Biochemical and Molecular Genetic Aspects of Floral Scents

Abstract

The chemical composition of floral scents has been extensively investigated for hundreds of years because of the commercial value of floral volatiles in perfumery. More recently, several ecological studies have examined the roles of floral scent in the biology of the plant. However, in contrast to the chemical emphasis of the perfumers and the organismal emphasis of the ecologists, until recently, there have been few studies concerning the biochemical synthesis of floral scent compounds and the enzymes and genes that control these processes. In fact, our recent investigation into the biogenesis of floral scent production in Clarkia breweri, an annual plant native to California that served as our model organism, and our even more recent work on the scent of the cultivated snapdragon (Antirrhinum majus) represent the only examples to date (to our knowledge) in which isolation of enzymes and genes involved in the de novo synthesis of scent compounds in the flower have been reported. In this Update we review the research leading to our work, report our findings, and discuss implications for future directions of the field. Many plants emit floral scents, and such scents can attract a variety of animal pollinators, mostly insects. When present, scent is often the dominant means of long-distance attraction, particularly in moth-pollinated flowers, which are searched out and visited at night. Floral fragrances vary widely among species in terms of the number, identity, and relative amounts of constituent volatile compounds. Although little is known about how insects respond to individual components found in floral scents, it is clear that insects are able to distinguish between complex floral scent mixtures, and that discriminatory visitation based on floral scent has important implications for plant reproductive success (Pellmyr, 1986). Since floral scent can be crucial in ensuring fertilization, and therefore in determining seed or fruit set, the presence or absence of a scent attractive to the locally available insect pollinators may have a substantial impact on the yield of agronomically important crops (Free, 1970). Plants imported into a new environment by humans may be especially disadvantaged in this regard, as they have not co-evolved with the local pollinators (Traub et al., 1942). Even if a local pollinator is attracted to the flowers, it may not be physically suitable to be an effective pollinator. On the other hand, pollinators that may have the appropriate physique (by chance) may not be successfully attracted to the plant (Herrera, 1987). Plants did not naturally evolve to produce their scent for the benefit of humans; nevertheless, it is clear that humans find an aesthetic value in certain types of floral scents, and the presence of floral scent may have contributed to the decision by humans to cultivate and propagate specific plant species. While there is certainly a wide variation in human taste, most people prefer the scents of bee-pollinated and, especially, moth-pollinated flowers, which they often describe as “sweet-smelling” (Knudsen and Tollsten, 1993). Unfortunately, very few plants are currently cultivated primarily for their scent. Moreover, a large number of commercial flower varieties have lost their scent during the selection and breeding processes due to, on the one hand, a focus on maximizing post-harvest shelf-life, shipping characteristics, and visual aesthetic values (i.e. color, shape), and on the other hand, to the lack of selection for the scent trait. Some volatile compounds found in floral scent have important functions in vegetative processes as well. They may function as attractants for the natural predators of herbivores (Pare and Tumlinson, 1997) or as airborne signals that activate disease resistance via the expression of defense-related genes in neighboring plants and in the healthy tissues of infected plants (Shulaev et al., 1997). They may also serve as repellents against herbivores (Gershenzon and Croteau, 1991). Pathways that lead to floral scent volatiles. Volatile compounds are shown with a yellow background; enzymes that have been identified in the synthesis of volatile compounds in vegetative tissues are shown in black; enzymes identified in floral tissues are shown in red. Not all reactions or enzymes have been identified. A, Sesquiterpenes (top) are synthesized in the cytosol. Biosynthesis of monoterpene (bottom) occurs in the plastids, although the location of the reactions leading to further modifications (e.g. linalool oxide) is not yet clear. B, The location of synthesis of most phenolpropanoids/benzenoids is not yet known, but is likely to be the cytosol and possibly the peroxisomes. IEMT, SAMT, and BAMT are methyltransferases that use SAM (not shown) as the methyl donor. BEAT is an acetyltransferase that uses acetyl-CoA (not shown) as the acetyl donor. The question of from which part of the flower the various scent components are emitted is a technically