Low expression of <i>SrMYB1</i> leads to the white flower phenotype in <i>Syringa reticulata</i> subsp. <i>amurensis</i> (Ruprecht) P. S. Green &amp; M. C. Chang

Syringa reticulata subsp. amurensis (Ruprecht) P. S. Green & M. C. Chang (S. reticulata subsp. amurensis) is a highly valued ornamental tree, however, its ornamental potential is constrained by its exclusive white flower phenotype, and the regulatory mechanisms underlying flower color formation remain largely unexplored. In this study, the detection of diverse anthocyanins and their derivatives in S. reticulata subsp. amurensis petals suggests the presence of a functional anthocyanin biosynthetic pathway. However, the total anthocyanin content in its petals was significantly lower than in Syringa oblata Lindl. (S. oblata). The expression of structural genes and transcription factors in the anthocyanin biosynthesis pathway was significantly downregulated in S. reticulata subsp. amurensis petals compared to S. oblata. We isolated SrMYB1 from S. reticulata subsp. amurensis, and its overexpression in tobacco led to a marked increase in anthocyanin accumulation across multiple tissues. SrMYB1 upregulates MYB and bHLH transcription factors expression in transgenic tobacco plants, subsequently enhancing structural genes (NtCHS, NtDFR, NtANS, NtUFGT, and NtANR) expression and promoting anthocyanin biosynthesis. Our findings demonstrate that the white flower phenotype in S. reticulata subsp. amurensis is attributed to insufficient anthocyanin accumulation, which is primarily due to low expression of SrMYB1, a key regulator of anthocyanin biosynthesis.