The precise control of physical properties of growing tissues is crucial for plant
morphogenesis. Sahaf and Sharon (pages 5509–5515 in this issue) examined the mechanics
of the expanding leaf and showed that plant tissues respond to stress by changing
their mechanical properties. A new method is proposed to distinguish reversible and
irreversible tissue deformation, an important step in understanding the physics of
a growing cell wall. Leaf blades could hold the key to understanding how plants regulate
their growth in different directions.
In recent years, considerable progress has been made in research on the molecular
and genetic basis of plant morphogenesis. However, organ growth is governed by physical
processes occurring at multiple scales. Plant cells grow due to the irreversible (or
plastic) deformation of their stiff cell wall under tension. The cell wall is a complex
hydrated gel composed mainly of pectins and hemicellulose reinforced by stiff cellulose
microfibrils. Different models of plant cell wall structure agree on the load-bearing
role of cellulose, but the mechanical function of other wall components remains unclear
(Cosgrove, 2015). Precise measurements of cell wall behavior under various mechanical
conditions are needed to further refine and validate structural models.
The mechanical force driving expansion of plant cells primarily results from turgor
pressure. In an isolated cell, the magnitude and orientation of tensile stresses is
determined only by its geometry and internal pressure. In an expanding tissue, local
differences in wall properties or stresses would in theory cause individual cells
to grow at different rates. Plant cells are, however, glued to their neighbors via
cell walls, forcing them to grow as a continuous tissue and creating residual stresses,
also termed tissue stresses (Baskin and Jensen, 2013). In modeling terms, this means
the specified growth (i.e. the expansion cells would exhibit if they did not have
neighbors) differs from the resultant growth, i.e. the only expansion we can actually
observe (Kennaway et al., 2011). The discrepancy between specified and resultant growth
depends on how tissues deal with residual stresses, e.g. reducing stresses by deforming
passively or building stresses up by resisting them. Since neither specified growth
nor mechanical stresses can be measured directly, the best way to investigate this
reaction is to apply an external force to a growing tissue.
Sahaf and Sharon (2016) show that tobacco leaves expand globally at a similar rate
in all directions under natural conditions, i.e. tissue growth is isotropic. However,
the leaf blade reacts to additional mechanical stress in a very remarkable way (Box
1). When subjected to a constant force for a short period of time, the tissue elongates
in the direction of imposed stretch and shrinks laterally due to the Poisson effect,
much as a piece of rubber would do. Over longer time scales an active response becomes
visible, which causes the elongation rate in the stretched direction to slow down
to the same values as in the unstretched part of the leaf and for growth to be normal
in the opposite direction despite the Poisson effect. In short, the leaf blade appears
to fight against external mechanical stresses, maintaining its specified growth in
both directions.
Box 1. Growth adapts to external mechanical constraints and correlates with tissue
elasticity
Applying a constant force (black arrows) results in fast tissue stretching (white
bar) in the direction of applied load and tissue contraction (red bar) in the perpendicular
orientation due to the Poisson effect (A). Growth under constant load is first oriented
along the applied force (B) and then becomes isotropic (C) as in the non-stretched
side (left). Releasing load causes growth reorientation, which becomes perpendicular
to the previously applied force (D) and turns back to isotropy only after several
hours (E). Before loading, leaf tissue is equally elastic in both orientations. Prolonged
load leads to tissue stiffening parallel to, and softening perpendicular to, the applied
force (F).
Plastic and elastic growth
What changes in mechanical properties underlie this coping mechanism? A possible candidate
would be modifications in elasticity (reversible deformation of the tissue). The link
between elasticity and growth is still unclear and even controversial (Cosgrove, 2015),
although it has been demonstrated in different studies (reviewed by Routier-Kierzkowska
and Smith, 2013). Part of the problem in measuring the reversible component of deformation
in a growing organ is that tissues are often visco-elastic, i.e. they take some time
to return to their original configuration (Nolte and Schopfer, 1997). To understand
the relationship between plastic and visco-elastic deformations Sahaf and Sharon used
the same experimental setup in two different ways. While they applied a constant force
over long periods of time to mimic growth stresses, visco-elasticity was assessed
by measuring tissue deformation under an oscillating load (Box 2).
Box 2. Measuring reversible vs plastic properties
A static load (red curve) can be used over long periods of time to mimic the effects
of tissue stresses and study the plastic deformation (green curve) of the leaf blade
(A). At first the tissue exhibits a fast linear deformation that is mainly reversible
(vertical segment of green curve) and later slowly creeps (plastic or irreversible
deformation) until reaching a steady growth rate. On a smaller timescale, visco-elasticity
is measured by applying an oscillating force (red curve) to the tissue (B). Loading
cycles occur sufficiently fast such that creep does not take place and the amplitude
of deformation (green curve) stays constant. The curve’s slope reflects tissue elasticity,
while the time shift between curves indicates viscous relaxation. The technique used
by Sharon and Sahaf probes tissue elasticity by adding mechanical stress to those
already existing. Measuring cell deformation upon osmotic treatments, on the other
hand, gives an insight into cellular elasticity at different levels of turgor-based
mechanical stress (Kierzkowski et al., 2012), as shown here in the shoot apical meristem
(C). Combining both approaches would provide an exciting new perspective on cell wall
modifications due to stress.
Sahaf and Sharon show that in the leaf blade changes in plastic behavior induced by
stress do correlate with modifications in visco-elastic properties. They observe a
clear increase in stiffness along the direction of imposed stretch. This behavior
is in accordance with previous findings that cortical microtubules orient according
to the direction of maximal stress, guiding the deposition of new cellulose microfibrils
(Hejnowicz et al., 2000; Hamant et al., 2008). Since microfibrils determine the direction
in which the cell wall is stiffer both in term of plastic and visco-elastic deformations
(Baskin and Jensen, 2013; Cosgrove, 2015), the alignment of newly deposited cellulose
fibres is a likely candidate for tissue stiffening in the direction of imposed stress.
More surprisingly, the tissues actually become elastically softer in the opposite
direction. In this case visco-elasticity also correlates with plastic behavior. Since
the tension in this direction was reduced due to the initial Poisson effect, normal
lateral growth rates can be interpreted as the effect of a higher plastic compliance.
This simultaneous softening and stiffening in opposite directions has not been shown
before, possibly because most studies of tissue elasticity have been conducted on
long and thin samples, such as young stems, which only allow measurements along one
axis. Paradoxically, leaves which normally do not exhibit a preferential growth axis
could provide new model systems to elucidate how mechanical properties are regulated
in different directions.
Back to the cell wall
Several scenarios explaining blade softening in the direction perpendicular to applied
stress could be investigated. The primary cell wall exhibits a polylamelate structure
in which cellulose microfibril orientation in-between successive layers can vary abruptly
(Zhang et al., 2016). Stress is distributed unevenly across wall layers and it is
possible that different layers are responsible for bearing transverse and longitudinal
stress (Hejnowicz and Borowska-Wykret, 2005). It is conceivable that selective softening
of older layers, in parallel with the deposition of a new layer reinforced in the
direction of applied stress, could explain the opposite regulation of tissue stiffness
in both directions. This could be investigated by using Atomic-force or Scanning Electron
microscopy to probe the modifications in nano-structure of cell wall layers (Zhang
et al., 2016) induced by constant load.
Leaves also provide an interesting system for exploring subcellular mechanical regulation
of growth. In Arabidopsis, despite a relatively uniform expansion of leaves at the
tissue scale (Remmler and Rolland-Lagan, 2012), puzzle-shaped epidermal cells often
display sharp local growth differences at the cell wall level (Elsner et al., 2012;
Armour et al., 2015). The complex shape of epidermal leaf cells also results in non-trivial
stress patterns (Sampathkumar et al., 2014). One could imagine that, within puzzle-shaped
cells, the orientation of individual lobes with respect to the applied tensile stress
could determine the local softening or stiffening of the cell wall. Such local modifications
could induce changes in mechanical anisotropy at the global scale. Testing such a
possibility would require leaf growth to be followed at very high resolution, for
example using confocal microscopy (Vlad et al., 2014), and combine stretching experiments
with tracking of subcellular deformations. Cellular and subcellular elasticity could
also be assessed using turgor manipulation (reviewed by Cosgrove, 2015), a technique
recently used to assess anisotropic elastic properties of single cells (Weber et al.,
2015; Hofhuis et al., 2016) as well as non-linear elasticity (Kierzkowski et al.,
2012).
Despite renewed interest in plant mechanics, fundamental questions regarding growth
regulation still need to be answered. Because they cannot be observed directly, the
different kinds of mechanical stresses driving cell expansion have often been omitted,
leading to apparent discrepancies between the observed growth and cell wall structure
(Baskin and Jensen, 2013). Using the leaf as a new model could help bridge the gap
between wall micro-mechanics, tissue stresses and growth.