Visualization, adhesiveness, and cytochemistry of the extracellular matrix produced by urediniospore germ tubes of Puccinia sorghi

Abstract

Adherence of germinating urediniospores of the common maize rust fungus (Puccinia sorghi Schw.) to substrata was studied by ultrastructural and cytochemical examination of extracellular matrix produced by germ tubes in conjunction with measurements of adhesion to plastic and glass surfaces. Copious amounts of extracellular matrix on germ tubes could consistently be visualized by scanning and transmission electron microscopy only when (i) a cationic detergent (cetylpyridinium chloride, polydiallyldimethylammonium chloride) or a cationic stain (ruthenium red, alcian blue, cuprolinic blue) was added to the fixation solutions, (ii) germ tubes were fixed by rapid-freezing and freeze-substitution and observed with a scanning electron microscope, or when (iii) germ tubes were observed in a frozen-hydrated state by low-temperature scanning electron microscopy. Incubation of germinated spores with dilute alkalies (NaOH, KOH), pronase E (nonspecific protease), and laminarinase (β-1,3 (1,3; 1,4-glucanase) removed the extracellular matrix and detached germ tubes from surfaces. Treatments with water, dilute acids, ionic and neutral detergents, organic solvents, hydrocarbons, and several polysaccharide-degrading enzymes did not remove the extracellular matrix and also did not detach germ tubes. These results, together with staining patterns obtained with lectins and other polysaccharide-specific reagents, indicate that the extracellular matrix is composed mainly of glycoproteins rich in acidic amino acids and β-1,3-glucan polymers, and that it is probably responsible for the adhesion of the rust germ tubes to the host leaf surfaces. Key words: Puccinia sorghi, germ tube adhesion, extracellular matrix, cytochemistry.