Abstract
Fruit color is an important economic trait. The color of red walnut cultivars is mainly attributed to anthocyanins. The aim of this study was to explore the differences in the molecular mechanism of leaf and peel color change between red and green walnut. A reference transcriptome of walnut was sequenced and annotated to identify genes related to fruit color at the ripening stage. More than 290 million high-quality reads were assembled into 39,411 genes using a combined assembly strategy. Using Illumina digital gene expression profiling, we identified 4568 differentially expressed genes (DEGs) between red and green walnut leaf and 3038 DEGs between red and green walnut peel at the ripening stage. We also identified some transcription factor families (MYB, bHLH, and WD40) involved in the control of anthocyanin biosynthesis. The trends in the expression levels of several genes encoding anthocyanin biosynthetic enzymes and transcription factors in the leaf and peel of red and green walnut were verified by quantitative real-time PCR. Together, our results identified the genes involved in anthocyanin accumulation in red walnut. These data provide a valuable resource for understanding the coloration of red walnut.
Highlights
Walnut (Juglans regia L.) is an economically important fruit tree that is cultivated worldwide.Walnuts are composed of the peel, shell, and kernel
The results suggested that the pathways of anthocyanin synthesis differ between the leaf and peel of red walnut, and confirmed that the difference in coloration between red and green walnut is due to anthocyanin accumulation
Its homolog in Arabidopsis is known to interact with GL, EGL and PAP to regulate anthocyanin biosynthesis [72]. These results indicate that members of the MYB, basic helix-loop-helix (bHLH) and WD40 transcription factor families play key roles in regulating anthocyanin biosynthesis in the leaf and peel of red and green walnut
Summary
Walnut (Juglans regia L.) is an economically important fruit tree that is cultivated worldwide. Walnuts are composed of the peel, shell, and kernel. Walnut kernels are rich in unsaturated fatty acids, protein, carbohydrates, cellulose, vitamins, calcium, phosphorus, iron, and a variety of polyphenols with antioxidant properties [2]. Both the leaf and peel of cultivated walnut varieties are green, and the kernel is yellowish-white or brown. Some red-kernel walnut varieties exist in China, and these may be rare and precious resources for walnut breeding [2].
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