Geranyl diphosphate synthase small subunit I (GPPS.SSU I) expression strongly correlates with the presence or absence of monoterpene emission from lily (Lilium spp.) flowers.

Abstract

Main conclusion: We reported the functional characterization of cDNAs encoding geranyl diphosphate synthase small subunit I (GPPS.SUU I) that determine the presence or absence of monoterpene emission from lily flowers. While monoterpene compounds are among the most important scent substances in lily flowers, their emission levels vary among lily cultivars. For example, Oriental hybrid cultivars emit sufficient monoterpenes, but Asiatic hybrid cultivars scarcely emit them. Geranyl diphosphate (GPP), a precursor of monoterpenes, is biosynthesized by GPP synthase (GPPS) in plastids. Angiosperm GPPSs are often heteromeric enzymes consisting of non-catalytic small subunits (GPPS.SSU) and catalytic large subunits (GPPS.LSU); the homomeric form of the latter functions as a geranylgeranyl diphosphate synthase (GGPPS). To determine which genes created large variations in monoterpene emission levels, we characterized Lilium hybrid GPPS.SSU I (LhGPPS.SSU I) and LhGGPPS/GPPS.LSU1, and compared their expression levels in scent and scentless lily flowers. Green fluorescent protein (GFP)-fused LhGGPPS/GPPS.LSU1 and LhGPPS.SSU I proteins were co-localized in the chloroplasts of tobacco cells, and LhGPPS.SSU I interacted physically with LhGGPPS/GPPS.LSU1 in the yeast two-hybrid system. LhGPPS.SSU I overexpression in tobacco (transient) and lily (stable) leaves promoted GPP biosynthesis. Thus, LhGPPS.SSU I interacted with LhGGPPS/GPPS.LSU1 to biosynthesize GPP. LhGPPS.SSU I expression in lily tepals occurred in Oriental but not in Asiatic hybrid cultivars, whereas LhGGPPS/GPPS.LSU1 expression occurred in both, irrespective of monoterpene emissions, indicating that LhGPPS.SSU I is a key determinant of monoterpene emissions from lily flowers. LhGPPS.SSU I was not expressed in seven wild species involved in Asiatic hybrid lily establishment but was expressed in Lilium cernuum, which can hybridize with Asiatic hybrid lilies, indicating that L. cernuum is a promising genetic resource for enhancing floral scent in Asiatic hybrids.