Name: single-cell analysis of the expression of pseudomonas syringae genes within the plant tissue
Uploaded: 2022-10-06T15:00:00
Description: Watch this Scientific Journal Video about Single-Cell Analysis of the Expression of Pseudomonas syringae Genes within the Plant Tissue at JoVE.com

This work was supported by Project Grant RTI2018-095069-B-I00 funded by MCIN/AEI/10.13039/501100011033/ and by &#34;ERDP A way of making Europe&#34;. J.S.R. was funded by Plan Andaluz de Investigaci&#243;n, Desarrollo e Innovaci&#243;n (PAIDI 2020). N.L.P. was funded by Project Grant P18-RT-2398 from Plan Andaluz de Investigaci&#243;n, Desarrollo e Innovaci&#243;n.

1. Plant preparation
<ol>
	<li>Prepare Arabidopsis Col-0 plants following the steps below.
	<ol>
		<li>Fill a 10 cm diameter pot with a 1:3 vermiculite-plant substrate mix (see Table of Materials), previously watered, and cover the pot with a 15 cm x 15 cm metal mesh with 1.6 mm x 1.6 mm holes. Adjust the metal mesh to the wet soil using a rubber band (Figure 1A).</li>
		<li>With a wet toothpick, sow Arabidopsis seeds into the holes of the metal mesh. Plac...

<ol>
	<li>Weigel, W. A., Dersch, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phenotypic+heterogeneity:+A+bacterial+virulence+strategy.">Phenotypic heterogeneity: A bacterial virulence strategy.</a> Microbes and Infection. 20 (9-10), 570-577 (2018).</li><li>Hare, P. J., LaGree, T. J., Byrd, B. A., DeMarco, A. M., Mok, W. W. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-cell+technologies+to+study+phenotypic+heterogeneity+and+bacterial+persisters.">Single-cell technologies to study phenotypic heterogeneity and bacterial persisters.</a> Microorganisms. 9 (11), 2277 (2021).</li><li>Mansfield, J., 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=Top+10+plant+pathogenic+bacteria+in+molecular+plant+pathology.">Top 10 plant pathogenic bacteria in molecular plant pathology.</a> Molecular Plant Pathology. 13 (6), 614-629 (2012).</li><li>Morris, C. E., 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+life+history+of+the+plant+pathogen+Pseudomonas+syringae+is+linked+to+the+water+cycle.">The life history of the plant pathogen Pseudomonas syringae is linked to the water cycle.</a> The ISME Journal. 2 (3), 321-334 (2008).</li><li>Rufi&#225;n, J. 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=Confocal+microscopy+reveals+in+planta+dynamic+interactions+between+pathogenic,+avirulent+and+non-pathogenic+Pseudomonas+syringae+strains.">Confocal microscopy reveals in planta dynamic interactions between pathogenic, avirulent and non-pathogenic Pseudomonas syringae strains.</a> Molecular Plant Pathology. 19 (3), 537-551 (2018).</li><li>Macho, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Subversion+of+plant+cellular+functions+by+bacterial+type-III+effectors:+Beyond+suppression+of+immunity.">Subversion of plant cellular functions by bacterial type-III effectors: Beyond suppression of immunity.</a> New Phytologist. 210 (1), 51-57 (2016).</li><li>Xiao, Y., Hutcheson, S. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+single+promoter+sequence+recognized+by+a+newly+identified+alternate+sigma+factor+directs+expression+of+pathogenicity+and+host+range+determinants+in+Pseudomonas+syringae.">A single promoter sequence recognized by a newly identified alternate sigma factor directs expression of pathogenicity and host range determinants in Pseudomonas syringae.</a> Journal of Bacteriology. 176 (10), 3089-3091 (1994).</li><li>Rufi&#225;n, J. 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=Generating+chromosome-located+transcriptional+fusions+to+fluorescent+proteins+for+single-cell+gene+expression+analysis+in+Pseudomonas+syringae.">Generating chromosome-located transcriptional fusions to fluorescent proteins for single-cell gene expression analysis in Pseudomonas syringae.</a> Methods in Molecular Biology. 1734, 183-199 (2018).</li><li>Rufian, J. 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=Pseudomonas+syringae+differentiates+into+phenotypically+distinct+subpopulations+during+colonization+of+a+plant+host.">Pseudomonas syringae differentiates into phenotypically distinct subpopulations during colonization of a plant host.</a> Environmental Microbiology. 18 (10), 3593-3605 (2016).</li><li>Katagiri, F., Thilmony, R., He, S. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Arabidopsis+thaliana-Pseudomonas+syringae+interaction.">The Arabidopsis thaliana-Pseudomonas syringae interaction.</a> Arabidopsis Book. 1, 0039 (2002).</li><li>Fouts, D. E., 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=Genomewide+identification+of+Pseudomonas+syringae+pv.+tomato+DC3000+promoters+controlled+by+the+HrpL+alternative+sigma+factor.">Genomewide identification of Pseudomonas syringae pv. tomato DC3000 promoters controlled by the HrpL alternative sigma factor.</a> Proceedings of the National Academy of Sciences of the United States of America. 99 (4), 2275-2280 (2002).</li><li>Huber, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Robust Statistics. , (1981).</li><li>Freed, N. E., 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+simple+screen+to+identify+promoters+conferring+high+levels+of+phenotypic+noise.">A simple screen to identify promoters conferring high levels of phenotypic noise.</a> PLoS Genetics. 4 (12), 1000307 (2008).</li><li>Hirano, S. S., Upper, C. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bacteria+in+the+leaf+ecosystem+with+emphasis+on+Pseudomonas+syringae-a+pathogen,+ice+nucleus,+and+epiphyte.">Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae-a pathogen, ice nucleus, and epiphyte.</a> Microbiology and Molecular Biology Reviews. 64 (3), 624-653 (2000).</li><li>Lindeberg, M., Myers, C. R., Collmer, A., Schneider, D. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Roadmap+to+new+virulence+determinants+in+Pseudomonas+syringae:+Insights+from+comparative+genomics+and+genome+organization.">Roadmap to new virulence determinants in Pseudomonas syringae: Insights from comparative genomics and genome organization.</a> Molecular Plant Microbe Interactions. 21 (6), 685-700 (2008).</li><li>Liu, X., 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=Bacterial+leaf+infiltration+assay+for+fine+characterization+of+plant+defense+responses+using+the+Arabidopsis+thaliana-Pseudomonas+syringae+pathosystem.">Bacterial leaf infiltration assay for fine characterization of plant defense responses using the Arabidopsis thaliana-Pseudomonas syringae pathosystem.</a> Journal of Visualized Experiments. (104), e53364 (2015).</li><li>O'Leary, B. M., Rico, A., McCraw, S., Fones, H. N., Preston, 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=The+infiltration-centrifugation+technique+for+extraction+of+apoplastic+fluid+from+plant+leaves+using+Phaseolus+vulgaris+as+an+example.">The infiltration-centrifugation technique for extraction of apoplastic fluid from plant leaves using Phaseolus vulgaris as an example.</a> Journal of Visualized Experiments. (94), e52113 (2014).</li><li>Tornero, P., Dangl, J. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+high-throughput+method+for+quantifying+growth+of+phytopathogenic+bacteria+in+Arabidopsis+thaliana.">A high-throughput method for quantifying growth of phytopathogenic bacteria in Arabidopsis thaliana.</a> The Plant Journal. 28 (4), 475-481 (2001).</li></ol>

The method presented here describes a non-invasive procedure that allows the infiltration of bacteria into the plant foliar tissue, allowing the rapid inoculation of large volumes while minimizing tissue disruption. One of the characteristics of the P. syringae species complex is the ability to survive and proliferate inside the plant apoplast and on the plant surface as epiphyte14. Thus, the possibility that the bacteria extracted using the present protocol come only from the plant apopl...

Pathogenic bacteria display differences in diverse phenotypes, giving rise to the formation of subpopulations within genetically identical populations. This phenomenon is known as phenotypic heterogeneity and has been proposed as an adaptation strategy during bacterial-host interactions1. Recent advances in the optical resolution of confocal microscopes, flow cytometry, and microfluidics, combined with fluorescent proteins, have fostered single-cell analyses of bacterial populations2.
The Gram-negative Pseudomonas syringae is an archetypal plant pathogenic bacteria due to both its academic and economic importance3. The life cycle of P. syringae is linked to the water cycle4. P. syringae enters the intercellular spaces between the mesophyll cells, the plant leaf apoplast, through natural apertures such as stomata or wounds5. Once within the apoplast, P. syringae relies on the type III secretion system (T3SS) and the type III-translocated effectors (T3E) to suppress plant immunity and manipulate plant cellular functions for the benefit of the pathogen6. The expression of T3SS and T3E depends on the master regulator HrpL, an alternative sigma factor that binds to the hrp-box motifs in the promoter region of the target genes7. 
By generating chromosome-located transcriptional fusions to fluorescent protein genes downstream of the gene of interest, one can monitor gene expression based on the fluorescence levels emitted at the single-cell level8. Using this method, it has been established that the expression of hrpL is heterogeneous both within bacterial cultures grown in the laboratory and within bacterial populations recovered from the plant apoplast8,9. Although gene expression analysis at the single-cell level is typically performed in bacterial cultures grown in laboratory media, such analyses can also be carried out on bacterial populations growing within the plant, thus providing valuable information on the formation of subpopulations in the natural context. A potential limitation for the analysis of bacterial populations extracted from the plant is that classic inoculation methods by syringe-pressure infiltration into the apoplast, followed by bacterial extraction by maceration of the leaf tissue, typically generate a large amount of cellular plant debris that interferes with downstream analysis10. Most cellular debris consists of autofluorescent fragments of chloroplasts that overlap with GFP fluorescence, resulting in misleading results. 
The present protocol describes the process of analyzing single-cell gene expression heterogeneity in two model pathosystems: the one formed by the P. syringae pv. tomato strain DC3000 and Arabidopsis thaliana (Col-0), and the other by the P. syringae pv. phaseolicola strain 1448A and bean plants (Phaseolus vulgaris cultivar Canadian Wonder). An inoculation method is proposed based on vacuum infiltration using a vacuum chamber and a pump, resulting in a fast and damage-free method to infiltrate whole leaves. Furthermore, as an improvement on conventional protocols, a gentler method is used to extract the bacterial population from the apoplast that significantly reduces tissue disruption, based on the extraction of apoplastic fluid by applying cycles of positive and negative pressure using a small amount of volume within a syringe.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>0.17 mm coverslip</td>
			<td></td>
			<td></td>
			<td>No special requirements</td>
		</tr>
		<tr>
			<td>1.6 x 1.6 mm metal mesh</td>
			<td>Buzifu</td>
			<td></td>
			<td>Fiberglass screen mesh</td>
		</tr>
		<tr>
			<td>10 cm diameter pots</td>
			<td></td>
			<td></td>
			<td>No special requirements</td>
		</tr>
		<tr>
			<td>140 mm Petri dishes</td>
			<td></td>
			<td></td>
			<td>No special requirements</td>
		</tr>
		<tr>
			<td>20 mL syringe</td>
			<td></td>
			<td></td>
			<td>No special requirements</td>
		</tr>
		<tr>
			<td>50 mL conical tubes</td>
			<td>Sarstedt</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Agarose</td>
			<td>Merk</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Ampicillin sodium</td>
			<td>GoldBio</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Bacteriological agar</td>
			<td>Roko</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Confocal Microscope Stellaris</td>
			<td>Leica Microsystems</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>FACSVerse cell analyzer</td>
			<td>BD Biosciences</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Fiji software</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Gentamycin sulfate</td>
			<td>Duchefa</td>
			<td>G-0124</td>
			<td></td>
		</tr>
		<tr>
			<td>Kanamycin monosulfate</td>
			<td>Phytotechnology</td>
			<td>K378</td>
			<td></td>
		</tr>
		<tr>
			<td>MgCl2</td>
			<td>Merk</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>NaCl</td>
			<td>Merk</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Parafilm</td>
			<td>Pechiney Plastic Packaging</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Plant substrate</td>
			<td></td>
			<td></td>
			<td>No special requirements</td>
		</tr>
		<tr>
			<td>Silwet L-77</td>
			<td>Cromton Europe Ltd</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Toothpicks</td>
			<td></td>
			<td></td>
			<td>No special requirements</td>
		</tr>
		<tr>
			<td>Tryptone</td>
			<td>Merk</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Tweezers</td>
			<td></td>
			<td></td>
			<td>No special requirements</td>
		</tr>
		<tr>
			<td>Vacuum chamber 25 cm diameter</td>
			<td>Kartell</td>
			<td>554</td>
			<td></td>
		</tr>
		<tr>
			<td>Vacuum pump</td>
			<td>GAST</td>
			<td>DOA-P504-BN</td>
			<td></td>
		</tr>
		<tr>
			<td>Vermiculite</td>
			<td></td>
			<td></td>
			<td>No special requirements</td>
		</tr>
		<tr>
			<td>Yeast Extract</td>
			<td>Merk</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

single-cell analysis of the expression of pseudomonas syringae genes within the plant tissue

A plethora of pathogenic microorganisms constantly attack plants. The Pseudomonas syringae species complex encompasses Gram-negative plant-pathogenic bacteria of special relevance for a wide number of hosts. P. syringae enters the plant from the leaf surface and multiplies rapidly within the apoplast, forming microcolonies that occupy the intercellular space. The constitutive expression of fluorescent proteins by the bacteria allows for visualization of the microcolonies and monitoring of the development of the infection at the microscopic level. Recent advances in single-cell analysis have revealed the large complexity reached by clonal isogenic bacterial populations. This complexity, referred to as phenotypic heterogeneity, is the consequence of cell-to-cell differences in gene expression (not linked to genetic differences) among the bacterial community. To analyze the expression of individual loci at the single-cell level, transcriptional fusions to fluorescent proteins have been widely used. Under stress conditions, such as those occurring during colonization of the plant apoplast, P. syringae differentiates into distinct subpopulations based on the heterogeneous expression of key virulence genes (i.e., the Hrp type III secretion system). However, single-cell analysis of any given P. syringae population recovered from plant tissue is challenging due to the cellular debris released during the mechanical disruption intrinsic to the inoculation and bacterial extraction processes. The present report details a method developed to monitor the expression of P. syringae genes of interest at the single-cell level during the colonization of Arabidopsis and bean plants. The preparation of the plants and the bacterial suspensions used for inoculation using a vacuum chamber are described. The recovery of endophytic bacteria from infected leaves by apoplastic fluid extraction is also explained here. Both the bacterial inoculation and bacterial extraction methods are empirically optimized to minimize plant and bacterial cell damage, resulting in bacterial preparations optimal for microscopy and flow cytometry analysis.

The expression of the type III secretion system is essential for bacterial growth within the plant. The timely expression of T3SS genes is achieved through intricate regulation, at the center of which is the extracytoplasmic function (ECF) sigma factor HrpL, the key activator of the expression of T3SS-related genes11. An analysis of the expression of hrpL was previously carried out using a chromosome-located transcriptional fusion to a downstream promoterless gfp gene and by foll...

Watch this Scientific Journal Video about Single-Cell Analysis of the Expression of Pseudomonas syringae Genes within the Plant Tissue at JoVE.com

Single-Cell Analysis of the Expression of Pseudomonas syringae Genes within the Plant Tissue

The present protocol describes a method that allows single-cell gene expression analysis on Pseudomonas syringae populations grown within the plant apoplast.

Single-Cell Analysis of the Expression of Pseudomonas syringae Genes within the Plant Tissue

A plethora of pathogenic microorganisms constantly attack plants. The Pseudomonas syringae species complex encompasses Gram-negative plant-pathogenic ...

Our protocol provides a gentle plant inoculation and extraction method that allows recovering bacterial populations grown in the leaf apoplast suitable for single-cell gene expression analysis such as flow cytometry. This method allows the extraction of a considerable amount of bacteria from the apoplast without significantly contaminating the samples with plant cellular debris. This optimized method for apoplastic bacteria recovery can be applied directly or with minor modifications to a number of plant bacterial systems comprising either plant pathogenic bacteria or beneficial endocytic bacteria.Demonstrating the procedure will be Nieves Lopez-Pagan, a Ph.D student from our laboratory. To begin, fill a 10-centimeter-diameter pot with previously watered 1:3 vermiculite plant substrate mix. Cover the pot with a perforated metal mesh and adjust the metal mesh to the wet soil using a rubber band.Next, sow Arabidopsis seeds into the holes of the metal mesh with a wet toothpick. Place three to four seeds in distant positions within the pot. Cover the pot with a plastic dome to maintain high relative humidity and incubate for stratification at four degrees Celsius for 72 hours.Transfer the pot to a plant growth chamber under short-day conditions. To uncover the pot, remove the plastic dome. Once the seeds are germinated, use the tweezers to remove most of the seedlings, keeping one seedling in each of the positions of the pot.Then prepare Phaseolus vulgaris bean plants by covering the bottom of a Petri dish with a wet piece of paper towel and placing the bean seeds on top of it. Seal the Petri dish with surgical tape and incubate at 28 degrees Celsius for three to four days. Transfer the germinated seeds into a 10-centimeter-diameter pot filled with wet 1:3 vermiculite plant substrate mix and incubate in a plant growth chamber under long-day settings.Streak out the glycerol stock of Pseudomonas syringae strain of interest from minus 80 degrees Celsius into an LB plate supplemented with the appropriate antibiotics. Incubate the plate at 28 degrees Celsius for 40 to 48 hours. Scrape the bacterial biomass and resuspend it in five milliliters of 10-millimolar magnesium chloride.Measure the optical density at 600 nanometers and adjust it to 0.1 by adding 10-millimolar magnesium chloride. Perform serial dilutions in 10-millimolar magnesium chloride to get a final inoculum concentration of five times 10 to the fifth CFU per milliliter. Then, prepare 200 milliliters of inoculum for the Arabidopsis plants and 50 milliliters for the bean plants.Before inoculation, add the surfactant Silwet L-77 to a final concentration of 0.02%for bean inoculation and 0.01%for Arabidopsis. For Arabidopsis infiltration, place two wood sticks forming an X over the pot and place the pot facing down over a 14-centimeter diameter Petri dish containing 200 milliliters of inoculum. For bean leaves inoculation, insert the plants and the inoculum solution into a vacuum chamber and introduce the leaf into a 50-milliliter conical centrifuge tube containing the inoculum.Give a pulse of 500 millibars for 30 seconds to infiltrate the leaves. Drain the excess inoculum solution with a piece of paper and return the plans to their corresponding growth chamber. Four days after inoculation, cut either the aerial part of the Arabidopsis plant or the inoculated leaf from the bean plant and place it into a 20-milliliter syringe without a needle.For bean leaves, roll the leaf on itself, leaving the abaxial face outward. Add enough volume of distilled water to cover the tissue. Then, insert the plunger holding the syringe in an upright position and remove the excess air and air bubbles inside the syringe by gently tapping the barrel until all the air is located near the tip, and, after removing the air, cover the tip of the syringe barrel with paraffin film.Now carefully press the plunger to generate positive pressure until the tissue turns darker. Then, pull the plunger to generate negative pressure. Remove the paraffin film and the plunger and collect the fluid containing the apoplast-extracted bacteria.For a single-cell analysis, prepare a 1.5%agarose solution in distilled water. Once melted, add enough volume to fill the space between two microscopy slides set side by side and place another slide on top. Let them drive for 15 minutes and remove the slide placed on the top carefully.Using a blade, cut the agarose pad into five millimeter by five millimeter pieces before use. Parallely, centrifuge one millimeter of the apoplast-extracted bacteria. Carefully remove the supernatant using a pipette and resuspend the pellet into 20 microliters of water to concentrate the cells.Place a two-microliter drop of the concentrated cells onto a 0.17-millimeter cover slip and cover the drop with a piece of agarose pad. Visualize the bacterial preparation under the confocal microscope to identify green fluorescent bacteria using specific lasers. Identify all the bacteria using the bright-field and merge both fields.To perform analysis by flow cytometry, take an aliquot of the apoplast-extracted bacteria suspensions. Analyze using the flow cytometer and acquire 100, 000 events. Expression of hrpL GFP is monitored by GFP fluorescence.The dot plot graphs show the fluorescence intensity distribution of GFP versus cell size in the population, while the histograms show the fluorescence intensity of GFP versus cell counts. The hrpL ON percentages were higher than the corresponding percentages of hrpL OFF. Fluorescence microscopy images show heterogeneous levels of GFP associated with the expression of hrpL.The bacteria displaying low or no GFP fluorescence are observed. For optimal results, be gentle during the bacteria extraction steps to avoid damaging the plant tissue and thus limit contamination from plant cellular content. The bacteria samples obtained can be used for other purposes where reducing plant contamination is an advantage, such as nucleic acid extractions for subsequent bacterial genomic or transcriptomic analysis.

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