Distinct and Overlapping Functions of Miscanthus sinensis MYB Transcription Factors SCM1 and MYB103 in Lignin Biosynthesis.

Cell wall recalcitrance is a major constraint for the exploitation of lignocellulosic biomass as a renewable resource for energy and bio-based products. Transcriptional regulators of the lignin biosynthetic pathway represent promising targets for tailoring lignin content and composition in plant secondary cell walls. However, knowledge about the transcriptional regulation of lignin biosynthesis in lignocellulosic feedstocks, such as Miscanthus, is limited. In Miscanthus leaves, MsSCM1 and MsMYB103 are expressed at growth stages associated with lignification. The ectopic expression of MsSCM1 and MsMYB103 in N. benthamiana leaves was sufficient to trigger secondary cell wall deposition with distinct sugar and lignin compositions. Moreover, RNA-seq analysis revealed that the transcriptional responses to MsSCM1 and MsMYB103 overexpression showed an extensive overlap with the response to the NAC master transcription factor MsSND1, but were distinct from each other, underscoring the inherent complexity of secondary cell wall formation. Furthermore, conserved and previously described promoter elements as well as novel and specific motifs could be identified from the target genes of the three transcription factors. Together, MsSCM1 and MsMYB103 represent interesting targets for manipulations of lignin content and composition in Miscanthus towards a tailored biomass.

The in vivo impact of MsLAC1, a Miscanthus laccase isoform, on lignification and lignin composition contrasts with its in vitro substrate preference.

BACKGROUND: Understanding lignin biosynthesis and composition is of central importance for sustainable bioenergy and biomaterials production. Species of the genus Miscanthus have emerged as promising bioenergy crop due to their rapid growth and modest nutrient requirements. However, lignin polymerization in Miscanthus is poorly understood. It was previously shown that plant laccases are phenol oxidases that have multiple functions in plant, one of which is the polymerization of monolignols. Herein, we link a newly discovered Miscanthus laccase, MsLAC1, to cell wall lignification. Characterization of recombinant MsLAC1 and Arabidopsis transgenic plants expressing MsLAC1 were carried out to understand the function of MsLAC1 both in vitro and in vivo. RESULTS: Using a comprehensive suite of molecular, biochemical and histochemical analyses, we show that MsLAC1 localizes to cell walls and identify Miscanthus transcription factors capable of regulating MsLAC1 expression. In addition, MsLAC1 complements the Arabidopsis lac4-2 lac17 mutant and recombinant MsLAC1 is able to oxidize monolignol in vitro. Transgenic Arabidopsis plants over-expressing MsLAC1 show higher G-lignin content, although recombinant MsLAC1 seemed to prefer sinapyl alcohol as substrate. CONCLUSIONS: In summary, our results suggest that MsLAC1 is regulated by secondary cell wall MYB transcription factors and is involved in lignification of xylem fibers. This report identifies MsLAC1 as a promising breeding target in Miscanthus for biofuel and biomaterial applications.

Plant protein inhibitors of invertases.

In higher plants, cell wall (CWI) and vacuolar invertases (VI) are important metabolic enzymes, but are also key players during wound and pathogen defense reactions and in several developmental transitions. These multiple functions are implemented by small gene families. While induction of CWI and VI activities usually operates via increased transcription of the corresponding isoform gene, the equally important silencing of invertase activity depends on post-translational mechanisms, including inactivation by specific inhibitor proteins. Recently, the first cDNAs for plant invertase inhibitors were cloned, NtCIF and NtVIF (cell wall/vacuolar inhibitor of beta-fructosidase). The encoded proteins have been expressed in E. coli for functional studies and transgenic tobacco and potato plants were generated to explore the inhibitor function(s) in vivo. Mining the Arabidopsis thaliana genome revealed an inhibitor protein family of limited sequence conservation, some members grouping with tobacco CIF and VIF, while others showing a closer similarity with a recently identified inhibitor of pectin methylesterase. In vitro studies have confirmed target enzyme specificity for invertase and pectin methylesterase inhibitors (PMEI), respectively. The current status of research on invertase inhibitors and the perspectives for their use in plant biotechnology will be discussed.