Mechanical stress in Arabidopsis leaves orients microtubules in a 'continuous’ supracellular pattern

Abstract

BackgroundCortical microtubules form a dynamic network and continuously undergo shrinking (catastrophe), pausing and rebuilding (rescue). The advantage of such a dynamic system is that it may mediate appropriate responses in a short time span. Microtubules are known to play a pivotal role in determining the orientation of the cellulose microfibril deposition in the plant cell wall. The latter is a solid exoskeleton surrounding the protoplast. It forms the physical framework that interconnects most cells and has to bear the tensile stresses within the tissue. Here we describe the effect of externally applied pressure on microtubule organization in growing Arabidopsis leaves.ResultsConfocal microscopy examination of transgenic plants bearing GFP-tagged TUA6 proteins led to the observation that application of an additional mechanical pressure on growing Arabidopsis leaves triggers an excessive bundling of microtubules within the individual cell. Besides, the microtubules seem to align in neighboring cells, creating a 'continuous’ supracellular pattern. This effect occurs within 3 hours after applied external force and is age-dependent, whereby only cells of leaves up to 19 days after sowing (DAS) are susceptible to the applied pressure.ConclusionsUpon externally applied pressure on developing Arabidopsis leaves, microtubules bundle and rearrange to form seemingly continuous supracellular patterns. As microtubules guide the cellulose synthase complexes, this observed reorganisation pattern probably affects the cellulose deposition, contributing to the reinforcement of the cell wall in a particular position to cope with the extra-applied pressure. The age-effect is reasonable, since younger cells, which are actively shaping their cell walls, are more vulnerable to altered mechanical stresses while in leaves older than 19 DAS, the walls are more robust and therefore can sustain the applied forces.

The kinematics and mechanics of leaf expansion: new pieces to the Arabidopsis puzzle

“…In a series of live cell imaging experiments it was shown that the subcellular organization of microtubules in pavement cells responds to and aligns with the predicted stress patterns [37 ]. This result was consistent with the results from another recent study that analyzed pavement cell microtubule alignment as a function of stress patterns [40 ]. Given that microtubules and cellulose microfibrils colocalize at indentations [41], and that a cellulose synthase mutant has a pavement cell phenotype [42 ], it is highly likely that these oriented microtubules align cellulose microfibrils that are parallel to stress directions.…”

A mechanically sensitive cell layer regulates the physical properties of the Arabidopsis seed coat

“…Microtubule orientation and bundling change in the presence of mechanical stresses in growing tissues and can therefore be used as a marker of stress patterns [12][13][14][15][16] . We documented microtubule behaviour in the cells of oi-ab and oi-ad before and after fertilization using lines expressing a GFP (green fluorescent protein) fusion to Tubulin Alpha 6 (ref.…”

Xyloglucan Deficiency Disrupts Microtubule Stability and Cellulose Biosynthesis in Arabidopsis, Altering Cell Growth and Morphogenesis

“…The application of external mechanical pressure on growing Arabidopsis leaves triggers MT bundling and creates continuous supracellular MT patterns (Jacques et al, 2013). To monitor MT dynamics in Col and xxt1 xxt2 seedlings expressing GFP-MAP4 under external pressure, we constructed chambers with double-sided tape on glass slides, mounted 3-d-old etiolated seedlings in these chambers, pressed coverslips onto the tape to apply gentle, even pressure, and collected confocal z-series of GFP-MAP4 fluorescence.…”

“…In general, cortical MT orientation responds to mechanical signals and can be altered by applying force directly to the shoot apical meristem (Hamant et al, 2008). The application of external mechanical pressure to Arabidopsis leaves also triggers MT bundling (Jacques et al, 2013). Kinesins, including KINESIN-13A (KIN-13A) and FRAGILE FIBER1 (FRA1), have been implicated in cell wall synthesis (Cheung and Wu, 2011;Fujikura et al, 2014).…”