Plant & Animal Genomes XIII Conference

January 15-19, 2005
Town & Country Convention Center
San Diego, CA

Candidate Genes For Mutations Causing Changes In Tocopherol (Vitamin E) Composition In Sunflower

Shunxue Tang¹ , Catherine Hass² , Steven J. Knapp^{1, 2}

¹  Center for Applied Genetic Technologies, University of Georgia, Athens, GA 30602, USA

²  Department of Crop and Soil Science, Oregon State University, Corvallis, OR 97331, USA

Sunflower (Helianthus annuus L.) is a naturally rich source of alpha-tocopherol, one of four tocopherol species found in seed oils. Several tocopherol composition variants have been described and are predicted to be caused by mutations in 2-methyl-6-phytydlplastoquinol methyl transferase (MPBQ-MT), gamma-tocopherol methyl transferase (gamma-TMT), or other genes in the tocopherol biosynthetic pathway. The goal of the present study was to further elucidate genetic mechanisms underlying three recessive mutations (Tph₁, Tph₂, and Tph₃). Previous forward genetic analyses identified MPBQ-MT as a candidate for Tph₁ and gamma-TMT as a candidate for Tph₂ and ruled out tocopherol cyclase (TC), MPBQ-MT, and gamma-TMT as candidates for Tph₃. RT-PCR analyses were performed on total RNA isolated from developing kernels of mutant homozygotes using gene specific primers for MPBQ-MT and gamma-TMT. MPBQ-MT transcripts accumulated in wildtype but not mutant homozygotes (tph₁tph₁). Tph₂ mutant homozygotes (tph₂tph₂) accumulated aberrant gamma-TMT transcripts caused by an intron splicing error. The latter was sequenced and found to carry a 119 bp fragment of the second gamma-TMT intron. The specific DNA polymorphisms causing the mutations have not yet been identified; however, sequence-tagged-site markers have been developed for MPBQ-MT and gamma-TMT and are diagnostic for the Tph₁ and Tph₂ mutations, respectively. The Tph₃ mutation seems to impair the activity of a delta-TMT. We speculate that delta-TMT catalyzes the synthesis of gamma- from beta-tocopherol and alpha- from delta-tocopherol in sunflower but not Arabidopsis and are presently screening candidate genes for delta-TMT.