An impedance immunosensor for the detection of the phytohormone abscisic acid

Electrochemical analysis of abscisic acid based on cytochrome P450 707A3

Cytochrome P450 707A3 (CYP707A3) from Arabidopsis thaliana is responsible for catalyzing hydroxylation of abscisic acid (ABA). In this study, the electrochemical analysis of ABA catalyzed by CYP707A3 protein were investigated. Direct electrochemical analysis of Fe3+/Fe2+ redox peaks in CYP707A3 was performed at a pyrolytic graphite electrode with a redox potential of approximately -0.5 V in an oxygen-free phosphate-buffered solution (PBS, pH 7.0). Under aerobic conditions, with the addition of ABA, the electrode continuously supplied electrons to the iron porphyrin in CYP707A3 protein, resulting in a continuous increase in the reduction peak current. The relationship between the change in current and the concentration of ABA exhibited typical characteristics of the Michaelis-Menten kinetic mechanism, and the apparent Michaelis constant (Kmapp) was calculated to be 77.08 nmol/L. The biosensor demonstrated a linear response to ABA within the range of 5 nM to 30 nM with a detection limit (LOD) of 4.85 nM (S/N = 3). The biosensor demonstrated high sensitivity, excellent reproducibility and good selectivity. It was applied to measure ABA content in the rice leaves under normal condition and drought stress, respectively.

Electrochemical sensors for plant signaling molecules

Plant signaling molecules can be divided into plant messenger signaling molecules (such as calcium ions, hydrogen peroxide, Nitric oxide) and plant hormone signaling molecules (such as auxin (mainly indole-3-acetic acid or IAA), salicylic acid, abscisic acid, cytokinin, jasmonic acid or methyl jasmonate, gibberellins, brassinosteroids, strigolactone, and ethylene), which play crucial roles in regulating plant growth and development, and response to the environment. Due to the important roles of the plant signaling molecules in the plants, many methods were developed to detect them. The development of in-situ and real-time detection of plant signaling molecules and field-deployable sensors will be a key breakthrough for botanical research and agricultural technology. Electrochemical methods provide convenient methods for in-situ and real-time detection of plant signaling molecules in plants because of their easy operation, high sensitivity, and high selectivity. This article comprehensively reviews the research on electrochemical detection of plant signaling molecules reported in the past decade, which summarizes the various types electrodes of electrochemical sensors and the applications of multiple nanomaterials to enhance electrode detection selectivity and sensitivity. This review also provides examples to introduce the current research trends in electrochemical detection, and highlights the applicability and innovation of electrochemical sensors such as miniaturization, non-invasive, long-term stability, integration, automation, and intelligence in the future. In all, the electrochemical sensors can realize in-situ, real-time and intelligent acquisition of dynamic changes in plant signaling molecules in plants, which is of great significance for promoting basic research in botany and the development of intelligent agriculture.

Melatonin: dual players mitigating drought-induced stress in tomatoes via modulation of phytohormones and antioxidant signaling cascades

Drought stress significantly retards the plant production. Melatonin is a vital hormone, signaling molecule, and bio-regulator of diverse physiological growth and development processes. Its role in boosting agronomic traits under diverse stress conditions has received considerable attention. However, the underlying molecular mechanism of action and how they increase drought stress tolerance has not been fully interpreted. The current study aimed to ascertain the protective role of melatonin in fortifying the antioxidant defense system, modulating the phytohormone profile, and improving agronomic traits of tomato seedlings under drought stress. After the V1 stage (1st leaf fully emerged), tomato seedlings were exposed to PEG-6000 to mimic drought-induced stress (DR 10% and DR 20%), followed by exogenous application of 100 µM soil drench. Drought-induced stress negatively impacted tomato seedlings by reducing growth and development and biomass accumulation, diminishing salicylic acid (SA) and chlorophyll levels, and dramatically lowering the antioxidant defense ability. However, melatonin protected them by activating the defense system, which decreased the oxidative burst and increased the activities of SOD, CAT, and APX. Administration of 100 µM melatonin by soil drench most remarkably downregulated the transcription factors of SlDREB3 and SlNCED3. This study has validated the moderating potential of melatonin against drought-induced stress by maintaining plant growth and development, enhancing hormone levels, elevating antioxidant enzyme activities, and suppressing the relative expression of drought-responsive genes. These findings also provide a basis for the potential use of MT in agricultural research and other relevant fields of study.

In Vivo Detection of Abscisic Acid in Tomato Leaves Based on a Disposable Stainless Steel Electrochemical Immunosensor

Abscisic acid (ABA) plays an important regulatory role in plants. It is very critical to obtain the dynamic changes of ABA in situ for botanical research. Herein, coupled with paper-based analysis devices, electrochemical immunoelectrodes based on disposable stainless steels sheet were developed for ABA detection in plants in situ. The stainless steel sheets were modified with carbon cement, ferrocene-graphene oxide-multi walled carbon nanotubes nanocomposites, and ABA antibodies. The system can detect the ABA in the range of 1 nM to 100 μM, with a limit of detection of 100 pM. The ABA content in tomato leaves under high salinity was detected in situ. The trend of ABA changes was similar to the expression of SlNCED1 and SlNCED2. Overall, this study offers an approach for in situ detection of ABA in plants, which will help to study the regulation mechanism of ABA in plants and to promote the development of precision agriculture.

Continuous monitoring of chemical signals in plants under stress

Time is an often-neglected variable in biological research. Plants respond to biotic and abiotic stressors with a range of chemical signals, but as plants are non-equilibrium systems, single-point measurements often cannot provide sufficient temporal resolution to capture these time-dependent signals. In this article, we critically review the advances in continuous monitoring of chemical signals in living plants under stress. We discuss methods for sustained measurement of the most important chemical species, including ions, organic molecules, inorganic molecules and radicals. We examine analytical and modelling approaches currently used to identify and predict stress in plants. We also explore how the methods discussed can be used for applications beyond a research laboratory, in agricultural settings. Finally, we present the current challenges and future perspectives for the continuous monitoring of chemical signals in plants.

Application of electrochemical methods for the detection of abiotic stress biomarkers in plants

Abiotic stress is the main cause of low productivity in plants. Therefore, it is important to detect stress and respond to it in a timely manner to avoid irreversible damage to plant productivity and health. The application of traditional methods in agriculture is limited by expensive equipment and cumbersome sample processing. More effective detection methods are urgently needed due to the trace amounts and low stabilities of plant biomarkers. Electrochemical detection methods have the unique advantages of high accuracy, a low detection limit, fast response and easy integration with systems. In this review, the application of three types of electrochemical methods to phytohormone assessment is highlighted including direct electrochemical, immunoelectrochemical, and photoelectrochemical methods. Research on electrochemical methods for detecting abiotic stress biomarkers, including various phytohormones, is also summarized with examples. To date, the detection limit of exogenous plant hormones can reach pg/mL or even lower. Nevertheless, more efforts need to be made to develop a portable instrument for in situ online detection if electrochemical sensors are to be applied to the detection of the endogenous hormones or the physiological state of plants. Additionally, plant-wearable sensors that can be directly attached to or implanted into plants for continuous, noninvasive and real-time monitoring are emphasized. Finally, rational summaries of the considered methods and present challenges and future prospects in the field of abiotic stress detection-based electrochemical biosensors are thoroughly discussed.

Toward Development of Fluorescence-Quenching-Based Biosensors for Drought Stress in Plants

Abscisic acid (ABA) is a drought stress signaling molecule, and simple methods for detecting its levels could benefit agriculture. Here, we present proof-of-concept detection for ABA in aqueous solutions by the use of a mixture of Cyanine 5.5 (Cy5.5) fluorophore- and BHQ3 quencher-conjugated endogenous ABA receptor pyrabactin resistance 1 like proteins (PYL3). These dye-conjugated PYL3 protein form dimers in solutions without ABA and monomerize upon ABA binding. When they are in dimers, fluorescence of Cy5.5 is either nearly completely quenched by the BHQ3 or 20% quenched by another Cy5.5. Consequently, mixtures of equal amounts of the two protein conjugates were used to detect ABA in aqueous solution. As the ABA concentration increased from <1 μM to 1 mM, the intensity of fluorescence detected at around 680 nm from the mixture was more than doubled as a result of ABA-induced monomerization, which leads to halt of quenching and recovery of fluorescence of Cy5.5 in monomers. Kinetic modeling was used to simulate the fluorescence response from the mixture and the results generally agree with the experimentally observed trend. This work demonstrates that fluorescence measurements of a single dissociation reaction in one spectral region are adequate to assess the ABA concentration of a solution.

A multi-channel localized surface plasmon resonance system for absorptiometric determination of abscisic acid by using gold nanoparticles functionalized with a polyadenine-tailed aptamer

A multi-channel localized surface plasmon resonance system is described for absorptiometric determination of abscisic acid (ABA). The system is making use of gold nanoparticles and consists of a broadband light source, a multi-channel alignment device, and a fiber spectrometer. The method is based on the specific interaction between an ABA-binding aptamer and ABA. This induces the growth of gold nanoparticles (AuNPs) functionalized with a polyadenine-tailed aptamer that act as optical probes. Different concentrations of ABA give rise to varied morphologies of grown AuNPs. This causes a change of absorption spectra which is recorded by the system. ABA can be quantified by measurement of the peak wavelength shifts of grown AuNPs. Under optimized conditions, this method shows a linear relationship in the 1 nM to 10 μM ABA concentration range. The detection limit is 0.51 nM. The sensitivity of the ABA assay is strongly improved compared to the method based on salt-induced AuNP aggregation. This is attributed to the use of a poly-A-tailed aptamer and the catalytic ability of AuNPs. In the actual application, the ABA concentration of ABA in fresh leaves of rice is measured with the maximum relative error of 8.03% in comparison with the ELISA method. Graphical abstractSchematic representation of an absorptiometric approach for determination of abscisic acid based on the growth of polyA-tailed aptamer-AuNPs probes and a multi-channel localized surface plasmon resonance system.

Electrochemical aptasensors for zeatin detection based on MoS2 nanosheets and enzymatic signal amplification

A simple and sensitive electrochemical aptasensor was constructed for zeatin detection, where MoS2 nanosheets were used as the immobilization matrix for gold nanoparticles (AuNPs), and AuNPs were employed as the immobilization matrix to probe DNA. After the aptamer DNA and assist DNA hybridized with probe DNA, Y-type DNA can be formed with two biotins at the terminals of aptamer DNA. Then, avidin modified alkaline phosphatase (Avidin-ALP) can be further modified on the electrode surface through the biotin and avidin interaction. Under the catalytic effect of ALP, p-nitrophenylphosphate disodium (PNPP) can be hydrolyzed to produce p-nitrophenol (PNP). However, in the presence of zeatin, the formed Y-type DNA can be destroyed due to the formation of the zeatin-aptamer conjugate, which further reduces the amount of PNP and leads to the decrease of the oxidation signal of PNP. Under the optimum conditions, the change of the oxidation peak current of PNP was inversely proportional to the logarithm value of zeatin concentration in the range of 50 pM-50 nM. The detection limit was calculated to be 16.6 pM. This electrochemical method also showed good detection selectivity and stability. The potential applicability of this method was proved by detecting zeatin in real samples.

Sensitive and high throughput quantification of abscisic acid based on quantitative real time immuno-PCR

BACKGROUND

Abscisic acid (ABA) functions as a stress phytohormone in many growth and developmental processes in plants. The ultra-sensitive determination of ABA would help to better understand its vital roles and action mechanisms.

RESULTS

We report a new sensitive and high throughput quantitative real time immuno-PCR (qIPCR) method based on biotin-avidin linkage system for ABA determination in plants. ABA monoclonal antibody (McAb) coated on the inner surface of PCR well pretreated with glutaraldehyde. The pre-prepared probe complex, including biotinylated McAb, biotinylated DNA and streptavidin linker, was convenient for high throughput operations. Finally, probe DNA was quantified by real-time PCR. The detectable ranges were from 10 to 40 ng/L with a limit of detection (LOD) of 2.5 fg. ABA contents in plant sample were simultaneously analyzed using LC-MS/MS to validate the qIPCR method. The results showed that qIPCR method has good specificity and repeatability with a recovery rate of 96.9%.

CONCLUSION

The qIPCR method is highly sensitive for ABA quantification for actual plant samples with an advantage of using crude extracts instead of intensively purified samples.

Trace analysis of multi-class phytohormones in Oryza sativa using different scan modes in high-resolution Orbitrap mass spectrometry: method validation, concentration levels, and screening in multiple accessions

Phytohormones are signaling and regulating metabolites involved in numerous plant processes, including growth, development, and responses to stress. Currently, the focus is on the analysis of multiple phytohormones in order to characterize crosstalk and hormone signaling networks. In this paper, representative phytohormones of the major classes are simultaneously determined in rice tissues by a generic solid-liquid extraction, followed by liquid chromatography and electrospray ionization high-resolution tandem mass spectrometry using a Q-Exactive™ instrument. After a thorough optimization of the sample preparation, the analytical method was fully validated toward the ultra-trace quantification of six a priori selected plant hormones using three scan modes of the quadrupole-Orbitrap instrument: full-scan high-resolution mass spectrometry, targeted single ion monitoring (t-SIM), and t-SIM followed by data-dependent tandem mass spectrometry. Overall, a similar quantitative performance was noticed for the different scan modes. The analytical method was successfully applied to measure basal phytohormone levels in six different rice accessions, comprising Oryza sativa ssp. japonica, indica, and Oryza glaberrima. Hormone concentrations were higher in shoots than in roots or at least similar. Except for a lower level of salicylic acid in shoots of O. glaberrima versus O. sativa, no other differences in hormone levels could be noticed that were dependent of the (sub)species assignment of the analyzed accessions. Making use of the benefits of full-scan high-resolution mass spectrometry, a first post-run suspect screening was performed, suggesting - based on accurate mass measurements and isotopic patterns - the possible presence of about 50 additional plant hormones in the rice tissues. Graphical abstract ᅟ.

Localized surface plasmon resonance-based abscisic acid biosensor using aptamer-functionalized gold nanoparticles

Abscisic acid (ABA) plays an important role in abiotic stress response and physiological signal transduction resisting to the adverse environment. Therefore, it is very essential for the quantitative detection of abscisic acid (ABA) due to its indispensable role in plant physiological activities. Herein, a new detection method based on localized surface plasmon resonance (LSPR) using aptamer-functionalized gold nanoparticles (AuNPs) is developed without using expensive instrument and antibody. In the presence of ABA, ABA specifically bind with their aptamers to form the ABA-aptamer complexes with G-quadruplex-like structure and lose the ability to stabilize AuNPs against NaCl-induced aggregation. Meanwhile, the changes of the LSPR spectra of AuNP solution occur and therefore the detection of ABA achieved. Under optimized conditions, this method showed a good linear range covering from 5×10-7 M to 5×10-5 M with a detection limit of 0.33 μM. In practice, the usage of this novel method has been demonstrated by its application to detect ABA from fresh leaves of rice with the relative error of 6.59%-7.93% compared with ELISA bioassay. The experimental results confirmed that this LSPR-based biosensor is simple, selective and sensitive for the detection of ABA. The proposed LSPR method could offer a new analytical platform for the detection of other plant hormones by changing the corresponding aptamer.

Zooming In on Plant Hormone Analysis: Tissue- and Cell-Specific Approaches

Plant hormones are a group of naturally occurring, low-abundance organic compounds that influence physiological processes in plants. Our knowledge of the distribution profiles of phytohormones in plant organs, tissues, and cells is still incomplete, but advances in mass spectrometry have enabled significant progress in tissue- and cell-type-specific analyses of phytohormones over the last decade. Mass spectrometry is able to simultaneously identify and quantify hormones and their related substances. Biosensors, on the other hand, offer continuous monitoring; can visualize local distributions and real-time quantification; and, in the case of genetically encoded biosensors, are noninvasive. Thus, biosensors offer additional, complementary technologies for determining temporal and spatial changes in phytohormone concentrations. In this review, we focus on recent advances in mass spectrometry-based quantification, describe monitoring systems based on biosensors, and discuss validations of the various methods before looking ahead at future developments for both approaches.

Kinetic Characterisation of a Single Chain Antibody against the Hormone Abscisic Acid: Comparison with Its Parental Monoclonal

A single-chain Fv fragment antibody (scFv) specific for the plant hormone abscisic acid (ABA) has been expressed in the bacterium Escherichia coli as a fusion protein. The kinetics of ABA binding have been measured using surface plasmon resonance spectrometry (BIAcore 2000) using surface and solution assays. Care was taken to calculate the concentration of active protein in each sample using initial rate measurements under conditions of partial mass transport limitation. The fusion product, parental monoclonal antibody and the free scFv all have low nanomolar affinity constants, but there is a lower dissociation rate constant for the parental monoclonal resulting in a three-fold greater affinity. Analogue specificity was tested and structure-activity binding preferences measured. The biologically-active (+)-ABA enantiomer is recognised with an affinity three orders of magnitude higher than the inactive (-)-ABA. Metabolites of ABA including phaseic acid, dihydrophaseic acid and deoxy-ABA have affinities over 100-fold lower than that for (+)-ABA. These properties of the scFv make it suitable as a sensor domain in bioreporters specific for the naturally occurring form of ABA.

Label-free impedimetric immunosensor for sensitive detection of fenvalerate in tea

In this experiment, fenvalerate antibodies were immobilised on the electrode by the crosslinking with glutaraldehyde modified on the glassy carbon electrode (GCE) via chitosan. Fenvalerate was measured by the increase of electron transfer resistance when the immune reaction occurred with Fe(CN)6(3-)/Fe(CN)6(4-) as the probe. Under optimal conditions, the change of resistance is in a linear relationship with the logarithm of the concentration in the range of 1.0×10(-3)∼1.0×10(1)mg/L (R=0.998) with a detection limit of 0.80 μg/L. This method bears such merits as simplicity of operation, high sensitivity, wide linear range, specificity, reproducibility and good stability. The immunosensor was applied in the detection of real samples of tea, achieving satisfactory results, and it could be regenerated after being placed alternately in 0.5 mol/L HCl and 0.5 mol/L NaOH solutions.

Indole-3-acetic acid biosensor based on G-rich DNA labeled AuNPs as chemiluminescence probe coupling the DNA signal amplification

A highly sensitive chemiluminescence (CL) method for detection of phytohormone indole-3-acetic acid (IAA) was developed by using G-rich DNA labeled gold nanoparticles (AuNPs) as CL probe coupling the DNA signal amplification technology. The IAA antibody was immobilized on carboxyl terminated magnetic beads (MBs). In the presence of IAA, antibody labeled AuNPs were captured by antibody functionalized MBs. The DNA on AuNPs is released by a ligand exchange process induced by the addition of DTT. The released DNA is then acted as the linker and hybridized with the capture DNA on MBs and probe DNA on AuNPs CL probe. The CL signal is obtained via the instantaneous derivatization reaction between a specific CL reagent, 3,4,5-trimethoxyl-phenylglyoxal (TMPG), and the G-rich DNA on AuNPs CL probe. IAA can be detected in the concentration range from 0.02 ng/mL to 30 ng/mL, and the limit of detection is 0.01 ng/mL.

Analytical methods for tracing plant hormones

Plant hormones play important roles in regulating numerous aspects of plant growth, development, and response to stress. In the past decade, more analytical methods for the accurate identification and quantitative determination of trace plant hormones have been developed to better our understanding of the molecular mechanisms of plant hormones. As sample preparation is often the bottleneck in analysis of plant hormones in biological samples, this review firstly discusses sample preparation techniques after a brief introduction to the classes, roles, and methods used in the analysis of plant hormones. The analytical methods, especially chromatographic techniques and immuno-based methods, are reviewed in detail, and their corresponding advantages, limitations, applications, and prospects are also discussed. This review mainly covers reports published from 2000 to the present on methods for the analysis of plant hormones.

Biosensors in plants

Biosensors come in an increasing array of forms and their development is defining the rate of advance for our understanding of many natural processes. Developmental biology is increasingly using mathematical models and yet few of these models are based on quantitative recordings. In particular, we know comparatively little about the endogenous concentrations or fluxes of signalling molecules such as the phytohormones, an area of great potential for new biosensors. There are extremely useful biosensors for some signals, but most remain qualitative. Other qualities sought in biosensors are temporal and spatial resolution and, usually, an ability to use them without significantly perturbing the system. Currently, the biosensors with the best properties are the genetically encoded optical biosensors based on FRET, but each sensor needs extensive specific effort to develop. Sensor technologies using antibodies as the recognition domain are more generic, but these tend to be more invasive and there are few examples of their use in plant biology. By capturing some of the opportunities appearing with advances in platform technologies it is hoped that more biosensors will become available to plant scientists.