|BOTANICAL ELECTRONIC NEWS|
No. 145 October 3, 1996
Tadeus Reichstein was born on 20th July 1897 in Wloclawek, at that time in Russian Poland. The family moved to Switzerland in 1906. During the 1920s he worked on the isolation of the volatile constituents of the flavour of roasted coffee, and in the 1930s he developed a method for the commercial synthesis of Vitamin C. For his work on adrenal cortical hormones he was awarded the Nobel Prize for Medicine and Physiology in 1950, together with E.C. Kendall and Philippe Hench. Since being made Emeritus Professor in 1967 he has worked full-time on ferns. His interests were wide-ranging and global. He was in correspondence with pteridologists from many parts of the world. His fascination was for polyploid fern complexes, but he also has spent many years working on the pteridophytes for Flora Iranica. He continued to use his expertise in organic chemistry in the investigation of phloroglucinols in Dryopteris. In all his fern work he was a great collaborator, and practically all his fern papers have been multi-authored. An exception is a favourite subject, his 1981 paper on "Hybrids in European Aspleniaceae (Pteridophyta)", Bot. Helv. 91: 89-139.
He died on 1st August 1996 at the age of 99.
The botanical landscape can be seen as a mycorrhizal landscape, a landscape where the roots of most plant species form interactive and beneficial partners with a variety of fungal species. In my talk I explored the astonishing structural complexity and hinted at the ecological significance of a variety of mycorrhizal symbioses found in B.C. and, indeed, worldwide. With the aid of a variety of microscopy techniques and numerous floristic examples, I explored and compared the significant features which make vesicular-arbuscular, ecto-, ectendo-, arbutoid, ericoid, monotropoid and orchid mycorrhizae.
Fungi involved in mycorrhizal relationships range from being very specific to their host (usually one plant genus), to being generalists, associating with an immense array of hosts, perhaps a clue to their numerous ecological contributions.
For example, several important genera of trees in B.C., such as Pinus, Picea, Pseudotsuga, but also smaller plants, such as Dryas and Kobresia, form ectomycorrhizae with their respective fungal symbionts (mostly basidiomycetes and ascomycetes). These ectomycorrhizae have a Hartig net (the functional interface between the fungus and the root cell) where presumably exchanges of metabolites take place, and a fungal sheath (or mantle) which surrounds the root and interfaces with the soil matrix, facilitating the uptake of nutrients and water by the plant. Worldwide, it is estimated that over 5000 species of ectomycorrhizal fungi has been described.
Ectendomycorrhizae, monotropoid and arbutoid mycorrhizae, repre- sent variations in some aspects of the ectomycorrhizal theme! Like ectomycorrhizae, ectendomycorrhizae have a Hartig net and a mantle, but also have intracellular coils, fungal hyphae that exist inside the root cells. The functional significance of these coils is unclear (after all, a Hartig net is already present). However, species, such as the genus Wilcoxina, is a fungus presumably involved worldwide with pine species. Arbutoid mycorrhizae are very similar to ectendo structurally, except that all fungal structures are restricted to the outer layer of root epidermal cells, as in other ericaceous plants (see below). The fungi involved with arbutoid plants are the same as those forming ectomycorrhizae, a phenomenon that could theoretically open the doors for "linkages" between hosts involved in these 2 classes. Demonstration of these ecological linkages will occupy the researcher well into the 21st century!
In the case of vesicular-arbuscular (VA) mycorrhizae, found on hundred of species in B.C., (from herbs to woody perennials, Liliaceae, Rosaceae, Asteraceae, Juglandaceae to name a few), it is the "haustoria-like" arbuscule (i.e. a tree-like fungal structure), which develop within root cells, that act as the functional interface for exchange between the plant and the fungus. The vesicles (i.e. intracellular flask-like fungal structures) act as the "warehouses" where the fungus can store lipids. The fungi involved in VA relationships are not diverse, perhaps 200 species worldwide, and exhibit low specificity for their hosts in general...one fungus can interact with most VA hosts. To date, no one has succeeded in growing these VA fungi in pure culture, emphasizing the obligate nature of these sym- biotic fungi.
The ericoid mycorrhizae involve numerous ericaceous hosts (Gaultheria, Rhododendron, Vaccinium, Calluna, etc.) and usually a "select" group of septate ascomycete fungi. They appear to be broad host ranging among ericaceous hosts but are restricted to them. Ericaceous host roots are very minute and the fungus usually interacts with the root epidermis, forming coils within each colonized cell. The mutual transfer of metabolites happens there!
The most curious mycorrhizae, which has intrigued plant physiologists for decades, involve monotropoid hosts (Monotropa, Pterospora, etc.) and possibly a subset of the vast ectomycorrhizal fungi pool. These plants are heterotrophic, obtaining carbohydrates from other sources, therefore it is hard to understand how fungi benefit by forming mycorrhiza with these plants. It seems the fungi are involved in a tripartite "contract", in which they first derive carbohydrates from a true autotrophic host (typically Picea, Pinus, etc.) and then transfer some to the monotropoid plant, apparently an "altruistic" transfer?!. It is unclear what the fungus gets in return but speculation abounds!
The last intriguing class, the orchid mycorrhizae, are even lesser known. With a worldwide diversity of orchid species exceeding the 20 thousands, and with a select group of basidiomycetes interacting with orchid tissues, either at the time of seed germination or at the time of symbiotic root formation, we find ourselves in the infancy of orchid mycorrhizae research! Here too, fungal coils invading cells are the presumed site of metabolite exchanges.
Our understanding of these complex mutualistic associations is still limited when one considers the vast undescribed diversity of fungi actually inhabiting the soil with their plant hosts! The living soil is truly one of our last frontiers...
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