Abstract
Bagged fruits of green apple cultivar “Granny Smith” have been found to turn cardinal red after debagging during fruit-ripening in the Loess Plateau region of China. To understand this phenomenon at post-transcriptional level, we have investigated the roles of microRNAs (miRNAs) in response to debagging. Three small RNA libraries were primarily constructed from peels of “Granny Smith” apples subjected to bagging followed by sunlight re-exposure treatments (0, 6 h, 1 day) (debagging), and from peels of apples without any bagging treatments (0, 6 h, 1 day). 201 known miRNAs belonging to 43 miRNA families and 220 novel miRNAs were identified via high-throughput sequencing. Some miRNAs were found to be differentially expressed after debagging, which indicated that miRNAs affected anthocyanin accumulation through their target genes in mature apple. To further explore the effect of debagging on miRNAs regulating the expression of anthocyanin regulatory genes, four miRNAs and their target genes regulating anthocyanin accumulation, miR156, miR828, miR858, and miR5072, were compared between green cultivar “Granny Smith” and red cultivar “Starkrimson.” Results showed that mdm-miR828 and mdm-miR858 regulated anthocyanin contents in both apple cultivars, while mdm-miR156 only affected anthocyanin accumulation in “Granny Smith,” and miR5072 affected anthocyanin accumulation in “Starkrimson.” Additional analysis of gene ontology for the differentially expressed miRNAs after debagging treatments and their predicted target genes showed that they were involved in photo-protective response after debagging from 0 h to 1 day; they might play important roles in fruit development and adaptation to high light stress.
Highlights
Apple skin coloration depends on the concentration and distribution of anthocyanins, chlorophyll and carotenoids (Lancaster, 1992)
Mature “Granny Smith” fruits peels from the debagging group were collected at 0, 6 h, and 1 day after bag removal, and peels were collected at the same time points in the unbagged group
Six small RNA libraries were constructed for deep sequencing
Summary
Apple skin coloration depends on the concentration and distribution of anthocyanins, chlorophyll and carotenoids (Lancaster, 1992). The red color of apple skin is attributed to the presence of anthocyanins and flavonols. Anthocyanin and flavonols content can be further increased in the bagged apples after subsequent debagging treatment due to increased light sensitivity of the apple fruits (Arakawa, 1988; Li and Cheng, 2008a; Chen et al, 2012). The expression levels of structural genes encoding the enzymes involved in the anthocyanin biosynthesis pathway are enhanced by UV-B treatments (Ubi et al, 2006; Wang et al, 2013). The expression level of MdMYB1 transcript factors, regulator genes involved in anthocyanin biosynthesis pathway, have been reported to be strongly light-inducible (Liu et al, 2013; Zhang et al, 2013)
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