In addition to structural polysaccharides (including cellulose, pectin and hemicellulose polymers), plant cell walls contain also structural proteins: hydroxyproline-rich glycoproteins, proline-rich proteins and glycine-rich proteins (Bradley et al. 1992). The composition of the cell wall can be markedly altered by various environmental stimuli. The transcription of defence genes encoding cell wall hydroxyproline-rich glycoproteins was markedly stimulated by exogenous GSH in bean cell suspension (Wingate et al. 1988). These proteins were shown to be induced also by a fungal elicitor. Some years later it was shown that the treatment of bean or soybean cells with fungal elicitor or GSH led to a rapid insolubilization of pre-existing proline-and hydroxyproline-rich structural proteins in the cell wall (Bradley et al. 1992). This response to elicitor or GSH could be mimicked by exogenous H202 and inhibited by simultaneous addition of catalase or ascorbate. This rapid toughening process of cell walls, which involved H202-mediated cross-linking, was complete during the initial stages of plant defence reactions (within 10 minutes) and preceded the transcription of defence genes (maximum rates of transcription were observed after 1-3 hours). The insolubilization process should markedly increase the effectiveness of cell wall as a protective barrier to microbial ingress. H202 was putatively generated at the plasma membrane. The optimal GSH concentration for the insolubilization was 50 |jM. By using 1 mM GSH this reaction is much slower which presumably reflects some H202 destruction as a competing reaction by the higher concentration of the reductant GSH (Bradley et al. 1992).
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