Transcriptomic analysis of the effects of γ-aminobutyric acid treatment on browning and induced disease resistance in fresh-cut apples
Introduction
Fresh-cut fruit and vegetables meet consumer demands as hygienic and fresh ready-to-eat foods. Furthermore, these products are convenient for consumption and considered healthy (Velderrain-Rodríguez et al., 2015). However, the processing and preservation of fresh-cut fruit and vegetables may lead to tissue damage, browning, softening, and lignification. Therefore, fresh-cut products are susceptible to microbial growth, which ultimately reduces the shelf life of these products (Oms-Oliu et al., 2010).
Owing to cutting-induced injury, apple tissue cells release phenolic compounds into the cell vacuoles. In the presence of oxygen, polyphenol oxidase (PPO) can catalyze the oxidation of o-diphenol to o-benzoquinone (Yoruk and Marshall, 2003), whereas peroxidase (POD) can oxidize guaiacol and catalyze the oxidation of quinone, glutathione, and ascorbic acid (Oliveira et al., 2021). Quinones interact with amino acids and proteins to form the high-molecular weigh polymer melanin, which induces the browning of fruit and vegetables (Queiroz et al., 2008). Phenylalanine ammonia lyase (PAL) is the first key enzyme in the synthesis of phenols; it catalyzes the conversion of l-phenylalanine to trans-cinnamic acid and phenols formed serve as substrates for the enzymatic browning reaction (Tomás-Barberán and Espín, 2010). PPO, POD, and PAL are the three key enzymes associated with the enzymatic browning of fresh-cut apples. The browning of fresh-cut apples changes their visual appeal, thereby reducing the acceptability of the product. Furthermore, previous studies have reported that outbreaks of foodborne diseases are closely related to the consumption of fresh-cut fruit and vegetables (Bhagwat et al., 2004). The most harmful microorganism detected in fresh-cut fruit and vegetables is Escherichia coli, followed by Salmonella (Anderson et al., 2011). Therefore, the identification of safe, environmentally friendly, and effective compounds that can serve as protective agents against browning and microbial contamination is crucial.
As a food additive, γ-aminobutyric acid (GABA) is generally recognized as safe (commonly known as “GRAS”) and has been widely employed as a bioactive compound in functional foods (Diana et al., 2014). GABA plays important roles in humans; it is an inhibitory neurotransmitter, exerts anti-anxiety effects, and improves brain activity (Siucinska, 2019). In recent years, the critical role of GABA in plants has been reported in multiple studies. GABA can regulate the pH of the plant cells environment, maintain the carbon and nitrogen balance for nutrition, and participate in the responses of plants to adverse stress (Gilliham and Tyerman, 2016). Previously, GABA treatment was found to inhibit PPO activity; enhance the activities of antioxidant enzymes, including catalase, POD, and superoxide dismutase (SOD); and induce the scavenging of redundant reactive oxygen species (ROS) (Gao et al., 2018). Additionally, GABA was found to activate PAL, chitinase (CHI), and β-1,3-glucanase (GLU) in fresh-cut apples inoculated with foodborne pathogens. However, the specific cellular pathways regulated by GABA in fresh-cut apples remain unclear.
The genome sequence of apple has been published previously (Velasco et al., 2010). Transcriptome sequencing is the chief method employed in transcriptome research currently owing to the development of new-generation high-throughput sequencing technology as well as the multiple advantages of the method, including the large throughput, low cost, high resolution, high sensitivity, wide detection range, simple operation process, and lack of necessity for cloning during its application (Qi et al., 2011). In recent years, transcriptome sequencing technology has been widely employed in the stuies on apple browning and disease prevention (Bonasera et al., 2006; Di Guardo et al., 2014). The genes involved in apple browning and the induction of disease resistance have been identified; these findings form the basis for further research on the relevant mechanisms underlying these processes.
To date, only a few studies have investigated the effect of protectants on fresh-cut apples using the transcriptomics approach. In this study, transcriptome sequencing was employed to analyze the changes induced by GABA treatment in the gene expression of fresh-cut apples. To provide a theoretical basis for extending the shelf life of fresh-cut apples, the genes related to browning and induced disease resistance were screened to explore the mechanism by which GABA prevents browning and induces resistance to foodborne pathogens in fresh-cut apples.
Section snippets
Fruit
‘Fuji’ apples (Malus pumila Mill.) were purchased from a local wholesale market in Shanghai, China. The browning process in Fuji apples is slower than that in other varieties of apple, and sellers prefer to use these as fresh-cut products. Apples were selected according to the uniformity in size, color, and maturity, and absence of defects. The apples were transported to the laboratory and immediately stored at 4 °C. Prior to the experiments, the apples were washed under running water to remove
GABA alleviats browning and pathogen growth in fresh-cut apples
As evident from the images of fresh-cut apples treated with distilled water, CA and GABA (Fig. S1), the degree of browning was the lowest in the GABA-treated fresh-cut apples. The L* values of Control 4 d, CW 4 d, and CA 4 d were significantly lower than those of GABA 4 d, whereas the a* and b* values were significantly higher than those of GABA (Table 1). The BI was 44.19 ± 2.03 in Control 4 d, 46.02 ± 1.24 in CW 4 d, and 44.45 ± 0.74 in CA 4 d. Meanwhile, the BI of GABA 4 d was 28.96 ± 0.70,
Discussion
In the present study, the treatment of fresh-cut apples with GABA not only slowed their browning during storage at low temperatures, but also inhibited the growth of E. coli O157:H7. Herein, we sought to explore, from multiple perspectives, the mechanisms by which GABA treatment prevents browning and induces disease resistance in fresh-cut apples, using a transcriptomics approach.
Conclusion
In the present study, GABA was demonstrated to regulate the expression level of browning-related enzymes and phenolic compound synthesis-related genes in fresh-cut apples. In addition, GABA increased the resistance of apples by inducing the gene expression of enzymes or proteins related to disease resistance. Notably, GABA was also found to maintain the integrity and stability of the cell wall, with respect to the composition and structure, in apple tissues (Fig. 4). The total phenolic, total
Author contributions
HG, ZQ, and PZ designed the experiments. PZ, WL, KW and CZ performed the experiments. PZ, WL, KW and CZ analyzed the data. HG, ZQ, PZ and WL drafted the manuscript. All authors read and approved the final manuscript.
Ethical approval
This article does not contain studies involving human participants or animals.
Declaration of Competing Interest
There are no conflicts of interest to declare.
Acknowledgements
This study was supported by the National Natural Science Foundation of China (No. 31972120), the Shanghai Science and Technology Committee (No. 18391901300).
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