Abstract
The strawberry species Fragaria nilgerrensis Schlechtendal ex J. Gay, renowned for its distinctive white, fragrant peach-like fruits and strong disease resistance, is an exceptional research material. In a previous study, an ethyl methane sulfonate (EMS) mutant library was established for this species, resulting in various yellow leaf mutants. Leaf yellowing materials are not only the ideal materials for basic studies on photosynthesis mechanism, chloroplast development, and molecular regulation of various pigments, but also have important utilization value in ornamental plants breeding. The present study focused on four distinct yellow leaf mutants: mottled yellow leaf (MO), yellow green leaf (YG), light green leaf (LG), and buddha light leaf (BU). The results revealed that the flavonoid content and carotenoid-to-chlorophyll ratio exhibited a significant increase among these mutants, while experiencing a significant decrease in chlorophyll and carotenoid contents compared to the wild type (WT). To clarify the regulatory mechanisms and network relationships underlying these mutants, the RNA-seq and weighted gene co-expression network (WGCNA) analyses were employed. The results showed flavonoid metabolism pathway was enriched both in MO and YG mutants, while the chlorophyll biosynthesis pathway and carotenoid degradation pathway were only enriched in MO and YG mutants, respectively. Subsequently, key structural genes and transcription factors were identified on metabolic pathways of three pigments through correlation analyses and quantitative experiments. Furthermore, a R2R3-MYB transcription factor, FnMYB4, was confirmed to be positively correlated with flavonoid synthesis through transient overexpression, virus-induced gene silencing (VIGS), and RNA interference (RNAi), accompanying by reoccurrence and attenuation of mutant phenotype. Finally, dual-luciferase (LUC) and yeast one-hybrid assays confirmed the binding of FnMYB4 to the FnFLS and FnF3H promoters, indicating that FnMYB4 positively regulates flavonoid synthesis. In addition, correlation analyses suggested that FnMYB4 also might be involved in chlorophyll and carotenoid metabolisms. These findings demonstrated the pivotal regulatory role of FnMYB4 in strawberry leaf coloration.
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