Wilt-inducing

Glucans and j8-Glucan Hydrolases in Plant Pathogenesis with Special Reference to Wilt-Inducing Toxins from Phytophthora Species... 

We have studied the wilt-inducing effect of macromolecular products of three species of Phytophthora chosen on the basis of their pathogenicity to the indicator plant Eucalyptus sieberi (50). These were the extremely pathogenic P. cinnamomiy the less pathogenic P. cryptogea and the non-pathogenic P. nicotianae. 

When the enzyme-treated preparations were tested for biological activity in the wilt induction assay it was found that the Rhizopus enzyme-treated preparation had lost its biological activity (Figure 4). The Bacillus enzyme-treated preparation retained its activity as might be expected from the inability of this enzyme to depolymerise the preparation. After treatment with the Euglena exo-hydrolase, which caused only partial degradation, the polymer retained most of its wilt-inducing activity. 

No clear-cut description of the molecular basis of initial events in the experimental, polysaccharide-induced wilting response can be given at present and its relationship to wilting induced by pathogens in vivo has not been established. 

Among the polysaccharide elicitors are 3,6-3-glucans from Phytophthora megasperma (84, 85) and yeast cell walls (59) and the algal glucan, laminarin (84)., which have structures comparable to those described for the wilt-inducing P. cinnamomi,... 

However, depolymerisation to an inactive form might have the advantage of localising the phytoalexin response (98) and other biological effects such as the hypersensitive reaction to the immediate region of the attack. Enzymic depolymerisation of wilt-inducing 3,6-3-glucans would presumably be a useful part of the plant s resistance mechanism. 

Milborrow (1972) has noted that high levels of ABA can prevent wilt-induced biosynthesis of ABA and he interpreted this as evidence for ABA-mediated feedback control of ABA biosynthesis. 

Heale, J.B., and Gupta, D.P. (1972). Mechanism of vascular wilt induced by Verticillium albo-atrum. Trans. Brit. Mycol. Soc., 58, 19-28. 

Rutherford, T.A., Mamiya, Y. and Webster, J.M. (1990) Nematode-induced pine wilt disease factors influencing its occurrence and distribution. Forest Science 36, 145-155. 

The tomatinase from the vascular wilt pathogen F. oxysporum f. sp. lycopersici has a different mechanism of action than the S. lycopersici tomatinase, releasing the intact p-lycotetraose group to give the aglycone tomatidine, Fig. (2) [35, 83]. The enzyme is extracellular and inducible by tomatine whereas breakdown products (tomatidine and sugars) fail to increase the level of tomatinase activity [35]. 

Wright, S.T.C. Hiron, R.W.P. (1969). (+)-Abscisic acid, the growth inhibitor induced in detached wheat leaves by a period of wilting. Nature (London) 224,719-720. 

Lagrimini, L.M., Bradford, S. Rothstein, S. (1990). Peroxidase-induced wilting in transgenic tobacco plants. The Plant Cell 2, 7-18. 

Guerrero, F.D., Jones, J.T. Mullet, J.E. (1990). Turgor-responsive gene transcription and RNA levels increase rapidly when pea shoots are wilted. Sequence and expression of three inducible genes. Plant Molecular Biology 15, 11-26. 

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