A novel R2R3-MYB transcription factor regulates light-mediated floral and vegetative anthocyanin pigmentation patterns in Lilium regale

Abstract

Color patterns in angiosperm flowers are produced by spatially and temporally restricted deposition of pigments. Many Lilium species show anthocyanin pigmentation of the whole tepal, which is regulated by the MYB12/basic helix-loop-helix 2 (bHLH2) transcription factor complex. However, in Lilium regale pigment deposition is restricted to light-exposed surfaces of the flower buds, leaves, and bracts. Investigation of how the unique color patterns of L. regale are regulated led to the identification and isolation of a novel R2R3-MYB gene, LrMYB15. In stable and transient transformation assays in tobacco, LrMYB15 stimulated transcription of anthocyanin biosynthesis genes, indicating that it is involved in the positive regulation of anthocyanins. In L. regale, anthocyanin biosynthesis genes and LrMYB15 were expressed in the outer tepals, leaves, and bracts that accumulated anthocyanin pigments. In contrast, LrbHLH2, another regulatory gene for anthocyanin biosynthesis, was transcribed in all organs irrespective of anthocyanin accumulation. These results indicate that LrMYB15 principally determines the unique anthocyanin color patterns of L. regale. In addition, LrMYB15 transcription ceased completely when plants were kept in shaded conditions and the colors of the flower buds faded, indicating that transcription of this gene is under the control of light. R2R3-MYB genes that regulate light-induced anthocyanin accumulation on exposed petal surfaces have been isolated in eudicots. The results here indicate that R2R3-MYB genes exhibiting similar (but not identical) functions are conserved between monocots and eudicots and that transcriptional regulation is a major mechanism for generating restricted pigment deposition in the flowers of a wide range of angiosperm species.