13
Intrabundle microtubule dynamics
311
Microtubule bundles are not static entities and individual microtubules exhibit vigorous dynamic
312
behavior within the crowded environment of a bundle similarly to individual microtubules (Shaw
313
& Lucas, 2011).
314
As shown previously, SIM offers superior contrast resolution, allowing the nearly linear titration
315
of bundled microtubule numbers by means of fluorescence intensity in the case of overlying
316
microtubules and by means of the Rayleigh criterion for laterally adjacent microtubules. Thus, it
317
was possible to follow and demonstrate intrabundle microtubule dynamics (Figs. 5A, I to K, L to
318
N) by kymographic analyses (Fig. 5J, M) showing that endwise growth and shrinkage rates are
319
comparable to those observed in free microtubules (Figs. 5P; Shaw & Lucas, 2011; Tables S3
320
and S4). Briefly, plus ends of intrabundle microtubules were growing and shrinking at 7.37
±3.27
321
μm/min (mean±SD; n=31; Fig. 5P; Tables S3 and S4) and 19.28±4.86 μm/min (mean±SD; n=33;
322
Fig. 5P; Tables S3 and S4) respectively. Accordingly the respective growth and shrinkage rates
323
of minus ends were 0.89
±0.83
μm/min (mean±SD; n=37; Fig. 5P; Tables S3 and S4) and
324
1.05
±1.10
μm/min (mean±SD; n=33; Fig. 5P; Tables S3 and S4). Similarly plus end
growth and
325
shrinkage rates for GFP-TUA6-labeled intrabundle microtubules were 8.08
±3.98 (mean±SD;
326
n=31; Fig. 5P; Tables S3 and S4) and 21.41
±7.95 (mean±SD; n=35; Fig. 5P; Tables S3 and S4).
327
For the minus ends that were observed the respective values were 0.83
±1.53 (mean±SD; n=31;
328
Fig. 5P; Tables S3 and S4) and 0.89
±1.05 (mean±SD; n=31; Fig. 5P; Tables S3 and S4).
329
From the kymographs of intrabundle microtubule dynamics it appeared that at the temporal
330
range of observations (which was between 3 and 5 min for all the time-lapsed acquisitions) there
331
were more catastrophe/rescue transitions in intrabundle microtubules than those observed in
332
individual microtubules. In this case
small length excursions, resulting from successive
333
rescue/catastrophe transitions, of ca. 200 nm visible by SIM (e.g., Figs. 5J, M, arrowheads) were
334
also considered and hence the overall catastrophe frequency of plus ends of intrabundle
335
microtubules was 0.032 events/s while the rescue frequency was 0.037 events/s (n=112
336
microtubules representing 321 minutes of observation).
337
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14
When visible, in most cases minus ends exhibited very slow dynamics and few transitions (Fig.
338
5J). Exceptionally, very few short microtubules exhibited vigorous minus end dynamics with
339
alternating short length growth and shrinkage resembling dynamic instability (Fig. 5M).
340
Microtubule dynamics of the mpk4 mutant
341
The bulk of cortical microtubules of hypocotyl epidermal cells of the
mpk4 mutant reside within
342
bundles therefore it was of interest to provide a preliminary characterization
of extra- and
343
intrabundle microtubule dynamics for this mutant.
344
Thus the dynamic behavior of free (Figs. 6A to E) and bundled microtubules (Figs. 7A to L) was
345
characterized by SIM. Quantitative analysis based on kymographs (Figs. 6C, D) showed that free
346
microtubule ends of GFP-MBD-labeled cortical microtubules grow and shrink at variable rates,
347
while frequently the same microtubule was showing different growth
and shrinkage rates during
348
successive events of rescue and catastrophe (Fig. 6D). This resulted in marginal lower rates of
349
growth compared to wild type at 4.14
±2.36
μm/min (mean±SD; n=29; Fig. 6E; Tables S1 and
350
S2, by comparison to plus end dynamics of GFP-MBD-labeled microtubules from wild type
351
cells, p=0.0012) but more pronounced decline in shrinkage
rates which were measured at
352
11.68
±5.63
μm/min (mean±SD; n=34; Fig. 6E; Tables S1 and S2; by comparison to plus end
353
dynamics of GFP-MBD-labeled microtubules from wild type cells, p=0.00056). By contrast, the
354
minus end growth and shrinkage rates were not significantly affected with elongation rated at
355
0.75
±0.75
μm/min (mean±SD; n=36; Fig. 6E; Tables S1 and S2; by comparison to plus end
356
dynamics of GFP-MBD-labeled microtubules from wild type cells, p=0.22) and shrinkage at
357
0.81
±0.83
μm/min (mean±SD; n=32; Fig. 6E; Tables S1 and S2; by comparison to plus end
358
dynamics of GFP-MBD-labeled microtubules from wild type cells, p=0.50).
359
Notably, the small length excursions which were reported for free ends of GFP-MBD-labeled
360
cortical microtubules from wild type cells as well as the overall growth/shrinkage transitions
361
appeared to be less frequent in the
mpk4 mutant. Therefore overall
catastrophe and rescue
362
frequencies were quantified and their respective values were 0.016 events/s (catastrophe
363
frequency; n=30 microtubules representing 82 minutes of observation) and 0.016 events/s
364
(rescue frequency; n=30 microtubules representing 82 minutes of observation).
365
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