Onion epidermis as a new model to study the control of growth anisotropy in higher plants

Abstract

To elucidate the role of cellulose microfibrils in the control of growth anisotropy, a link between their net orientation, in vitro cell wall extensibility, and anisotropic cell expansion was studied during development of the adaxial epidermis of onion (Allium cepa) bulb scales using polarization confocal microscopy, creep tests, and light microscopy. During growth the net cellulose alignment across the whole thickness of the outer epidermal wall changed from transverse through random to longitudinal and back to transverse relative to the bulb axis. Cell wall extension in vitro was always higher transverse than parallel to the net cellulose alignment. The direction of growth anisotropy was perpendicular to the net microfibril orientation and changed during development from longitudinal to transverse to the bulb axis. The correlation between the degree of growth anisotropy and the net cellulose alignment was poor. Thus the net cellulose microfibril orientation across the whole thickness of the outer periclinal epidermis wall defines the direction but not the degree of growth anisotropy. Strips isolated from the epidermis in the directions perpendicular and transverse to a net cellulose orientation can be used as an extensiometric model to prove a protein involvement in the control of growth anisotropy.

Diffuse Growth of Plant Cell Walls

“…It is commonly accepted that cell walls expand preferentially in the direction at right angle to the net direction of cellulose alignment (Baskin, 2005;Suslov et al, 2009). Multicellular tissues present additional structural complications beyond the scope of this review (Crowell et al, 2010;Baskin and Jensen, 2013).…”

“…Epidermal peels from onion scales have been used in numerous studies to connect tissue-level mechanics, growth, and net cellulose orientation (Wilson et al, 2000;Hepworth and Bruce, 2004;Suslov et al, 2009;Beauzamy et al, 2015a). In previous studies, whole epidermal layers were prepared with intact (living) cells.…”

Cellulose Synthesis and Cell Expansion Are Regulated by Different Mechanisms in Growing Arabidopsis Hypocotyls

“…Cell expansion is controlled by changes in cell wall properties, which have been extensively studied under different conditions (Cosgrove and Li, 1993;Schopfer et al, 2002;Suslov et al, 2009;Peaucelle et al, 2015). More specifically, cell expansion is linked to cell wall extensibility, which can be defined as the ability of the cell wall to increase in surface area irreversibly during growth (Cosgrove, 2016a).…”

Cellular Force Microscopy for in Vivo Measurements of Plant Tissue Mechanics

“…Using a simple model of a pressurized tissue with homogeneous cells walls, the surface above anticlinal walls was always stiffer upon indentation, indepen- ) for the first four maps but different for the last stiffness map (6-22 N m dently of the internal pressure. We could, however, reproduce the transition from stiffer to softer at the cell junctions by using a more realistic structural model based on cross-sections of onion epidermis (Suslov et al, 2009;Fig. 5).…”

“…S10). However, in order to fully reproduce the experimental results, the gusset material has to be much softer than the rest of the upper cell wall, which is supported by the fact that it does not react with a cellulose stain (Suslov et al, 2009). As the cell surface bulges out due to the increasing turgor pressure, the thick upper cell wall surface becomes increasingly tense and stiff in the cell center.…”