Unexpected behavior of coniferin in lignin biosynthesis of Ginkgo biloba L.

To gain insight into the behavior of monolignol glucoside in Ginkgo biloba L., we examined glucosides potentially involved in lignin biosynthetic pathway. Coniferin (coniferyl alcohol 4O-beta-D-glucoside) is a strong candidate for the storage form of monolignol. Coniferaldehyde glucoside may also have a role in lignin biosynthesis; this was examined with tracer experiments using labeled glucosides fed to stem segments. A series of tracer experiments showed that coniferin and coniferaldehyde glucoside were modified into coniferyl alcohol and then efficiently incorporated into lignin under the experimental conditions used. Interestingly, more than half of the administered coniferin underwent an oxidation to the aldehyde form before its aglycone; coniferyl alcohol was polymerized into lignin. This suggests that there is an alternative pathway for coniferin to enter the monolignol biosynthetic pathway, in addition to the direct pathway beginning with the deglucosylation of coniferin catalyzed by beta-glucosidase. Enzymatic assays revealed that coniferaldehyde glucoside was produced enzymatically from coniferin, and that coniferaldehyde glucoside can be deglucosylated to yield coniferaldehyde, which could be fated to become coniferyl alcohol . Albeit the findings cannot be taken as proof for the in-planta functioning, these results present a possibility for the existence of alternative pathway in which some of the stored coniferin is oxidized to coniferaldehyde glucoside, which is deglucosylated to generate coniferaldehyde that joins the monolignol biosynthesis pathway.