Sulfur nutrition and plant defense against pathogens

Whole plants do not accumulate nor remobilize S-reserves (Mengel and Kirkby 1982). In the past, the sulfur used by crop plants resulted from sulfur-containing fertilizers and/or sulfur in rainfall (Jolivet 1993). Due to environmental policies both these sources were significantly reduced in the last 25 years: atmospheric sulfur deposition significantly decreased and many of the currently used mineral fertilizers lack sulfur (Blake-Kalff et al. 2000). Recent studies indicate that sulfur deficiency can be a limiting factor to crop yield and quality (Saito 2004, Hawkesford 2005). Therefore, former research on plant adaptation to excessive inputs of sulfur due to aerial pollution moved now into the effects of S-deficiencies.

A positive influence of sulfur nutrition on plant health was demonstrated in western Europe in the fourth quarter of last century, when the decrease in atmospheric sulfur emissions was accompanied by an increase in the incidence of fungal diseases of annual crops including oilseed rape, a high S-demanding specie (Bloem et al. 2007). Elemental sulfur (S0) is probably the oldest pesticide, with references as old as 1000 BC (Williams and Cooper 2004). Unexpectedly for eukaryotes, it was unravelled that some plant species produce S0 as a component of the defense system against vascular pathogens (Williams et al. 2002). S0 was then identified as the only inorganic phytoalexin recorded to date (Williams and Cooper 2004).

In addition, several S-secondary plant metabolites play key roles in defense against pathogens (Hell and Kruse 2007), namely glucosinolates and alli-ins (Schnug 1997). Glucosinolates are S-containing glucosides produced mostly by members of the Brassicaceae. In response to plant tissue damage, glucosinolates are hydrolyzed by myrosinases releasing volatile isothiocyanates which have a wide range of biological effects; they are inhibitors of microbial growth, attractants for specialist insects, herbivore toxins and feeding repellents (Mik-kelsen et al. 2003). Plant defensins are small (45 to 54 aminoacids), basic pep-tides that have a characteristic three-dimensional folding pattern stabilized by eight disulfide-linked cysteines (Thomma et al. 2002). Several defensin gene sequences from different plant species are now available; y-thionin represents an example of a plant defensin structurally related to defensins in general, including human defensins. Most plant defensins exhibit activity against a broad range of fungi, including various plant pathogens, and also against insect pests. Although defensins accumulate in response to pathogens and their elicitors (Thomma et al. 2002), they can also be developmentally regulated. For the first time in grapes, genes encoding defensins were differentially expressed among cultivars, when using ESTs derived from flower-berry libraries: one sequence which encodes a putative y-thionin is expressed exclusively in Cabernet Sauvi-gnon ripening berries, while it is expressed at pre-veraison, veraison, and ripening berries of Chardonnay, suggesting distinct patterns of gene expression between genotypes (Goes da Silva et al. 2005).

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