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.
Highlights
More than 40% of the fruit and vegetables harvested worldwide are lost; a large percentage of that loss is related to fungal pathogens that cause postharvest decay [1]
The fruits treated with 8 mM phenylalanine had a significantly smaller decay area than the nontreated controls and the fruits treated with other phenylalanine concentrations (Figure 1a)
Stem-end rot caused by L. theobromae in mango fruits treated with 8 mM phenylalanine covered a smaller area in comparison to controls and fruits treated with other phenylalanine concentrations (Figure 1c)
Summary
More than 40% of the fruit and vegetables harvested worldwide are lost; a large percentage of that loss is related to fungal pathogens that cause postharvest decay [1]. Plant activators are compounds that act on the fruit by triggering defense mechanisms and increasing their tolerance to fungal pathogens [6]. This pathway is known to be triggered in response to various stresses and to play a key role in the induced defense response in plants [7]. The amino acid phenylalanine was shown to induce a plant defense response against Botrytis cinerea in petunia and tomato leaves [14]
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