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.