22
other cytoskeletal proteins involved in various aspects of microtubule organization and dynamics
557
may be targeted and regulated by MAPK-dependent phosphorylation.
558
Concluding remarks and perspectives
559
Successful applications in live cell biology studies lag behind current advancements of
560
superresolution microscopy, owing to physical burdens imposed by the size and contour of plant
561
cells as well as by optical properties of cell walls. Herein one promising superresolution
562
technique, SIM, is quantitatively assessed and shows the potential to follow spatio-temporal
563
evolutions of cortical microtubules.
564
Present findings indicate the potential of SIM to unravel subdiffraction details
of plant cortical
565
microtubule organization. Using SIM with high resolution optics and imaging setup, individual
566
microtubules were resolved well below Abbe’s limit. It was possible to describe the complexity
567
of microtubule bundle structure and characterize extra- and intrabundle microtubule dynamics
568
with length fluctuations below the limits of other techniques such as WF, CLSM, TIRF and SD.
569
Live SIM imaging will allow to shed new light on interactions of microtubules with MAPs that
570
induce bundling and bias the parallel arrangement of cortical arrays (Tulin et al., 2012, Stoppin-
571
Mellet et al., 2013), but also to track microtubule nucleation processes in plant acentrosomal
572
cells (Binarová et al., 2006, Nakamura et al., 2010, Kirik et al., 2012, Stoppin-Mellet et al.,
573
2013). Finally, the resolution of dynamic features of microtubule structures such as the
574
preprophase microtubule band (Müller et al., 2009), the mitotic spindle (Zhang &Dawe, 2011),
575
and the phragmoplast (Smertenko et al., 2011) could be the challenging task for future live SIM
576
imaging in the plant field.
577
578
www.plantphysiol.org
on July 21, 2018
- Published by
Downloaded from
Copyright © 2014 American Society of Plant Biologists. All rights reserved.
23
Materials and Methods
579
Plant material and sample preparation for microscopy
580
Seedlings of
Arabidopsis thaliana ecotype Columbia (Col-0) carrying either GFP-MBD or GFP-
581
TUA6 constructs as well as
mpk4 mutant (in Col-0 background)
carrying GFP-MBD construct
582
were used. Plants were stably transformed by the floral dipping method (Clough & Bent, 1998)
583
with either a cauliflower mosaic virus 35S promoter driven GFP fusion of the microtubule
584
binding domain of the mammalian microtubule associated protein MAP4 (
proCaMV35S::GFP-
585
MBD) (Marc et al., 1998), or with a similarly expressed fusion of GFP with the TUA6 alpha-
586
tubulin isoform (
proCaMV35S::GFP-TUA6) provided by Dr. Sidney Shaw. Seeds were surface
587
sterilized and placed on half-strength MS culture medium (pH 5.7) without
vitamins containing
588
1% (w/v) sucrose and 0.4% (w/v) phytagel. The plates were stored at 4°C for 48 h to break
589
dormancy, and then kept vertically under 22°C in darkness for 5 days. Seedlings expressing
590
fusion constructs visualizing microtubules were transferred to microscopic slides that had been
591
modified into thin chambers using high precision and low thickness tolerance Nexterion round
592
cover slips (SCHOTT CR, a.s. Zašovská 850, 75701 Valašské Mezi
říčí, Czech Republic, HI,
593
D=0.17 mm ± 0.003 mm, diameter=25mm for objective "Plan-Apochromat" 100x/NA1.57 Oil,
594
DIC Corr, 000000-1787-996, very dense flint N-SSK2, refractive index n = 1.62229, Abbe
595
numbers Vd = 53.27, Ve = 52.99, reflectance at 0.5876 µm R = 0.05632). The chambers were
596
filled with the liquid half-strength MS medium either using one layer of parafilm as spacer or
597
without this spacer. Microchambers with seedlings were sealed with silicone paste to prevent
598
evaporation of liquid medium and stabilize samples during microscopy examinations.
599
Microscopy setup, optics and image acquisition
600
For WF, SIM, CLSM and TIRF all samples were examined in a Zeiss Axioimager Z.1 platform
601
equipped with the Elyra PS.1 super-resolution system for SR SIM and the LSM780 module for
602
CLSM using Zeiss objectives Alpha Plan Apochromat 63x/NA1.40 oil objective (tot. mag.
603
1008x and Alpha Plan Apochromat 100x/NA1.57 oil objective (tot. mag. 1600x) with
604
appropriate oils (Immersol 518F with refractive index of 1.518 for 63x/NA1.40 objective, and
605
Immersol HI with refractive index of 1.66 for 100x/NA1.57 objective). In few cases longer
606
imaging was done on a Zeiss LSM710 platform with a 63x/NA1.40 objective. Light
source for
607
www.plantphysiol.org
on July 21, 2018 - Published by
Downloaded from
Copyright © 2014 American Society of Plant Biologists. All rights reserved.