The fine structure of haustoria, intracellular hyphae and intercellular hyphae of Puccinia poarum

Callose deposition during the interaction between cowpea (Vigna unguiculata) and the monokaryotic stage of the cowpea rust fungus (Uromyces vignae)

The interaction between the race 1 of the cowpea rust fungus (Uromyces vignae Barclay) and the resistant cowpea (Vigna unguiculata (L.) Walp.) ev. Queen Anne is characterized by the deposition of callose around intracellular fungal structures. Ultrastructural examination of the early stages of infection by basidiospores of the fungus revealed two types of deposits induced by intracellular hyphae: a non-callose collar at the penetration site and a callose encasement around the invasion hypha. The callose encasement was developed from the site where the fungus encountered the inside of the plant cell wall and was separated from the fungus by the extrahyphal membrane and an extension of plant plasma membrane. The incidence of encasements was reduced in plants treated with inhibitors of transcription (actinomycin D), protein synthesis (cycloheximide), protein glycosylation (tunicamycin) and microfilament polymerization (cytochalasin E). Inhibitors of Golgi-associated vesicle transfer (monensin, brefeldin A) and anti-microtubule agents (colchicine, oryzalin) had no effect. When the fungus was killed by heat treatment in either the resistant or the susceptible cultivar, callose was deposited at various locations along the fungus, mostly in the extrahyphal matrix. The data suggest that unlike the extrahaustorial membrane surrounding D-haustoria of this fungus, the extrahyphal membrane is capable of generating callose. Since this process does not normally occur in the susceptible or the resistant cv. even when callose is deposited in the latter by regions of the plasma membrane not associated with the fungus, we conclude that the deposition of callose by the extrahyphal membrane is inhibited by the living fungus.

Comparative ultrastructure of aecial and telial infections of the autoecious rust fungus Puccinia tuyutensis

This study demonstrates morphological differences between aecial and telial stages of the autoecious rust Puccinia tuyutensis. The aeciospores possess 'verrucose' ornamentation while the teliospores have smooth surfaces. The aecial and telial haustoria of this rust produced in the mesophyll of Cressa cretica differ morphologically in the following respects:(1) the haustorial mother cell of telial haustorium is more differentiated than that of aecial haustorium and its wall at the penetration site is composed of 4 layers; (2) the aecial haustorium is filamentous in appearance and slightly constricted at the point of entry into the host cell, while the telial haustorium is clavate and possesses a narrow neck with a densely staining neckband and swollen body; (3) the neck of the telial haustorium is always associated with numerous vesicles while that of the aecial haustorium is not. Vascular tissue of host leaves is heavily invaded by aecial haustoria but not by telial haustoria.

Structure and function of the interfaces in biotrophic symbioses as they relate to nutrient transport

In this review we compare the structure and function of the interfaces between symbionts in biotrophic associations. The emphasis is on biotrophic fungal parasites and on mycorrhizas, although necrotrophic parasitic associations and the Rhizobium/legume symbiosis are mentioned briefly. We take as a starting point the observations that in the parasitic associations nutrient transport is polarized towards the parasite, whereas in mutualistic associations it is bidirectional. The structure and function of the interfaces are then compared. An important common feature is that in nearly all cases the heterotrophic symbiont (whether mutualistic or parasitic) is located topologically outside the cytoplasm of the host cells, in an apoplastic compartment. This means that nutrient movements across the interface must involve transport into and out of this apoplastic region through membranes of both organisms. Basic principles of membrane transport in uninfected cells are briefly reviewed to set the scene for a discussion of transport mechanisms which may operate in parasitic and mycorrhizal symbioses. The presence and possible roles of ATPases associated with membranes at the interfaces are discussed. We conclude that cytochemical techniques (used to demonstrate the activity of these enzymes) need to he extended and complemented by biochemical and biophysical studies in order to confirm that the activity is due to transport ATPases. Nevertheless, the distribution of activity appears to he in accord with polarized transport mechanisms in some pathogens and with bidirectional transport in mycorrhizas. The absence of ATPases on many fungal membranes needs re-examination. We emphasize that transport mechanisms between mycorrhizal symbionts cannot be viewed simply as the exchange of carbon for phosphate. Additional features include provision for transport of carbon and nitrogen as amino acids or amides and for ions such as K⁺ and H⁺ involved in the maintenance of charge balance and pH regulation, processes which also occur in parasitic associations. Interplant transport of nutrients via mycorrhizal hyphae is discussed in the context of these complexities. Some suggestions for the directions of future work are made. CONTENTS Summary 1 I. Introduction 2 II. The availability of nutrients to the symbionts 3 III. Structure of interfaces between symbionts 4 IV. Identity of nutrients transferred: an overview 12 V. Membrane transport: basic principles 14 VI. Transport at the interface of biotrophic symbioses 15 VII. Regulation of pH in biotrophic symbioses 25 VIII. Conclusions: 26.