Deciphering the enigma of lignification: precursor transport, oxidation, and the topochemistry of lignin assembly

Towards uncovering the roles of switchgrass peroxidases in plant processes

Herbaceous perennial plants selected as potential biofuel feedstocks had been understudied at the genomic and functional genomic levels. Recent investments, primarily by the U.S. Department of Energy, have led to the development of a number of molecular resources for bioenergy grasses, such as the partially annotated genome for switchgrass (Panicum virgatum L.), and some related diploid species. In its current version, the switchgrass genome contains 65,878 gene models arising from the A and B genomes of this tetraploid grass. The availability of these gene sequences provides a framework to exploit transcriptomic data obtained from next-generation sequencing platforms to address questions of biological importance. One such question pertains to discovery of genes and proteins important for biotic and abiotic stress responses, and how these components might affect biomass quality and stress response in plants engineered for a specific end purpose. It can be expected that production of switchgrass on marginal lands will expose plants to diverse stresses, including herbivory by insects. Class III plant peroxidases have been implicated in many developmental responses such as lignification and in the adaptive responses of plants to insect feeding. Here, we have analyzed the class III peroxidases encoded by the switchgrass genome, and have mined available transcriptomic datasets to develop a first understanding of the expression profiles of the class III peroxidases in different plant tissues. Lastly, we have identified switchgrass peroxidases that appear to be orthologs of enzymes shown to play key roles in lignification and plant defense responses to hemipterans.

Impact of the absence of stem-specific β-glucosidases on lignin and monolignols

Monolignol glucosides are thought to be implicated in the lignin biosynthesis pathway as storage and/or transportation forms of cinnamyl alcohols between the cytosol and the lignifying cell walls. The hydrolysis of these monolignol glucosides would involve β-glucosidase activities. In Arabidopsis (Arabidopsis thaliana), in vitro studies have shown the affinity of β-GLUCOSIDASE45 (BGLU45) and BGLU46 for monolignol glucosides. BGLU45 and BGLU46 genes are expressed in stems. Immunolocalization experiments showed that BGLU45 and BGLU46 proteins are mainly located in the interfascicular fibers and in the protoxylem, respectively. Knockout mutants for BGLU45 or BGLU46 do not have a lignin-deficient phenotype. Coniferin and syringin could be detected by ultra-performance liquid chromatography-mass spectrometry in Arabidopsis stems. Stems from BGLU45 and BGLU46 mutant lines displayed a significant increase in coniferin content without any change in coniferyl alcohol, whereas no change in syringin content was observed. Other glucosylated compounds of the phenylpropanoid pathway were also deregulated in these mutants, but to a lower extent. By contrast, BGLU47, which is closely related to BGLU45 and BGLU46, is not implicated in either the general phenylpropanoid pathway or in the lignification of stems and roots. These results confirm that the major in vivo substrate of BGLU45 and BGLU46 is coniferin and suggest that monolignol glucosides are the storage form of monolignols in Arabidopsis, but not the direct precursors of lignin.