Department of Microbiology and Plant Pathology, University of Natal, P Bag X01, Scottsville 3209, South Africa

Background and objectives
In most rust infections the first haustorium is reportedly formed between 12 and 48 h post-inoculation (h.p.i.). Many authors, however, have commenced their examinations 1 day p.i. The formation of the substomatal vesicle, the primary hypha, the first haustorial mother cell and the first haustorium in rust fungi, have been poorly documented. The objective of this study was to determine when the post-appressorium infection structures of Puccinia sorghi form in leaves of near isogenically susceptible and resistant maize (Zea mays) and how the host responds.

Materials and methods
Urediospores were produced on susceptible maize seedlings. The abaxial surface of the third leaf of three young plants of each of the near-isogenic maize line B37 rp (susceptible) and the resistant B37 Rp, was inoculated with urediospores suspended in Soltrol 130. Inoculated plants were incubated in a dew chamber at 20C until sampled at 7, 8 and 10 h.p.i. Specimens for scanning and transmission electron microscopy were fixed in glutaraldehyde and osmium, and dehydrated in an alcohol series. Material for transmission electron microscopy was embedded in Spurr's resin.

Results and conclusions
Seven h.p.i., in both susceptible and resistant plants, some of the substomatal vesicles (SSVs) examined had already given rise to two primary infection hyphae, each of which, almost synchronously, on contact with a mesophyll cell, had cut off a haustorial mother cell (HMC). At the tip of each HMC, after host cell penetration, a young first haustorium with dense cytoplasmic contents and an irregularly lobed outline, was produced. Seven h.p.i. in the resistant host, fewer infections than those observed in the susceptible host, had developed to the haustorium stage. Some attempts at cell penetration had aborted and were associated with papilla formation, in which case the HMC was dead while the host cell cytoplasm appeared normal. Alternatively, a small, lobed haustorium had formed at the tip of a penetration tube with little evidence of a papilla. In the latter case, the cytoplasm of the haustorium, its HMC and the penetrated host cell were nearly always disorganized, although rarely the haustorial and HMC cytoplasm appeared normal, while the cytoplasm of the penetrated host cell appeared disorganized. The SSV cytoplasm in the resistance reaction at 7, 8 and 10 h.p.i., and that of the host cells surrounding the penetrated cell, appeared unaffected. The expression of host resistance as early as 7 h.p.i. appears to be unique to this host-fungus relationship. In the Sr6 reaction of wheat to Puccinia graminis f.sp. tritici, haustorial necrosis usually "occurred in association with, or was evident before there was any indication of, host necrosis" [1]. In the present study it was clear that resistance expressed itself mainly in response to cell penetration and that only fungal and host structures involved in penetration were necrotic by 7-10 h.p.i. No cell degeneration was associated within the intercellular structures of the rust fungus. Based on the smaller number of HMCs and haustoria seen in the resistant reaction, intercellular development by the fungus appeared to be somewhat impeded. These observations therefore support the conclusion of Harder et al. [1] that "significant events determining incompatibility occur during haustorium formation and host plasmalemma invagination".

1. Harder DE, Samborski DJ, Rohringer R et al., 1979. Canadian Journal of Botany 57, 2626-2634.