Characterization and Transcriptional Profiling of Ginkgo biloba Mevalonate Diphosphate Decarboxylase Gene (GbMVD) Promoter Towards Light and Exogenous Hormone Treatments

Abstract

Mevalonate diphosphate decarboxylase (GbMVD) of Ginkgo biloba catalyzes the final step of mevalonic acid pathway by converting the six-carbon mevalonate diphosphate into isopentenyl diphosphate, a precursor of ginkgolides and bilobalide collectively termed terpene trilactones (TTLs). This paper presents the isolation of a 1.2-kb fragment (designated as GbMVDpro) of the 5′-flanking region of GbMVD. Extensive sequence analysis revealed the presence of evolutionarily conserved putative cis-acting elements in light-regulated transcription, hormone signaling (abscisic acid, jasmonate, and salicylic acid), and plant defense (W-box/WRKY) in the GbMVD promoter region. Electrophoretic mobility shift analysis suggested possible involvement of W-box in GbMVD promoter function. To assess the organ-specific and environmentally responsive characteristics of GbMVD expression, GbMVDpro was fused to GUS reporter gene. GbMVDpro::GUS was introduced into tobacco. Histological analysis of the transgenic tobacco showed that GUS mainly localized in the leaf epidermis, stem secondary xylem, and root vasculature. These observations closely correlated with the previously reported presence of GbMVD transcripts in the roots, stems, and leaves. Further functional characterizations in transiently transformed tobacco leaves allowed us to identify the region that can be considered as the minimal promoter. Light and hormonal signals (i.e., ethephon, salicylic acid, and methyl jasmonate) induced the activity of GbMVDpro-driven GUS in transgenic tobacco and enhanced the transcription of GbMVD as well as TTL production in ginkgo seedlings. These results, together with the bioinformatic analysis of GbMVDpro, suggest that the GbMVD promoter harbored a numerous TTL biosynthesis-related cis-elements that regulate the flux of terpenoid precursors.