Th e long s earch f or a "t oxi n" produced by plant path og ens met with dis appoi nt­ ment unti l att enti on was cent ered on a h ost-s pecifi c t oxi n (HST). Th e dis­ appoi nt ment th at s panned almost h alf a cent ury res ult ed most ly f rom th e st udy of t oxi ns th at f uncti on i n vari ous t ypes of s ympt om development, but are not th e i niti al i nciti ng ag ents of dis eas e. "Mi cro org anis m X (but not oth ers) produces s ubst ance Y whi ch damag es plant or plant g roup A but not oth ers , and only A is parasiti zed." This st at ement by Wood (174) at th e 1971 L ong Asht on Symposi um on F ung al P ath og eni city and th e P lant's Res pons e sh ows th e s eri ous need f or t oxi n st udy . Th e fi ndi ng of a g roup of high ly h ost -s electi ve HSTs , valua ble both as t ools f or academi c res earch on plant h ostparasit e i nt eracti ons and as markers f or epi demi ologi cal s urv ey of path og ens i n fi elds, confi rms th at th e s earch f or t oxi ns is an i mport ant part of modem plant path olog y . Th e pres ent knowledg e of HSTs h as come almost enti rely f rom th e s o-called "s aproph ytic path og ens." This g roup i ncludes f ami li ar f ungi , s uch as Alternar­ ia and Helminthosporium, and oth ers less well known. It is just withi n th e last 15 years that we h ave really beg un t o work with HSTs i n Alternaria s peci es . Th eref ore, th e pres ent st at us of our kno wl edg e is sti ll pri miti ve and much work remai ns to be do ne. Unti l recent ly, o ur att entio n fo r th e to xi ns was bi as ed t oward th e ph ysi ologi cal or bi och emi cal mech anis ms of th ei r s electi vit y, and

Host-specific toxins and chemical structures from alternaria species

Siderophores and Biological Systems: An Overview.- Iron Uptake Systems in Fungi.- Absorption, Transport and Metabolic Significance of Iron in Plants.- Iron-Efficiency Reactions of Monocotyledonous and Dicotyledonous Plants.- Siderophore Involvement in Plant Iron Nutrition.- The Effect of Pseudomonas Siderophores on Iron Nutrition of Plants.- The Facilitation of Iron Uptake in Bacteria and Plants by Substituted Catechols.- Diverse Effects of some Bacterial Siderophores on the Uptake of Iron by Plants.- Characterization and Structural Analysis of the Siderophore Produced by the PGPR Pseudomonas putida strain WCS358.- Physical, Biological and Host Factors in Iron Competition in Soils.- Importance of Siderophores in Microbial Interactions in the Rhizosphere.- Suppression of Root Diseases of Wheat by Fluorescent Pseudomonads and Mechanisms of Action.- Biological Control of Fusarium Wilts by Pseudomonasputida and its Enhancement by EDDHA.- Pyoverdine-Facilitated Iron Uptake among Fluorescent Pseudomonads.- Bacterial Siderophores: Structure of Pyoverdins and Related Compounds.- Methods of Studying Plant Growth Stimulating Pseudomonads: Problems and Progress.- Emergence-Promoting Rhizobacteria: Description and Implications for Agriculture.- Nature of Intrageneric Competition between Pathogenic and Non-Pathogenic Fusarium in a Wilt-Suppressive Soil.- Antagonism and Siderophore Production by Biocontrol Agents, Plant Growth Promoting Organisms and the General Rhizosphere Population.- Herbicide-Induced Interactions between Cereal Roots and Fluorescent Pseudomonas spp.- Iron as a Factor in Disease Development in Animals.- Stimulation of Disease Development by Siderophores and Inhibition by Chelated Iron.- A New Siderophore in Aeromonas hudrophila: Possible Relationship to Virulence.- Ferrous Complexes and Chelating Compounds in Suppression of Fungal Diseases of Cereals.- The Effect of Chelating Agents on a Rust Fungus Developing on Induced Resistant Plants.- High Affinity Iron Transport in Ustilago maydis.- Production of Fusarinine and Iron Assimilation by Pathogenic and Non-pathogenic Fusarium.- Cell Surface Mutants of Erwinia chrysanthemi: Possible Involvement of Iron Acquisition in Phytopathogenicity.- Iron and Phytotoxins as Exemplified by Stemphyloxins and other Toxins.- Characterization of Cell Envelope Protein Patterns of Crop Yield Increasing Root-Colonizing Pseudomonas spp.- A Saga of Siderophores.- Genetic Analysis of the Iron-Uptake System of Two Plant Groups Promoting Pseudomonas Strains.- Genotypic Diversity of Fluorescent Pseudomonads as Revealed by Southern Hybridization Analysis with Siderophore-Related Gene Probes.- Biological Control of Phytopathogens by Pseudomonas spp.: Genetic Aspects of Siderophore Production and Root Colonization.- Participants.

Iron, Siderophores, and Plant Diseases

(a ) Predominantly biotrophic hemibioc-rophs . (b ) Predominantly necrotrophic hemibiotrophs . ( I ) Klebs’ Laws . . . . . . . ( 2 ) Parasite host and environment . . . . (a ) Host reproductive physiology . . . (b) Necrogenesis, ripening and senescence . VIII . Known pathosystems . . . . . . ( I ) Advantages of parasitism . . . . . (2) Pathosystems avoiding necrogenesis . . ( 3 ) Hemibiotrophic pathosystems . . . . (a ) Biotrophic phase predominant . . . VII . Regulation of behaviour in plant parasitic fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 476 477 477 478 478

Trophism and the ecology of fungi associated with plants

Bacterial and fungal plant pathogens are known to produce diffusible toxins in infected plants. These phytotoxins are harmful to plants in very low concentrations, and many reproduce at least some of the symptoms of the relevant bacterial or fungal disease. Several phytotoxins have been shown to be involved in pathogenesis. Recent years have seen substantial progress in our knowledge about the nature, structure and mode of action of phytotoxins, and this is briefly reviewed. Finally, possible applications in fields other than plant pathology are mentioned.

Phytotoxins and their involvement in plant diseases. Introduction

The purpose of this chapter is to acquaint the reader with the chemical structures of various plant growth regulators and reports of their detection from plant sources. In general only plant growth substances from higher plants are considered. Growth substances from algae have been recently reviewed by Augier (see Augier, 1978). Kochert (1978) has reviewed sexual pheromones in algae and fungi. This topic will be dealt with in detail also in a forthcoming volume of this Encyclopedia on Interactions Between Plant Cells. Secondary metabolites from bacteria and fungi that affect the growth of higher plants have been reviewed by Strobel (1974, 1977), Patil (1974), Rudolph (1976), Harborne (1977) and Tamura (1979). Letham (1978 b) has reviewed growth substances (other than the principal hormones) from both higher and lower plants.

Plant Hormones and Other Growth Substances — Their Background, Structures and Occurrence

It was claimed by GAumann (1954) that microorganisms are pathogenic only if they are toxigenic. Whether or not this proposition agrees with the scientific progress of the last 20 years, depends on our understanding of toxins in plant pathology. In the case of host-specific toxins (see Scheffer in this Volume) their role in the “primary determination of disease” seems obvious. This Chapter, however, deals with the large group of other phytotoxic compounds of plant pathogenic origin which are not host-specific; that is, they are toxic to hosts and non-hosts of the pathogen. Since these toxins cannot explain host specificity they have been defined as “secondary determinants of disease”. Nevertheless, some of these toxins seem to be necessary for a successful infection of the host plant, whereas others play only a secondary role. It is one aim of this review to compilate the available data from non-host-specific toxins in regard to their possible role in disease. Therefore, the main emphasis is placed on plant physiological and plant pathological aspects, contained in the subchapters “Mode of action of phytotoxic compounds” (see Section 8) and “Toxins in relation to pathogenesis” (see Section 9). Also the “Chemistry of phytotoxic compounds” (see Section 7) is reviewed as far as possible, since the chemical characterization can contribute to an understanding of the physiological role of a toxin.

K. Rudolph

Papers

Non-specific Toxins