Plants are exposed to various pathogens in nature. To counter microbial attack, they have evolved multiple and complex defence strategies. Most of these microbes can be dealt with by the plant basal defence system that limits the growth of non-host pathogens. Plant disease resistance can be induced via host recognition of pathogen elicitors (also designated as pathogen-associated molecular patterns or PAMPs; Nürnberger et al., 2004). Bacterial flagellin is an excellent example of a general elicitor that induces resistance through interaction with a plasma membrane-localized receptor kinase (Zipfel et al., 2004). This mode of pathogen recognition resembles pathways of innate immunity response (Gomez-Gomez and Boller, 2002; Navarro et al., 2004). The plant innate immune response is initiated when a plant resistance (R) gene product either directly or indirectly recognizes specific effector molecules or proteins produced by pathogen avirulence (Avr) genes (Martin et al., 2003). This type of recognition triggers defined signal transduction cascades within the infected plant cell, leading to the generation of endogenous signalling coumpounds and to the subsequent biosynthesis of antimicrobial proteins locally as well as systemically, i.e. in distant parts of the plant, a phenomenon termed systemic acquired resistance (SAR) (Ryals et al., 1996). Often, plant resistance is accompanied by rapid localized and programmed cell death at the infection site, termed the hypersensitive response (HR), which is preceded by physiological disturbances (ion fluxes, pH changes, membrane depolarization, and oxidative burst; Nürnberger and Scheel, 2001). HR and SAR are accompanied by the accumulation of defence molecules and proteins, resulting in cell wall modifications and the production of antimicrobial molecules (phytoalexins) and a large group of pathogenesis-related (PR) proteins, some of which have also antimicrobial activities (Van Loon and Van Strien, 1999; Narasimhan et al., 2005).
Elicitor- or pathogen-activated transcription factors play an important role in controlling defence gene expression and plant resistance responses. Five major families of plant transcription factors (bZIP, WRKY, MYB, EREBF, and homeodomain proteins) have been shown to participate in the regulation of plant defence responses (Rushton and Somssich, 1998). It is generally assumed that WRKY transcription factors act as major regulatory proteins by binding to the W-box, a common promoter element contained in several SAR gene promoters (Maleck et al., 2000). This class of transcription factors belongs to a large family of proteins mainly present in plants, and is characterized by their highly conserved DNA-binding region termed the WRKY domain. In plants, many WRKY proteins are involved in the defence against attack by phytopathogens such as bacteria (Dellagi et al., 2000; Asai et al., 2002; Chen et al., 2002; Deslandes et al., 2002; Dong et al., 2003), fungi (Beyer et al., 2001; Asai et al., 2002; Chen et al., 2002; Kalde et al., 2003), and viruses (Wang et al., 1998; Yang et al., 1999; Chen et al., 2002; Yoda et al., 2002; Liu et al., 2004). Furthermore, a role in various physiological processes has also been suggested, including embryogenesis, seed coat and trichome development, senescence, regulation of biosynthetic pathways, and hormonal signalling (Ulker and Somssich, 2004; Lagace and Matton, 2004; Xu et al., 2004; Zhang et al., 2004; Zou et al., 2004; Xie et al., 2005).
Cultivated grapevines (Vitis vinifera L.) are susceptible to many pathogens such as phytoplasmas, viruses, bacteria, and fungi (Galet, 1996; Martini et al., 2002; Ferreira et al., 2004). Among them, the most important are grey mould (Botrytis cinerea), powdery mildew (Erysiphe necator), and downy mildew (Plasmopara viticola), which cause extensive loss of quantity and quality in harvested berries. Consequently, wine growing requires extensive use of phytochemicals. For instance, in France, 50% of the total mass of these products are used in vineyards, which represent only 3.7% of farmed surfaces. Intensive use of chemicals has led to the development of microorganisms resistant to certain types of fungicides, and some products are now prohibited due to their high toxicity. Nowadays, several research strategies are being pursued so that wine growers may produce healthy fruits right up to maturity with a minimum use of chemical treatments.
The aim of this study was to identify components of the grape defence mechanisms, and we focus on a WRKY transcription factor expressed in grape berries because fruits, rich in sugar and other nutrients, provide an ideal target for pathogens. To this end, a full-length cDNA, VvWRKY1, was isolated from a Vitis vinifera grape berry library. The ability of the corresponding protein to bind specifically to W-box DNA elements was demonstrated. Then, its expression was characterized during the development of healthy plants, berries, and leaves, and under stress conditions. The biological role of VvWRKY1 was assessed by its overexpression in Nicotiana tabacum. These transgenic plants exhibited decreased susceptibility towards several fungal pathogens compared with the wild type, implicating this transcription factor in plant defence responses.