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Abstract
The morphogenesis of organs necessarily involves mechanical interactions and changes in mechanical properties of a tissue. A long standing question is how such changes are directed on a cellular scale while being coordinated at a tissular scale. Growing evidence suggests that mechanical cues are participating in the control of growth and morphogenesis during development. We introduce a mechanical model that represents the deposition of cellulose fibers in primary plant walls. In the model both the degree of material anisotropy and the anisotropy direction are regulated by stress anisotropy. We show that the finite element shell model and the simpler triangular biquadratic springs approach provide equally adequate descriptions of cell mechanics in tissue pressure simulations of the epidermis. In a growing organ, where circumferentially organized fibers act as a main controller of longitudinal growth, we show that the fiber direction can be correlated with both the maximal stress direction and the direction orthogonal to the maximal strain direction. However, when dynamic updates of the fiber direction are introduced, the mechanical stress provides a robust directional cue for the circumferential organization of the fibers, whereas the orthogonal to maximal strain model leads to an unstable situation where the fibers reorient longitudinally. Our investigation of the more complex shape and growth patterns in the shoot apical meristem where new organs are initiated shows that a stress based feedback on fiber directions is capable of reproducing the main features of in vivo cellulose fiber directions, deformations and material properties in different regions of the shoot. In particular, we show that this purely mechanical model can create radially distinct regions such that cells expand slowly and isotropically in the central zone while cells at the periphery expand more quickly and in the radial direction, which is a well established growth pattern in the meristem.
Highlights
Mechanical forces are integral part of any living system and recent data is confirming their importance as signaling cues in animal and plant development [1,2,3]
It has recently been shown that mechanical cues can guide these fibers
We developed detailed mechanical models to investigate how fiber directions may be responding to mechanical cues and what consequences this may have for positional and directional growth patterns
Summary
Mechanical forces are integral part of any living system and recent data is confirming their importance as signaling cues in animal and plant development [1,2,3]. This may be especially important for plants which have to sustain large loads while executing a developmental program that is optimal in their habitat [4]. The walls are composed of a network of cellulose microfibers interconnected by polysaccharides and xyloglucans [5,6,7] They constitute the structurally strong element of plant tissue providing support against turgor pressure and internal tension. From a mechanical point of view, the walls can be considered to be thin visco-elastic elements
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