Abstract
Skin color is a key quality attribute of fruits and how to improve fruit coloration has long been a major concern. 5-Aminolevulinic acid (ALA), a natural plant growth regulator, can significantly increase anthocyanin accumulation in fruit skin and therefore effectively improve coloration of many fruits, including apple. However, the molecular mechanism how ALA stimulates anthocyanin accumulation in fruit skin remains unknown. Here, we investigated the impact of ALA on apple skin at the protein and mRNA levels. A total of 85 differentially expressed proteins in apple skins between ALA and water treatment (control) were identified by complementary gel-based and gel-free separation techniques. Most of these differentially expressed proteins were up-regulated by ALA. Function analysis suggested that 87.06% of the ALA-responsive proteins were associated with fruit ripening. To further screen ALA-responsive regulators, we constructed a subtracted cDNA library (tester: ALA treatment; driver: control) and obtained 104 differentially expressed unigenes, of which 38 unigenes were indicators for the fruit ripening-related genes. The differentially changed proteins and transcripts did not correspond well at an individual level, but showed similar regulated direction in function at the pathway level. Among the identified fruit ripening-related genes, the expression of MdMADS1, a developmental transcription regulator of fruit ripening, was positively correlated with expression of anthocyanin biosynthetic genes (MdCHS, MdDFR, MdLDOX, and MdUFGT) in apple skin under ALA treatment. Moreover, overexpression of MdMADS1 enhanced anthocyanin content in transformed apple calli, which was further enhanced by ALA. The anthocyanin content in MdMADS1-silenced calli was less than that in the control with ALA treatment, but higher than that without ALA treatment. These results indicated that MdMADS1 is involved in ALA-induced anthocyanin accumulation. In addition, anthocyanin-related verification in apple calli suggested that the regulation of MdMADS1 on anthocyanin biosynthesis was partially independent of fruit ripening process. Taken together, our findings provide insight into the mechanism how ALA regulates anthocyanin accumulation and add new information on transcriptase regulators of fruit coloration.
Highlights
Skin color is a key quality attribute of apple fruit, and one of the most important factors determining apple market acceptance
Since the time course of anthocyanin accumulation in apple skin showed that the promotion of Aminolevulinic acid (ALA) on anthocyanin accumulation initiated after 24 h light irradiation (See the “Results”), to identify the early upstream regulators of anthocyanin biosynthesis induced by ALA, we chose skins of apples that exposed to light for 24 h for the proteomics and suppression subtractive hybridization (SSH) analysis
To confirm the effect of ALA on apple fruit coloration and provide more detailed information on this process, we investigated the time course of anthocyanin accumulation in apple skin after ALA treatment
Summary
Skin color is a key quality attribute of apple fruit, and one of the most important factors determining apple market acceptance. The traditional fruit production practices contain paper bagging (Ju, 1998) and covering the orchard floor with reflecting films (Meinhold et al, 2010). These methods demand a mass of manpower, material resources, and time, or even bring orchard pollution. ALA has been demonstrated to be effective for the promotion of fruit coloration in several fruit crops, including apple ALA can simultaneously improve fruit coloration and fruit interior quality without any detrimental effects, suggesting great application prospect in fruit production
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