by Robert G. Franks
North Carolina State University, Raleigh, NC., USA
How organ size and shape are determined in developing organisms remains a key question of interest to developmental biologists. An understanding of the relationships between gene expression and the organ shape requires analyses of relationships between the subcellular, cellular, and organ levels and is often approached through mathematical modeling techniques (Boudon et al., 2015; Heer and Martin 2017; Eder et al., 2017). One mechanism that links these multiple organizational scales is the biomechanics of the cells and tissues of the organ itself. In the developing plant it has become clear that the biomechanical properties of the cells, and particularly the cell walls, can be key determinants of both cell and organ shape. However, the relationships between gene expression and cell wall properties and organ shape currently are poorly understood.
In a new article Robinson et al. (2017) describe a new method to measure the mechanical properties of plant tissues. This method has several advantages over previously utilized methods; the measurements can be made in live tissues, have cellular scale resolution, and can be measured in the plane of growth. To measure the mechanical properties of an organ or tissue, the deformation of the tissue must be measured in response to a known, externally applied force. One method to achieve this is to use extensometers to apply a known force to a structure and then measure the deformation. One disadvantage of this method is that the mechanical properties can only be measured at the tissue level. Robinson et al. have paired a micro-extensometer with a confocal microscope to measure the mechanical properties of the cells within an organ at single-cell resolution. They call this system Automated Confocal Micro-Extensometer or ACME. The authors have made available custom software and instructions for the 3D printing of custom parts required for outfitting a confocal microscope with the ACME system.
Robinson and colleagues use the ACME system to measure the changes in the mechanical properties of the individual cells of the developing hypocotyl in an Arabidopsis seedling upon the application of the plant hormone gibberellic acid that promotes growth in the hypocotyl (Robinson et al. 2017). Changes in the mechanical properties of the hypocotyl cells were found to occur in a gradient along the main axis of the hypocotyl in a pattern that correlated with the observed cellular growth patterns. The experimental determination of the mechanical properties of cells is critical for the successful construction of mathematical models of plant organ growth that are useful for understanding the complex interactions between the cellular and tissue scales that can significantly affect the shape of the mature organ.
The application of this technology to developing floral organs is promising. Although the relatively small size of the Arabidopsis flower may make it unsuitable for mounting on this system without modifications, species that generate larger flowers would provide suitable material for analysis of mechanical properties of cells of the developing floral organs. The quantitative biophysical data at cellular resolution that can be resolved with the ACME technology is well suited for developing mathematical models of floral organ size and shape determination.
Boudon F, Chopard J, Ali O, Gilles B, Hamant O, Boudaoud A, Traas J, Godin C. 2015. A computational framework for 3D mechanical modeling of plant morphogenesis with cellular resolution. PLoS Computational Biology 11 (1), e1003950. https://doi.org/10.1371/journal.pcbi.1003950
Eder D, Aegerter C, Basler K. 2017. Forces controlling organ growth and size. Mechanisms of Development 144, 53–61. https://doi.org/10.1016/j.mod.2016.11.005
Robinson S, Huflejt M, Barbier de Reuille P, Braybrook SA, Schorderet M, Reinhardt D, Kuhlemeier C. 2017. An automated confocal micro-extensometer enables in vivo quantification of mechanical properties with cellular resolution. The Plant Cell. https://doi.org/10.1105/tpc.17.00753.