Different Routes for Conifer- and Sinapaldehyde and Higher Saccharification upon Deficiency in the Dehydrogenase CAD1
Plant Physiology, 175 (3) 1018-1039, https://doi.org/10.1104/pp.17.00834
Van Acker R, Déjardin A, Desmet S, Hoengenaert L, Vanholme R, Morreel K, Laurans F, Kim H, Santoro N, Foster C, Goeminne G, Légée F, Lapierre C, Pilate G, Ralph J, Boerjan WA
Collaboration INRA (Val de Loire, Versailles, AgroParisTech), VIB/Univ Gent (Belgique), Univ. Wisconsin-Madison (USA), Wisconsin Energy Institute (USA).
XYLOBIOTECH technical support :Confocal microscopy
In search for renewable energy sources, genetic engineering is a promising strategy to improve plant cell wall composition for biofuel and bioproducts generation. Lignin is a major factor determining saccharification efficiency and is therefore a prime target. Here, lignin content and composition were modified in poplar (Populus tremula×P. alba) by specifically downregulating CINNAMYL ALCOHOL DEHYDROGENASE1 (CAD1) by a hairpin-RNA-mediated silencing approach. These transgenic lines showed no biomass penalty despite a 10% reduction in Klason lignin content and severe shifts in lignin composition. NMR spectroscopy and thioacidolysis revealed a strong increase in sinapaldehyde incorporation into lignin, whereas coniferaldehyde was not markedly increased. Accordingly, ultrahigh-performance liquid chromatography-mass spectrometry-based phenolic profiling revealed a more than 24,000-fold accumulation of a newly identified compound made from 8-8 coupling of two sinapaldehyde radicals. However, no additional cinnamaldehyde coupling products could be detected. Instead, the CAD1-deficient lines accumulated a range of hydroxycinnamate-derived metabolites of which the most prominent accumulation was observed for a compound that was identified by purification and NMR as syringyl lactic acid hexoside. Our data suggest that upon downregulation of CAD1, coniferaldehyde is converted into ferulic acid and derivatives, whereas sinapaldehyde is either oxidatively coupled into S′(8-8)S′ and lignin, or converted to sinapic acid and derivatives. The most prominent sink of the increased flux to hydroxycinnamates is syringyl lactic acid hexoside. Furthermore, low-extent saccharification assays, under different pretreatment conditions, showed strongly increased glucose and xylose release, suggesting that downregulating CAD1 is a promising strategy for improving lignocellulosic biomass for the 'sugar platform' industry.