Tansley Review No. 18 Mechanisms of resistance and pathogenic specialization in rust-wheat interactions.

Forms of resistance and pathogenic specialization in interactions between wheat and its stem and leaf rust Fungi have been selected for analysis with two hypotheses in mind. One is that phytoalexin formation is a universal defence mechanism in plants; the other is that resistance is elicited specifically by products of avirulent strains of pathogens. A search for phytoalexins in wheat, using a range of extraction and separation procedures and a number of forms of expression of resistance, has indicated the presence of several inhibitors of fungal development but has yielded very little evidence for post-infectional increases in the amounts of these inhibitors. No evidence for the formation of phytoalexins in wheat has been obtained. Hypersensitivity has been confirmed as being closely associated with the expression of most forms of resistance studied. Recent work with some very rapidly expressed forms of resistance to rust fungi has implicated lignification as a component of active defence in wheat, a finding in agreement with those of other workers. The expression of resistance conferred by the Lr20 gene towards avirulent strains of the leaf rust fungus was the principal subject of explorations for elicitors. Histological observations of the sequence of events in fungal and wheat cells suggested the action of rust strain and Lr20-specific elicitors in bringing about hypersensitivity. Experimental manipulations involving heat treatments or use of epidermis-free segments of leaves incubated on infected mesophylls gave results consistent with this interpretation. Attempts to extract specific elicitors from infected leaves failed but yielded non-specific elicitors. Non-specific elicitors have been obtained in intercellular washing fluids of leaves infected with strains of both the leaf and stem rust fungi. They have also been obtained from extracts of germ tubes of both species of fungus. Although not specific for any known resistance gene, the elicitors are selective for a character determined by a factor on chromosome 5 A in some wheat cultivars. A role for the non-specific elicitors as incompatibility factors between rust fungi and wheat is suggested. A revised hypothesis is presented in order to explain the evolution of pathogenesis in rust fungi towards wheat and it is discussed in relation to concepts of basic compatibility between pathogens and their hosts, as proposed also by others. CONTENTS Summary 233 I. Introduction 234 II. Working Ideas and Hypotheses 234 III. The Search for Phytoalexins 235 IV. Elicitation of Resistance 237 V. Revised Hypothesis 240 Acknowledgements 242 References 242.