Effect of postharvest acibenzolar-S-methyl dipping on phenylpropanoid pathway metabolism in muskmelon (Cucumis melo L.) fruits
Introduction
Muskmelon (Cucumis melo L.) is among the most important fruits in northwestern China due to its fragrant sweet taste and economical value (Ren et al., 2012). However, these fruits are quite perishable after harvest. Various types of pathogens cause fruit decay, which leads to severe losses during transportation and storage (Bi et al., 2010). Pink rot caused by Trichothecium roseum is one of the main fruit diseases that seriously degrades quality (Ge et al., 2008). Studies have shown that synthetic fungicides such as imazalil (Aharoni et al., 1992), iprodione, and azoxystrobin (Ma et al., 2004) can control muskmelon diseases. However, due to public concern about fungicide residues, the development of fungicide resistance by pathogens, and harmful effects on the environment, new alternative methods have been proposed (Bi et al., 2010, Tripathi and Dubey, 2004).
Acibenzolar-S-methyl (ASM; Bion®, Actigard), a functional analog of salicylic acid, has been developed for the systemic induction of disease resistance in many fruits (Bi et al., 2007). Pre- or postharvest ASM treatment can suppress the multiple postharvest diseases affecting mango, strawberry, tomatoes, orange, and banana (Cao et al., 2010, Iriti et al., 2007, Moscoso-Ramírez and Palou, 2013, Tang et al., 2010, Zhu et al., 2008). An earlier study in our lab indicated that postharvest ASM dipping decreased lesion diameters of muskmelon fruit inoculated with T. roseum and Alternaria alternata (Ge et al., 2008, Ren et al., 2012, Wang et al., 2008).
ASM-induced resistance in fruit is related to the accumulation of reactive oxygen species, production of pathogenesis-related proteins, and activation of the phenylpropanoid pathway to accumulate lignin, phenolics, and flavonoids (Ge et al., 2012). The phenylpropanoid metabolic pathway influences many crucial disease resistance traits through the synthesis of phenolic compounds, flavonoids, lignin, phytoalexin, and alkaloid, which contribute to plant defense reactions (Jin et al., 2009).
Phenylalanine ammonia lyase (PAL), tyrosine ammonia lyase (TAL), cinnamate-4-hydroxylase (C4H), and 4-coumarate/coenzyme A ligase (4CL) are the key enzymes of the phenylpropanoid pathway that affect the accumulation of phenylpropanoids in plants and fruits (Ferrer et al., 2008, Röther et al., 2002). Cao et al. (2010) reported that ASM treatment enhanced the PAL, TAL, C4H, and 4CL activities in strawberry. ASM also increased PAL activity to suppress blue mould in peach (Cao et al., 2011a, Liu et al., 2005). It was also demonstrated that postharvest ASM treatment increased the contents of total phenolic compounds, flavonoids, and lignin in peach, pear, mango, and strawberry (Cao et al., 2005, Cao et al., 2011a, Cao et al., 2011b, Liu et al., 2005, Terry and Joyce, 2004, Zhu et al., 2008). However, limited information is available about the effect of ASM on the enzyme activities and phenylpropanoid metabolic pathway in muskmelon fruit.
The objectives of this study were to investigate the effect of 0.1 g/L ASM dipping on: the activities of the main enzymes in the phenylpropanoid pathway, including PAL, TAL, 4CL, C4H, cinnamate-4-hydroxylase (CAD), peroxidase (POD), and laccase; the contents of p-coumaric acid, caffeic acid and ferulic acid, total phenolic compounds, flavonoids and lignin in muskmelon fruit; and the possible role of the phenylpropanoid pathway in ASM-induced disease resistance in harvested muskmelon fruit.
Section snippets
Materials
Muskmelon fruit (Cucumis melo L. cv. Yujinxiang) were harvested at 35 days after full blossom in Minqin county, Gansu Province, China. ASM (solid particle, ai. 50%, Bion®) was provided by Syngenta Company.
ASM treatment
Uniform fruit without mechanical injuries or diseases were cleaned, disinfected, and subsequently immersed in 0.1 g/L ASM solution (containing 0.05% [v/v] Tween-80) for 10 min. Fruit treated with distilled water were considered controls. All fruits were air-dried, kept in cartons, and stored at
Effects of ASM on PAL, TAL, C4H, and 4CL activities
The PAL activity in the control fruit stayed at a low level at all assay times. ASM treatment significantly increased (P ≤ 0.05) PAL activity throughout the assay times except the first 2 days after treatment (Fig. 1A) and peaking at 4 and 10 days after treatment. TAL activity in the ASM-treated fruit was significantly (P ≤ 0.05) higher than that in the control group during the entire storage time. The TAL activity in ASM-treated fruit peaked at 6 days after treatment, which was 6.04-fold higher
Discussion
The phenylpropanoid pathway plays an important role in plant and fruit disease resistance (Ge et al., 2012). The shikimate pathway is the gateway to phenylpropanoid biosynthesis (Röther et al., 2002). PAL and TAL are two rate-limiting enzymes in the phenylpropanoid pathway that increase PAL and TAL activities and are associated with the biosynthesis of active metabolites such as phytoalexins, phenolics, flavonoids, and lignin (Stadnik and Buchenauer, 2000). l-Phenylalanine and l-tyrosine are
Acknowledgement
This work was supported by the National Natural Science Foundation of China (31160405).
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