Explosive seed dispersal

Lignin developmental patterns and Casparian strip as apoplastic barriers: A review

Lignin and Casparian strips are two essential components of plant cells that play critical roles in plant development regulate nutrients and water across the plants cell. Recent studies have extensively investigated lignin diversity and Casparian strip formation, providing valuable insights into plant physiology. This review presents the established lignin biosynthesis pathway, as well as the developmental patterns of lignin and Casparian strip and transcriptional network associated with Casparian strip formation. It describes the biochemical and genetic mechanisms that regulate lignin biosynthesis and deposition in different plants cell types and tissues. Additionally, the review highlights recent studies that have uncovered novel lignin biosynthesis genes and enzymatic pathways, expanding our understanding of lignin diversity. This review also discusses the developmental patterns of Casparian strip in roots and their role in regulating nutrient and water transport, focusing on recent genetic and molecular studies that have identified regulators of Casparian strip formation. Previous research has shown that lignin biosynthesis genes also play a role in Casparian strip formation, suggesting that these processes are interconnected. In conclusion, this comprehensive overview provides insights into the developmental patterns of lignin diversity and Casparian strip as apoplastic barriers. It also identifies future research directions, including the functional characterization of novel lignin biosynthesis genes and the identification of additional regulators of Casparian strip formation. Overall, this review enhances our understanding of the complex and interconnected processes that drive plant growth, pathogen defense, regulation and development.

Development and diversity of lignin patterns

Different patterns of lignified cell walls are associated with diverse functions in a variety of plant tissues. These functions rely on the stiffness and hydrophobicity that lignin polymers impart to the cell wall. The precise pattern of subcellular lignin deposition is critical for the structure-function relationship in each lignified cell type. Here, we describe the role of xylem vessels as water pipes, Casparian strips as apoplastic barriers, and the role of asymmetrically lignified endocarp b cells in exploding seed pods. We highlight similarities and differences in the genetic mechanisms underpinning local lignin deposition in these diverse cell types. By bringing together examples from different developmental contexts and different plant species, we propose that comparative approaches can benefit our understanding of lignin patterning mechanisms.

Hydration-induced reversible deformation of the pine cone

The scales of pine cones undergo reversible deformation due to hydration changes in order to optimize seed dispersal. This improves the survivability of the pine. The reversible flexing of the scales is caused by two tissue layers arranged in a sandwich configuration: a layer composed of sclereid cells and a sclerenchyma layer. They expand differentially upon hydration (and contract upon dehydration) due to differences in the structure that are analyzed here for Torrey pine (Pinustorreyana) cones. In addition to this well-known mechanism by which the cellulose microfibrils in the scales vary their angle with the wood cell axis, we confirm the presence of a porosity gradient in the sclereid cells and calculate, using a model consisting of three layers, the stresses generated upon dehydration taking into account the effect of hydration on the elastic modulus. Our quantitative analysis reveals that this gradient structure can significantly decrease the stress concentrations due to the mismatch between the two layers, and show that this is an ingenious design to increase the interfacial toughness to improve the robustness of pine cone scales. We also show that each individual layer of sclereid cells and sclerenchyma fibers undergoes bending when hydrated separately, and suggest that the two layers operate synergistically to effect the required deformation for seed release. A synthetic bioinspired analog consisting of hydrogels with different porosities is used to confirm this principal actuation mechanism. These findings may inspire the materials science and mechanical engineering communities to develop more robust, biocompatible and energy-efficient actuation systems. STATEMENT OF SIGNIFICANCE: Some biological structures can exhibit reversible deformation enabled by water inflow and outflow of their structure. We analyse the reversible motion of pine cone scales. The dehydration produces their flexure and opening, resulting in the release of seeds and their dispersal, when conditions are right. This process is reversible, and rehydration of the pine cone recloses the scales. The processes of flexing and straightening are governed by shrinking and swelling which are directed by differences in the arrangement of cellulose microfibrils in a bilayer construct. We demonstrate that the scales are more complex than a simple bilayer structure and that they actually have gradients, which significantly reduce the internal stresses and ensure their integrity. We analyse the process of opening and closing of the scales for a gradient structure in the Torrey pine cone using a simple idealized trilayer model. The results demonstrate a significant decrease in internal stresses produced by the gradient structure. Using the lessons learned from the pine cone, we produce a bilayer junction using hydrogels with different porosities which exhibit the same reversible bending response.

A water drop-shaped slingshot in plants: geometry and mechanics in the explosive seed dispersal of Orixa japonica (Rutaceae)

BACKGROUND AND AIMS

In angiosperms, many species disperse their seeds autonomously by rapid movement of the pericarp. The fruits of these species often have long rod- or long plate-shaped pericarps, which are suitable for ejecting seeds during fruit dehiscence by bending or coiling. However, here we show that fruit with a completely different shape can also rely on pericarp movement to disperse seeds explosively, as in Orixa japonica.

METHODS

Fruit morphology was observed by hard tissue sectioning, scanning electron microscopy and micro-computed tomography, and the seed dispersal process was analysed using a high-speed camera. Comparisons were made of the geometric characteristics of pericarps before and after fruit dehiscence, and the mechanical process of pericarp movement was simulated with the aid of the finite element model.

KEY RESULTS

During fruit dehydration, the water drop-shaped endocarp of O. japonica with sandwich structure produced two-way bending deformation and cracking, and its width increased more than three-fold before opening. Meanwhile the same shaped exocarp with uniform structure could only produce small passive deformation under relatively large external forces. The endocarp forced the exocarp to open by hygroscopic movement before seed launching, and the exocarp provided the acceleration for seed launching through a reaction force.

CONCLUSIONS

Two layers of water drop-shaped pericarp in O. japonica form a structure similar to a slingshot, which launches the seed at high speed during fruit dehiscence. The results suggest that plants with explosive seed dispersal appear to have a wide variety of fruit morphology, and through a combination of different external shapes and internal structures, they are able to move rapidly using many sophisticated mechanisms.

Form, Structure, and Function: How Plants vs. Animals Solve Physical Problems

Plants and animals have evolved solutions for a wide range of mechanical problems, such as adhesion and dispersal. Several of these solutions have been sources for bio-inspiration, like the Lotus Effect for self-cleaning surfaces or Velcro for adhesion. This symposium brought together plant and animal biomechanics researchers who tackle similar problems in different systems under the unifying theme of structure-function relations with relevance to bio-inspiration. For both communities it holds true that the structural systems, which have evolved in the respective organisms to address the mechanical challenges mentioned above, are often highly complex. This requires interdisciplinary research involving "classical" experimental biology approaches in combination with advanced imaging methods and computational modeling. The transfer of such systems into biomimetic technical materials and structures comes with even more challenges, like scalability issues and applicability. Having brought all these topics under one umbrella, this symposium presented the forefront of biophysical basic and application-oriented international research with the goal of facilitation knowledge transfer across systems and disciplines.

Why plants make puzzle cells, and how their shape emerges

The shape and function of plant cells are often highly interdependent. The puzzle-shaped cells that appear in the epidermis of many plants are a striking example of a complex cell shape, however their functional benefit has remained elusive. We propose that these intricate forms provide an effective strategy to reduce mechanical stress in the cell wall of the epidermis. When tissue-level growth is isotropic, we hypothesize that lobes emerge at the cellular level to prevent formation of large isodiametric cells that would bulge under the stress produced by turgor pressure. Data from various plant organs and species support the relationship between lobes and growth isotropy, which we test with mutants where growth direction is perturbed. Using simulation models we show that a mechanism actively regulating cellular stress plausibly reproduces the development of epidermal cell shape. Together, our results suggest that mechanical stress is a key driver of cell-shape morphogenesis.