Transcriptomic analysis of the effects of γ-aminobutyric acid treatment on browning and induced disease resistance in fresh-cut apples

Highlights

• GABA affected the expression level of genes related to browning enzyme in apples.

• GABA increased of gene expression of disease resistant enzyme or protein in apples.

• GABA maintain the cell wall composition and structural integrity of apples.

• GABA is a potential fruit protective agent for fresh-cut apples.

Abstract

γ-Aminobutyric acid (GABA) has several physiological functions and broad application prospects. In this study, as a tool used for transcriptomics, RNA-seq sequencing was employed to determine the mechanism by which GABA slows browning and induces resistance against the growth of bacterial in fresh-cut apples. GABA altered the expression level of genes related to the synthesis of browning enzymes and phenolic substances in fresh-cut apples, which slowed the browning process during storage. Additionally, GABA promoted the gene expression of enzymes or proteins related to disease resistance, which resultantly induced disease resistance in fresh-cut apples. Notably, GABA maintained the integrity and stability of the cell wall composition and structure in apple tissues to reduce browning and induce disease resistance. Our findings revealed the mechanism by which GABA acts as a protective agent against browning and bacterial growth in for 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.