Expression analysis of γ-tocopherol methyl transferase genes and α-tocopherol content in developing seeds of soybean [<i>Glycine max</i> (L.) Merr.]

Abstract

Soybean, <i>Glycine max</i> (L.) Merr., besides being rich source of protein and oil, is a good source health promoting substances <i>viz</i>., isoflavones, anthocyanins and tocopherols, the latter are composed of four isoforms α-, β-, γ- and δ-tocopherols. Due to the potent antioxidant property and nutraceutical relevance of α-tocopherol, increasing its levels endogenously by modulating γ-tocopherol methyl transferase (<i>γ-TMT</i>) gene expression has gained significant attention in recent years. Despite having γ-tocopherol as the predominant form, its conversion into highly active α-tocopherol is the limiting step in soybean seeds; the step which is catalyzed by γ-tocopherol methyl transferase enzyme. Based on the differences in amino acid sequence at NH2-domain, the <i>γ-TMT</i> is classified as <i>γ-TMT1, γ-TMT2</i> and <i>γ-TMT3</i>. Expression of these <i>γ-TMT</i> genes was studied in developing soybean seeds of two contrasting genotypes with respect to α-tocopherol content, namely Bragg (‘high’) and DS-2706 (‘low’). Among <i>γ-TMT</i>, significantly higher (<i>P</i> < 0.05) expression (2-4.5 fold) of <i>γ-TMT3</i> was observed in Bragg genotype than in DS-2706 at 50 days after flowering (DAF). A positive correlation was observed between <i>γ-TMT3</i> gene expression and α-tocopherol accumulation (14 µg/g seed) in Bragg at 50 DAF than in DS-2706. The study may prove useful in greater understanding of temporal differences in the α-tocopherol accumulation and <i>γ-TMT3</i> gene expression in seeds and the distinct roles of <i>γ-TMT</i> genes in soybean.