Physiological studies on cultivar-specific resistance of tomato plants to Cladosporium fulvum

Ultrastructural and physiological aspects of cultivar-specific resistance of tomato against Cladosporium fulvum (syn. Fulvia fulva) are subject of this thesis.The ultrastructural study described in the first paper was meant as an introduction to a physiological study of cultivar-specific resistance. Light, fluorescence and scanning electron microscopy revealed no differences between virulent and avirulent races of Cladosporium fulvum with respect to conidial germination and stomatal penetration. In incompatible interactions fungal growth was arrested very soon after stomatal penetration and the fungus became confined to stomata and surrounding cells, which showed callose deposition, cell browning and cell collapse. In addition, extracellular material was deposited on the outer surface of mesophyll cells. In compatible interactions, however, fungal growth was not inhibited and the above- mentioned host reactions did hardly occur.In compatible and incompatible interactions differential changes in soluble leaf proteins were found which might be correlated with cultivar-specific resistance (second paper). A de novo synthezised protein of host origin appeared more rapidly in incompatible than in compatible interactions. After some time this protein also appeared in non-inoculated leaflets opposite the inoculated ones. Although the appearance of the protein was strongly associated with the hypersensitive response in incompatible interactions, its presence, at the concentrations tested, was not correlated with inhibition of Cladosporium fulvum in tomato leaf.In the third paper, a possible mechanism of fungal growth inhibition of Cladosporium fulvum in incompatible interactions is described. Two phytoalexins were found in leaves which accumulated more rapidly in incompatible than in compatible interactions. Fungal growth inhibition in incompatible interactions coincided with accumulation of phytoalexins. However, in fruits neither differential accumulation of these nor of the sesquiterpene rishitin took place.The fourth paper describes the identification of the two above-mentioned phytoalexins of inoculated tomato leaves and fruits and one, which only occurred in tomato fruits, as being falcarindiol, falcarinol and cis -tetradeca-6-ene-1,3diyne-5,8-diol, respectively. Falcarindiol and falcarinol had not been found in Solanaceae before, while cis -tetradeca-6-ene-1,3-diyne-5,8-diol was a completely novel compound. Although the level to which the polyacetylenes accumulated in vivo nearly reached the ED 50 -values of hyphal and mycelial growth in vitro it is difficult to conclude that these phytoalexins can explain fungal growth inhibition in incompatible interactions completely.In the fifth paper, the induction of phytoalexins was studied, mainly of rishitin which can be readily quantified. In culture filtrates of young cultures of Cladosporium fulvum high-molecular weight glycoproteins were found which were inducers of rishitin accumulation. These elicitors contained many glucose and only a few mannose and galactose residues. They appeared not to be host-specific, as they induced also glyceollin and pisatin in soybean cotyledons and pea pods, respectively.In the sixth paper, elicitors occurring in the cell wall of Cladosporium fulvum were compared with those in culture filtrates of varying age. Elicitors present in culture filtrates of young cultures are probably other macromolecules than those in culture filtrates of old cultures. The former contain nearly only glucose, while the latter, in addition to glucose, contain many mannose and galactose residues, which are likely derived from the cell wall.Rishitin-inducing activity always appeared to be positively correlated with the mannose and galactose content of the elicitor. The elicitor from the cell wall is a peptido galactoglucomannan which occurs on the cell surface and is very similar to the peptido phosphogalactorunnan, proteo galactoglucomannan, and galactomannan of Cladosporium werneckii, Piricularia oryzae and Penicillium charlesii, respectively. Presence of terminal mannose and/or galactose residues of the peptido galactoglucomannan is likely a prerequisite for its rishitin and necrosis-inducing activity. Like the culture filtrate elicitor, the cell wall elicitor is neither race, nor cultivar or host-specific. Although the accumulation of phytoalexins in leaves was correlated with the incompatible interaction (third paper), no evidence was obtained for the induction of phytoalexins being race or cultivar-specific.Probably elicitors of phytoalexin accumulation are merely inducers of a general defence reaction (including callose deposition, lignification and cell necrosis). This mans that in the future we should rather look for substances which specifically suppress the resistance response than for those which specifically induce such a reaction.