how the mechanical and geometric properties of plant tissues contribute to growth, and how they are developmentally regulated.
Plant growth involves coordinated expansion of multiple tissue layers; where imbalances in growth rates occur, this can lead to significant mechanical stress and tissue cracking. In the plant stem, the outer epidermal wall is thought to be a load-bearing layer under tension, confining expansion of the inner tissues that are under compression. Both biochemical and biomechanical cues are believed to be involved in coordinating growth between these layers such that the stem remains intact. Ali Ferjani and colleagues set out to test this model, using a mutant Arabidopsis strain, clv3-8 det3-1, that displays spontaneous cracking of the stem. The clv3-8 mutation promotes proliferation, while det3-1 mutants show reduction in cell wall stiffness. The authors provide evidence that the higher proliferation in these mutants correlates with increased mechanical stress in the epidermal layer and propose that this – combined with the weaker cell walls – leads to a loss of tissue integrity and cracking. Subsequently, the cracks relieve the tension, allowing increased expansion of the inner layer. Thus, these data support the idea that the epidermal layer acts as a load-bearing layer that constrains growth, and that mechanical feedback between inner and outer layers acts to coordinate growth of the plant stem.
Plant root tips must be able to efficiently penetrate soil without losing mechanical integrity. The domed shape of root tips appears highly conserved across species, suggesting that their shape may be under evolutionary constraint. Using morphometric analysis and mathematical modelling, Tatsuaki Goh, Koichi Fujimoto and colleagues investigate how root tip shape is defined – from both a geometric and a developmental perspective. They first show that the shape of primary and lateral root tips in Arabidopsis, and in several other species, can best be described by a catenary curve: the curve described by a free-hanging chain suspended between two points. Mechanically, generating such a curve in a growing structure requires the existence of a sharp boundary between a proliferating and non-proliferating region at the lateral edge of the root primordium, along with spatially uniform unidirectional tissue growth within the growing tip. Arabidopsis root primordia display both these characteristics, and mutants that disrupt either requirement lead to a deviation from the catenary shape. The authors further show – computationally – that mechanical force is uniformly distributed across the surface of a root tip with a catenary curve, and propose that this may aid efficient penetration into the soil.