A PRESUMED ADHESIVE MATERIAL ASSOCIATED WITH PRIMARY GERM TUBES AND APPRESSORIA ON BARLEY LEAVES INFECTED BY ERYSIPHE GRAMINIS HORDEI
Department of Biological Sciences, Western Illinois University, Macomb, IL 61455, USA
Background and objectives
Formation of the primary haustorium is essential for infection by the barley powdery mildew fungus. Success or failure is determined between 8 and 12 h when the appressorium is formed and the host cell wall breached by the infection peg. I will describe the formation of a presumed adhesive material (PAM) associated with appressoria and primary germ tubes (PGT) that appears during this critical time.
Materials and methods
The abaxial surface of primary leaf segments of Hordeum vulgare cv. Manchuria were inoculated with conidia of Erysiphe graminis hordei race CR3, and incubated in either 12L/12D or 24D. Samples were fixed in clearing solution [ethanol:chloroform (75:25, v/v), 0.15% trichloroacetic acid (w/v)], stained in hot lactophenol and observed with DIC in acetic acid-glycerol-water (5:20:75, v/v/v). Tissues were observed with standard SEM procedures, either fixed in clearing agent or chloroform and directly critically-point dried and sputter coated, or tissues air dried and then sputter coated.
Resuits and conclusions
Tissues fixed in clearing solution contained PAM surrounding the tip of primary germ tubes where they made contact with the host cuticle 2-4 h after inoculation in either incubation regime. Similarly positioned PAM occurred in appressoria developed in the dark at 8-12 h, whereas light-developing appressoria contained PAM between 10 and 16 h. In both PGT and appressoria, PAM disappeared by 24 h. PAM was not present in tissues fixed in FAA or ethanol-acetic acid. PAM is an amorphous material with a pin-cushion appearance that stains dark blue and is assumed to stick the PGT and appressorium to the host surface, but it may also provide a structure for targeting fungal enzymes. In the light, PAM formation shows a similar 4-6 h delay to that previously demonstrated for haustorial development .
1. Edwards HH, 1993. Physiological and Molecular Plant Pathology 42, 299-308.