Regulating plant physiology with organic electronics

Edited by David C. Martin, University of Delaware, Newark, DE, and accepted by Editorial Board Member John A. Rogers March 8, 2017 (received for review October 26, 2016)
April 18, 2017
114 (18) 4597-4602

Significance

Hormones play a crucial role in the coordination of the physiological processes within and between the cells and tissues of plants. However, due to a lack of capable technologies, direct and dynamic interactions with plants’ hormone-signaling systems remains limited. Here, we demonstrate the use of an organic electronic device—the organic electronic ion pump—to deliver the plant hormone auxin to the living root tissues of Arabidopsis thaliana seedlings, inducing differential concentration gradients and modulating plant physiology. Electronically regulated transport of aromatic structures such as auxin in an organic electronic device was achieved by synthesis of a previously unidentified class of dendritic polyelectrolyte. Such bioelectronic technology opens the door for precise, electronically mediated control of a plant’s growth and development.

Abstract

The organic electronic ion pump (OEIP) provides flow-free and accurate delivery of small signaling compounds at high spatiotemporal resolution. To date, the application of OEIPs has been limited to delivery of nonaromatic molecules to mammalian systems, particularly for neuroscience applications. However, many long-standing questions in plant biology remain unanswered due to a lack of technology that precisely delivers plant hormones, based on cyclic alkanes or aromatic structures, to regulate plant physiology. Here, we report the employment of OEIPs for the delivery of the plant hormone auxin to induce differential concentration gradients and modulate plant physiology. We fabricated OEIP devices based on a synthesized dendritic polyelectrolyte that enables electrophoretic transport of aromatic substances. Delivery of auxin to transgenic Arabidopsis thaliana seedlings in vivo was monitored in real time via dynamic fluorescent auxin-response reporters and induced physiological responses in roots. Our results provide a starting point for technologies enabling direct, rapid, and dynamic electronic interaction with the biochemical regulation systems of plants.
The coordination of plants’ physiological activity is regulated by a complex array of chemical signals within and between their cells, tissues, and organs. Although plants do not possess a central nervous system, fluxes and gradients of chemical hormone compounds play a central role in the overall management of growth, response to environment, and homeostasis (1, 2). Among the hormones that are generally conserved across the plant kingdom, auxin (indole-3-acetic acid, or IAA) was the first discovered, is perhaps the best characterized, and is certainly one of the most crucial (3). Auxin plays an important role in a multitude of physiological processes and is involved in many aspects of plant development from the single-cell level (endocytosis and morphogenesis) to macroscopic phenomena (embryogenesis and organ formation). It is understood that the presence of tightly controlled auxin gradients within cells and tissues is essential for regulating physiology throughout the life of the plant (4). Precise regulation of cell-to-cell auxin gradients and their role in plant development can be found in a variety of tissues, such as the base of the developing embryo (5, 6), the inner apical hook of young seedlings (7), at the tips of the developing cotyledons (5, 8), at the primary root tip (9), and at the primordia of organs such as lateral roots, leaves, and flowers (8). The cellular scale of auxin activity is clearly demonstrated by the isolated effects of its application on single cells or small cell groups in certain tissues. For example, auxin application affects the emergence of root hairs from specific epidermal cells (10) and modulates K+ channel currents within individual stomatal guard cells (11). As such, deciphering auxin’s molecular and cellular modes of action is of fundamental importance for the elucidation of plant biology (12).
Researchers have traditionally conducted studies of hormone effects in plants via exogenous application. A wide range of chemical compounds is routinely used for probing plant hormone biology (13, 14). The applied compounds passively diffuse and/or are actively imported by the plant into the target tissues, where their effects can be observed. Commonly used methods include spraying or soaking of the plant (15), as well as applying gels, paraffin, or polymer beads (10, 16) that have been soaked in known concentrations of compound or have been allowed to absorb compounds from the plants themselves. For more localized studies, application of hormone-containing microdroplets via microscope-guided micromanipulators has been demonstrated (17). Others have used micro- or nanofluidic systems capable of fluidic transport and delivery of a variety of chemical species (1820). Finally, some have turned to nanoscale functional systems for directed introduction of materials and molecules within plant cells and tissues (21). As with similar techniques for in vitro and in vivo animal studies, these methods all suffer from poor dynamic control, for example in the case of bead or nanoparticle-based delivery, or from cumbersome liquid transport that disrupts native concentration gradients or introduces undesirable stresses on cells and tissues. The shortcomings of currently available localized delivery methods, combined with the cellular-scale effects of auxin in particular, point toward an unmet technological need. The development of a method allowing controlled, localized delivery of hormones and other compounds at the tissue and cellular scale would thus represent a significant advance for the plant research community.
In recent years, a range of organic electronic tools has been developed (22) that enable precise dynamic delivery of small ionic molecules. The organic electronic ionic pump (OEIP) is one of these technologies and was developed primarily as an application for mammalian systems to enable diffusive synapse-like delivery of neurosignaling compounds (alkali ions and neurotransmitters) with high spatiotemporal resolution. Recently, OEIP devices have been demonstrated for a variety of in vitro (23, 24) as well as in vivo applications (25), including therapy in awake animals (26). OEIPs are electrophoretic delivery devices that leverage the unique ionic and electronic properties of conducting polymers and polyelectrolytes to convert electronic signals into ionic fluxes. The OEIP’s polymer delivery channel (i.e., electrophoresis channel) is composed of a polycationic (or polyanionic) material with a high density of fixed charge groups that allows for the selective transport of anions (or cations). The electrophoretic transport used by OEIP devices is flow-free—only the intended molecules are delivered to the target region, not additional liquid or oppositely charged counter ions that may be present in the source solution. The selective electrophoretic transport of the desired molecular species through an OEIP device results in high concentration gradients localized at the OEIP outlet (24), on the scale of ∼100 µm to 1 mm. Additionally, electronic addressing to the OEIP enables the molecular delivery to be rapidly switched on and off, and, importantly, the electrical driving current can be directly correlated with the ionic delivery rate. These device characteristics allow for the precise control of chemical concentration gradients with high spatial and temporal resolution.
However, the materials used for all previous OEIP-based technologies pose a significant limitation. Controlled transport through OEIPs and similar “iontronic” devices has been demonstrated only for atomic ions or the smallest of linear molecules (2326). However, many biological processes—and bioelectronic application scenarios—require transport of larger compounds. The number of available polyelectrolyte materials suitable for OEIP device technologies is limited. One class of materials—indeed, the ones used in all previous OEIPs—is cross-linked semirandom networks of linear polyelectrolytes, such as poly(styrenesulfonate) or poly(vinylbenzylchloride) (qPVBC) (27). However, such linear polymers have not yet demonstrated the capability to transport larger and more rigid molecular compounds, and there exist inherent challenges for further optimization.
First, it is difficult to synthesize linear polyelectrolytes from prefunctionalized monomers bearing both cross-linkable and ionic groups because any cross-linkable groups also tend to inadvertently polymerize during polymerization. Second, linear polyelectrolytes are challenging to postfunctionalize to a high degree owing to their immiscibility to most postsynthetic methods. Thus, it is difficult to control key structural characteristics of linear polymer networks relevant to ion transport properties, namely the size and distribution of fixed charges and void fraction, the effective porosity of the bulk, and the degree of swelling of the polymer network during hydration (2830). Indeed, the capability to transport IAA using OEIPs based on the polyelectrolyte qPVBC was initially investigated. According to mass spectroscopy analysis, qPVBC-based devices were found to deliver only negligible quantities of IAA (Fig. S1A). Further, as described below, similar testing of qPVBC-based OEIPs to deliver IAA to Arabidopsis thaliana plant models was unsuccessful.
Fig. S1.
MS measurements of IAA and oxIAA delivered via OEIP. Total (summed) OEIP-delivered IAA or oxIAA vs. time (e.g., 30-min time point is the sum of 15-min measurement plus 30-min measurement, as marked by dotted lines). Error bars indicate SD. (A) IAA delivered by qPVBC-based OEIP. (B) oxIAA deliverd by dendrolyte-based OEIP.
To address the need for OEIP technologies capable of transporting larger ionic compounds, we investigated hyperbranched polymers (31) as the foundation for a previously unidentified class of polyelectrolyte materials. Hyperbranched polymers have generally spherical or globular structures and possess a high number of terminal functional groups that define their customizable physiochemical properties (32). Here, we present a dendritic polyelectrolyte material system using highly branched polyglycerols as the base unit, phosphonium chloride as the ionic charge component, and allylic groups for cross-linking. These “dendrolyte” materials enable the density of ionic and cross-linking groups to be tuned during synthesis (33) instead of during postfunctionalization. In this way, fundamental limitations of previous OEIPs can be addressed: swelling and rigidity of the polymer network can be controlled by cross-linking, and transport of “larger” or rigid aromatic substances can be facilitated by tuning the void fraction distribution and effective porosity of the bulk. Importantly, dendrolytes enable processing from a “one-pot” three-component miscible mixture of functionalized dendritic polyglycerols, cross-linker, and photoinitiator. One-pot mixtures enable a homogeneous distribution of bulk charge and cross-linking in the membrane and further offer a high degree of compatibility with a variety of patterning processes such as printing or lithographic techniques (30).

Results

In this paper we report on the cross-over of molecular delivery technology to plant applications and the capability of transporting aromatic compounds by an OEIP device, enabled by the dendrolyte material system (Fig. 1 B and D). OEIP devices were prepared by photolithographic patterning of the cationically functionalized dendrolyte film (2 µm thick) on a flexible polyethylene terephthalate (PET) plastic substrate. The shape and dimensions of the resulting OEIP device structure are illustrated and pictured in Fig. 1 A and C.
Fig. 1.
De novo design of an OEIP delivering IAA in vitro. Schematic diagrams of (A) OEIP device materials and geometries and (B) conceptualization of the cationic dendrolyte membrane. Anionic species such as IAA are selectively transported and migrate through the ion conducting channel in proportion to the applied potential gradient. (C) Photograph of the fully fabricated OEIP device. (D) Dendritic polyglycerol-based polyelectrolyte system (green) showing cross-linkages (black) and terminal groups (blue) with positive charge group (red). (E) OEIP mounted to a motorized micromanipulator and Arabidopsis seedlings positioned vertically on agar-growth plates. (F) OEIP positioned in proximity to the seedling root apical meristem (AM) and elongation zone (EZ). (G) OEIP delivery tip and root cross-section shown submerged in the agar-growth gel. Electrical current source, voltage meter (V), and electrode arrangement illustrated. Delivery of IAA is pictured as a diffusive concentration gradient from the OEIP delivery tip through the agar gel and exogenous to the root tissue.
Mass spectrometry was used to quantify the capability of dendrolyte-based OEIPs to transport IAA. In this regard, IAA played the dual role of biologically relevant plant hormone and model aromatic substance. The OEIP was operated continuously at 1 µA and samples were collected at 15-min intervals for 135 min (Fig. 2A). Under these conditions, OEIPs achieved an averaged IAA delivery rate of 0.45 ± 0.16 pmol⋅min−1. Using a finite element analysis method (24), based on this measured delivery rate and a basic diffusion model for IAA [that uses the diffusion rate of IAA (34) but neglects potentially biologically relevant parameters such as exogenous uptake and transport within the root], we calculated the expected concentration evolution of delivered IAA as a function of distance from the OEIP outlet. This calculation shows that micromolar [IAA] is rapidly established in close proximity to the OEIP delivery tip. Hypothetically, plant tissue located 50 μm away from the delivery tip would be exposed to 30 μM IAA after 60 s whereas tissue located at a distance of 200 μm would be exposed to 5 μM. Further, it can be observed from the calculation that a near-linear concentration gradient across the lateral position of the root is formed within 5–15 min of OEIP operation (Fig. 2B). These results indicate that the cationic dendrolyte material system is capable of transporting IAA in biologically active quantities (35). Trace amounts of 2-oxindole-3-acetic acid (oxIAA), a known IAA catabolite (36), were also detected during mass spectrometry measurements, typically in concentrations 100–1,000 times lower than the measured IAA (Fig. S1A). The oxIAA detected was likely formed by nonenzymatic oxidation of IAA during the OEIP experiments. However, oxIAA has been reported to be inactive in bioassays (36).
Fig. 2.
OEIP-mediated delivery of IAA. (A) MS measurements of IAA delivered via OEIP operated continuously at 1 μA, total (summed) IAA vs. time ± SD, corresponding to an averaged IAA delivery rate of 0.45 ± 0.16 pmol⋅min−1. (B) Calculated IAA delivery concentrations as functions of distance from the OEIP outlet (x axis) and time (various color lines) using the above measured delivery rate and a basic diffusion model for IAA. The approximate (Approx.) horizontal position of the root is highlighted in gray.
We proceeded to use the dendrolyte-based OEIPs for in vivo experiments on a highly accessible model plant system suitable for live-cell imaging in the intact organism. Specifically, the apical root meristem and early elongation zone of 5-d-old Arabidopsis seedlings positioned on agar gel were targeted for delivery of IAA via the OEIP. Root tips were monitored using a horizontally oriented spectral macroconfocal laser-scanning microscope system schematically illustrated in Fig. 1E. In this arrangement, seedlings were positioned and imaged vertically. Using the OEIP devices we targeted the root apical meristem of Arabidopsis seedlings with IAA (Fig. 1 F and G). It is known that IAA can either stimulate or suppress processes such as organ growth in plants, depending on its concentration and the tissue in question (4). Root growth was used as a rapidly accessible parameter to demonstrate the physiological activity of OEIP-delivered IAA, because it is well established that high IAA concentrations inhibit root elongation (35, 37). Additionally, as a negative control, benzoic acid (38) was delivered by the OEIP device operated in the same configuration.
Fig. 3A shows bright-field images taken of the OEIP device and seedling root tips at the beginning and after 60 min of delivery of IAA or benzoic acid. Root tip position was measured at 15-min intervals and averaged over five trials, and the growth rate of roots targeted with IAA was compared with benzoic acid negative control and nontargeted Arabidopsis seedlings. For seedlings targeted with IAA, a rapid decrease in growth rate was observed starting at 15 min of delivery, from 4.7 ± 1.0 µm⋅min−1 to 2.4 ± 0.7 µm⋅min−1 after 60 min, whereas both benzoic acid and nontargeted control seedlings maintained their growth rates (Fig. 3B). The reduction in growth rate of plant seedlings by delivery of IAA via the OEIP is consistent with previous findings on exogenous application of IAA (35, 37) (images of IAA and benzoic acid growth rate trials are available in Figs. S2 and S3).
Fig. 3.
OEIP-mediated delivery of IAA. (A) Bright-field images of Arabidopsis root tips at different time intervals during continuous OEIP delivery of IAA. The position of the OEIP’s 25-µm-wide polyelectrolyte delivery channel is highlighted in green. Reduction in growth rate is observed during delivery of IAA compared with benzoic acid negative control over the same time interval. Start and end root tip positions are indicated with blue circles, image area matching Fig. 4A highlighted. (Scale bar, 250 µm.) (B) The growth rate of A. thaliana root tips are plotted as a function of OEIP delivery time (averages ± SEM from n = 5 independent treatments are displayed from a time interval of 15 min) of IAA, benzoic acid, and for nontargeted control.
Fig. S2.
IAA delivered via OEIP. (AE) Starting and ending bright-field images during 60-min continuous delivery at 1 μA of IAA while positioned and operated in close proximity to apical meristems of 5-d-old Arabidopsis seedling root tips. Root tip positions at 15-min intervals are indicated with colored circles: green, 0 min; light green, 15 min; yellow, 30 min; orange, 45 min; red, 60 min.
Fig. S3.
Benzoic acid delivered via OEIP. (AE) Starting and ending bright-field images during 60-min continuous delivery at 1 μA of benzoic acid while positioned and operated in close proximity to apical meristems of 5-d-old Arabidopsis seedling root tips. Root tip positions at 30-min intervals are marked with colored circles: green, 0 min; yellow, 30 min; red, 60 min.
To detect, visualize, and monitor IAA delivery in near real time we used two widely used engineered transgenic Arabidopsis lines expressing the semiquantitative 35S::DII-Venus (39) reporter or DR5rev::GFP (5) marker, both of which show a dynamic fluorescence response in the presence of IAA. DII-Venus is a negative reporter; IAA causes quenching of the Venus yellow fluorescent protein, leading to an inverse relationship between fluorescence signal and IAA concentration. Conversely, in DR5rev::GFP, IAA triggers transcription of new GFP, yielding a direct relationship between fluorescence signal and transcriptional response to IAA, which might be correlated to IAA levels. The relative IAA abundance is therefore visualized faster and more accurately by DII-Venus than by DR5 (38), because the DII-Venus signal relies on a protein degradation mechanism in direct correlation with IAA concentration rather than the slower transcriptional and translational production mechanisms of DR5.
Using Arabidopsis DII-Venus seedlings, we monitored fluorescent signal intensity and observed onset of strong fluorescence reduction between 30–60 min (Fig. 4A). Similar roots targeted with the control molecule benzoic acid preserved their fluorescence (Fig. S4). Quantitative analyses comparing normalized fluorescent intensities of DII-Venus seedlings targeted with IAA or benzoic acid, as well as nontargeted controls, revealed a strong and significant decrease in fluorescence only after IAA delivery via the OEIP (Fig. 4B).
Fig. 4.
Live imaging of auxin delivered via OEIP using Arabidopsis DII-Venus seedlings. (A) Confocal fluorescent image sequence of the root tip of DII-Venus reporter seedling at intervals 0, 30, and 60 min (benzoic acid control can be found in Fig. S4). (B) Fluorescence intensity of DII-Venus reporter seedlings plotted for OEIP delivery of IAA, benzoic acid, and nontargeted (Control). Averages ± SEM from n = 5 independent treatments are displayed. (Scale bar, 50 µm.) Images are representative of five roots treated.
Fig. S4.
Benzoic acid delivered via OEIP. OEIP positioned and operated with benzoic acid at 1 μA in close proximity (100–150 μm) to the apical meristem of 5-d-old Arabidopsis DII-Venus reporter seedlings. (Scale bar, 250 μm.)
In the second experiment we used the dendrolyte-based OEIP to target the elongation zone of DR5rev::GFP reporter seedlings with IAA (Fig. 5A). Confocal images of the root elongation zone cells revealed the onset of fluorescence in plant tissues after 1 h and the signal continued to increase between 2 and 3 h (Fig. 5B). From the image sequence and lateral intensity profile, significant variation in the lateral fluorescent intensity of the roots can be observed—with cells on the left side (OEIP side) of the root being brighter than those on the right. This lateral intensity variation is consistent with the [IAA] gradients calculated from the diffusion model (Fig. 2B) and was observed in most DR5 trials (Fig. S5). Roots targeted with the control molecule benzoic acid did not display alterations in fluorescent intensity of the DR5 reporter.
Fig. 5.
Live imaging of auxin delivered via OEIP using the Arabidopsis DR5rev::GFP reporter line. (A) Elongation zone of DR5rev::GFP seedlings targeted with OEIP, with image area matching B highlighted. (Scale bar, 250 µm.) (B) Confocal fluorescent image sequence of the elongation zone of DR5rev::GFP reporter seedlings at intervals 0, 1, 2, and 3 h. Image intensities were summed from 16 z-stack layers with 3-µm spacing. Lateral fluorescent intensity across the root elongation zone is summed vertically, normalized, and superimposed. (Scale bar, 50 µm.)
Fig. S5.
IAA delivered via OEIP. (AE) Confocal fluorescent image sequence of the root elongation zone of DR5rev::GFP reporter seedlings at intervals 0, 1, 2, and 3 h with the OEIP, operated with IAA at 1 μA, positioned at the left side and in close proximity (100–150 μm) to the roots. Image intensities were summed from 16 z-stack layers with 3-µm spacing. Lateral fluorescent intensity across the root elongation zone is summed vertically, normalized, and superimposed. (Scale bars, 50 µm.)

Discussion

Using a dendrolyte-based OEIP device, we were able to demonstrate delivery of an aromatic compound—the plant-signaling hormone IAA (auxin)—to a living plant model. Induction of dynamic auxin-response alterations was visualized in near real time using two different fluorescent auxin reporters in transgenic Arabidopsis seedlings. With this method, we elicited rapid physiological changes in the growth rate of developing Arabidopsis roots and observed induced differential lateral [IAA] gradients across root tissues.
These results were made possible by the dendrolyte material, a hyperbranched dendritic core-shell polyelectrolyte system that addresses many of the previous limitations of OEIPs and other iontronic technologies. The hyperbranched polyglycerol dendrolyte system, used as the ion transport channel of the OEIP, enables the controlled transport of larger and more rigid ionic compounds while overcoming the limited control of important polyelectrolyte materials parameters such as porosity, swelling, and processability. Specifically, in addition to the natural auxin hormone IAA, the capability of the dendrolyte-based OEIP devices to deliver other aromatic substances was also verified using the synthetic auxin analog 1-NAA (Fig. S6).
Fig. S6.
The 1-NAA delivered via OEIP. OEIP positioned and operated with 1-NAA at 1 μA in close proximity (100–150 μm) to the apical meristem of 5-d-old Arabidopsis DII-Venus reporter seedlings. (Scale bar, 250 μm.)
Although the dendrolyte system was rationally designed to address materials parameters such as porosity, swelling, and processability, in this study no systematic optimizations were made of the many tailorable parameters of the polyelectrolyte (i.e., size of the dendritic core shell, length of the cross-linking polymer, degree of cross-linking, and degree or type of functionalization). We fully expect that these parameters will have a significant role in the polyelectrolyte’s transport characteristics and corresponding influence on organic electronic device performance. Still, given that the majority of plant hormones such as abscisic acid, brassinosteroids, gibberellins, and cytokinins are all of comparable size and similar cyclic or aromatic molecular structure, this work provides the foundation for organic electronic devices that are capable of delivering a wide assortment of biomolecules to directly interact with many fundamental chemical signaling systems in plants.
Seedling root growth rate was used as an easily accessible physiological parameter to demonstrate OEIP-mediated delivery of IAA. Subsequent studies can leverage the larger ion transport capabilities afforded by the dendrolyte materials for more detailed investigations of the role that auxin plays during many growth and behavioral process such as cellular morphogenesis, cell elongation and planar polarity, regulation of cytoskeletal organization, vesicular trafficking, and cell wall formation. Further, by coupling dendrolyte materials with other recent advancements in OEIP technology, such as multiple addressing points and rapid on–off speeds (40), it will become possible to create more sophisticated tools to produce complex, electronically controlled hormone concentration gradients with unprecedented spatial and temporal resolution.
OEIP-based technologies were envisioned and developed primarily for mammalian systems, ultimately as therapeutics for humans. We hope that this study serves as a reminder that chemical signaling plays a fundamental role in all biological systems, and such an anthropocentric focus has overlooked many complimentary and potentially important application areas for organic electronics. We anticipate this technology to be the starting point for precise regulation of chemical signaling networks in—and between—plants and other living systems.

Materials and Methods

Hyperbranched Dendritic Polyglycerol (Dendrolyte) Synthesis.

HyPG [molecular weight (Mw) 10,000 g⋅mol−1, 135 hydroxyl groups per molecule] was purchased from Nanopartica GmbH. All other chemicals were obtained from Sigma-Aldrich and used as received. Dimethylformamide (DMF) was dried over 4-Å molecular sieves before use. Reactions were run at room temperature unless otherwise specified. Equivalents of reagents means molar equivalents relative to number of –OH groups in the dendrolyte (10 kDa = 135 –OH groups).
NMR spectra were recorded on a Varian 300-MHz instrument using deuterium oxide (D2O), methanol-d4 (MeOD), or chloroform-d (CDCl3) as solvent [NMR spectra (41) are available in Fig. S7]. Internal solvent peaks were used as reference. Concentrations were performed under diminished pressure (1–2 kPa) at bath temperatures of 40–60 °C. For purification by dialysis, Spectra/Por Regenerative Cellulose (RC) membranes with 3.5-kDa molecular weight cutoff (MWCO) were used and purchased from Spectrum Laboratories.
Fig. S7.
NMR spectrum of dendrolyte compounds: 1H NMR (D2O, 300 MHz). (A) Compound A: dendritic polyglycerol-allyl. (B) Compound B: dendritic polyglycerol-allyl-butyl chloride. (C) Compound C: dendritic polyglycerol-allyl-butyltrimethylphosphonium chloride.
A fully detailed description of the synthesis and chemical verification can be found in Supporting Information.

Dendrolyte Membrane and OEIP Device Preparation.

Circular PET substrates (Policrom screens) with diameter 101.6 mm (4 inch) were washed with acetone and water and subsequently dried at 110 °C for 10 min before they were treated with 02 plasma (150 W for 60 s). The activated substrates were spin-coated with a 2-mL solution of 5% (3-glycidyloxypropyl)trimethoxysilane (GOPS; Alfa Aesar) in water at 500 rpm (Photo Resist Spinner Model 4000; Electronic Micro Systems) for 30 s and allowed to rest in open air for 15 min. The surfaces were washed with ethanol (EtOH) and dried at 110 °C for 10 min. Treated surfaces were spin-coated with 2 mL MeOH stock solution containing 264 mg of dendrolyte material [Compound C in Dendritic Polyglycerol-Allyl-Butyltrimethylphosphonium Chloride (Compound C)], 18 mg Thiocure 1300 (Bruno Bock Chemische Fabrik GmbH & Co), and 18 mg Irgacure 2959 (Sigma-Aldrich). UV cross-linking was carried out under nitrogen atmosphere inside a glove box and the films were exposed to UV light (254 nm) for 10 min.
Ion channels were patterned using photolithography of Microposit S1818 photoresist and developed for 60 s in MF319 (both supplied by Shipley). Unpatterened, cross-linked dendrolyte material was removed using a CF4 + O2 reactive ion etch (150 W for 90 s) and remaining photoresist was removed with acetone. To facilitate ion exchange, patterned wafers were soaked in 1 M NaCl(aq) for 5 min. OEIPs were encapsulated with 2× 10-μm bar-coated DuPont 5018 UV curing ink. Individual OEIP devices were cut out and packaged in ADW-400 heat-shrink tubing containing sealant glue (Kacab Teknik AB). The OEIP delivery tips were shaped by hand using a scalpel.
To hydrate the dendrolyte channel, OEIP devices were soaked and stored in deionized water before use. Additionally, to reduce the amount of unreacted polymers and chemical compounds remaining in the polyelectrolyte after the above processing steps, OEIP devices were preconditioned by operating the device with 0.1 M KCl(aq) in both the target and source reservoirs. Following the KCl flushing, OEIP devices underwent a loading phase to exchange the Cl ions in the polyelectrolyte with IAA; OEIPs were operated continuously at 250 nA until steady voltage characteristics were observed (∼12 h).

Plant Material and Growth Conditions.

A. thaliana seedlings expressing the auxin-responsive fluorescent markers 35S::DII-Venus (39) or DR5rev::GFP (5) were used to monitor the response to IAA delivered via OEIP. To this end, seeds of both genotypes and wild-type Col-0 were surface-sterilized with 70% ethanol for 1 min, incubated in pool cleaner (550 mg/g trichloroisocyanuric acid, one tablet per 2,000 mL H2O; Biltema) for 12 min and washed four times with sterile, distilled water (dH2O). Seeds were plated on 1/2 Murashige and Skoog (MS) (Duchefa Biochemie), 0.5 g/L MES (Sigma-Aldrich), 1% sucrose, and 0.7% plant agar (Duchefa) growth medium, pH 5.6 (120- × 120-mm square Petri-dishes; Gosselin), and vernalized for 3 d at 4 °C in the dark. The plates were then placed in a growth chamber in vertical orientation and the seedlings were grown at 23 °C with 16 h of light per day. Twelve hours before the start of the experiments, 5 d postgermination, seedlings were positioned onto fresh plates of identical MS media composition additionally supplemented with 0.01 M KCl.

Fluorescent Imaging Protocol.

A custom reoriented macro confocal laser-scanning microscope with a vertical stage was used to acquire images. The macro confocal consisted of a horizontally placed AZ100 macroscope (Nikon and Bergman-Labora) adapted with a specially built XYZ motorized stage (Prior Scientific fitted by Bergman-Labora) and supplied with diascopic white light and episcopic fluorescence light (Nikon). A climate enclosure with passive humidification was designed to surround the stage area to keep plants in a humid, dark environment. The AZ100 macroscope was connected to a C2+ confocal laser scanning system (Nikon) equipped with lasers for 405-nm, 457/488/514-nm, and 561-nm excitation and a transmitted light detector. A coarse manipulator, MM-89 (Narishige), was attached to the stage for placing and keeping the OEIP at a specific position. An agar-growth plate from which the lid was removed was placed vertically on the macroconfocal stage and an OEIP loaded with IAA was placed with the delivery outlet in the growth agar in close proximity of the seedlings. For imaging, a 2× AZ Plan Fluor objective (N.A. 0.2, WD 45 mm; Nikon) or a 5× AZ Plan Fluor objective (N.A. 0.5, WD 16 mm; Nikon) was used. Excitation was at 488 nm and emission detected with a 525/50 filter. DII-Venus fluorescence intensities from a single confocal image layer were normalized to initial intensity, averaged, and compared (standard deviation of the mean). DR5rev::GFP image intensities were summed from 16 z-stack layers with 3-µm spacing. The lateral fluorescent intensity, plotted at the bottom of each image (Fig. 5C), was summed along the y axis and normalized to the maximum intensity of the image sequence (the image sequence for all root trials is shown in Fig. S5). We tested the OEIP’s ability to deliver the synthetic auxin 1-naphthalene acetic acid (1-NAA) and observed similar dynamic fluorescence quenching in DII-Venus reporter seedlings (Fig. S6).

OEIP Operation.

Immediately before the experiments, the OEIPs were operated in a target solution of 10−5 M KCl(aq). A Keithley 2612b SourceMeter (Keithley Instruments Inc.) and custom LabVIEW (National Instruments Corp.) software were used to source current and record voltage. The OEIP device was turned on immediately before the first imaging sequence and operated at a constant electrical current of 1 μA. For these experiments, the OEIP delivery tip was submerged in the MS media in close proximity (100–200 μm) to the root epidermal tissue (Fig. 3 B and C) and was held at a fixed position in the growth MS media for the duration of each trial.

Mass Spectrometry Measurement Protocol.

The OEIP reservoir was loaded with 80 µL of 10% methanol in dH2O containing IAA at 10−5 M concentration. The OEIP outlet tip was submerged in target solution of 50 µL of 1/2 MS media, pH 5.7 (Plant Material and Growth Conditions) without agar and containing 0.01 M KCl. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrodes on a PET substrate (cut from Orgacon F-350 film; AGFA-Gevaert) were used in the source reservoir and the target solution. The OEIP pump was operated sourcing a constant current of 1 µA using a Keithley 2612b SourceMeter and custom LabVIEW software. The target solution was collected and replaced with fresh solution every 15 min, at time intervals of 15, 30, 45, 60, 75, 90, 105, 120, and 135 min. In between the analyses, the pump was washed with methanol/dH2O and stored in dH2O. This was repeated five times with the same OEIP.
To estimate the amount of IAA (and oxIAA) pumped into the target solution, IAA quantification was performed according to Novák et al. (42) with minor modifications. Briefly, 20 µL from each target solution was mixed with 500 µL of dH2O and purified by solid phase extraction using hydrophilic–lipophilic balance reversed-phase sorbent columns (Oasis HLB, 1 cc/30 mg; Waters). Before purification, 4 pmol of [13C6]-labeled IAA and 4 pmol of [13C6]-labeled oxIAA were added to each sample as internal standards to validate the quantification. Purified samples were analyzed using an LC-MRM-MS (liquid chromatography–multiple reaction monitoring–mass spectrometry) system. The LC-MS system consisted of 1290 Infinity Binary LC System coupled to 6490 Triple Quad LC/MS System with Jet Stream and Dual Ion Funnel technologies (Agilent Technologies). Chromatograms were analyzed using MassHunter software version B.05.02 (Agilent Technologies). A Milli-Q deionization unit (Millipore) was used for preparation of the purified water for mobile phases and solutions. The 2-oxo-[indole-13C6]-IAA was obtained from Olchemim Ltd., and [indole-13C6]-IAA was obtained from Cambridge Isotope Laboratories. All other chromatographic solvents and chemicals were of analytical grade or higher purity from Sigma-Aldrich Chemie GmbH.

Dendrolyte Polymer Synthesis and Analysis

Dendritic polyglycerol (NanoPartica GmbH) with Mw and degree of branching (DB) of 10 kDa and 53.6%, respectively, was used as the starting material for all synthetic compounds. All other chemicals were obtained from Sigma-Aldrich and used as received. DMF was dried over 4-Å molecular sieves before use. Reactions were run at room temperature unless otherwise specified. Equivalents of reagents means molar equivalents relative to number of –OH groups in dendrolyte (10 kDa = 135 –OH groups).
NMR spectra were recorded on a Varian 300-MHz instrument using D2O, MeOD, or CDCl3 as solvent. Internal solvent peaks were used as reference (41). Concentrations were performed under diminished pressure (1–2 kPa) at bath temperatures of 40–60 oC. For purification by dialysis, Spectra/Por RC membranes with 3.5-kDa MWCO were used and purchased from Spectrum Laboratories.

Dendritic Polyglycerol-Allyl (Compound A)

Dendritic polyglycerol (1 g, 13.5 mmol) was dissolved in dry DMF (27 mL). NaH (270 mg, 11.26 mmol) was then added to solution and stirred for 1 h at room temperature. Allyl bromide (584.7 μL) was then added to the solution above. The mixture was stirred for 48 h at room temperature. The product was purified by dialysis against methanol. After the solvent was removed under vacuum, the white-yellowish, sticky compound was obtained as a final product (1.05 mg, 84% yield); 1H NMR (CD3OD) 300 MHz, δ (ppm): 0. 8–1.2(t, CH2 of TMP), 3.2–3.9(m, 10H), 4.0–4.13(d, 2H), 5.06–5.33(s, d, 2H), 5.9(m, 1H).

Dendritic Polyglycerol-Allyl-Butyl Chloride (Compound B)

Compound A (300 mg, 3.23 mmol) was dissolved in dry DMF (6.5 mL) then NaH (0.129 g, 5.38 mmol) was added to the solution and stirred for 1 h. Then, 1 bromo-4-chloro butane (1.106 mL) was added to the solution above. The mixture was stirred for 48 h at room temperature. After reducing the DMF volume under vacuum and the product was purified by dialysis (MWCO = 3.5 kDa) against ethanol. Compound B was obtained as a sticky white compound (320 mg, 76% yield); 1H NMR (CD3OD) 300 MHz, δ (ppm):0. 8,1.2(t, CH2 forTMP),1.66(s,2H),1.8(s,2H),3.23.9(m,14H),4.0,4.12(d,2H),5.06–5.32(s,d,2H),5.9(m,1H).

Dendritic Polyglycerol-Allyl-Butyltrimethylphosphonium Chloride (Compound C)

Compound B (300 mg, 2.3 mmol) was dissolved in DMF and a solution of NaI (1.008 g, 6.72 mmol) in acetonitrile was mixed together. Trimethylphosphite (401 μL, 4.6 mmol) was then added. The mixture was refluxed at 80 °C for 48 h. After the evaporation, the polymer was purified by dialysis against methanol. An aqueous solution of NaCl (1 M, 3 mL) was then added. The mixture was then stirred at room temperature for 3 h and the final product purified by dialysis against methanol. A sticky, colorless compound (0.25 g, yield 66%) was obtained as the final product; 1H NMR (CD3OD) 300 MHz, δ (ppm):1.6–1.9(br,s,4H), 1.9–2.2(br,s,9H),2.2–2.6(br,s,2H),3.4–4(br,m,12H),4–4.3(d,2H),5.1–5.4(d,s,2H),5.8–6.1(br,s,1H).

qPVBC MS Measurement Protocol

The OEIP reservoir was loaded with 80 µL of 10% methanol in dH2O containing IAA at 10−5 M concentration. The OEIP outlet tip was submerged in target solution of 50 µL of 0.1% acetic acid. PEDOT:PSS electrodes on a PET substrate (cut from Orgacon F-350 film; AGFA-Gevaert) were used in the source reservoir and the target solution. The OEIP pump was operated sourcing a constant current of 1 µA using a Keithley 2612b SourceMeter and custom LabVIEW software. The target solution was collected and replaced with fresh solution every 30 min, at time intervals of 30, 60, 90, 120, and 150 min. In between the analyses, the pump was washed with methanol/dH2O and stored in dH2O. This was repeated six times with the same OEIP.
To estimate the amount of IAA pumped into the target solution, IAA quantification was performed according to Novák et al. (42) by an injection of 20 µL from target solution directly into an LC-MRM-MS system. The LC-MS system consisted of 1290 Infinity Binary LC System coupled to 6490 Triple Quad LC/MS System with Jet Stream and Dual Ion Funnel technologies (Agilent Technologies). Chromatograms were analyzed using MassHunter software version B.05.02 (Agilent Technologies). A Milli-Q deionization unit (Millipore) was used for preparation of the purified water for mobile phases and solutions. All other chromatographic solvents and chemicals were of analytical grade or higher purity from Sigma-Aldrich Chemie GmbH.

Acknowledgments

We thank Rishikesh Bhalerao and Henrik Jönsson for valuable discussions and Ove Nilsson for helping to initiate this collaboration. We thank Nanopartica GmbH for NMR spectra and analysis of the dendritic polyglycerol and the Swedish Metabolomics Centre for the use of instrumentation. This work was supported by Knut and Alice Wallenberg Foundation ShapeSystems project Grant KAW 2012.0050, with additional support from the Swedish Foundation for Strategic Research (Project Grant RMA-11:0104).

Supporting Information

Supporting Information (PDF)

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Information & Authors

Information

Published in

Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 114 | No. 18
May 2, 2017
PubMed: 28420793

Classifications

Submission history

Published online: April 18, 2017
Published in issue: May 2, 2017

Keywords

  1. auxin
  2. Arabidopsis thaliana
  3. dendritic polymer
  4. bioelectronics
  5. polyelectrolyte

Acknowledgments

We thank Rishikesh Bhalerao and Henrik Jönsson for valuable discussions and Ove Nilsson for helping to initiate this collaboration. We thank Nanopartica GmbH for NMR spectra and analysis of the dendritic polyglycerol and the Swedish Metabolomics Centre for the use of instrumentation. This work was supported by Knut and Alice Wallenberg Foundation ShapeSystems project Grant KAW 2012.0050, with additional support from the Swedish Foundation for Strategic Research (Project Grant RMA-11:0104).

Notes

This article is a PNAS Direct Submission. D.C.M. is a guest editor invited by the Editorial Board.

Authors

Affiliations

David J. Poxson1
Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, 601 74 Norrköping, Sweden;
Michal Karady1
Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden;
Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, 601 74 Norrköping, Sweden;
Department of Physics, Chemistry and Biology, Linköping University, 581 83 Linköping, Sweden;
Aziz Y. Alkattan
Department of Physics, Chemistry and Biology, Linköping University, 581 83 Linköping, Sweden;
Anna Gustavsson
Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, 901 87 Umeå, Sweden;
Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden;
Stéphanie Robert
Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden;
Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, 901 83 Umeå, Sweden;
Markus Grebe
Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, 901 87 Umeå, Sweden;
Plant Physiology, Institute of Biochemistry and Biology, University of Potsdam, 14476 Potsdam, Golm, Germany
Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, 601 74 Norrköping, Sweden;
Magnus Berggren
Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, 601 74 Norrköping, Sweden;

Notes

2
To whom correspondence should be addressed. Email: [email protected].
Author contributions: D.J.P., A.G., S.R., K.L., M.G., D.T.S., and M.B. designed research; D.J.P., M.K., R.G., A.Y.A., A.G., and S.M.D. performed research; D.J.P., M.K., and S.M.D. analyzed data; and D.J.P. wrote the paper.
1
D.J.P., M.K., and R.G. contributed equally to this work.

Competing Interests

The authors declare no conflict of interest.

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    Regulating plant physiology with organic electronics
    Proceedings of the National Academy of Sciences
    • Vol. 114
    • No. 18
    • pp. 4561-E3749

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