Location: Dairy Forage ResearchTitle: Structural features of alternative lignin monomers associated with improved digestibility of artificially lignified maize cell walls
|DAVIDSON, CHRISTY - University Of Wisconsin|
|TOBIMATSU, YUKI - Kyoto University|
|KIM, HOON - University Of Wisconsin|
|LU, FACHUANG - South China University Of Technology|
|ZHU, YIMIN - Pennsylvania State University|
|OPIETINIK, MARTINA - Lenzing Ag|
|SANTORO, NICHOLAS - University Of Michigan|
|FOSTER, CLIFF - Michigan State University|
|YUE, FENGXIA - South China University Of Technology|
|RESS, DINO - University Of Wisconsin|
|PAN, XUEJUN - University Of Wisconsin|
|RALPH, JOHN - University Of Wisconsin|
Submitted to: Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2019
Publication Date: 3/13/2019
Citation: Grabber, J.H., Davidson, C., Tobimatsu, Y., Kim, H., Lu, F., Zhu, Y., Opietinik, M., Santoro, N., Foster, C.E., Yue, F., Ress, D., Pan, X., Ralph, J. 2019. Structural features of alternative lignin monomers associated with improved digestibility of artificially lignified maize cell walls. Plant Science. https://doi.org/10.1016/j.plantsci.2019.02.004.
Interpretive Summary: Structural polysaccharides and lignin are the main components of cell walls in plants. Unfortunately lignin limits the digestion of structural polysaccharides in forage crops fed to livestock and in biomass crops that are used for the production of biofuels and biochemicals. Therefore a number of plant bioengineering programs are trying to modify lignin to make it less inhibitory to structural polysaccharide digestion. To help guide these bioengineering efforts, we artificially formed lignin polymers in corn cell walls using normal lignin monomers plus a wide array of alternative monomers that are natural plant antioxidants but not typical components of lignin. We identified several types of alternate monomers that readily formed lignin while permitting extensive digestion of structural polysaccharides by microorganisms located in the forestomach (rumen) of cattle and by commercial enzymes used to digest biomass crops following mild chemical pretreatments. Our findings will enable plant molecular biologists to more efficiently develop new varieties of highly digestible forage and biomass crops for livestock, biofuel, and biochemical production. Biomimetic cell wall model studies assessed 21 structurally diverse alternative monomers for forming lignin and for lessening the inhibitory effects of lignin on cell wall digestibility. Several monomers that impaired lignification generally improved digestibility, this presumably would diminish the functional value of lignin in plants. Monomers designed to moderately alter hydrophobicity or introduce readily cleaved acetal, amide, or ester functionalities into the polymer often readily formed lignin, but most failed to improve digestibility. Fortunately, several alternative monomers containing multiple ester-linked catechol or pyrogallol units were identified as desirable bioengineering targets because they efficiently formed lignin and improved digestibility, presumably because they blocked cross-linking of lignin to structural polysaccharides via quinone-methide intermediates and ferulate esters and they promoted lignin fragmentation during chemical pretreatment.
Technical Abstract: Lignification of cell walls limits the digestibility of forage and biomass crops. To identify promising targets for lignin bioengineering, we artificially lignified maize (Zea mays L.) cell walls with normal monolignols plus 21 structurally diverse alternative monomers to assess their suitability for lignification and for improving cell wall digestibility. Lignin formation and structure were assessed by mass balance, Klason lignin, acetyl bromide lignin, gel-state 2D-NMR and thioacidolysis procedures, and cell wall digestibility was evaluated with rumen microflora and from glucose production by fungal enzymes following mild acid or base pretreatments. Highly acidic or hydrophilic monomers proved unsuitable for lignin modification because they severely depressed cell wall lignification. By contrast, monomers designed to moderately alter hydrophobicity or introduce cleavable acetal, amide, or ester functionalities into the polymer often readily formed lignin, but most failed to improve digestibility, even after chemical pretreatment. Fortunately, several alternative monomers containing multiple ester-linked catechol or pyrogallol units were identified here and in prior work as desirable genetic engineering targets because they efficiently formed lignin and improved the ruminal fermentation and enzymatic saccharification of cell walls, presumably by blocking cross-linking of lignin to structural polysaccharides and by promoting lignin fragmentation during mild acidic and especially alkaline pretreatment.