Poster Juliette Roué .pdf


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Role of the Arabidopsis thaliana root cap in the response
to mechanical stimuli
J. Roué (1, 2), H. Chauvet (1, 2), E. Kolb (3), M.B. Bogeat-Triboulot (4,5), F. Bizet (1, 2), and V. Legué (1, 2)
(1) Université Clermont Auvergne, UMR 547 PIAF, F-63000 Clermont-Ferrand, France (2) INRA, UMR 547 PIAF, F-63039 Clermont-Ferrand, France
(3) PMMH, ESPCI, CNRS UMR 7636, F-75231 Paris Cedex 5, France
(4) INRA, UMR EEF, F-54280 Champenoux, France (5) Université de Lorraine, UMR EEF, F-54506 Vandoeuvre-lès-Nancy, France

Introduction

juliette.roue@gmail.com

Root growth and development are largely modulated by mechanical soil properties such as soil density, texture and structure.
Root responses to increase of soil mechanical resistance:

Root responses to contact with impenetrable obstacles:

Root buckling
(Silverberg et al., 2012)
Root tip reorientation
(Massa and Gilroy, 2003)

Radial expansion
(Iijima and Kato, 2007)

Lateral root organogenesis
(Richter et al., 2009)

Root cap:
A major sensing and integration site for environmental cues.
In permanent renewal thanks to cell generation, differentiation
and sloughing.
Promotes root penetration in soils through mucilage production.

CONTACT

Root meristem re-organisation (Potocka et al., 2011)

Is the root cap an overriding site of perception
and/or integration of mechanical signals ?

?

GRAVITY

Results:

 Mimicking a mechanical
stimulus in a transparent growth
medium

Root growth rate (µm/min)

Experimental methods:

 Root growth in one-layer phytagel medium
Root grow at an average rate of 3µm/min in a 0.2% phytagel medium

4.5
4
3.5
3
2.5
2
1.5
1
0.5
0

a
ab
b
b

0.20%

0.30%

0.40%

b

0.50%

0.80%

Phytagel concentration

An increasing of medium hardness limits significantly root growth rate
Root tip diameter
(µm)

150
100

Root tip diameters are not affected by the hardness of phytagel medium.

50
0
0.20%

0.30%

0.40%

0.50%

0.80%

Phytagel concentration

Arabidopsis thaliana seeds are
cultivated in two categories of MS
growth media:
- One-layer phytagel medium: 0.2%,
0.3%, 0.4%, 0.5% and 0.8%
Phytagel-containing media.
Medium hardness is linked to
phytagel rate.
- Two-layers medium of different
phytagel concentration

 Root growth responses in two-layers phytagel medium

-240 min

0.5%

0.5%

0.2%

0 min

90 min

-- 240
240 min
min

240 min

00 min
min

12h
12h

240
240 min
min

1 mm

4

5
Root growth rate (µm/min)

Root growth rate (µm/min)

6
3.5

4
3

3

2.5

2

2

1.5

1
0
-120

Time lapse imaging of primary roots
2 days after germination

0.2%

-70

-20

30

80

130

180

230

280
Time (min)

 Reorientation of root growth
None of the roots are able to penetrate the 0.5% phytagel layer
A rapid buckling forms in the elongation zone 1h after contact
with harder layer
 Inhibition of root growth in response to mechanical
stimulus: the root growth rate drops sharply upon contact with
harder layer

1

0.5
0
-150

-100

-50

0

50

100

150

200

250

300
Time (min)

 90% of roots are able to penetrate the 0.2% phytagel lower layer
The roots still exhibit a slight change of trajectory after penetration
 Fast transient inhibition of root growth rate as the roots
encounter the lower layer surface

 Phenotypes of mca1-null mutant roots in two-layers phytagel medium
MCA1 encodes a mechanosensitive Ca2+-permeable ion channel (Nakagawa et al., 2007). mca1-null roots behavior in response to
mechanical stimulations might deepen the study of root mechanoperception.
Optimisation of images quality using
Rawtherapee

- 240
min

Measurement of root length and
root tip angle using home-made
program Root Stem Extractor

240 min

90 min

3.5
3
2.5
2
1.5
1
0.5
0

Root growth rate
(µm/min)

30% of the mca1-null roots are able to
penetrate directly or indirectly the 0.5%
lower layer

0 min

-120

-70

-20

30

80

130

180

230

280
Time (min)

The other 70% behave like WT roots
 mca1-null roots seem to penetrate more easily in a hard medium layer than WT roots
 The perception of a hard medium layer by the root may provoke an avoidance reaction which implies the establishment
of a bending

Conclusion and further studies
The change of medium hardness is perceived as a mechanical signal which impacts root growth and orientation.
Roots that don’t penetrate the lower medium layer adopt a step-like structure due to establishment of two bendings
- In elongation zone: passive bending due to root buckling
- At the root tip: putative active process maintaining the root cap in permanent contact with the hard medium
Hypothesis: important role of root cap in the integration of mechanical signals.
To confirm this hypothesis, the ability of several defective root cap mutants in response to mechanical stimuli will be analysed.


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