Concluding Remarks

Ongoing research is attempting to unravel the Vitis primary sulfur pathway, from root sulfate uptake to cysteine synthesis. The expression of genes of some sulfate transporters has been analyzed for the first time (Tavares et al 2008) and genomics of the assimilatory enzymes is in progress.

Recently, the release of grapevine genome (Jaillon et al. 2007, Velasco et al. 2007) disclosed large families of genes associated with primary sulfur metabolism, sulfate uptake and assimilation, methionine and GSH metabolism, as well as with secondary metabolism pathways directly connected with sulfur availability. The role of GSH in photosynthetic organisms and the emerging functions for glutaredoxins was reviewed recently (Rouhier et al. 2008). GSH is the most abundant low-molecular-weight thiol in cells and it constitutes a redox buffer that keeps the intracellular environment reduced. It can be oxidized to GSSG by some reactive oxygen species (ROS), such as H2O2; thus playing a major role in the detoxification of ROS. GSH also participates in the glu-tathione/ascorbate cycle, in which GSH allows regeneration of reduced ascor-bate, the other major antioxidant in plant cells. GSH also provides electrons to peroxidases of the peroxiredoxin family, which use GSH alone or with glutare-doxin. Other GSH-dependent enzymes, such as glutathione S transferases (GST) form GSH conjugates. In grapevine, stress conditions which up-regulate anthocyanin biosynthesis can lead to anthocyanin levels toxic to the plant cells. This toxic effect can be avoided by the transport of anthocyanins into the vacuole after conjugation with GSH by GSTs (Boss and Davies 2009, Xiang et al. 2001).

Fig. 9. Relative expression of VvATP-Sl and VvAPSR in S-depleted cells transferred to + S (1.5 mM) or receiving 1 mM GSH or 1 mM cysteine for 24 h, analyzed by real-time PCR as in Fig. 2 (Tavares et al. unpublished).

Phenylpropanoid compounds are induced in plants by biotic and abiotic stresses. Low nitrogen is related with the accumulation of flavonoids, low phosphorous with anthocyanins and low iron with phenolic acids (Dixon and Paiva 1995). Anthocyanins are secondary metabolites present in almost all higher plants, namely in the skin of red grapes. It is reported that nitrogen supply affects the metabolic pathway of anthocyanins: berries from high N supplied vines had a lower anthocyanin content which decreased significantly during maturation (Hilbert et al. 2003). Stilbenes are stress-induced phenylpropanoids which result from the condensation of p-coumaroyl-CoA or cinnamoyl-CoA with malonyl-CoA by the action of stilbene synthase. In grapevine, resveratrol is a low molecular weight stilbene acting as a phytoalexin (Bavaresco et al. 2009). Its synthesis is elicited in leaves, berries and cell suspensions in response to biotic, abiotic and nutritional stress. N supply decreased resveratrol in leaves and berries (Bavaresco et al. 2001). However, the effects of sulfur availability on phenylpropanoid compounds, including anthocyanins and resveratrol, have not been elucidated. These compounds have important roles in the resistance of grapevine to biotic and abiotic stresses and can confer antioxidant properties to red wine; therefore the cross talk between sulfur and phenylpropanoid metabolism is an important research avenue to investigate.

A point deserving future attention is the hormonal control of sulfur assimilation and the effect of sulfur status mediated by hormonal signalling. Arabidopsis transcriptome analysis of sulfur-deficiency stress suggests that auxin is involved in the activation of nitrilase at expenses of glucosinolate biosynthesis leading to changes in root morphology (Kutz et al. 2002, Hirai and Saito 2004). Cytokinin is reported as a negative regulator of sulfur acquisition (Maruyama-Nakashita et al. 2004). Based on the hypothesis of evolutionary conservation of cytokinin signalling pathway (Müller and Sheen 2007) the effect of S-depletion on the genomics of cytokinin transduction signalling was tested in grapevine cells. The expression of VvCyt, the grapevine ortholog to the histidine kinase cytokinin receptor CRE1/AHK4, is not affected by S-depletion while a tendency for a down-regulation of VvHP, a histidine phophotransmitter and VvRRa, an isoform of A-type RR transcription factor, was observed after 57 days in S-deficiency (Fernandes and Tavares, unpublished results).

The basic knowledge obtained in the last ten years with the model plant Arabidopsis is now applied to crop plants (Saito 2004). The grapevine genome sequencing (Jaillon et al. 2007, Velasco et al. 2007) and the high number of Vi-tis ESTs deposited at database (Adam-Blondon et al. 2007) makes it possible to investigate the coordination between sulfur primary metabolism and secondary metabolic pathways and, last but not least, to obtain further information on the fine-tuning of sulfur genome regulation at the level of promoter cis elements, transcription factors and small RNAs.

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