Environmentally driven transcriptomic and metabolic changes leading to color differences in "Golden Reinders" apples

Abstract

Apple is characterized by its high adaptation to diverse growing environments. However, little is still known about how different environments can regulate at the metabolic or molecular level specific apple quality traits such as the yellow fruit peel color. In this study, changes in carotenoids and chlorophylls, antioxidants as well as differences in the transcriptome were investigated by comparing the peel of "Golden Reinders" apples grown at different valley and mountain orchards. Mountain environment favored the development of yellow color, which was not caused by an enhanced accumulation of carotenoids but rather by a decrease in the chlorophyll content. The yellow phenotype was also associated to higher expression of genes related to chloroplast functions and oxidative stress. Time-course analysis over the last stages of apple development and ripening, in fruit from both locations, further revealed that the environment differentially modulated isoprenoids and phenylpropanoid metabolism and pointed out a key role for H₂O₂ in triggering apple peel degreening. Overall, the results presented herein provide new insights into how different environmental conditions regulate pigment and antioxidant metabolism in apple leading to noticeable differences in the apple peel color.

Keywords: Malus domestica (Bork.); antioxidant metabolism; color; environment; isoprenoids; oxidative stress; phenylpropanoids.

Figure 1

Principal component analysis (PCA) characterization of the apple peel from four different locations (two orchards from the valley: Alcanó and Vilanova; and two orchards from the mountain: Llesp and Gotarta) using 16 variables including Hue angle (H°) individual pigments and tocopherols, antioxidant capacity, TPC and MDA (A). Total carotenoids (B), total chlorophylls (C), α-tocopherol (D), and γ-tocopherol (E) in the peel of apples grown in different locations. Error bars represent the standard error (n = 4). Letters, whenever available, indicate significant differences between locations.

Figure 2

Number of genes upregulated and down-regulated (A) in Gotarta apples (yellow phenotype) compared with the other 3 orchards (green phenotypes). The 20 most frequent GO upregulated in Gotarta were displayed as % Total GO counted (B). Gene functional categories enriched in Gotarta apples (C) provided by REVIGO’s “Scatterplot view” (http://revigo.irb.hr/). Disc color indicates enrichment value of p (the softer the color the highest the value) and size represents the frequency of the category. Thickness of grey lines represent semantic similarity between categories while spatial arrangement of discs corresponds to grouping of categories by semantic similarity. Those related to photosynthesis are circled in green, and terms associated with oxidative stress are circled in blue.

Figure 3

Comparison of carotenoid metabolism in “Golden Reinders” apples grown in mountain (Gotarta-) or valley (Vilanova-) orchards during fruit development/ripening. Dashed lines indicate that some steps have been omitted. Quantified carotenoid compounds are represented in bar plots, including minor carotenoid isomers marked with the superscript “iso.” Error bars represent the standard error (n = 4). Letters, whenever available, indicate significant differences for the interaction location*developmental stage according to Tukey test (p ≤ 0.05). The corresponding transcript levels of the main genes are represented as heatmaps. Letters within the heatmaps indicate significant differences (p ≤ 0.05) between development stages, while asterisk denote differences among locations at each specific development stage.

Figure 4

Comparison of phenolic compounds metabolism in “Golden Reinders” apples grown in (Gotarta-) or valley (Vilanova-) orchards during fruit development/ripening. Dashed lines indicate that some steps have been omitted. Phenolic compounds are represented in plots. Error bars represent the standard error (n = 4). Letters, whenever available, indicate significant differences for the interaction location*developmental stage according to Tukey’s test (p ≤ 0.05). The corresponding transcript levels of the main genes are represented as heatmaps. Letters within the heatmaps indicate significant differences (p ≤ 0.05) between development stages, while asterisk denote differences among locations at each specific development stage.

Figure 5

Comparative overview of the ROS-scavenging metabolism and ascorbate recycling pathway of “Golden Reinders” apples grown in mountain (Gotarta-) or valley (Vilanova-) orchards during fruit development/ripening. The activity of the main antioxidant enzymes, as well as the content of ascorbic acid and H₂O₂, is represented in bar plots. Error bars represent the standard error (n = 4). Letters, whenever available, indicate significant differences for the interaction location*stage according to Tukey test (p ≤ 0.05). The corresponding transcript levels of the main genes are represented as heatmaps. Letters indicate significant differences (p ≤ 0.05) between development stages, while asterisk denote differences among locations at each specific development stage.