- Open Access
In vivo monitoring of plant small GTPase activation using a Förster resonance energy transfer biosensor
- Hann Ling Wong†1, 2,
- Akira Akamatsu†1, 3,
- Qiong Wang†4, 5,
- Masayuki Higuchi1,
- Tomonori Matsuda1,
- Jun Okuda1,
- Ken-ichi Kosami4,
- Noriko Inada6,
- Tsutomu Kawasaki1, 7,
- Takako Kaneko-Kawano8,
- Shingo Nagawa9, 10,
- Li Tan9,
- Yoji Kawano1, 4, 11Email authorView ORCID ID profile and
- Ko Shimamoto^1
© The Author(s) 2018
- Received: 20 January 2018
- Accepted: 29 June 2018
- Published: 7 July 2018
Small GTPases act as molecular switches that regulate various plant responses such as disease resistance, pollen tube growth, root hair development, cell wall patterning and hormone responses. Thus, to monitor their activation status within plant cells is believed to be the key step in understanding their roles.
We have established a plant version of a Förster resonance energy transfer (FRET) probe called Ras and interacting protein chimeric unit (Raichu) that can successfully monitor activation of the rice small GTPase OsRac1 during various defence responses in cells. Here, we describe the protocol for visualizing spatiotemporal activity of plant Rac/ROP GTPase in living plant cells, transfection of rice protoplasts with Raichu-OsRac1 and acquisition of FRET images.
Our protocol should be adaptable for monitoring activation for other plant small GTPases and protein–protein interactions for other FRET sensors in various plant cells.
- Small GTPase
- FRET sensor
- Plant immunity
Scientists have wanted for many years to observe events in living cells at the molecular level, and fluorescent proteins offer powerful tools for doing so. Förster Resonance Energy Transfer (FRET) is a physical phenomenon that can be sensitive to changes in molecular conformation and association typically in the 1–10 nm range [1, 2]. In FRET applications, a pair of donor and acceptor fluorescent proteins is used to monitor a variety of biological events including protein–protein interactions, protein kinase activities, post-translational modifications and the dynamics of second messengers [3–6]. When the emission spectrum of a donor fluorescent protein overlaps with the absorption spectrum of an acceptor fluorescent protein, and the distance between the two proteins is less than 10 nm, non-radiative transfer of energy occurs from the donor to the acceptor.
Small GTPases exhibit both GDP/GTP-binding activity and GTPase activity, and work as molecular switches by cycling between GDP-bound inactive and GTP-bound active forms. Rac/ROP family small GTPases are master regulators, controlling various signalling systems in plants such as those governing pollen tube growth, root hair development, auxin signaling, and disease resistance [7–10]. Therefore, monitoring the activation state of a Rac/ROP GTPase within living cells is critical to further understanding its functions. Until the 1990s, activation states of small GTPases were measured in vivo by labelling cells with inorganic [32P] phosphate, followed by immunoprecipitation of small GTPases and thin-layer chromatography to obtain quantitative data for their associated GDP and GTP levels. However, this method required not only large amounts of radioisotope but also a time-consuming procedure. Currently, two alternative non-radioactive techniques are available for monitoring the in vivo activation of Rac/ROP: a PAK CRIB pull-down assay, and FRET sensors [5, 11–13]. These methods exploit the selective interaction between the Cdc42/Rac interactive binding (CRIB) domain of the Rac-effector PAK1 (PAK CRIB) and plant and animal Rac GTPases. PAK CRIB shows a high affinity only for the active GTP-bound form of Rac, and not for the inactive GDP-bound form [11, 13]. PAK CRIB binding suppresses the intrinsic GTPase activity of Rac, and this function provides a useful tool for isolating active GTP-bound forms of Rac/ROP from crude cell lysates and for monitoring the activation state of Rac/ROP in living cells. Currently, kits using GST-tagged PAK CRIB for a PAK CRIB pull-down assay are available from several manufacturers (e.g., Cytoskeleton). This is a widely used method for the semi-quantitative measurement of Rac/ROP activity from plants to animals [11, 12, 14]. However, the PAK CRIB pull-down assay only provides a snapshot, and cannot detect the spatial and temporal dynamics of intracellular signaling in living cells. To solve this problem, FRET biosensors for small GTPases have been developed.
FRET biosensors that have been developed to visualize small GTPase activity in living cells can be classified into two types: intramolecular (or unimolecular) and intermolecular (or bimolecular) biosensors. Both types exploit the ability of two different proteins to interact with each other when one of them is activated. Intramolecular biosensors have both donor and acceptor fluorescent proteins in one molecule, whereas a donor and an acceptor fluorescent protein are expressed separately in intermolecular biosensors. The first report on in vivo monitoring of a small GTPase involved intermolecular biosensors, namely the combination of animal cultured cells expressing green fluorescent protein (GFP)-Rac as the donor and microinjection of PAK CRIB labeled with Alexa 546 dye as the acceptor, which binds selectively to the GTP-active form of Rac . In plants, an intermolecular FRET biosensor was built to analyze ROP2 activity. With this sensor, which used a downstream effector protein of the small GTPase ROP2, CFP-RIC4 (donor), and YFP-ROP2 (acceptor), it was shown that ROP2 is preferentially activated in lobe-forming regions of the cell cortex .
However, there are several disadvantages in systems involving intermolecular biosensors. For example, to obtain reliable data from FRET analyses, we have to control the expression levels of donors and acceptors . The expression level of the acceptor should be higher than that of the donor; moreover, a high proportion of activated donors must associate with acceptors to obtain sufficiently high signal/noise ratios to prevent FRET signals from being buried under noise. In plants, particle bombardment of plasmids into leaf epidermal pavement cells of Arabidopsis is often carried out for FRET analyses, but it is technically difficult to obtain appropriate levels of expression of donors and acceptors in Arabidopsis using intermolecular biosensors. Moreover, excessive expression of acceptors can cause abnormal activation or inhibition of downstream molecules.
To overcome these disadvantages, Matsuda and his colleagues have developed excellent intramolecular FRET biosensors for small GTPases in animal cells, collectively naming them Raichu (Ras superfamily and interacting protein chimeric unit). Raichu was initially developed to study activation of the small GTPases Ras and Rap1 following growth factor stimulation in animal cells [5, 18]. The original Raichu contains a donor (cyan-emitting fluorescent protein; CFP), an acceptor (yellow-emitting fluorescent protein; YFP), and the Ras-binding domain of Raf (RBD), which is a downstream effector and binds specifically to active Ras. Therefore, the molar ratio of the individual component proteins is the same irrespective of expression level. Accordingly, this intramolecular FRET biosensor eliminates the problem of variability in the expression levels of donor and acceptor fluorescent proteins and is an ideal sensor for monitoring the activation states of small GTPases.
We have previously revealed that the small GTPase OsRac1 is an important regulator controlling rice immunity [9, 10, 20], and monitoring its activation within living cells is therefore the next key step in further elucidating how plants trigger immunity. To monitor activation states of OsRac1, we have developed a plant version of the Raichu-Rac1 system by combining the modification of human Raichu-Rac1 and a rice protoplast transfection system. Protoplasts do not possess a cell wall, and this enables direct live imaging of events both within the cell and at the cell surface, simultaneously and with no time delay in the response. Rice protoplasts also display a high growth rate and a high transfection rate, and we can control the expression levels of FRET sensors in plant cells without difficulty. Our work has pioneered the monitoring of spatiotemporal activities of plant small GTPases in living cells, which had been impossible by conventional biochemical methods . We have observed the resistance (R) protein Pit, an immune receptor that activates OsRac1 at the plasma membrane [13, 21], and reported that OsRac1 is activated within 3 min after treatment with a fungal cell wall component, chitin , which is a microbe-associated molecular pattern (MAMP) that triggers plant immunity.
2,4-Dichlorophenoxyacetic acid (2,4-D) (C8H6Cl2O) (Sigma, Cat. no. D7299).
Agarose, low gelling temperature (Sigma, Cat. no. A9414).
Ammonium sulphate ((NH4)2SO4) (Sigma, Cat. no. A3920).
Boric acid (H3BO3) (Sigma, Cat. no. B6768).
Calcium chloride dihydrate (CaCl2·2H2O) (Sigma, Cat. no. C5080).
Cellulase RS (Yakult, Cat. no. C8260).
Chitin (N,N′,N″,N″′,N″″,N″′″-hexaacetylchitohexaose) (Carbosynth., Cat. no. OH07433).
Copper(II) sulphate pentahydrate (CuSO4·5H2O) (Sigma, Cat. no. C8027).
d-mannitol (C6H14O) (Sigma, Cat. no. M1902).
Deionized, distilled water (ddH2O).
Ethylenediaminetetraacetic acid disodium salt dihydrate (Na2EDTA) (Sigma, Cat. no. E5134).
Glycine (C2H5NO) (Sigma, Cat. no. G7126)
Iron(II) sulphate heptahydrate (FeSO4·7H2O) (Sigma, Cat. no. F8633).
Macerozyme R10 (Yakult, Cat. no. C1003).
Magnesium chloride (MgCl2) (Sigma, Cat. no. M8266).
Magnesium sulphate heptahydrate (MgSO4·7H2O) (Sigma, Cat. no. 63138).
Manganese(II) sulphate tetrahydrate (MnSO4·4H2O) (Alfa Aesar, Cat. no. B22081)! CAUTION It is toxic and dangerous for the environment. Handle it with care, and wear gloves and eye protection.
MES monohydrate (C6H13NO4S·H2O) (Fluka, Cat. no. 69889).
Murashige and Skoog (MS) Vitamin (Sigma, Cat. no. M3900).
Myo-inositol (C6H12O6) (Sigma, Cat. no. I7508).
Nicotinic acid (C6H5NO2) (Sigma, Cat. no. N0761).
Polyethylene glycol (PEG 4000) (Sigma, Cat. no. 81240).
Potassium chloride (KCl) (Sigma, Cat. no. P9541).
Potassium nitrate (KNO3) (Sigma, Cat. no. P6083).
Pyridoxine hydrochloride (C8H12ClNO3) (Sigma, Cat. no. 47862).
Plasmid Maxi Kit (Qiagen, Cat. no. 12165).
Sodium chloride (NaCl) (Sigma, Cat. no. S5886).
Sodium hydroxide (NaOH) (Sigma, Cat. no. S8045)! CAUTION NaOH is corrosive. Handle it with care, and wear gloves and eye protection.
Sodium molybdate dihydrate (Na2MoO4·2H2O) (Sigma, Cat. no. M1003).
Sodium phosphate monobasic dihydrate (NaH2PO4·2H2O) (Sigma, Cat. no. 71505).
Sucrose (C12H22O11) (Sigma, Cat. no. S0389).
Thiamine hydrochloride (C12H18Cl2N4OS) (Sigma, Cat. no. T1270).
Zinc sulphate heptahydrate (ZnSO4·7H2O) (Sigma, Cat. no. Z0251).
0.2-μm filter 250 ml (CORNING 431096).
440-nm diode laser (iFLEX 2000, Point Source).
60× oil-immersion objective lens (UPlanSApo 60×/1.42, Olympus).
Centrifuge with a swinging bucket rotor (Eppendorf: 5804R and S-4-72).
Confocal scanner (Yokogawa CSU22).
Cooled charge-coupled device camera (Hamamatsu Photonics, EM-CCD C9100-02).
Cover glass (Matsunami Glass, Small: 24 × 32 mm, 0.13–0.17 mm, C024321, Large: 24 × 60 mm, 0.13–0.17 mm, C024601).
Dual-View image splitter (DualView: Optical Insights).
FRET filter for dual-emission imaging (Omega Optical, CFP: 480 ± 15 nm and Venus: 535 ± 20 nm).
Inverted fluorescence microscope (Olympus IX-81).
MetaMorph software (Universal Imaging).
Microscope slide printed with highly water-repellent mark (Matsunami Glass, Cat. no. TF0215).
Nylon mesh (40 μm, Kyoshin Rikoh, Cat. no. PP-40n).
Leica SMD FLCS microscope.
- 20 mg/ml 2,4-D:
Dissolve 0.2 g of 2,4-D in 10 ml of 70% (v/v) Ethanol. Store the solution at 4 °C in dark for up to 6 months
- 1M CaCl2:
Dissolve 14.7 g of CaCl2·2H2O in 100 ml of ddH2O and autoclave. Store it at RT for up to 12 months.
- Cellulase solution:
Add 140 mg of CaCl2·2H2O, 0.3 g of MES·H2O, 12.0 g of cellulase RS and 3.0 g of Macerozyme R10 to 200 ml of ddH2O and stir gently for 10–30 min. Next, add 18.2 g of d-mannitol and adjust the final volume to 300 ml with ddH2O. Filter the solution through three layers of filter paper. Adjust the pH to 5.6 and sterilize the solution with a 0.2-μm filter. Finally, divide into 20-ml aliquots in 50-ml tubes and store at − 20 °C.
- 1 M FeEDTA:
Dissolve 375 mg of Na2EDTA and 275 mg of FeSO4·7H2O in 80 ml of ddH2O. Adjust the final volume to 100 ml. Store at 4 °C.
- 40 mM KNO3:
Dissolve 0.4 g of KNO3 in 100 ml of ddH2O. Store at RT for up to 6 months.
- 0.8 M Mannitol:
Dissolve 14.6 g of d-mannitol in 100 ml of ddH2O and autoclave. Store at RT for up to 6 months.
- 1 mM MgSO4:
Dissolve 25 mg of MgSO4·7H2O in 100 ml of ddH2O and autoclave. Store at RT for up to 6 months.
- MMG solution:
Dissolve 14.6 g of d-mannitol, 0.6 g of MgCl2 and 170 mg of MES·H2O in 160 ml of ddH2O. Adjust the pH to 5.7 with 5 N NaOH and dilute to 200 ml with ddH2O. Autoclave and divide into 15-ml Falcon tubes. Store at RT for up to 1 month.
- 2 mM NaH2PO4:
Dissolve 31 mg of NaH2PO4·2H2O in 100 ml of ddH2O. Store at RT for up to 6 months.
- 2.5 mM (NH4)2SO4:
Dissolve 33 mg of MgSO4·7H2O in 100 ml of ddH2O. Store at RT for up to 6 months.
- PEG solution:
Prepare just before use. Dissolve 4.0 g of PEG4000 in 3 ml of ddH2O. Add 2.5 ml of 0.8 M d-mannitol and 1 ml of 1 M CaCl2. Adjust volume to 10 ml with ddH2O. Dissolve and sterilize with a 0.2-μm filter.
- R2 Macro 20× solution:
Dissolve 80.0 g of KNO3, 6.7 g of (NH4)2SO4, 5.0 g of MgSO4·7H2O, 3.0 g of CaCl2·2H2O and 5.5 g of NaH2PO4·2H2O in 800 ml of ddH2O. Adjust the volume to 1 l with ddH2O. Store at 4 °C.
- R2 Micro 1000× solution:
Dissolve 160 mg of MnSO4·4H2O, 220 mg of ZnSO4·7H2O, 13 mg of CuSO4·5H2O, 0.6 g of H3BO3 and 13 mg of Na2MoO4·2H2O in 800 ml of ddH2O. Adjust the volume to 1 l with ddH2O and store at 4 °C.
Dissolve 30.0 g of sucrose in 800 ml of ddH2O. Add 50 ml of 20× R2 Macro, 2 ml of FeEDTA, 1 ml of 1000× R2 Micro, 1 ml of MS Vitamin and 0.1 ml of 2,4-D. Adjust the pH of the solution to 5.7 with NaOH, and the volume to 1 l with ddH2O. Autoclave and store this medium at 4 °C for up to 1 month.
- W5 solution:
Dissolve 1.8 g of NaCl, 2.8 g of CaCl2·2H2O, 70 mg of KCl and 90 mg of MES·H2O in 160 ml of ddH2O. Adjust the pH of the solution to 5.7 with 5 N NaOH, and the volume to 200 ml with ddH2O. Autoclave and store at RT for up to 1 month.
Construction of Raichu-OsRac1
We first tested the binding specificity of the CRIB domain of PAK for OsRac1 using a pull-down assay. PAK CRIB specifically bound to the constitutively active (G19V) form of OsRac1 (OsRac1 CA) and not to the dominant negative (T24N) form (OsRac1 DN), indicating that the interaction between PAK CRIB and OsRac1 occurs only when OsRac1 is activated . The backbone of our Raichu-OsRac1 is essentially identical to that made by Itoh et al. , except that human Rac1 and the C-terminal polybasic region and CAAX box of human Ki-Ras are replaced by rice OsRac1 and the corresponding elements of OsRac1, respectively . The choice of C-terminal polybasic region and post-translational modification site is also important, because subcellular localization of small GTPases is strongly influenced by these functions . To determine whether the activation of OsRac1 increases FRET efficiency in vivo, we also prepared CA and DN forms of Raichu-OsRac1.
Cell culture and preparation of Oc cells—time: 3 days
We used Oryza sativa L. C5924 (Oc) cells to generate rice protoplasts for transfection assays of Raichu-OsRac1, because their transfection efficiency is much higher than that of protoplasts derived from other rice cultivars .
*All procedures should be done on a clean bench to avoid contamination
Use fully saturated suspension cells cultured in fresh R2S medium for 3 days.
Isolation of protoplasts from Oc cells—time: 6 h
We developed a rice protoplast isolation and transfection method, according to Yoo et al., with minor modifications . Cell walls of Oc cells are removed using cellulase and the protoplasts transfected with Raichu-OsRac1 vectors by a PEG method.
Prepare a conical flask with a funnel and nylon mesh filter, and autoclave (Fig. 3c).
Irradiate a clean bench with ultraviolet light for 10–20 min.
Pre-warm cellulase solution at 30 °C to thaw.
Completely remove R2S medium from a flask containing a 3-day culture (Fig. 3a); add 20 ml of cellulase solution and cover the flask with aluminium foil.
Incubate the cells at 30 °C with shaking at 50 rpm for 3–5 h (Fig. 3d).
Add 20 ml of W5 solution and filter the cells through a sheet of 40-μm nylon mesh (Fig. 3e).
* All procedures should be conducted on ice.
Transfer the filtered cells to a 50-ml Falcon tube.
Centrifuge the tube at 120g for 4 min using a swinging bucket rotor.
Decant and discard the supernatant.
Add 20 ml of W5 solution to the protoplast pellet and suspend it by gentle swirling.
Centrifuge at 120g for 4 min using a swinging bucket rotor.
Discard the supernatant and add 2 ml of W5 solution.
Keep the suspension on ice for 30 min.
Dilute 10 μl of the protoplast suspension with 90 μl of W5 solution to measure the density of protoplasts using a haemocytometer, and estimate the total number of protoplasts.
Centrifuge at 120g for 4 min using a swinging bucket rotor and discard the supernatant.
Resuspend the protoplasts at 2 × 106 cells/ml with MMG.
*CRITICAL STEP For general experiments such as localization studies and analyses of gene expression, protoplasts can be kept at 4 °C for up to 24 h, but freshly prepared protoplasts are essential for Raichu-OsRac1 experiments.
Transfection of protoplasts with Raichu-OsRac1 plasmids
All steps are carried out at 25 °C (room temperature).
Prepare plasmid DNA for transfection and fresh PEG solution.
Add the required plasmids to a 1.5-ml tube.
Add 100 μl of protoplast suspension (2 × 105 cells).
Add 110 μl of PEG solution and mix gently by inverting the tube five times.
Incubate the transfection mixture for 20 min at room temperature.
Add 1 ml of W5 solution to the tube and mix gently by inverting five times.
Centrifuge at 120g for 10 min and discard the supernatant.
Add 100 μl of W5 solution.
Suspend the protoplasts gently and place the tube in a light-resistant box on a slant.
Incubate in the dark at 30 °C for 10–16 h.
FRET imaging and data processing
Raichu-OsRac1 proteins begin to be expressed 8 h after transfection, and it usually takes until about 10 h after transfection to achieve sufficient expression of the biosensors. The optimal observation time is 10–16 h after transfection. FRET efficiencies may vary depending on the timing of observation, due to differences in the time required for maturation of donor and acceptor fluorescent proteins, and it is better to avoid observing cells beyond 16 h after transfection in our condition. The expression levels of Raichu-OsRac1 are different in individual cells and it is important to select cells displaying plasma membrane localization  and comparable levels of Raichu-OsRac1 .
Mount 10 μl of the transfected rice protoplasts on a microscope slide.
Open MetaMorph software and obtain images of Raichu-OsRac1.
Subtract background and make ratio images.
Merge processed CFP and Venus images into a single image and measure the Venus/CFP ratio.
For statistical analyses, corrected FRET (cFRET) followed by normalized FRET (nFRET) are calculated, according to Sorkin et al.,  with minor modifications. Fluorescence in the Venus channel consists of FRET and non-FRET fluorescence derived from the cross-over of Venus and CFP fluorescence through the FRET filters. Cross-over between Venus and CFP fluorescence divides into two factors: CFP emission detected in the Venus channel (xCFP) and Venus emission induced by CFP excitation (yVenus). cFRET is calculated as follows: cFRET = total Venus − xCFP – yVenus. The extent of cross-over (x and y) is a characteristic value in each optical system and must be obtained in advance using cells expressing either Venus or CFP and the following formulae:
To examine whether the activation of OsRac1 increased FRET efficiency in living cells, we transfected rice protoplasts with Raichu-OsRac1 WT, DN and CA. OsRac1 has been shown previously to localize to the plasma membrane . Consistent with this, Raichu-OsRac1 was localized mainly at the plasma membrane. The ratio of Venus/CFP fluorescence in protoplasts expressing Raichu-CA-OsRac1 was higher than those in protoplasts expressing either Raichu-WT-OsRac1 or Raichu-DN-OsRac1 (Fig. 5a), indicating that the interaction between CRIB and OsRac1 occurs only when OsRac1 is activated. These results demonstrate that the ratios of Venus/CFP fluorescence for Raichu-OsRac1 reflect the activation state of OsRac1 in rice protoplasts.
We have previously found that a PRONE-type GEF, OsRacGEF1, displays GEF activity towards OsRac1 in vitro and plays an important role in the activation of OsRac1 in chitin-induced immune responses . To extend that observation, we next tried to monitor the activation of OsRac1 by OsRacGEF1 in living cells. In control GUS-transfected cells, the ratio of Venus/CFP fluorescence of Raichu-OsRac1 was low, but when we expressed the PRONE domain of OsRac1GEF1 (OsRacGEF1 PRONE), the ratio increased (Fig. 5b), showing that OsRacGEF1 PRONE indeed activates OsRac1 in rice protoplasts.
To elucidate the spatiotemporal regulation of OsRac1 activation in MAMP-triggered immunity, we treated the rice protoplasts with chitin, which is a component of pathogenic and non-pathogenic fungi and is one of the best-studied MAMPs. Chitin markedly and rapidly induced OsRac1 activation in the rice protoplasts; in contrast, the OsRac1 activation level was negligible in cells treated with buffer (Fig. 6). Together, these results show that OsRac1 is rapidly activated at the plasma membrane of rice protoplasts after chitin elicitor treatment.
Troubleshooting (also see Table 1)
Low protoplast yield and low transfection efficiency
Condition of suspension cells
Suspension cell condition is the most important factor for obtaining successful results. If you have problems with a transfection, prepare new suspension cells from your stock or rice seeds
Insufficient cellulase digestion
Optimize the period of cellulase treatment or prepare fresh cellulase solution
Problem with transfection reagents
Prepare new transfection reagents. PEG solution can not be stored, and you should prepare fresh PEG for each experiment
Low DNA quality
Use an animal transfection grade of DNA purification kit
Decay of FRET signal during imaging
Photobleaching by the excitation
Minimize exposure time and duration of laser excitation
Plasmid purity affects the transfection efficiency and we highly recommend the use of animal transfection grade plasmid purification kits or CsCl gradient purification.
In this study, we established a FRET sensor to monitor activation of small GTPase OsRac1. Moreover, using the same protocol, we have previously monitored the interaction between OsRac1 and its effector protein OsRbohB by FRET sensor and revealed that cytosolic Ca2+ concentration regulates the OsRac1-OsRbohB interaction in a dynamic manner . Recently, Xie et al. employed the modified version Raichu-OsRac1 and bimolecular fluorescence complementation (BiFC)-based FRET to demonstrate the ternary repressor complexes that modulate histone deacetylation and/or methylation . Since the principle of the intramolecular FRET biosensor appears to have universal applications for all of the small GTPases, we believe this Raichu system will become used in diverse research areas for understanding the spatiotemporal regulation of plant small GTPases and protein–protein interactions and this protocol can be adapted for for other FRET in various plants.
It was important for us to address the issue of whether OsRac1 is activated following pathogen recognition in planta. We therefore tried to make transgenic rice plants expressing Raichu-OsRac1. Unfortunately, we have not yet been able to observe Raichu-OsRac1 fluorescence in planta. This may be due to homology-dependent silencing of YFP and CFP expression, a problem that has been encountered by other researchers endeavouring to develop stably expressed FRET biosensors in planta . Further work is necessary to resolve this issue.
HLW, AA, YK, and KS designed the study; HLW, AA, QW, MH, TM, JO, KK, QW and YK performed experiments and analyzed data; YK, TK, and KK wrote the manuscript; MH, NI, TK, SN, LT, and KS gave technical support and conceptual advice. All authors read and approved the final manuscript.
We thank Dr. Michiyuki Matsuda for providing the Raichu-Rac1 vector. We appreciate Ms. Tomoko Aoi for excellent technical assistance. We also thank the members of the Laboratory of Plant Molecular Genetics at NAIST and the Laboratory of Signal Transduction and Immunity at PSC for invaluable support and discussions. Figure 6 is reproduced from Akamatsu et al., Cell Host Microbe 2013, 13(4):465–476 by copyright permission of the Cell Press. Dedicated to Ko Shimamoto, who passed away on September 28, 2013.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Consent for publication
Ethics approval and consent to participate
Yoji Kawano was supported by National Natural Science Foundation in China (31572073 and 31772246), Chinese Academy of Sciences Hundred Talents Program (173176001000162114), Chinese Academy of Sciences, Shanghai Institutes for Biological Sciences, Shanghai Center for Plant Stress Biology, CAS Center of Excellence for Molecular Plant Sciences, JSPS KAKENHI, and the Takeda Science Foundation. Akira Akamatsu was supported by a fellowship from JSPS.
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