Silencing CAFFEOYL SHIKIMATE ESTERASE Affects Lignification and Improves Saccharification in Poplar

Abstract

Caffeoyl shikimate esterase (CSE) was recently shown to play an essential role in lignin biosynthesis in Arabidopsis (Arabidopsis thaliana) and later in Medicago truncatula. However, the general function of this enzyme was recently questioned by the apparent lack of CSE activity in lignifying tissues of different plant species. Here, we show that down-regulation of CSE in hybrid poplar (Populus tremula 3 Populus alba) resulted in up to 25% reduced lignin deposition, increased levels of p-hydroxyphenyl units in the lignin polymer, and a relatively higher cellulose content. The transgenic trees were morphologically indistinguishable from the wild type. Ultra-high-performance liquid chromatography-mass spectrometry-based phenolic profiling revealed a reduced abundance of several oligolignols containing guaiacyl and syringyl units and their corresponding hydroxycinnamaldehyde units, in agreement with the reduced flux toward coniferyl and sinapyl alcohol. These trees accumulated the CSE substrate caffeoyl shikimate along with other compounds belonging to the metabolic classes of benzenoids and hydroxycinnamates. Furthermore, the reduced lignin amount combined with the relative increase in cellulose content in the CSE down-regulated lines resulted in up to 62% more glucose released per plant upon limited saccharification when no pretreatment was applied and by up to 86% and 91% when acid and alkaline pretreatments were used. Our results show that CSE is not only important for the lignification process in poplar but is also a promising target for the development of improved lignocellulosic biomass crops for sugar platform biorefineries.

Plant cell wall sugars: sweeteners for a bio‐based economy

“…The reduction in lignin content increased the saccharification efficiency fourfold as compared to WT plants. CSE orthologs are present in poplar, and downregulation of CSE in poplar improved saccharification efficiency, as anticipated (Saleme et al ). However, there is no indication for the presence of a proper CSE ortholog in maize making it unlikely that the positive effects on saccharification yield can be translated to this crop (Ha et al ).…”

Structural and biochemical approaches uncover multiple evolutionary trajectories of plant quinate dehydrogenases

“…The NAD + ‐dependent QDHs, on the other hand, are most probably involved in diverting quinate away from CGA biosynthesis and in utilizing the quinate that is released from CGA during its breakdown (Figure a) (Ding et al ., ). Consistent with this model, silencing of a caffeoyl shikimate esterase in poplar results in buildup of caffeoyl/feruloyl quinate (Saleme et al ., ). The identification of CGA‐specific esterase is the focus of future studies.…”

Secondary cell wall biosynthesis

“…Conversion of caffeoyl shikimate to caffeoyl CoA may also be carried out by an alternative route, first to caffeic acid by caffeoyl shikimate esterase (CSE) and then to caffeoyl CoA by 4CL (Fig. 7), as demonstrated in Arabidopsis, poplar and Medicago truncatula by mutation or down-regulation of CSE resulting in a reduction in lignin biosynthesis (Vanholme et al, 2013;Ha et al, 2016;Saleme et al, 2017). However, no CSE orthologs were found in the genomes of Brachypodium distachyon and corn, and little CSE activities were detected in these species (Ha et al, 2016), suggesting that the role of CSE in lignification may not be universal in all plant species.…”