INTRODUCTION .. RECOGNITION . STRUcruRE AND ACTIVITY OF CELL SURFACE POLYSACCHARIDES .. Exopolysaccharides and Capsular Polysaccharides . 7Jt:t:;,:��.�������.:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: EARLY EVENTS IN THE INFECTION PROCESS . Attachment . Location of Infectible Root Cells . Infective Cell Type of Rhizobia . ROOT HAIR CURLING AND INFECTION THREAD INITIATION . Root Hair Curling . Mechanism of Curling . Infection Thread Formation . Alternate Modes of Infection . Role of Cell Wall Degrading Enzymes . REGULATION OF INFECTION AND NODULATION . CONCLUDING REMARKS .

Infection of Legumes by Rhizobia

An elicitor of phytoalexin accumulation (endogenous elicitor) is solubilized from purified cell walls of soybean (Glycine max [L.] Merr., cv. Wayne) by extracting the walls with hot water or by subjecting the walls to partial acid hydrolysis. The endogenous elicitor obtained from soybean cell walls binds to an anion exchange resin. The elicitor-active material released from the resin contains oligosaccharides rich in galacturonic acid; small amounts of rhamnose and xylose are also present. The preponderance of galacturonic acid in the elicitor-active fragments suggests that the elicitor is, in fact, a fragment of a pectic polysaccharide. This possibility is supported by the observation that treatment of the wall fragments with a highly purified endopolygalacturonase destroys their ability to elicit phytoalexin accumulation. This observation, together with other evidence presented in this paper, suggests that galacturonic acid is an essential constituent of the elicitor-active wall fragments. Endogenous elicitors were also solubilized by partial hydrolysis from cell walls of suspension-cultured tobacco, sycamore, and wheat cells.

Host-Pathogen Interactions : XIX. THE ENDOGENOUS ELICITOR, A FRAGMENT OF A PLANT CELL WALL POLYSACCHARIDE THAT ELICITS PHYTOALEXIN ACCUMULATION IN SOYBEANS.

This chapter examines the quiescence period during different stages of fungal attack of postharvest pathogens: quiescence during spore germination and initial hyphal development, during and after appressorium formation, and quiescence of germinated appressorium and subcuticular hyphae. The different mechanisms for quiescence are reviewed: factors affecting quiescence of germinated spores, appressoria formation and germination, and fungal colonization. Special emphasis is given to mechanisms of quiescence involving fungal colonization: 1. the pathogen’s nutritional requirements, 2. preformed antifungal compounds, 3. the elicitation of phytoalexins and preformed compounds, and 4. the activation of factors in fungal pathogenicity.

Pathogen quiescence in postharvest diseases.

The separation ofSclerotinia sclerotiorum (Lib.) de Bary, 5.trifoliorum Erikss., andS. minor Jagger into three distinct species has been based on traditional morphological and physiological criteria such as gross cultural characteristics, sclerotial size, ascus and ascospore dimensions, time of apothecial development in the field, and host association. However, these characteristics tend to be variable and some workers have concluded that the three fungi should be included in one species, 5.sclerotiorum. Recently, new data have been published on morphological, cultural, physiological, ontogenetic, enzyme pattern, mycelial interaction, and cytological characteristics of isolates ofSclerotinia spp. This information supplements, but does not replace, that available from more traditional taxonomic methods and helps to resolve the controversy on the taxonomy and nomenclature of these fungi.

The biology of Sclerotinia sclerotiorum, S. trifoliorum, and S. minor with emphasis on specific nomenclature.

A gene (PGN1) encoding extracellular endopolygalacturonase was isolated from the fungal maize pathogen Cochliobolus carbonum race 1. A probe was synthesized by polymerase chain reaction using oligonucleotides based on the endopolygalacturonase amino acid sequence. Genomic and cDNA copies of the gene were isolated and sequenced. The corresponding mRNA was present in C. carbonum grown on pectin but not on sucrose as carbon source. The single copy of PGN1 in C. carbonum was disrupted by homologous integration of a plasmid containing an internal fragment of the gene. Polygalacturonase activity in one transformant chosen for further analysis was 10% or 35% of the wild-type activity based on viscometric or reducing sugar assays, respectively. End product analysis indicated that the residual activity in the mutant was due to an exopolygalacturonase. Pathogenicity on maize of the mutant lacking endopolygalacturonase activity was qualitatively indistinguishable from the wild-type strain, indicating that in this disease interaction endopolygalacturonase is not required. Either pectin degradation is not critical to this interaction or exopolygalacturonase alone is sufficient.

Endopolygalacturonase is not required for pathogenicity of Cochliobolus carbonum on maize.

One characteristic feature of many phytopathogenic organisms is their ability to produce an array of enzymes capable of degrading the complex polysaccharides of the plant cell wall (Bateman and Millar, 1966; Wood, 1967; Albersheim et al., 1969; Wood, 1973) and membrane constituents (Porter, 1966; Tseng and Bateman, 1968). These enzymes usually are produced inductively. Generally, they are extracellular, highly stable and present in infected host tissues. In most plant diseases caused by microbial agents, cell walls are penetrated, tissues are colonized, and permeability of host cells is altered. A brief summary of our understanding of cell wall and membrane structure, coupled with knowledge of the enzymes capable of degrading the components of these structures, and an analysis of the association of these enzymes with diseased tissue, should enable us to make an appraisal of their involvement in pathogenesis and point the way to an objective consideration of this area of disease physiology.

H. G. Basham

Papers

Degradation of Plant Cell Walls and Membranes by Microbial Enzymes