Báo cáo lâm nghiệp: "Amplification of root—fungus interface in ectomycorrhizae by Hartig net architecture"

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Amplification of root—fungus interface in ectomycorrhizae by Hartig net architecture I. Kottke F. Oberwinkler FR.G Universität Institut für Botanik, Spezielle Botanik und Mykologie, Tübingen, Tübingen, hyphae become dilated but remain in Introduction close contact with the surface of the cortical cells. These characteristics can be In previous investigations, it was found found when the Hartig net is in an active that the Hartig net is formed quite similarly state. The bidirectional active exchange of in ectomycorrhizae of different fungal spe- solutes between fungus and root is cies with diverse trees, despite charac- underlined by the results obtained from teristic differences in mantle structure cytochemical proof of ATPase activities at (Mangin, 1910; Blasius et al., 1986; Kottke this stage (Lei and Dexheimer, 1988). In and Oberwinkler, 1986; Haug, 1987). The ageing mycorrhizae, cortical cells are the question arose whether there was any first to die and their active uptake of functional benefit for the development of solutes is no longer possible. At this the observed Hartig net architecture stage, hyphae of the Hartig net can imposing evolutionary pressure to equa- lize the development in different mycor- rhizal types. Transmission electron microscopy studies revealed that the scarce septation and the intimate juxtaposition of the hyphae result in a transfer cell-like struc- ture of the Hartig net, amplifying the inter- symbiont surface (Fig. 1; Kottke and Oberwinkler, 1987). Hyphae do not pene- trate separately but as a lobed front between the cortical cells. The tips of the fan-like branched hyphal system are characterized by a large number of mito- chondria and high amounts of rough endoplasmic reticulum. Cytoplasm in this region contains many ribosomes and lacks large vacuoles. The hind parts of the
Materials and Methods Models were delineated from micrographs of the Hartig net and from possible alternatives in growth of hyphae (Fig. 3). Area, perimeter and length of hyphal walls of the models were mea- sured with Mop-Videoplan, an analytic system (Zeiss-Kontron), using standard software. The surface/volume ratios of the different systems were calculated on the basis of an average 3 pm diameter of hyhae. Results Three different models of hyphal growth in the intercellular spaces have been de- signed (Fig. 3a, b, c) and the surface/ volume ratio of hyphae calculated. The first model is delineated from the real be found from each frequently separated occurring Hartig net structure, the second other. from presumed broadly dilated hyphae Hyphal growth of an active Hartig net is and the third from presumed separately totally different from hyphal growth on growing hyphae. Measurements from solid surfaces, e.g., agar media. Hyphal these models of area and perimeter of growth on agar media shows apical hyphal complex and length of hyphal walls dominance of the parent hyphae, regular, are presented in Table I. but not too frequent branching in cor- The results show that the surface/ relation to septation, and negative auto- volume ratio of the fungus is the best, tropism between neighboring hyphae when hyphae grow separately through the (Trinci, 1984; Prosser, 1983). The result is intercellular space. However, the hyphal pinnate growth of scattered hyphae, a area in close contact with the cortical cell spreading quickly over a large surface is considerably larger with broadly dilated (Fig. 2). It is also well documented that, in hyphae. The surface of dilated hyphae is fungal colonies, only the hyphal tips again enlargedl by compartmentation into contain dense cytoplasm, whereas the Hartig net lobes. other parts of the hyphal system are highly It can be concluded from the calculation vacuolated. Although regulators are still of the surface/volume ratio that the lobed unknown, the strongly modified growth Hartig net structure is only the most pattern of hyphae with Hartig net forma- favorable as long as an active bidirectional tion is most probably elicited by the transport is present. When hyphae take up cortical cells. It is therefore presumed, that ions and molecules from the intercellular the growth pattern is beneficial for the space, the absorptive surface of hyphae is cortical cells as well as for the fungus. By larger with free, separately penetrating modeling we try to show that the Hartig hyphae, a structure that can be observed net architecture is the most effective only after the death of cortical cells, when as long as bidirectional transport of sol- active absorption is restricted to the utes is assumed.
ion The intimate contact with the cell influence release fungus. tree root may (Pohleven, 1988). Transport to the cortical surface and the transfer cell-like structure cell walls can easily occur as there seems of the Hartig net must therefore be to be no physical barrier to ion or molecule considered as not only promoting absorp- diffusion between the cell walls of the tion by but also releasing solutes from the fungus and the cortex. A greater outflow of fungus. The solute efflux may be active or solutes from the hyphae will thus enable by leakage. Cytokinins secreted by the __L.- .....- I’L ...... 1’.....I
Kottke I. & Oberwinkler F. (1986) Mycorrhiza of easier uptake by the cortical cells, an forest trees - structure and function. Trees 1, 1- the plasmalemma of the cortical although 24 cells is not enlarged. Kottke I. & C!berwinkler F. (1987) Cellular structure and function of the Hartig net: coenocytic and transfer cell-like organization. Nord. J. Bot 7,135-95 Lei J. & Dexheimer J. (1988) Ultrastructural Acknowledgments localization of ATPase activity in the Pinus sylvestrisltaccaria laccata ectomycorrhizal as- sociation. New P 1 O8, 329-334 7yfo/. / The have been investigations supported by grants from the Deutsche Forschungsgemein- Mangin L. (19’10) Introduction a 1’6tude des mycorhizes des arbres forestiers. Nouv. Arch. schaft and the Projekt Europ5isches For- Mus. Nist Nat. 5 Ser 2, 245-276 schungszentrum. Pohleven F. (1988) The influence of cytokinin 2i PA on growth, ion transport and membrane fluidity in mycelia of the mycorrhizae fungus Suillus variegatus. 2nd European Symposium References on mycorrhizae, 14-20 August 1988. Abstr. 80 Prosser J.1. (1983) Hyphal growth patterns. In: Fungal Differentiation: a Contemporary Syn- Blasius D., Feil W., Kottke I. & Oberwinkler F. thesis (Smith E., ed.). Mycol. Ser. Vol. 4. Marcel (1986) Hartig net structure and formation in fully Dekker, New York, pp. 357-396 ensheathed ectomycorrhizas. Nord. J. Bot. 6, Trinci A.P. (1984) Regulation of hyphal 837-842 branching and hyphal orientation. In: The Ecology and Physiology of the Fungal Haug 1. (1987) Licht- und elektronen- Mycelium (Jennings D.H. & Rayner A.D., eds.). mikroskopische untersuchungen mykor- an Symp. Br. Mycol. Soc. 1983. Cambridge rhizen von fichtenbestanden im Schwarzwald. PhD. dissertation, Tubingen. F.R.G. University Press, Cambridge, pp. 23-52