MYB-mediated upregulation of lignin biosynthesis in <i>Oryza sativa</i> towards biomass refinery

Abstract

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 T 1 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.

Disrupting Flavone Synthase II Alters Lignin and Improves Biomass Digestibility

“…3D), S lignin signals were clearly depleted over G lignin signals in the fnsII mutant cell wall spectra. Besides the typical aromatic signals from the monolignol-derived lignins, the HSQC spectra of wildtype cell walls displayed the characteristic set of aromatic signals from lignin-bound tricin units (T); the chemical shifts of all the C-H correlations from the flavone aromatic system (T 3 , T 8/6 , and T 29/69 ) are in total agreement with literature data (Del Río et al, 2012;Lan et al, 2015;Koshiba et al, 2017). In contrast, all these tricin signals were strikingly depleted to undetectable levels (less than 1%) in the spectra of fnsII mutant cell walls (Fig.…”

“…Subsequently, extensive surveys have revealed that tricin-bound lignins exist abundantly, particularly in the monocot family Poaceae, which comprises grasses including cereals. They also have been found in some commelinid monocot families outside Poaceae, such as Arecaceae (palms) and Bromeliaceae (pineapples and relatives), the noncommelinid family Orchidaceae (the orchids), particularly in the genus Vanilla, and also in certain dicots (Rencoret et al, 2013;Wen et al, 2013;Del Río et al, 2015;Lan et al, 2015Lan et al, , 2016aLan et al, , 2016bKoshiba et al, 2017).…”

Lignin characterization of rice CONIFERALDEHYDE 5‐HYDROXYLASE loss‐of‐function mutants generated with the CRISPR/Cas9 system

“…A few previous studies have also implicated the possible existence of such alternative monolignol pathway(s) or metabolic pool(s) associated with the grass‐specific lignin substructures. For example, heterologous expression of a lignin‐associated MYB transcriptional factor in rice increased lignin content mainly by enriching grass‐specific lignin subunits, including p CA and tricin (Koshiba et al ., ). Barros et al .…”

“…In addition, S lignins may be beneficial to the conversion of lignin‐derived aromatic chemicals because S lignins produce higher yields of aromatic monomers in chemical degradations compared with yields from G lignins (Rinaldi et al ., ; Shuai et al ., ; Lan et al ., ). On the other hand, G lignins may enhance the combustion properties of biomass as they exhibit lower degrees of lignin methoxylation and higher frequencies of carbon–carbon inter‐monomeric linkages, and therefore may show greater heating values during biomass combustion compared with those of S lignins (Koshiba et al ., ; Takeda et al ., ; Umezawa, ). Overall, we consider that an advanced understanding of lignin biosynthesis unique to grasses will increase our capability to manipulate grass biomass for sustainable production of biofuels and biomass‐based materials.…”

“…To mitigate such recalcitrance of lignin for polysaccharide utilization, many studies of lignin metabolic engineering have aimed to reduce lignin content or to alter its chemical structure (Chiang, ; Vanholme et al ., ; Weng et al ., ; Ragauskas et al ., ; Wang et al ., ; Mottiar et al ., ; Rinaldi et al ., ). On the other hand, lignin has a higher carbon content, and thus elevated heating values, compared with those of polysaccharides, therefore lignocellulosic biomass with a high lignin content is considered to be beneficial for pellet fuel and firewood uses (Koshiba et al ., ; Umezawa, ). In addition, lignin has been considered to be an important feedstock for aromatic products.…”