1. IntroductionProcessing tomato is a cultivated type of common tomatoes that received its name from its thick skins, which is resistant to transport damage and suitable for processing. Xinjiang, the valley region of California, and the Mediterranean region of Europe are known as the three major centers of tomato cultivation and processing in the world. However, during the growth and storage processes, processing tomatoes are susceptible to attack by various pathogenic and spoilage micro-organisms, which reduce their yield and quality during the growth period [1,2]. Among these fungi, Alternaria spp. can infect processing tomatoes and contribute to devastating fungal diseases, such as tomato early blight and black spot disease [3]. The latter disease mainly occurs after the coloring period. A. alternata enter the host tissue (tomato fruit) through wounds or natural openings during the harvest or pre-harvest periods [4], lurk for several days, and then appear as black spots causing Alternaria rot [5]. These spots appear as sunken lesions, and are mostly observed near the blossom-end or peduncle of the fruit, leading to fruit spoilage that limits the product’s marketability, in addition to causing considerable post-harvest losses [6].Alternaria spp. is not only a filamentous and spoilage fungus that survives in a wide range of temperatures and attacks a wide range of economically important plants (for example, apple [7,8], tomato [8,9], pear [7,10], wheat [11], etc.), but also produces a variety of secondary metabolites with toxic properties, named as Alternaria toxins [12], including alternariol (AOH), alternariol monomethyl ether (AME), tenuzonic acid (TeA), tentoxin (TEN), etc [13]. In addition, these toxins can primarily be bound to sulfates or glucosides, forming the conjugated mycotoxins present in the plant host [14]. Additionally, these conjugated mycotoxins release the free toxin after being hydrolyzed during metabolism, potentially endangering the health of human beings [15]. The research conducted on Alternaria toxins dates back to the 1960s–1970s, when some metabolites produced by Alternaria spp. Were first reported to exert toxic effects [16]. A limited in vivo study showed that TeA exerted mild toxic effects on mammalian cells [17,18]. Both AOH and AME have repeatedly been reported to possess cytotoxic and, of particular concern, genotoxic properties in micromolar concentrations [19,20]. In 2011, according to the EFSA, the tolerable upper intake level, using threshold of toxicological concern (TTC) was 2.5 ng/kg body weight (b.w.) for AOH and AME and 1500 ng/kg b.w. for TeA and TEN [13]. In 2022, the European Union (EU) issued proposal 202/553 to amend regulation (EC) No 401/2006 for the monitoring of Alternaria toxins in food, and set limits for the toxins present in processed tomato products, in which AOH, AME, and TeA do not exceed 10, 5, and 500 μg/kg, respectively [21].A survey conducted in Brazil revealed that neither AME nor AOH was detected in 80 samples, but TeA was observed in 7 tomato pulp (39~111 μg/kg) and 4 tomato puree (29~76 ng/kg) samples [22]. In contrast, AOH was found at levels of 13 μg/kg with high frequency in tomato (93% of 44 samples) products, reported by Ackerman et al. [23]. Approximately 60% of Argentinian tomato pulp samples were contaminated with TeA, AME, and AOH at levels up to 4021 μg/kg (29% of 80 samples), 1734 μg/kg (26% of 80 samples), and 8756 μg/kg (6% of 80 samples), respectively [24]. A survey conducted on the Swiss market in 2010 showed that TeA was found most frequently in tomato products (81 out of 85 samples) and in the highest levels of up to 790 μg/kg, while AOH and AME were found in lower concentrations, ranging from 25]. In an expanded follow-up survey, conjugated mycotoxins AOH-3-sulfate (AOH-3-S) and AME-3-sulfate (AME-3-S) were detected in 9% and 34% of tomato sauces collected in retail markets in Austria, Croatia, and Italy, and their contents were up to 2.1 μg/kg and 17.5 μg/kg, accounting for 7~100% of their parent toxin concentrations [26]. A total of 17 Alternaria toxins, including AOH-3-glucoside (AOH-3-Glc), AOH-9-glucoside (AOH-9-Glc), AOH-3-S, AME-3-glucoside (AME-3-Glc), and AME-3-S, were investigated in tomato sauce, sunflower seed oil, and wheat flour, and interestingly, the results determined that concentrations of AOH-9-Glc and AME-3-S were in similar to their parent toxins in a naturally contaminated tomato sauce sample [27]. These observations highlight the importance to include Alternaria toxins in analytical methods for food surveillance, and due to the detection of excessive Alternaria toxins in most tomato products, attention should be paid to the production of Alternaria toxins in tomato fruits.There are only a few studies addressing the production and transformation of mycotoxins. An experiment about the infection process of Fusarium culmorum in wheat spikes after spray and single spikelet inoculations was presented by Kang and Buchenauer [28], who observed that the pathogen was extended in the rachis in upward and downward directions by inter- and intra- cellular growths inside and outside of the vascular bundles of the rachis. Wang et al. [29] investigated the interaction between Penicillium expansum and wounded apple fruit tissues during the early stages of the infection, using a Pannoramic MIDI slide scanner to determine the key time points to collect samples for the transcriptomic analysis. In 2018, Xie et al. [30] simultaneously quantified the pathway metabolites of aflatoxin biosynthesis in culture medium and revealed the dynamic changes in the biosynthesis pathway by orbitrap fusion mass spectrometry and a D-optimal mixture design method. To date, in relation to the complex mechanisms in plants leading to various factors affecting the production of mycotoxins, different types of culture medium have been used to study the transformation and metabolism of toxins. Generally, ultra-performance liquid chromatography-high resolution mass spectrometry (UPLC-HRMS) is used to identify the metabolic intermediates of mycotoxins.

Over the years, most of the current studies on the toxicity and mechanism of Alternaria toxins have mainly focused on the in vivo investigations and generation mechanism, and the transformation pathway of A. alternata in the host has not been determined. This is mainly limited by instrument conditions and toxin standards. Many standards of Alternaria toxins, including conjugated toxins, are lacking developed research and availability of commercial products, which require UPLC-HRMS to perform a qualitative analysis. Therefore, in this study, the potential of Alternaria toxins production in processing tomatoes is evaluated via ultra-high-performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS) and ultra-performance liquid chromatography–ion mobility quadrupole time-of-flight mass spectrometry (UPLC-IMS QToF MS). Moreover, the changes in Alternaria toxin species and contents are compared in processing tomatoes during the growth and storage periods. This study provides the basic data for the further study of the production and metabolism of Alternaria toxins in processing tomatoes, which is of great significance to control the quantity of Alternaria toxins in tomato products.

5. Material and Methods 5.1. Chemical and Reagents

Anhydrous magnesium sulfate (MgSO4) and sodium chloride (NaCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). LC-MS grades of acetonitrile, formic acid, and acetic acid were purchased from Thermo Fisher Scientific (Waltham, MA, USA). Ultra-pure water was provided by the Watsons Group (Hong Kong) Ltd. Standard TeA, AOH, AME, and TEN were purchased from Romar Labs Division Holding GmbH (Getzersdorf, Austria), and individual standard solutions of each mycotoxin were prepared at ~100 μg/mL in acetonitrile and stored at −20 °C. Leucine enkephalin was acquired from Waters Corporation (Milford, MA, USA).

5.2. Instruments

The samples were weighed on an XSE 204 balance (Mettler-Toledo, Greinfesee, Switzerland), homogenized with a T13 basic ultraturrax (IKA, Staufen, Germany), and mixed in an automatic horizontal shaker (Hannuo Instruments, Shanghai, China). Centrifugation was performed using a Sorvall biofuge Stratos system (Thermo Fisher Scientific, Waltham, MA, USA).

5.3. Tomato, A. alternata and Spore Suspension

H1015 tomatoes were grown in an experimental field in Changji, Xinjiang Province. Sowing was performed on 1 May 2021, and a single test field that consisted of 70 to 80 6-m rows spaced 40 cm apart was used. Seeds were sown 80 cm apart in the row, resulting in a population of approximately 600 plants. A. alternata isolate H10, being isolated from diseased tomatoes and identified, was obtained by single-spore isolation in our laboratory and cultivated in potato dextrose agar (PDA). To obtain a spore suspension of A. alternata isolate H10, sterile distilled water was added to the fully overgrown PDA plates and the spore suspension was adjusted to 1 × 105 conidia/mL using a hemocytometer. Suspension was used directly after preparation for the different experiments.

5.4. Field Experiment

The tomatoes were inoculated approximately 15 days before harvest, washed, and sanitized with 75% ethanol. Using a sterilized toothpick, uniform cuts 3 mm deep and 3 mm wide were made in both sides of the epidermis of the fruit in the equatorial region. A quantity of 15 μL of spore suspension was then inoculated into each cut, and tomatoes inoculated with sterile water were used as controls. Samples were obtained the following day after inoculation, 7 times in total.

5.5. In Vivo Experiment

Healthy, non inoculated, and uniform-sized tomatoes free of disease spots were selected for the experiment. After being washed with water and sanitized with 75% ethanol, using a sterilized toothpick, uniform cuts 3 mm deep and 3 mm wide were made on both side of the epidermis of the fruit in the equatorial region. A quantity of 15 μL of a spore suspension and 15 μL of sterile water were inoculated into each cut of the experimental and control groups. The fruits were stored at 25 °C (±2 °C) in sterilized, breathable boxes for 13 d, and the samples were obtained the following day after inoculation.

5.6. Lesion Diameter Measurement

A total of 7 samples were obtained from the experimental and control groups respectively, in both the field test and in vivo experiment, and the diameters of the spots were measured.

5.7. Extraction of Alternaria Toxins

The homogenized samples (5 g) were placed in 50 mL centrifugal tubes, followed by the successive addition of 10 mL of water and 10 mL of acetonitrile containing 1% acetic acid. The mixtures were shaken on an automatic horizontal shaker at 2500 rpm for 5 min to fully disperse the sample. Subsequently, 4 g anhydrous MgSO4 and 1 g NaCl were immediately added while vigorously shaking the tube to prevent the agglomeration of the salts. After centrifugation at 5000× g for 5 min, the supernatant was evaporated to near dryness (approximately 1 mL of residue remained) under a stream of nitrogen at 40 °C. Finally, 1 mL of the combined solution (acetonitrile/methanol/formic acid, 70:29:1, v/v/v) was added to the residue, which was vortexed, filtered through a 0.22 μm PTEE filter, and injected into the UPLC-IMS QToF MS system.

5.8. Alternaria Toxins Detection

The detection of Alternaria toxins (TeA, AOH, AME, and TEN) was performed on a Waters Acquity UPLC tandem quadrupole (TQD) mass spectrometer (Waters, Milford, MA, USA), which contained an Acquity UPLC HSS C18 (1.7 μm, 2.1 × 100 mm) column for separation. Column temperature was set at 40 °C. The mobile phase comprised methanol as eluent A and 0.1 mM ammonium carbonate as eluent B. A gradient elution was applied as follows: 20% A was initially used and linearly increased to 100% within 4 min, then maintained for 1.5 min, then decreased to 20% within 0.5 min, then maintained for 2 min, after which, column re-equilibration occurred, leading to a total run time of 8 min. The flow rate was set at 0.3 mL/min.

The MS/MS analysis was operated in the negative mode at a capillary voltage of 2.5 kV, a desolvation temperature of 600 °C, a source block temperature of 125 °C, a desolvation gas of 1000 L h−1, and a cone nitrogen gas flow of 150 L h−1. The ion chromatogram for each Alternaria toxin was obtained in MS2 mode of the full-scan chromatogram. The accurate masses of TeA, AOH, AME, and TEN were 197.10, 258.05, 272.07, and 414.18, respectively, in the negative mode; the ionized form of Alternaria toxins in [M−H]−; and the accurate masses of these ionized Alternaria toxins were 196.10, 257.05, 271.07, and 413.18, respectively. From the scan filter, each mycotoxin was detected, along with the peak times: TeA, 1.8 min; AOH, 3.98 min; AME, 5.42 min; and TEN, 5.00 min.

The four conjugated Alternaria toxins (AOH-3-S, AME-3-S, AOH-9-Glc, and AME-3-Glc) were screened using a Waters Acquity UPLC VIONTM ion mobility quadrupole time-of-flight mass spectrometer (Water, Milford, MA, USA), which contained an Acquity UPLC HSS T3 (1.8 μm, 2.1 × 100 mm) column for separation. The column temperature was set at 40 °C. The mobile phase comprised acetonitrile, containing 0.1% (v/v) formic acid as eluent A, and ultra-pure water, containing 0.1% (v/v) formic acid, as eluent B. A gradient elution was applied as follows: 5% of mobile phase A was initially used and linearly increased to 80% within 2.5 min and 90% within 2 min and then maintained for 1.5 min. Then, column re-equilibration was performed, resulting in a total run time of 6 min. The flow rate was set to 0.3 mL/min.

The IMS QToF MS analysis for the conjugated toxins was conducted in negative ion mode (ESI-) at a capillary voltage of 2.5 kV, a desolvation temperature of 350 °C, a source temperature of 125 °C, a desolvation gas flow of 800 L/h, and a cone nitrogen gas flow of 50 L/h. Argon was used as the collision gas at a pressure of 4 × 10−3 mbar. The injection volume was 3 μL. High-definition mass spectrometry (HDMSE) was used as the acquisition mode with low- and high-collision energies set to 6 eV and 30–60 eV, respectively. The data were acquired between 50 and 1000 m/z. Lock mass correction was performed by infusing a leucine enkephalin solution (0.2 ng/L) into the ion source every 5 min (m/z 554.2615 used as the reference ion in the negative mode). Ion mobility and mass calibration were determined using a Major Mix IMS/ToF Calibration Kit (Waters Corporation). The other parameters for data acquisition and processing were set according to the manufacture’s guidelines and performed using UNIFI 1.8.2 software (Waters Corporation).

5.9. Statistical Analysis

The results of this study were analyzed with Microsoft Office Excel 2010, and a line chart was plotted using Origin 2018. The data shown in the results are the means of triplicate (or more) values and expressed as ± SD (standard deviation) for the lesion diameter and mycotoxin content. Statistical analysis was performed using SPSS statistical package 18.0. One-way analysis of variance (ANOVA) and Turkey’s HSD test were performed to determine the significance of the main factors and their interactions. p < 0.01 was considered statistically significant.