Foliar iron application delays flavonoid degradation during the later grain-filling stage and regulates sugar metabolism in colored rice

Introduction: Colored rice is recognized for nutritious function due to its flavonoid compounds. Flavonoid aglycones are conjugated with various sugar moieties. Iron (Fe) is an essential mineral nutrient for plants; however, the regulatory mechanisms by which Fe influences flavonoid accumulation dynamics and sugar metabolism remain unclear. Methods: was foliar-applied to black rice cultivar Nanheinuo and red rice cultivar Yuhongdao 5815 to study regulatory mechanisms by which Fe influences flavonoid accumulation dynamics and sugar metabolism. Results and discussion: The content of anthocyanins, proanthocyanidins, and total flavonoids in the grains decreased throughout the later filling stages. Fe application delayed their degradation during this phase. The differential expression of numerous genes associated with flavonoid transformation may be the primary reason. Genes annotated as key enzymes involved in phenylpropanoid and flavonoid biosynthesis were significantly upregulated in Nanheinuo. Additionally, foliar Fe application significantly impacted sugar and sugar alcohol content by regulating the expression of genes involved in polysaccharide decomposition, phosphorylation and dephosphorylation processes, and sugar interconversion. A close relationship was observed between sugar content and flavonoid-related compound content in the grains of colored rice. Multiple hub genes annotated as glycoside hydrolases in KEGG occupied central nodes within the co-expression network, suggesting that glycoside hydrolases may mediate the crosstalk between sugar and flavonoid metabolism in colored rice grains under Fe treatment, although glycosyltransferases primarily function in flavonoid glycosylation. Foliar Fe application delays flavonoid degradation during the later filling stage and regulates sugar metabolism in colored rice, thereby influencing flavonoid enrichment and eating quality.