Electrocatalytic activity of salicylic acid on Au@Fe3O4 nanocomposites modified electrode and its detection in tomato leaves infected with Botrytis cinerea

A novel elicitor protein phosphopentomutase from Bacillus velezensis LJ02 enhances tomato resistance to Botrytis cinerea

The loss of tomatoes caused by Botrytis cinerea (B. cinerea) is one of the crucial issues restricting the tomato yield. This study screened the elicitor protein phosphopentomutase from Bacillus velezensis LJ02 (BvEP) which improves the tomato resistance to B. cinerea. Phosphatemutase was reported to play a crucial role in the nucleoside synthesis of various microorganisms. However, there is no report on improving plant resistance by phosphopentomutase, and the related signaling pathway in the immune response has not been elucidated. High purity recombinant BvEP protein have no direct inhibitory effect on B. cinerea in vitro,and but induce the hypersensitivity response (HR) in Nicotiana tabacum. Tomato leaves overexpressing BvEP were found to be significantly more resistant to B. cinerea by Agrobacterium-mediated genetic transformation. Several defense genes, including WRKY28 and PTI5 of PAMP-triggered immunity (PTI), UDP and UDP1 of effector-triggered immunity (ETI), Hin1 and HSR203J of HR, PR1a of systemic acquired resistance (SAR) and the SAR related gene NPR1 were all up-regulated in transgenic tomato leaves overexpressing BvEP. In addition, it was found that transient overexpression of BvEP reduced the rotting rate and lesion diameter of tomato fruits caused by B. cinerea, and increased the expression of PTI, ETI, SAR-related genes, ROS content, SOD and POD activities in tomato fruits, while there was no significant effect on the weight loss and TSS, TA and Vc contents of tomato fruits. This study provides new insights into innovative breeding of tomato disease resistance and has great significance for loss reduction and income enhancement in the tomato industry.

Disposable stainless steel working electrodes for sensitive and simultaneous detection of indole-3-acetic acid and salicylic acid in Arabidopsis thaliana leaves under biotic stresses

The detection of phytohormones in real time has attracted increasing attention because of their critical roles in regulating the development and signaling of plants, especially in defense against biotic stresses. Herein, stainless steel sheet electrodes modified with carbon cement were coupled with paper-based analysis devices for direct and simultaneous detection of salicylic acid (SA) and indole-3-acetic acid (IAA) in plants. We demonstrated that the excellent conductivity of stainless steel sheet electrodes enabled us to simultaneously differentiate IAA and SA at a level of 10 nM. With our approach, the content of IAA and SA in Arabidopsis thaliana leaves infected or not infected with Pst DC3000 could be rapidly quantified at the same time. Our experimental results on differentiation of IAA and SA at different time points showed that there were antagonistic interactions between the IAA and SA after infection of Arabidopsis leaves with Pst DC3000. By offering a cost-effective approach for rapid and sensitive detection of IAA and SA, this study suggests that electrochemical detection can be used in the study and development of precision agriculture technology.

Electrochemical sensor formed from poly(3,4-ethylenedioxyselenophene) and nitrogen-doped graphene composite for dopamine detection

In this study, an electrochemical sensor for dopamine (DA) detection has been developed by a composite of poly(3,4-ethylenedioxyselenophene) (PEDOS) and nitrogen-doped graphene (PEDOS/N-Gr) using an in situ polymerization method. Its structure and properties were then compared with those of the composites of poly(3,4-ethylenedioxythiophene) (PEDOT)/nitrogen-doped graphene (PEDOT/N-Gr), which were prepared by the same methods. FT-IR, Raman, UV-vis, XPS, mapping and SEM investigated the structure and morphology of these composites. These revealed that PEDOS/N-Gr had a higher conjugation degree than PEDOT/N-Gr. The synergetic effect between PEDOS and N-Gr was beneficial for the formation of a homogenous surface coating. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods were conducted for electrochemical detection of DA. Compared with PEDOT/N-Gr, the PEDOS/N-Gr displayed an enhanced sensitivity and electrocatalytic performance for DA detection with linear ranges of 0.008-80 μM (PEDOT/N-Gr: 0.04-70 μM) and limits of detection (LOD) of 0.0066 μM (S/N = 3) (PEDOT/N-Gr: 0.018 μM (S/N = 3)).

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.

A flexible rGO electrode: a new platform for the direct voltammetric detection of salicylic acid

Flexible sensors are of considerable interest for the development of wearable smart miniature devices. This work reported a flexible electrochemical platform based on reduced graphene oxide (rGO) for the detection of salicylic acid (SA). The free-standing and flexible rGO electrode was prepared via a simple extruded process. Dynamic mechanical deformation and bending studies illustrated the resilience and compliance of the flexible electrode against extreme mechanical deformations. Quantitative analysis of SA was performed by using differential pulse voltammetry (DPV) with this flexible rGO electrode. Linearity ranges for SA were obtained from 1.0 × 10^-10 M to 1.0 × 10^-5 M with the detection limit of 2.3 × 10^-11 M (S/N = 3). This strategy provided a new insight into the design and application of flexible electrodes. It will extend the applications of rGO in sensing, bio-electronics and lab-on-chip devices.

Flow-Batch Sample Preparation for Fractionation of the Stress Signaling Phytohormone Salicylic Acid in Fresh Leaves

Salicylic acid (SA) is an important stress signaling phytohormone and plays an essential role in physiological processes in plants. SA fractionation has been carried out batchwise, which is not compatible with the high analytical demand in agronomical studies and increases susceptibility to analytical errors. In this context, a novel flow-batch sample preparation system for SA fractionation on fresh plant leaves was developed. It was based on microwave-assisted extraction with water and conversion of the conjugated species to free SA by alkaline hydrolysis. Free and total SA were quantified by fluorimetry after separation by sequential injection chromatography in a C18 monolithic column. The proposed procedure is directly applicable to plant leaves containing up 16 mg kg^-1 SA, with a limit of detection of 0.1 mg kg^-1 of SA, coefficient of variation of 3.0% (n = 10), and sampling rate of 4 samples h^-1. The flow-batch sample preparation system was successfully applied to SA fractionation in sugarcane, corn, and soybean leaves without clogging or increasing in backpressure. The proposed approach is simple, less time-consuming, and more environmentally friendly in comparison to batchwise procedures.

Systematic evaluation of sample preparation for fractionation of phytohormone salicylic acid in fresh leaves

The determination of salicylic acid (SA), an important phytohormone responsible for stress signaling in plants, is of great importance in agricultural studies. However, a critical evaluation of the procedures for the extraction of the analytes and hydrolysis of the conjugated forms of SA is lacking in the literature and the available alternatives are complex, time-consuming, and laborious. In this study, the sample preparation methods for SA fractionation were critically evaluated to develop a simpler and faster alternative procedure. Microwave-assisted extractions were carried out with 2.0 g of fresh leaves and 8.0 mL of a 75% v/v ethanol:water solution at 40 °C for 10 min, followed by alkaline hydrolysis using 100 μL of 0.1 mol L-1 NaOH at 80 °C for 60 min. Free and total SA were determined in crude and hydrolyzed extracts, respectively, by fluorimetry after chromatographic separation of the sample matrix under isocratic elution (25% v/v acetonitrile/phosphate buffer) using a C18 column. Recovery experiments using methyl salicylate and acetylsalicylic acid model compounds demonstrated that the soft microwave-assisted extraction did not decompose the SA derivatives and that alkaline hydrolysis was quantitative. The proposed procedure was successfully applied for fractionation of SA in sugarcane, corn, and soybean leaves with extraction and hydrolysis yields up to 70 and 20% higher than those achieved in previously proposed approaches, respectively. The developed procedure is a simple, fast, and reliable alternative for SA fractionation in crude extracts without sample clean-up, and utilizes dilute reagents and green solvents.

Electro-Oxidation and Simultaneous Determination of Indole-3-Acetic Acid and Salicylic Acid on Graphene Hydrogel Modified Electrode

A selective and sensitive electrochemical sensor was developed for simultaneous detection of phytohormones indole-3-acetic acid (IAA) and salicylic acid (SA). The sensing interface was fabricated on a porous, three-dimensional networked graphene hydrogel (GH) modified glassy carbon electrode (GCE). The electrocatalytic behavior of IAA and SA on the surface of the modified electrode (GH/GCE) was investigated by cyclic voltammetry and linear sweep voltammetry. Results show that the oxidation reactions of IAA and SA occur at different potentials, which enable their simultaneous detection at the sensing interface. Under optimal conditions, the GH/GCE exhibited good selectivity and stability and its response, unaffected by various interferents, was linear in the range of 4 to 200 μM of IAA and SA. The limit of detection (S/N = 3) achieved were 1.42 μM for IAA and 2.80 μM for SA. The sensor performance was validated by measuring for IAA and SA in real vegetable samples with satisfactory results.

Gold nanoparticle-doped three-dimensional reduced graphene hydrogel modified electrodes for amperometric determination of indole-3-acetic acid and salicylic acid

Three-dimensional (3D) networked nanomaterials have attracted great interest because of their unique porous and 3D-networked structures. In this work, a series of gold nanoparticle (AuNP) doped graphene hydrogel nanocomposites (AuNP-GHs) were synthesized through hydrothermal reaction under various conditions. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the AuNP-GH. The AuNP-GH was used to modify the glassy carbon electrode (GCE) for the detection of indole-3-acetic acid (IAA) and salicylic acid (SA) using chronoamperometric measurements. Under optimum conditions, the AuNP-GH/GCE exhibited linear response to IAA in the ranges of 0.8-4 μM and 4-128 μM, and to SA in the ranges of 0.8-8.4 μM and 8.4-188.4 μM. The detection limits (S/N = 3) were calculated to be 0.21 μM for IAA and 0.22 μM for SA. The proposed sensor showed good sensitivity and stability and hence it was applied in the detection of IAA and SA in spiked samples with satisfactory results.

A Selective Joint Determination of Salicylic Acid in Actinidia chinensis Combining a Molecularly Imprinted Monolithic Column and a Graphene Oxide Modified Electrode

A new combination between selective polymer monolith microextraction (PMME) and sensitive differential pulse voltammetry (DPV) was developed for the determination of the phytohormone salicylic acid (SA) in Actinidia chinensis. A molecularly imprinted monolithic column (MIMC) thermally in-situ polymerized in a micropipette tip by using SA as a template, 4-vinyl pyridine (4-VP) as a functional monomer and ethylene glycol dimethacrylate (EGDMA) as a cross-linker in the mixed porogen of toluene and dodecanol, was employed for the microextraction of SA. The prepared MIMC was characterized by a Fourier transform infrared spectrometer (FI-TR), scanning electron microscope (SEM) and thermo gravimetric analysis (TGA). The results confirmed the binary continuous structure of the porous network. The extracted SA was determined by DPV on a graphene oxide (GO) modified electrode. The joint conditions between MIMC and DPV were investigated practically. Under the optimum conditions, SA could be determined selectively and sensitively in a linear range from 0.1 to 60.0 μg g^-1. The limit of detection was 0.03 μg g^-1 and the recoveries were between 86.2 and 105.2%. The proposed joint method was successfully used to determine SA in Actinidia chinensis.

Determination of four salicylic acids in aloe by in vivo solid phase microextraction coupling with liquid chromatography-photodiode array detection

In recent years, great concerns have been raised about salicylic acid (SA) and its derivatives as plant regulators. Therefore, precise determination of the distribution of SAs in the living plants is necessary for not only fundamental researches but also the regulating mechanisms. In this study, a custom-made solid phase microextraction (SPME) fiber based on diallyl dimethyl ammonium chloride-assembled graphene oxide-coated C18 composite (C18@GO@PDDA) was proposed for in vivo detection of salicylic acid, acetylsalicylic acid (ASA), 4-methyl salicylic acid（4-SA）and 3-methyl salicylic acid (3-SA) in aloe plants. Under the optimized conditions, the analytical performance evaluated in homogenized aloe plant tissues exhibited low detection limits (1.8-2.8 μg g^-1), wide linear ranges (10-5000 μg g^-1), and satisfactory reproducibility (relative standard deviations less than 8.4% and 9.3% for inter-fiber and intra-fiber assays, respectively). Under cadmium stress, the developed method was applied for the in vivo tracing of four salicylic acids in aloe plants. A 48-h in vivo tracing revealed that salicylic acids were involved in the pathway of cadmium stress tolerance. To our best knowledge, it is the first effort to realize the in vivo analysis of SA and its derivatives in plants, and it has a made a great step forward in the area of plant hormone analysis.

In situ detection of salicylate in Ocimum basilicum plant leaves via reverse iontophoresis

We report an in situ quantitative method to measure the concentration of salicylates, from intact, living Ocimum basilicum plant leaves. This simple, non-invasive method utilises iontophoresis in combination with cyclic voltammetry at disposable screen-printed electrodes.