Afrothismia winkleri develops fleshy rhizomes, densely covered with small root tubercles, narrowing to filiform roots with age. The exclusively intracellular mycorrhizal fungus has distinct morphologies in different tissues of the plant. In the filiform root the hyphae grow straight and vesicles are borne on short hyphal stalks. The straight hyphae are present in the epidermis of the root tubercles, but change to loosely coiled and swollen hyphae in the rhizome tissue. No penetration from epidermis to root cortex was found. From the rhizome, a separating cell layer permits only one or rarely two hyphal penetrations into the cortex of each root tubercle. The hyphae proceed apically within the root hypodermis in a spiral row of distinctively coiled hyphae, branches of which colonize the inner root cortex. In the inner root cortex the hyphal coils degenerate to amorphous clumps. In older roots the cortex itself also deteriorates, but epidermis, hypodermis, endodermis and central cylinder persist. The mycorrhizal pattern in A. winkleri is interpreted as an elaborate exploitation system whereby the fungus provides carbon and nutrients to the plant and, simultaneously but spatially distinct, its hyphae are used to translocate and store the matter within the plant. Several features indicate that the endophyte is an arbuscular mycorrhizal fungus.
Triuris hyalina Miers, an unusual achlorophyllous plant, was investigated for subterranean morphology, root anatomy, and mycotrophy. Stems with scale leaves extend subterraneously to a depth of 15 cm. Pairs of adventitious roots develop at the scale leaves and clumps of apparently radiating roots, formed by accumulations of side shoot and scale leaf developments, occur. Roots consist of epidermis, short cell exodermis, three distinct layers of cortex parenchyma, endodermis, and an extremely reduced central cylinder with one or two central tracheidal xylem elements. The fungus associated with T. hyalina roots exhibits thick-walled, 6-9 µm thick, aseptate external hyphae. It penetrates the epidermis by developing appressoria and enters the cortex solely through the short cells of the exodermis. In the cortex cells, the aseptate hyphae start to coil. In the outer cortex layer, hyphae are thin, frequently branched, and most densely coiled. In the middle cortex layer they are thicker and less densely coiled, and mostly appear degenerated to clumps of amorphous fungal material. The inner cortex layer rarely becomes colonized. Vesicles occur in the outer and the middle cortex layers. This mycorrhizal pattern is interpreted as an adaption to attain a sustainable use from the endophyte. It is suggested that the mycorrhiza in Triuris hyalina be interpreted as a type of arbuscular mycorrhiza (AM). Implications for systematics and ecology are discussed.Key words: Triuris, Triuridaceae, root structure, anatomy, arbuscular mycorrhiza, myco-heterotrophy.
Roots of Voyria truncata retain the primary root structure even though they can grow as thick as 2 mm in diameter. These root diameters are due to a retained capability for cell division in the cortex parenchyma. This is explained as a vital adaptation to its life form. Based on the extraradical mycelium, the mode of penetration, the structurally incompatible intraradical phase, the presence of intercellular vesicles in the root cortex, and the occurrence of immediate hyphal bridges from arbuscular mycorrhizal roots of neighbouring plants, the mycorrhiza of V. truncata is described as an arbuscular mycorrhiza (AM), although the characteristic arbuscles are missing. Special features of the AM in V. truncata are interpreted as an improved efficiency in taking advantage of the mycorrhiza. Root connections with roots of neighbouring plants are common and preferred locations for fungal infections. An evolutionary tendency towards parasitism of higher plants is discussed.
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