MYB-mediated upregulation of lignin biosynthesis in Oryza sativa towards biomass refinery.

Lignin encrusts lignocellulose polysaccharides, and has long been considered an obstacle for the efficient use of polysaccharides during processes such as pulping and bioethanol fermentation. However, lignin is also a potential feedstock for aromatic products and is an important by-product of polysaccharide utilization. Therefore, producing biomass plant species exhibiting enhanced lignin production is an important breeding objective. Herein, we describe the development of transgenic rice plants with increased lignin content. Five Arabidopsis thaliana (Arabidopsis) and one Oryza sativa (rice) MYB transcription factor genes that were implicated to be involved in lignin biosynthesis were transformed into rice (O. sativa L. ssp. japonica cv. Nipponbare). Among them, three Arabidopsis MYBs (AtMYB55, AtMYB61, and AtMYB63) in transgenic rice T₁ lines resulted in culms with lignin content about 1.5-fold higher than that of control plants. Furthermore, lignin structures in AtMYB61-overexpressing rice plants were investigated by wet-chemistry and two-dimensional nuclear magnetic resonance spectroscopy approaches. Our data suggested that heterologous expression of AtMYB61 in rice increased lignin content mainly by enriching syringyl units as well as p-coumarate and tricin moieties in the lignin polymers. We contemplate that this strategy is also applicable to lignin upregulation in large-sized grass biomass plants, such as Sorghum, switchgrass, Miscanthus and Erianthus.