White campion, Silene latifolia (Caryophyllaceae), emits a diverse array of volatiles to attract sphingid, geometrid, and noctuid moths for pollination [1–4]. This species also shows a pronounced day-night rhythm in odor emission, with the key compounds predominately emitted during the night [4–7]. Among commonly known pollinators for this species, Hadena bicruris, a noctuid moth, is a specialist nursery pollinator and obligate seed predator [8, 9]. Female H. bicruris are not only attracted for nectaring but also for oviposition into female S. latifolia flowers . The larvae nurture on developing seeds  and consume almost one fourth of the fruits developed [12, 13]. Available experimental evidence indicates that the Hadena-Silene relationship can swing in between mutualism and antagonism .
Recently, the scent composition of S. latifolia and related species has been identified and studied for behavioral activity in the pollinators [4, 10, 14]. A large set of volatile compounds has been found in the S. latifolia floral odor bouquet [6, 7, 13, 15] and these compounds comprise three major categories: fatty acid derivatives, aromatics, and terpenoids [6, 7]. Using wind-tunnel bioassays, Dötterl et al.  investigated pollinators interaction with individual scent compound and uncovered that only seven (veratrole, decanal, linalool, guaiacol, phenylacetaldehyde, isopentylaldoxime, and lilac aldehydes) out of total produced compounds in S. latifolia flowers showed behavioral activity in H. bicruris. A further study based on scent composition analysis revealed that veratrole and lilac aldehydes emission is reduced four-folds after pollination, while other behaviorally active and non-active compounds remain unaltered . Therefore, apart from being involved in pollinator attraction, the decrease in veratrole and lilac aldehyde emission may slow down oviposition and subsequent seed predation by Hadena following pollination. Phenylacetaldehyde, one of the most abundant behaviorally active compounds, is involved in floral isolation of S. latifolia from the closely related species S. dioica. These investigations altogether imply that veratrole, lilac aldehydes, and phenylacetaldehyde are key odor compounds that play a central role in pollinator attraction and floral isolation [6, 7, 10, 14]. It is presently unclear which compounds induce oviposition by Hadena females into female S. latifolia flowers. H. bicruris rarely oviposits into S. dioica. However, the qualitative difference in floral volatile organic compounds (VOCs) between S. dioica and S. latifolia involves only few compounds. Veratrole, guaiacol, and benzyl benzoate are produced only in S. latifolia but a fatty acid derivative, nonanal is only emitted by S. dioica. Therefore, besides quantitative scent differences , three compounds produced in S. latifolia are involved in species differentiation and presumably in maintaining the Hadena-Silene latifolia relationship.
During the past two decades, molecular research on Silene has primarily focused on sex-determination [16–19], the evolution of heteromorphic sex chromosomes [20–24], hybridization [25, 26], and EST sequencing for species differentiation or marker development [27, 28]. The production of copious amounts of behaviorally active volatile compounds also makes Silene an ideal system for investigating genes underlying volatile biosynthesis. At present, though, scent biosynthetic pathways remain uncharacterized in Silene. Among several scent enzymes known so far, the plant O-methyltransferase (OMT) family of enzymes performs a prominent role in secondary metabolism and eliminates a methyl group from S-adenosyl-L-methionine to the hydroxyl group of the substrate . Besides playing a role in lignin biosynthesis [30, 31], anthocyanin biosynthesis [32, 33], and disease resistance [34–36], these OMTs are also involved in volatile biosynthesis [37–40]. For instance, eugenol O-methyltransferase (EOMT) and chavicol O-methyltransferase (CVOMT) methylate the substrates in order to synthesize methyleugenol and methylchavicol, respectively [41, 42]. Studies in roses reported the functional characterization of orcinol O-methyltransferases (OOMT1 and OOMT2) genes that are involved in the formation of 3-hydroxy 5-methoxytoluene and 3, 5–dimethoxytoluene (DMT), two key scent compounds of rose varieties [43, 44]. Until now, several plant methyltransferases have been functionally characterized owing to their involvement in floral scent biosynthesis and flavoring properties [42, 45, 46].
As part of an ongoing research project to characterize key genes involved in floral scent biosynthesis in Silene species, we have recently developed a S. latifolia floral EST resource of 3,072 sequences by constructing one standard and two subtraction cDNA libraries (Gupta et al. in prep). The analysis of these sequences allowed us to characterize a wide range of candidate genes including several OMTs with high similarities to functionally characterized OMTs in other species. Here we show that two full-length coding cDNAs derived from these libraries represent S. latifolia guaiacol O-methyltransferase1 (SlGOMT1) and S. latifolia guaiacol O-methyltransferase2 (SlGOMT2) genes and address the following questions: 1) Do heterologously expressed proteins catalyze the formation of veratrole in S. latifolia and S. dioica? 2) How do differences in veratrole emission between day and night in S. latifolia controlled? 3) Are SlGOMT genes differentially expressed between floral and leaf tissues, and between sexes? 4) Do SlGOMT and S. dioica O-methyltransferase (SdOMT) genes show evidence for selection?