The more relevant Vitaceae phytoalexins comprise a group of molecules belonging to the stilbene family [1, 2], which are derivatives of the trans-resveratrol structure (3,5,4'-trihydroxystilbene). In addition to trans-resveratrol derived molecules, other oligomers produced by its oxidation and generically known as viniferins have been found as the result of infection or stress [3]. Different naturally occurring stilbenes like resveratrol, pterostilbene, piceatannol and resveratrol glucoside derivatives [4] are known to be strong antioxidants. In particular, the potential benefits of resveratrol on human health have made it one of the most thoroughly studied phytochemical molecules [5]. See de la Lastra and Villegas [6] for a review of the reported resveratrol effects.

Stilbenes are synthesized via the phenylpropanoid/malonate pathway from phenylalanine that, in turn, is converted into cinnamic acid by phenylalanine ammonia lyase (PAL). The consecutive action of cinnamate 4-hydroxylase (C4H) and 4-coumarate CoA ligase (4CL) transform cinnamic acid into p-coumaryl-CoA. Derived compounds, collectively referred to as polyphenols, are originated at this branching point through the action of enzymes chalcone synthase (CHS) and stilbene synthase (STS) for flavonoids and stilbenoids, respectively [2].

Vitis vinifera cell cultures have been used in several studies to explore the factors involved in the induction and regulation of stilbene biosynthesis and metabolism [7, 8]. Jasmonic acid (JA) and its more active derivative methyljasmonate (MeJA) have been proposed as key compounds of the signal transduction pathway involved in the elicitation of secondary metabolite biosynthesis which takes part in plant defence reactions [9]. Application of MeJA on grapevine leaves and plant cell suspension cultures can induce the accumulation of stilbenes [10, 11]. However, the reported amount of stilbenes secreted to the medium in MeJA-treated cell cultures is negligible [8, 12].

Cyclodextrins (CDs) are naturally occurring cyclic oligosaccharides derived from starch. Addition of 2,6 dimethyl-β-ciclodextrin (DIMEB) to grapevine cell cultures induces both resveratrol biosynthesis and its accumulation in the culture media [13]. Among the differently modified β-cyclodextrins, the methylated and hydroxypropylated caused the highest production of this phytoalexin, which is translocated to the cell walls and accumulates in the culture media [14, 15].

The aim of this study was to evaluate the elicitor effect of joint applications of CDs and MeJA on grapevine cell cultures by carrying out a quantitative analysis of their role on resveratrol production. We also monitored the expression of several genes encoding key enzymes in the phenylpropanoid pathway, including those involved in resveratrol biosynthesis, to determine the relationship between resveratrol accumulation in the medium after elicitation and the regulation of gene expression. A synergistic interaction between CD and MeJA on resveratrol production and on the expression of stilbene biosynthesis related genes is discussed.

The results described here show that the combined addition of MeJA and CD to V. vinifera cv. Monastrell albino cell cultures yields a much higher resveratrol accumulation than the sum of the individual additions. In our experiments, the combined treatment increased seven times the yield of resveratrol when compared to CDs alone. This final resveratrol level (1600 μmole gDW^-1) represents an increase between 10- and more than 1000-fold with respect to previous reports [8, 12, 16]. The expression analysis of this response shows that both elicitors stimulated the expression of PAL, C4H, 4CL and STS independently of the anthocyanins/isoflavonoids (CHS) and lignins (CCR) pathways and therefore both induce stilbene biosynthetic genes in a highly specific way, in agreement to results reported by Saigne-Soulard et al.[17]. Furthermore, the synergistic interaction of both elicitors on resveratrol production (Figure 2) seems to be the result of their synergistic effect on the expression of biosynthetic genes (Figure 3).

In MeJA-treated cells, a significant reduction in cell growth was observed (Figure 1) in parallel with a strong induction of the general phenylpropanoid pathway (Figure 3), as recently reported for Arabidopsis cell suspension cultures [18]. However, although STS expression was highly induced (Figure 3), no significant amounts of resveratrol were detected in the spent media (Figure 2). Such discrepancy could be due to either post-transcriptional and/or post-translational regulatory mechanisms [12, 16, 19].

CD elicited cells effectively produced significant amounts of resveratrol (Figure 2) in correlation with a transient expression of the central phenylpropanoid enzymes and STS genes (Figure 3). Furthermore, most (or all) the resveratrol synthesized up to 72 h remained in the culture medium until the end of the assay (Figure 2). CDs are able to form inclusion complexes with stilbene compounds, such as trans- resveratrol and diethylstilbestrol [13, 20], which could protect resveratrol from oxidation or glucosylation. This could explain the observation that, although STS expression dropped after 72 h in the CD treatment (Figure 3), the amount of resveratrol stayed constant (Figure 2). In contrast to the effect of MeJA, CD treated cells were not altered in their growth, displaying a similar biomass growth curve as control cells (Figure 1). Since both complexed and uncomplexed CD molecules remain in the culture medium during the whole assay, the transient gene expression and the high but limited production of resveratrol must be the result of additional regulatory mechanisms. Given the regular growth curve of the CD-treated cell cultures it is tentative to propose that engagement of cells in active division could somehow compete with further production of resveratrol after a transient elicitation response. In fact, Naill & Roberts [21] observed that most metabolite productive cells in Taxus cuspidata suspension cultures were in the G₀/G₁ phase of the cell cycle (i.e. non-cycling cells) and this stage was suggested as the most specialised for accumulation of secondary metabolites.

In agreement with the previous hypothesis, when both CD and MeJA were simultaneously added to the culture medium, they caused a significant reduction in cell growth (Figure 1) as well as a sustainable maximum expression of STS and central phenylpropanoid genes, even after 168 h (Figure 3), which was paralleled by a maximum resveratrol accumulation in the medium (Figure 2). We believe that the blockage in cell division and metabolic rearrangement likely caused by MeJA [18] could place the cells in a non-cycling state [21] allowing a sustained elicitation by CD.

It has been suggested that MeJA may induce a subset of secondary metabolite biosynthetic genes which could modulate expression of genes and accumulation of compounds induced by elicitors [22, 23]. Although we cannot completely discard this possibility, we show that the synergistic effect observed on resveratrol production is related with a synergistic effect on the expression of the same set of stilbene biosynthetic genes induced by CD (Figure 3). The observed effects of MeJA on cell suspension growth and the recent characterization of MeJA effects on Arabidopsis cell cultures open the possibility to propose an alternative hypothesis to explain this synergy based on the combined effect of MeJA on cell cycle together with a true and strong elicitor like CD. Further experiments will be required to confirm this possibility on the interaction between cell cycle and secondary metabolite biosynthetic gene expression.