Genetic variation of γ-tocopherol methyltransferase gene contributes to elevated α-tocopherol content in soybean seeds

<p>Abstract</p> <p>Background</p> <p>Improvement of α-tocopherol content is an important breeding aim to increase the nutritional value of crops. Several efforts have been conducted to improve the α-tocopherol content in soybean [<it>Glycine max </it>(L.) Merr.] through transgenic technology by overexpressing genes related to α-tocopherol biosynthesis or through changes to crop management practices. Varieties with high α-tocopherol content have been identified in soybean germplasms. The heritability of this trait has been characterized in a cross between high α-tocopherol variety Keszthelyi Aproszemu Sarga (KAS) and low α-tocopherol variety Ichihime. In this study, the genetic mechanism of the high α-tocopherol content trait of KAS was elucidated.</p> <p>Results</p> <p>Through QTL analysis and fine mapping in populations from a cross between KAS and a Japanese variety Ichihime, we identified <it>γ-TMT3</it>, which encodes <it>γ</it>-tocopherol methyltransferase, as a candidate gene responsible for high α-tocopherol concentration in KAS. Several nucleotide polymorphisms including two nonsynonymous mutations were found in the coding region of <it>γ-TMT3 </it>between Ichihime and KAS, but none of which was responsible for the difference in α-tocopherol concentration. Therefore, we focused on transcriptional regulation of <it>γ-TMT3 </it>in developing seeds and leaves. An F₅line that was heterozygous for the region containing <it>γ-TMT3 </it>was self-pollinated. From among the progeny, plants that were homozygous at the <it>γ-TMT3 </it>locus were chosen for further evaluation. The expression level of <it>γ-TMT3 </it>was higher both in developing seeds and leaves of plants homozygous for the <it>γ-TMT3 </it>allele from KAS. The higher expression level was closely correlated with high α-tocopherol content in developing seeds. We generated transgenic Arabidopsis plants harboring GUS gene under the control of <it>γ-TMT3 </it>promoter from KAS or Ichihime. The GUS activity assay showed that the activity of <it>γ-TMT3 </it>promoter from KAS was higher than that of Ichihime.</p> <p>Conclusions</p> <p>The genetic variation in <it>γ-TMT3</it>, which plays a major role in determining α-tocopherol concentration, provides significant information about the regulation of tocopherol biosynthesis in soybean seeds. This knowledge will help breeding programs to develop new soybean varieties with high α-tocopherol content.</p>