Abstract

Anthocyanin is the primary determinant of visual quality in red-fleshed apples. Iron (Fe) contributes to alleviating chlorosis and improving fruit quality, including anthocyanin accumulation. However, the mechanism linking Fe nutrition to anthocyanin content remains unclear, particularly in red-fleshed cultivars. Here, we evaluated how foliar Fe application influenced anthocyanin and soluble sugar contents in apple flesh and investigated the associated transcriptional responses using RNA-seq. Fe increased leaf chlorophyll content and promoted the accumulation of soluble sugars and anthocyanins in fruits grown in calcareous soil. RNA-seq analysis identified 323 differentially expressed genes (DEGs) in response to Fe treatment. Specifically, multiple DEGs were associated with sugar biosynthesis and transport, anthocyanin biosynthesis, and phytohormone biosynthesis. Based on RNA-seq and physiological analyses, we proposed that Fe may promote anthocyanin accumulation through three putative mechanisms: (i) elevating sugar levels via increased leaf chlorophyll content, thereby providing substrates and signals; (ii) activating key anthocyanin structural genes and related transcription factors; and (iii) stimulating ethylene and brassinosteroid biosynthesis and signaling pathways involved in anthocyanin accumulation. This work provides new insight into the Fe-mediated regulatory network underlying anthocyanin accumulation, offering a practical basis to refine fertilization strategies for fruit quality improvement in red-fleshed apples.

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