Isolation of the floral morph-related genes in heterostylous flax (Linum grandiflorum): the genetic polymorphism and the transcriptional and post-transcriptional regulations of the S locus.

Heterostylous species have two types of flowers, thrum and pin morphs, and these are controlled by a single diallelic locus designated the 'S locus'; fertilization between these two types of flowers is successful. The S gene and the molecular mechanism by which it operates remain to be uncovered, although heterostyly has been studied since the time of Darwin. We compared transcripts and proteins of the thrum and pin flowers of heterostylous flax (Linum grandiflorum) to characterize the molecular differences between them and to elucidate the molecular machinery of heterostyly. Twelve floral morph-related genes were eventually isolated by an integrated study of subtraction and 2D-PAGE analyses, and four genes, TSS1, LgAP1, LgMYB21 and LgSKS1, were predicted to be related to heterostyly. TSS1, a thrum style-specific gene, showed some features suitable for the S gene. Although its biological function is unclear, TSS1 was expressed only in the thrum style and is probably linked to the S locus. LgMYB21, another thrum style gene, would be involved in floral morphogenesis. LgMYB21 was highly expressed in the thrum style, which is shorter than the pin style, and its overexpression in Arabidopsis reduced pistil length. Furthermore, a comparison of transcript and protein accumulations showed no differences in the mRNA accumulation of some thrum-specific proteins, including LgSKS1, suggesting that these are regulated by floral morph-specific post-transcriptional regulation. The Linum S locus regulates not only S specificity but also many floral phenotypes. Dynamic regulation of transcripts and proteins would be necessary for the pleiotropic function of the Linum S locus.