Active defence processes and in vitro selection for resistance to pathogens

In vitro selection for resistance to pathogens has been based on the isolation of toxin tolerant genotypes, a method which is often unsuccessful. Within the frame of a programme aimed at the development of selection techniques based on active defense processes, tomato cells (cv. Red River) were selected for increased peroxidase activity in the presence of Fusarium oxysporum f.sp. lycopersici using a guaiacol in vivo staining procedure and for high polysaccharide content on a Calcofluor-containing medium. High polysaccharide containing cell clones were shown to inhibit the pathogen in dual cultures, to possess high inducible peroxidase activity, high polysaccharide content and to tolerate fusaric acid better than the parental cultivar. On the other hand high peroxidase clones maintained control values for all these parameters. On the basis of these and previous results the hypothesis is put forward that in the system studied the cascade of events leading to active defense may be triggered by fungal recognition and high polysacchaide content, peroxidase activation being a secondary step in the hierarchical organisation of resistance.