Phenylalanine: A Promising Inducer of Fruit Resistance to Postharvest Pathogens

Abstract

More than 40% of harvested fruit is lost, largely due to decay. In parallel, restrictions on postharvest fungicides call for eco-friendly alternatives. Fruit's natural resistance depends mainly on flavonoids and anthocyanins-which have antioxidant and antifungal activity-synthesized from the phenylpropanoid pathway with phenylalanine as a precursor. We hypothesized that phenylalanine could induce fruit's natural defense response and tolerance to fungal pathogens. The postharvest application of phenylalanine to mango and avocado fruit reduced anthracnose and stem-end rot caused by Colletotrichum gloeosporioides and Lasiodiplodia theobromae, respectively. The postharvest application of phenylalanine to citrus fruit reduced green mold caused by Penicillium digitatum. The optimal phenylalanine concentrations for postharvest application were 6 mM for citrus fruits and 8 mM for mangoes and avocadoes. The preharvest application of phenylalanine to strawberries, mangoes, and citrus fruits also reduced postharvest decay. Interestingly, citrus fruit resistance to P. digitatum inoculated immediately after phenylalanine application was not improved, whereas inoculation performed 2 days after phenylalanine treatment induced the defense response. Five hours after the treatment, no phenylalanine residue was detected on/in the fruit, probably due to rapid phenylalanine metabolism. Additionally, in vitro testing showed no inhibitory effect of phenylalanine on conidial germination. Altogether, we characterized a new inducer of the fruit defense response-phenylalanine. Preharvest or postharvest application to fruit led to the inhibition of fungal pathogen-induced postharvest decay, suggesting that the application of phenylalanine could become an eco-friendly and healthy alternative to fungicides.

Keywords: fungal pathogen; induced resistance; phenylalanine; postharvest application; postharvest decay; preharvest application.

Figure 1

Effect of postharvest treatment with phenylalanine (Phe) on the decay area caused by C. gloeosporioides or L. theobromae in mango fruit. “Shelly” mango fruits were treated with different concentrations (1, 2, 4, 8, 16, or 32 mM) of Phe or water (control) and, 2 days later, inoculated with either C. gloeosporioides or L. theobromae. (A) Area of anthracnose decay caused by C. gloeosporioides during 12 days post inoculation (dpi). (B) Representative picture of mango fruits treated with 8 mM Phe and controls, 10 dpi. (C) Area of stem-end rot caused by L. theobromae during 9 dpi. (D) Representative picture of mango fruit treated with 8 mM Phe and controls, 9 dpi. Values are mean ± SE. Different letters indicate a significant difference between treatments according to one-way ANOVA, p ≤ 0.05.

Figure 2

Effect of postharvest treatment with phenylalanine (Phe) on decay caused by C. gloeosporioides, A. alternata, or L. theobromae in avocado fruit. “Fuerte” avocado fruits were treated with different concentrations of Phe (1, 2, 4, 8, 16, or 32 mM) or water (control) and, 2 days later, inoculated with C. gloeosporioides, A. alternata, or L. theobromae. (A) Area of anthracnose decay caused by C. gloeosporioides. (B) Representative picture of avocado fruits treated with 8 mM Phe and controls, 13 days post inoculation (dpi). (C) Area of decay caused by A. alternata. (D) Representative picture of avocado fruits treated with 8 mM Phe and controls, 15 dpi. (E) Area of stem-end rot caused by L. theobromae. (F) Representative picture of avocado fruits treated with 8 mM Phe and controls, 8 dpi. Values are mean ± SE. Different letters indicate a significant difference between treatments according to one-way ANOVA, p ≤ 0.05.

Figure 3

Effect of preharvest phenylalanine treatments on postharvest decay in mango. A “Keitt” mango orchard was sprayed preharvest (Pre) with water or 8 mM phenylalanine 2, 4, or 7 days (d)—or their combination—before harvest, or fruits were treated with 8 mM phenylalanine postharvest (Post). All the fruits were cold-stored (CS) at 12 °C for 14 d and then shelf life (SL)-stored at 22 °C for 10 d. (A) Incidence of natural decay (percentage). (B) Severity of natural decay (index, 0–10). (C) Representative pictures of mango fruits treated with 8 mM phenylalanine preharvest (7 + 2 d preharvest) or postharvest and controls, after shelf-life storage. Values are mean ± SE. Different letters indicate a significant difference between treatments at a specific time point (upper case letters for CS; lower case letters for SL) according to one-way ANOVA, p ≤ 0.05.

Figure 4

Effect of preharvest application of phenylalanine on postharvest decay in strawberry and mandarin. “Malach” or “Gilli” strawberry plants were sprayed preharvest with water or 6 mM phenylalanine 1, 2, or 3 weeks preharvest (WP) (or their combination). After harvest, the fruits were stored at 10 °C for 12 days. (A) Decay incidence (percentage of rotten fruit) and (B) decay severity (index, 0–5) in strawberries cv. Gilli. (C) Decay incidence and (D) decay severity in strawberries cv. Malach. (E,F) “Michal” mandarin orchard was sprayed with water or 6 mM phenylalanine 3 days before harvest. (E) Decay incidence and (F) decay severity in harvested fruit. Values are mean ±  SE. Different letters indicate a significant difference (p ≤ 0.05) according to t-tests or one-way ANOVA.

Figure 5

Induced resistance by phenylalanine application in mandarin fruit. “Michal” mandarin fruits were treated with water (Control) or 6 mM phenylalanine. Then, the fruit was either immediately inoculated with P. digitatum (early infection) or inoculated 2 days after the phenylalanine application (late infection) and stored at 22 °C. Decay was evaluated 5 days later. (A) Decay incidence (percentage of rotten fruit). (B) Decay severity (index, 0–5). (C) Representative picture of mandarin fruit treated with 6 mM phenylalanine compared to controls, under early- and late-infection protocols. Values are mean ± SE. Asterisk (*) represents a statistically significant difference between the treatment and control according to t-tests p ≤ 0.05.