Downregulation of Cinnamoyl-Coenzyme A Reductase in Poplar: Multiple-Level Phenotyping Reveals Effects on Cell Wall Polymer Metabolism and Structure

Department of Plant Systems Biology, Flanders Institute for Biotechnology, 9052 Gent, Belgium.
The Plant Cell (Impact Factor: 9.34). 12/2007; 19(11):3669-91. DOI: 10.1105/tpc.107.054148
Source: PubMed


Cinnamoyl-CoA reductase (CCR) catalyzes the penultimate step in monolignol biosynthesis. We show that downregulation of CCR in transgenic poplar (Populus tremula x Populus alba) was associated with up to 50% reduced lignin content and an orange-brown, often patchy, coloration of the outer xylem. Thioacidolysis, nuclear magnetic resonance (NMR), immunocytochemistry of lignin epitopes, and oligolignol profiling indicated that lignin was relatively more reduced in syringyl than in guaiacyl units. The cohesion of the walls was affected, particularly at sites that are generally richer in syringyl units in wild-type poplar. Ferulic acid was incorporated into the lignin via ether bonds, as evidenced independently by thioacidolysis and by NMR. A synthetic lignin incorporating ferulic acid had a red-brown coloration, suggesting that the xylem coloration was due to the presence of ferulic acid during lignification. Elevated ferulic acid levels were also observed in the form of esters. Transcript and metabolite profiling were used as comprehensive phenotyping tools to investigate how CCR downregulation impacted metabolism and the biosynthesis of other cell wall polymers. Both methods suggested reduced biosynthesis and increased breakdown or remodeling of noncellulosic cell wall polymers, which was further supported by Fourier transform infrared spectroscopy and wet chemistry analysis. The reduced levels of lignin and hemicellulose were associated with an increased proportion of cellulose. Furthermore, the transcript and metabolite profiling data pointed toward a stress response induced by the altered cell wall structure. Finally, chemical pulping of wood derived from 5-year-old, field-grown transgenic lines revealed improved pulping characteristics, but growth was affected in all transgenic lines tested.

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    • "Moreover, little is known about their correlations with mandatory selection criteria such as growth and straightness , or with other end-use properties such as wood density, modulus of elasticity or fibre-related traits (Pot et al. 2002; Da Silva Perez et al. 2007). Recent studies involving transgenic poplars and radiata pines suggest that the capacity for lignin reduction is limited: lignin contents reduced by less than 10% do not appreciably change tree growth characteristics (Pilate et al. 2002; Wagner et al. 2009; Voelker et al. 2010), whereas reductions in lignin content by about 20% can dramatically affect productivity (Leple et al. 2007; Wagner et al. 2009; Voelker et al. 2010). "

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    • "Along with cellulose and hemicellulose, lignin is a main component of secondary cell walls (Zhong et al. 2011). In grass species, lignin comprises approximately 20% of the secondary cell wall, filling pores between the polysaccharides (Leple et al. 2007; Vogel 2008). Lignin deposition begins when cell differentiation is completed and secondary thickening occurs in the walls (Rogers and Campbell 2004; Wang et al. 2013). "
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    • "Remarkably, the CCR gene was first cloned in Eucalyptus gunnii (EguCCR) and its identity was confirmed unambiguously by the enzymatic activity of the corresponding recombinant protein (Lacombe et al., 1997). EguCCR cDNA was then used as a probe to clone its orthologs in tobacco (Nicotiana tabacum) (Piquemal et al., 1998), Arabidopsis thaliana (Lauvergeat et al., 2001) and Populus (Lepl e et al., 2007). In line with its key role in controlling lignin content and composition, EguCCR was later shown to co-localize with a quantitative trait locus (QTL) for S : G lignin ratio (Gion et al., 2011). "
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