We thank H. Tsutsui and D. Kurihara for providing us with transgenic plants, including the T. fournieriRPS5Ap::H2B-tdTomato line and the A. thaliana UBQ10pro::H2B-mClover line, respectively. This work was supported by the Institute of Transformative Bio-Molecules of Nagoya University and the Japan Advanced Plant Science Network. Financial support for this work was provided by grants from the Japan Science and Technology Agency (ERATO project grant no. JPMJER1004 for T.H.), a Grant-in-Aid for Scientific Research on Innovative Areas (Nos. JP16H06465 and JP16H06464 for T.H.), and Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for challenging Exploratory Research (grant no. 26600061 for N.Y. and grant nos. 25650075 and 15K14542 for Y.S.).

1. Fabrication of the PDMS Microdevice to Examine Growing Pollen Tubes and Moss Protonemata
NOTE: We used a maskless photolithography instrument to prepare PDMS molds on silicon wafers. The details regarding the operation of the system are omitted in this manuscript. A standard photolithography technique9 using a photomask may also be used to create the PDMS molds described in this manuscript.
<ol>
	<li>Pour 11&#160;g of pre-polymer PDMS mixture (elastomer base:curing...

<ol>
	<li>Higashiyama, T., Takeuchi, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Mechanism+and+Key+Molecules+Involved+in+Pollen+Tube+Guidance.">The Mechanism and Key Molecules Involved in Pollen Tube Guidance.</a> Annu. Rev. Plant. Biol. 66, 393-413 (2015).</li><li>Vogler, H., Shamsudhin, N., Nelson, B. J., Grossniklaus, U., Obermeyer, G., Feij&#243;, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+cytomechanical+forces+on+growing+pollen+tubes.">Measuring cytomechanical forces on growing pollen tubes.</a> Pollen Tip Growth. , 65-85 (2017).</li><li>Kehr, J., Wagner, C., Willmitzer, L., Fisahn, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+modified+carbon+allocation+on+turgor,+osmolality,+sugar+and+potassium+content+and+membrane+potential+in+the+epidermis+of+transgenic+potato+(Solanum+tuberosum+L.)+plants.">Effect of modified carbon allocation on turgor, osmolality, sugar and potassium content and membrane potential in the epidermis of transgenic potato (Solanum tuberosum L.) plants.</a> J. Exp. Bot. 50 (334), 565-571 (1999).</li><li>Benkert, R., Obermeyer, G., Bentrup, F. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+turgor+pressure+of+growing+lily+pollen+tubes.">The turgor pressure of growing lily pollen tubes.</a> Protoplasma. 198, 1-8 (1997).</li><li>Vogler, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+pollen+tube:+a+soft+shell+with+a+hard+core.">The pollen tube: a soft shell with a hard core.</a> Plant J. 73 (4), 617-627 (2013).</li><li>Shamsudhin, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Massively+parallelized+pollen+tube+guidance+and+mechanical+measurements+on+a+Lab-on-a-Chip+platform.">Massively parallelized pollen tube guidance and mechanical measurements on a Lab-on-a-Chip platform.</a> PloS one. 11 (12), e0168138 (2016).</li><li>Cheung, A. Y., Boavida, L. C., Aggarwal, M., Wu, H. M., Feij&#243;, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+pollen+tube+journey+in+the+pistil+and+imaging+the+in+vivo+process+by+two-photon+microscopy.">The pollen tube journey in the pistil and imaging the in vivo process by two-photon microscopy.</a> J Exp Bot. 61 (7), 1907-1915 (2010).</li><li>Metzner, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+comparison+of+MRI+and+X-ray+CT+technologies+for+3D+imaging+of+root+systems+in+soil:+Potential+and+challenges+for+root+trait+quantification.">Direct comparison of MRI and X-ray CT technologies for 3D imaging of root systems in soil: Potential and challenges for root trait quantification.</a> Plant Methods. 11 (1), 1-11 (2015).</li><li>Xia, Y., Whitesides, G. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Soft+Lithography.">Soft Lithography.</a> Angew Chemie Int Ed. 37 (5), 550-575 (1998).</li><li>Massalha, H., Korenblum, E., Malitsky, S., Shapiro, O. H., Aharoni, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Live+imaging+of+root-bacteria+interactions+in+a+microfluidics+setup.">Live imaging of root-bacteria interactions in a microfluidics setup.</a> P Natl Acad Sci. 114 (17), 4549-4554 (2017).</li><li>Grossmann, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Time-lapse+Fluorescence+Imaging+of+Arabidopsis+Root+Growth+with+Rapid+Manipulation+of+The+Root+Environment+Using+the+RootChip.">Time-lapse Fluorescence Imaging of Arabidopsis Root Growth with Rapid Manipulation of The Root Environment Using the RootChip.</a> J Vis Exp. (65), (2012).</li><li>Busch, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+microfluidic+device+and+computational+platform+for+high-throughput+live+imaging+of+gene+expression.">A microfluidic device and computational platform for high-throughput live imaging of gene expression.</a> Nature Methods. 9, 1101-1106 (2012).</li><li>Parashar, A., Pandey, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plant-in-chip:+Microfluidic+system+for+studying+root+growth+and+pathogenic+interactions+in+Arabidopsis.">Plant-in-chip: Microfluidic system for studying root growth and pathogenic interactions in Arabidopsis.</a> Appl. Phys. Lett. 98, 263703 (2011).</li><li>Bascom, C. S., Wu, S. -. Z., Nelson, K., Oakey, J., Bezanilla, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Long-Term+Growth+of+Moss.">Long-Term Growth of Moss.</a> in Microfluidic Devices Enables Subcellular Studies in Development. Plant Physiol. 172 (1), 28-37 (2016).</li><li>Higashiyama, T., Kuroiwa, H., Kawano, S., Kuroiwa, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Guidance+in+vitro+of+the+pollen+tube+to+the+naked+embryo+sac+of+Torenia+fournieri.">Guidance in vitro of the pollen tube to the naked embryo sac of Torenia fournieri.</a> Plant Cell. 10, 2019-2032 (1998).</li><li>Nishiyama, T., Hiwatashi, Y., Sakakibara, K., Kato, M., Hasebe, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tagged+mutagenesis+and+gene-trap+in+the+moss,+Physcomitrella+patens+by+shuttle+mutagenesis.">Tagged mutagenesis and gene-trap in the moss, Physcomitrella patens by shuttle mutagenesis.</a> DNA Res. 7, 9-17 (2000).</li><li>Maruyama, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Independent+Control+by+Each+Female+Gamete+Prevents+the+Attraction+of+Multiple+Pollen+Tubes.">Independent Control by Each Female Gamete Prevents the Attraction of Multiple Pollen Tubes.</a> Dev. Cell. 25 (3), 317-323 (2013).</li><li>Rensing, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Physcomitrella+genome+reveals+evolutionary+insights+into+the+conquest+of+land+by+plants.">The Physcomitrella genome reveals evolutionary insights into the conquest of land by plants.</a> Science. 319, 64-69 (2008).</li><li>Yanagisawa, N., Sugimoto, N., Arata, H., Higashiyama, T., Sato, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capability+of+tip-growing+plant+cells+to+penetrate+into+extremely+narrow+gaps.">Capability of tip-growing plant cells to penetrate into extremely narrow gaps.</a> Sci. Rep. 7 (1), 1403 (2017).</li><li>Dolan, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Clonal+relationships+and+cell+patterning+in+the+root+epidermis+of+Arabidopsis.">Clonal relationships and cell patterning in the root epidermis of Arabidopsis.</a> Development. 120, 2465-2474 (1994).</li><li>Denais, C. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nuclear+envelope+rupture+and+repair+during+cancer+cell+migration.">Nuclear envelope rupture and repair during cancer cell migration.</a> Science. 352 (6283), 353-358 (2016).</li></ol>

Several critical steps in the protocol need to be followed precisely to obtain the results presented above. First, the PDMS layer and glass bottom dish surfaces must both be treated with plasma for a sufficient amount of time before bonding. Otherwise, the PDMS layer may locally detach from the glass surface while tip-growing cells are crossing the microgaps. Another crucial step in the root hair and moss protonemata protocol is the sterilization of the microdevice. Normally, root hairs and moss protonemata cells need to...

After pollen grains germinate on a stigma, each grain produces a single pollen tube that carries sperm cells to the egg cell and the central cell in the ovule for double fertilization. Pollen tubes elongate through the style and eventually reach the ovule by sensing multiple guidance cues along their way1. During the elongation, pollen tubes encounter a series of physical barriers; the transmitting track is filled with cells, and pollen tubes must enter the minute micropylar opening of the ovule to reach their target (Figure 1A)2. Therefore, pollen tubes must have the ability to penetrate physical obstacles, while tolerating the compressive stress from their surroundings. Root hairs are another type of tip-growing plant cell that must withstand physical obstacles in the environment, in the form of packed soil particles (Figure 1B).
Various mechanical properties of the pollen tube have been studied, including turgor pressure and stiffness of the cell&#39;s apical region, which can be measured using the incipient plasmolysis method3,4 and cellular force microscopy (CFM)5,6, respectively. However, these methods alone do not reveal whether pollen tubes are capable of elongating through physical barriers along their growth paths. An alternative technique that allows pollen tube elongation to be monitored in vivo is two photon microscopy7. However, with this method, it is difficult to observe the morphological changes in individual pollen tubes deep inside the ovule tissue. Additionally, root hair growth in soil can be visualized using X-ray computed tomography (CT) and magnetic resonance imaging (MRI)8, albeit with low resolution. Here, we present a method that can be used to acquire high-resolution images of a cell&#39;s deformation process on a conventional microscope.
The overall goal of the method described here is to visualize the elongation capability of tip-growing plant cells, including pollen tubes, root hairs, and moss protonemata, in extremely small spaces. As the poly-dimethylsiloxane (PDMS) microdevices presented in this manuscript are optically transparent and air permeable, we can culture living cells inside the device and observe their growth behaviors under a microscope. It is also possible to create micro ~ nanometer scale spaces by the soft lithography technique9 with the use of molds. These features allow us to study the elongation capability of tip-growing plant cells in a physically confined environment.
In this work, we constructed 1 &#181;m wide gaps (4 &#181;m in height) in microfluidic devices and examined the ability of pollen tubes to penetrate these artificial obstacles that are much smaller than the diameter of the cylindrical pollen tube (approximately 8 &#181;m). This experimental platform enables us to visualize the pollen tube&#39;s response to microgaps and capture time-lapse images of the response, which track the cell&#39;s deformation process. We also developed the microdevices that can be used to investigate the penetration capability of root hairs and moss protonemata. Several microdevices have been reported to date that enable the visualization of plant root10,11,12,13 and moss protonemata14 growth at high resolution. In our device, a series of root hair growth channels are perpendicularly connected to a root growth chamber, and individual root hairs (approximately 7 &#181;m in diameter) are guided to fluidic channels with a 1 &#181;m wide gap. We also cultured moss protonemata (approximately 20 &#181;m in diameter) in a microdevice containing microgaps to examine their responses to these physical barriers. The proposed microfluidic-based approach allows us to explore the capability of various tip-growing plant cells to elongate through extremely small spaces, which cannot be examined by any other currently available method.

<table border="0" cellpadding="0" cellspacing="0" width="927">
	<tbody>
		<tr height="18">
			<td height="18" style="height:19px;width:216px;">PDMS</td>
			<td style="width:165px;">Dow Corning Co.</td>
			<td style="width:119px;">Sylgard184</td>
			<td style="width:427px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Murashige &#38; Skoog Medium</td>
			<td style="width:165px;">Wako Pure Chemical</td>
			<td style="width:119px;">392-00591</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">MES</td>
			<td style="width:165px;">Dojindo</td>
			<td style="width:119px;">345-01625</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sucrose</td>
			<td style="width:165px;">Wako Pure Chemical</td>
			<td style="width:119px;">196-00015</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">50 mm glass-bottom dish</td>
			<td style="width:165px;">Matsunami Glass</td>
			<td style="width:119px;">D210402</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">35 mm glass-bottom dish</td>
			<td style="width:165px;">Iwaki&#160;</td>
			<td style="width:119px;">3971-035</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Surgical blade</td>
			<td style="width:165px;">Feather</td>
			<td style="width:119px;">No.11</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">biopsy punches</td>
			<td style="width:165px;">Harris</td>
			<td style="width:119px;"></td>
			<td>Uni-Core</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Gel loading tips</td>
			<td style="width:165px;">Bio-Bik</td>
			<td style="width:119px;">124-R-204</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Inverted Microscope</td>
			<td style="width:165px;">Olympus</td>
			<td style="width:119px;">IX83</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:22px;width:216px;">CSU-W1</td>
			<td style="width:165px;">Yokogawa Electric</td>
			<td style="width:119px;"></td>
			<td>No Catalog number is avairable for this customized microscope</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">MetaMorph imaging software</td>
			<td style="width:165px;">Molecular Devices</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
	</tbody>
</table>

development of microfluidic devices to study the elongation capability of tip-growing plant cells in extremely small spaces

In vivo, tip-growing plant cells need to overcome a series of physical barriers; however, researchers lack the methodology to visualize cellular behavior in such restrictive conditions. To address this issue, we have developed growth chambers for tip-growing plant cells that contain a series of narrow, micro-fabricated gaps (~1 &#181;m) in a poly-dimethylsiloxane (PDMS) substrate. This transparent material allows the user to monitor tip elongation processes in individual cells during microgap penetration by time-lapse imaging. Using this experimental platform, we observed morphological changes in pollen tubes as they penetrated the microgap. We captured the dynamic changes in the shape of a fluorescently labeled vegetative nucleus and sperm cells in a pollen tube during this process. Furthermore, we demonstrated the capability of root hairs and moss protonemata to penetrate the 1 &#181;m gap. This in vitro platform can be used to study how individual cells respond to physically constrained spaces and may provide insights into tip-growth mechanisms.

As illustrated in Figure 1, tip-growing plant cells encounter a series of physical barriers along their growth paths in vivo. The microfluidic in vitro cell culture platforms presented in this study enabled the examination the of tip-growing process in three types of plant cells (pollen tubes, root hairs, and moss protonemata) through 1 &#181;m artificial gaps (Figure 3, Figure 4, <...

Watch this Scientific Journal Video about Development of Microfluidic Devices to Study the Elongation Capability of Tip-growing Plant Cells in Extremely Small Spaces at JoVE.com

Development of Microfluidic Devices to Study the Elongation Capability of Tip-growing Plant Cells in Extremely Small Spaces

We describe a method to investigate the capability of tip-growing plant cells, including pollen tubes, root hairs, and moss protonemata, to elongate through extremely narrow gaps (~1 &#181;m) in a microfluidic device.

In vivo, tip-growing plant cells need to overcome a series of physical barriers; however, researchers lack the methodology to visualize cellular behavior ...

This method can help answer key questions in the plant cell biology field, such as how tip-growing plant cells penetrate cell physical barriers. The main advantage of this technique is the ability to acquire high-resolution images that rebuilds a cell's deformation process at the micrometer scale gap under a conventional microscope. To make a device for examination of the growing pollen tubes and moss protonemata, first make approximately 11 grams of a PDMS, and load it into a four-inch mold.Then, degas the mold for 20 minutes in a vacuum chamber. Next, cure the mold at 65 degrees Celsius for 90 minutes. Once cured, peel off the PDMS layer from the mold, and open the access holes to the channel within the PDMS using a biopsy punch tool.Next, expose the PDMS and the five-centimeter glass dish to air plasma for 50 seconds. To seal off the microfluidic network, press the PDMS layer onto the glass dish, and cure it for 30 minutes at 65 degrees Celsius. To make a microdevice designed for root hair examination, two molds are loaded and cured.When punching out the channel holes, use a two-millimeter punch. After exposing both layers to air plasma for 50 seconds, join them together, including a coverslip, under a stereomicroscope. Use the custom-made aligner tool to accomplish this.Cure the assembly for 30 minutes in the oven to seal off the microfluidic network. After curing, remove the coverslip from the device, and transfer it to a five-centimeter glass dish. Before using the pollen tube microdevice, degas it in a vacuum chamber for 20 minutes.Add growth medium to the pistil inlet using a fine-tipped micropipette. Load the other wells with the same medium. Allow a few minutes for the channels to fill.Meanwhile, place a moist paper towel into the dish to help maintain local humidity. Now, collect pollen grains from a T.fournieri blue and white flower. Transfer the grains onto the stigma using a dissecting needle.Next, cut a one-centimeter long pollinated style lengthwise, and then insert the cut style into the inlet of the microfluidic device. When the cut style is inserted into the inlet of microfluidic device with tweezers, it is important not to hold the style very tightly because it may damage the style. Then, secure a lid onto the dish using tape, and transfer the dish to an incubator at 28 degrees Celsius where it must remain in the dark for five to six hours.Working under a laminar flow hood, begin by sterilizing transgenic A.thaliana Columbia seeds. Soak them in cleaning solution for five minutes. Then, rinse the seeds in autoclaved water thoroughly.Once sterilized, store the seeds at four degrees Celsius in darkness for 48 hours. A few days later, degas the microdevice for 20 minutes before using it. Next, load the appropriate growth medium into the wells of the device using a finely tipped pipette, and wait a few minutes for the solution to be pulled through all the channels.Also, load some moist paper towel onto the dish to maintain humidity. Now, transfer a prepared seed into the device's inlet. Then, secure the lid on using tape, and transfer the dish to a 22 degree Celsius incubator with continuous light.Before its use, sterilize the microdevice under UV overnight. The following day, degas the microdevice for 20 minutes. Once degassed, load the microwells with the appropriate growth medium.While the channels gradually fill, surround the microdevice with some autoclaved water to maintain humidity. Transfer a small piece of moss protonemata tissue to the inlet of the microdevice. Now, culture the device at 25 degrees Celsius under continuous light.After two to three weeks of growth, take bright-field images of the results under a microscope. Tip-growing plant cells encounter a series of physical barriers along their growth paths in vivo, such as in the transmitting tract and at the micropyle, not to mention the path of root hairs in the soil. The presented microfluidic in vitro cell culture platforms enable the examination of tip-growing process in three types of plant cells, pollen tubes, root hairs, and moss protonemata.Viewing is accomplished through one-micron gaps in the devices. Because micro-gaps of this size are fragile, sometimes they will close, especially after repeated use. Thus, it is important to verify that the gaps are intact before performing the experiments.For the pollen tube study, live-cell imaging was used to monitor morphological changes in the apical region of pollen tubes, as well as the vegetative nucleus and sperm cells in response to encountering an extremely small space. After its development, this technique paved the way for researchers in the field of plant cell biology to explore the elongation capability of tip-growing plant cells, such as pollen tubes, root hairs, and moss protonemata in an extremely small space.

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7.2K Aufrufe.  Nagoya University. This method can help answer key questions in the plant cell biology field, such as how tip-growing plant cells penetrate cell physical barriers. The main advantage of this technique is the ability to acquire high-resolution images that rebuilds a cell's deformation process at the micrometer scale gap under a conventional microscope. To make a device for examination of the growing pollen tubes and moss protonemata, first make approximately 11 grams of a PDMS, and load it into a four-inch mold....