Cloning and characterization of a lupeol synthase involved in the synthesis of epicuticular wax crystals on stem and hypocotyl surfaces of Ricinus communis

Pentacyclic triterpenoids are a large group of secondary metabolites found in many different plant species, either as glycoside conjugates or as aglycones. The latter in many cases accumulate to high amounts in the cuticular wax and hence at the surface of plant organs. In the present work, the cuticle-specific formation of triterpenoids was investigated in Ricinus communis stems, combining analytical and molecular genetic methods. Two phenotypes of castor bean could be distinguished based on the glaucous or glossy appearance of the surfaces of all stem portions including the hypocotyls, and were due to the presence or absence of thread-shaped epicuticular wax crystals, respectively. Comparative studies showed that these crystals are formed by the triperpenoid lupeol, present in high amounts on all stem surfaces. On the hypocotyl portion of stems, lupeol was found to accumulate rapidly during early development of the surface (10–15 days after emergence). Mature hypocotyls of glossy individuals were covered with 12.5 μg/cm2 of wax containing approximately 1% of lupeol, whereas the glaucous phenotype had a wax load of 51.9 μg/cm2 with 56% of lupeol. Two oxidosqualene cyclases from castor bean were cloned, functionally expressed in yeast, and characterized as a cycloartenol synthase (RcCAS) and a lupeol synthase (RcLUS). Phylogenetic analyses revealed that RcLUS is similar to two clades of known lupeol synthases, but also exhibits some similarities with β-amyrin synthases. Both the organ-specific expression of RcLUS and the expression pattern during hypocotyl development exactly matched the accumulation of cuticular lupeol in castor bean. In contrast, RcCAS was constitutively expressed in all organs at various times. We conclude that the RcLUS enzyme is responsible for formation of the cuticular lupeol, and thus for the characteristic surface properties of R. communis stems.