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Four Hundred-Million-Year-Old Vesicular Arbuscular Mycorrhizae (Endomycorrhiae/Symbiosis/Fossil Ft /Mut ) WINFRIED REMY*, THOMAS N

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Four Hundred-Million-Year-Old Vesicular Arbuscular Mycorrhizae (Endomycorrhiae/Symbiosis/Fossil Ft /Mut ) WINFRIED REMY*, THOMAS N Proc. Natl. Acad. Sci. USA Vol. 91, pp. 11841-11843, December 1994 Plant Biology Four hundred-million-year-old vesicular arbuscular mycorrhizae (Endomycorrhiae/symbiosis/fossil ft /mut ) WINFRIED REMY*, THOMAS N. TAYLORt*, HAGEN HASS*, AND HANS KERP* *Forschungsstelie ffr Paliobotanik, Westfalische Wilhelms-Universitit, MOnster, Germany; and tDepartment of Plant Biology, Ohio State University, Columbus, OH 43210 Contributed by Thomas N. Taylor, August 24, 1994 ABSTRACT The discovery of arbuscules in Aglaophyton In this paper we report evidence of arbuscules of VAM major, an Early Devonian land plant, provides unequivocal fungi from plants preserved in the Rhynie chert. The fossils evidence that mycorrhlizae were estabised >400 million years are preserved as permineralizations at this Lower Devonian ago. Nonseptate hyphae and arbuscules occur in a specialized (Siegenian) locality. Since the fossil plants containing the meristematic region ofthe cortex that continually provided new fungi are preserved in silica, it is necessary to prepare thin cells for fungal infection. Arbuscules are morphologically sections by cementing pieces of the rock matrix to micro- identical to those ofliving arbuscular mycorrhizae in consisting scope slides and then grinding the rock to a thickness of of a basal trunk and repeatedly branched bush-like tuft within 50-150 ,Am with silicon carbide powder. Fungi were photo- the plant cell. Although interpretations of the evolution of graphed under oil immersion objectives directly on the pol- mycorrhizal mutualisms continue to be speulative, the exis- ished surface of the rock. Slides have been deposited in the tence of arbuscules in the Early Devonian indicates that collection of W.R. nutrient transfer mutualism may have been in existence when The fungi occur in all regions of the axes of Aglaophyton plants invaded the land. major, an enigmatic plant that possesses features found in both vascular plants and bryophytes (9). Aglaophyton axes have a uniformly thick epidermis of rectangular cells over- Perhaps the most widespread, and certainly significant, mu- lying a two- to five-cell-thick hypodermis. Inside ofthis layer tualism between plants and fungi is the root symbiosis termed is a one- to four-cell-thick zone of thin-walled cells that vesicular arbuscular mycorrhiza (VAM) or arbuscular my- appear similar to palisade parenchyma. To date this tissue corrhiza. These fungal endosymbionts, which are nearly has been observed only in Rhynia and Aglaophyton. It is in universal in their association with flowering plants, are this region ofthe axis that the arbuscules occur (Fig. 1), even represented by >100 species ofZygomycetes included in the though the intraradical mycelium is extensively developed Glomales (1). Not only are they an important component of within the intercellular spaces in the remaining cortical the mycota, but, because of their interrelationships with tissues. The nonseptate hyphae range up to 4.0 ,um in angiosperms, they are an integral component of terrestrial diameter and often show both Y- and H-shaped anastomoses. ecosystems throughout the world (2). Members of the Glo- In extant VAM fungi arbuscules are formed when intercel- maceae are believed to have been present as early as the lular hyphal branches penetrate the cell wall of the host but Cambrian period (3). Arbuscular mycorrhizae have been do not rupture the plasmalemma (10). The hyphal trunk ofthe reported in extant bryophytes, pteridophytes, and gymno- arbuscule is =1.3 Am wide (Fig. 2) and branches repeatedly sperms; however, the physiological interrelationships are not to form a "bush-like" structure within the cell (Fig. 3). well understood in these instances. While it is well known Secondary branches range from 0.7-2.0 pum wide, and the that VAMs considerably increase the uptake of specific ultimate ones are beyond the resolving power of light mi- nutrients by the host, other functions attributed to the fungi croscopy (in the size range 0.2-0.5 pum). Fig. 4 shows the top include production of plant growth hormones, protection of ofan arbuscule indicating the configuration ofthe more distal host roots from pathogens, and increased solubility of soil branches. In the arbuscules of extant VAMs, the walls are minerals (4). osmiophilic and decrease in thickness to <20 nm near the The widespread geographic and biologic distribution of tips. In some arbuscules the tips may be slightly swollen, and modern arbuscular mycorrhizae suggested to some the an- this condition also occurs in the fossils. tiquity of this symbiosis (5), while others hypothesized that In extant VAMs, the arbuscules are ephemeral and begin this kind of mutualism was pivotal in the origin of the to break down at the tips ofthe smallest branches in 4-6 days, terrestrial flora (6). This hypothesis is based on the assump- ultimately forming an amorphous mass within the cell (11). tion that the various fungal structures found in the anatom- We have no way of gauging the longevity of the fossil ically preserved Early Devonian plants represent compo- arbuscules except to note that various stages of arbuscule nents of an endosymbiont. These include terminal thick- morphology are present in a single section, including those walled spores that morphologically resemble VAM that have collapsed and deteriorated. Boullard (12) suggested chlamydospores, nonseptate mycelia, coiled hyphae, and that in some living ferns the frequent presence of clumps irregularly shaped, thin-walled spheres that resemble vesi- indicates a short functional span of an arbuscule. Several cles (7). Although it is difficult to document a physiological authors working with modern VAM fungi reported the for- function based solely on morphology, the presence of arbus- mation of a subapical septum that separates the functional cules currently provides a reliable clue to the mutualistic and nonfunctional portion of an arbuscule during deteriora- nature of the association. To date no arbuscules have been tion. We have not observed this structure in any of the fossil identified in any Paleozoic plant, although convincing exam- arbuscules to date. The fossil arbuscules appear morpholog- ples are known from the roots of a Triassic cycad (8). ically identical to those of many extant VAMs (13). There is far less information available about the structural and phys- The publication costs ofthis article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" Abbreviation: VAM, vesicular arbuscular mycorrhiza. in accordance with 18 U.S.C. §1734 solely to indicate this fact. fTo whom reprint requests should be addressed. 11841 Downloaded by guest on September 25, 2021 11842 Plant Biology: Remy et al. Proc. Natl. Acad. Sci. USA 91 (1994) FIG. 1. Transverse section of A. major axis showing tissue FIG. 3. Lateral view showing arbuscule. (x600.) preservation. Arrow indicates zone of arbuscule-containing cells. (x 15.) is there a morphological correspondence between the fossil and extant arbuscules, but the presence ofappositions in the iological interrelationships between mycorrhizal fungi and cells ofAglaophyton also indicates that these host cells were their fern and bryophyte hosts, although both groups are alive and responded by the formation of thickenings, similar characterized as having VAMs (10). In both, a major arbus- to those of their modern counterparts. cular trunk initially dichotomizes and each of the branches As more anatomical and morphological information is divides repeatedly to form smaller and smaller units. These assembled about the Rhynie chert plants, it is becoming are quite different morphologically from the arbuscules in a increasingly clear that these organisms are structurally com- Triassic cycad, which are less branched and more robust (8). plex. For example, Aglaophyton and Rhynia are the only Many of the Rhynie chert arbuscules are also accompanied plants in the Rhynie chert that possess a zone ofpalisade-like by a slight thickening in the cell wall. In modern VAMs this cells beneath the hypodermis in which arbuscules develop. thickening represents an apposition that forms at the point of Many ofthe cells in this layer contain remnants ofarbuscules penetration ofthe host cell wall by the hypha. Thus, not only (Fig. 1). These host cells also appear collapsed and disasso- ciated, while arbuscule-free cells look normal. Since many of the cells in this zone are organized in files, we speculate that the arbuscule-containing zone may be meristematic, perhaps throughout the life of the plant, to produce new cells that provide sites for the infection of VAM fungi. This appears to be unlike the situation in modern VAM infections (14), where Aii FIG. 2. Cortical cell showing arbuscule trunk (arrow) extending FIG. 4. View from the top of an arbuscule showing extensive into the cell lumen. (x1500.) branching. (x 1500.) Downloaded by guest on September 25, 2021 Plant Biology: Remy et al. Proc. Natl. Acad. Sci. USA 91 (1994) 11843 nonfunctioning arbuscules are absorbed by the host cells. tive pressures that they had on their hosts. In the presence of Although it is not understood how this takes place metabol- modem endophytes, plants are known to increase the amount ically, once resorption is completed the host cells continue to of vascular tissue and the lignification ofthe xylem (22). The function. If our assumption is accurate, it means that the presence of clusters of transfusion tracheids in Aglaophyton meristematic zone in Aglaophyton
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