The mycorrhizal relationship of multinucleate rhizoctonias from non-orchids with Microtis (Orchidaceae)

Serendipita restingae sp. nov. (Sebacinales): an orchid mycorrhizal agaricomycete with wide host range

The Serendipitaceae family was erected in 2016 to accommodate the Sebacinales 'group B' clade, which contains peculiar species of cultivable root-associated fungi involved in symbiotic associations with a wide range of plant species. Here we report the isolation of a new Serendipita species which was obtained from protocorms of the terrestrial orchid Epidendrum fulgens cultivated in a greenhouse. This species is described based on phylogenetic analysis and on its microscopic and ultrastructural features in pure culture and in association with the host's protocorms. Its genome size was estimated using flow cytometry, and its capacity to promote the germination of E. fulgens seeds and to associate with roots of Arabidopsis thaliana was also investigated. Serendipita restingae sp. nov. is closely related to Serendipita sp. MAFF305841, isolated from Microtis rara (Orchidaceae), from which it differs by 14.2% in the ITS region and by 6.5% in the LSU region. It produces microsclerotia formed of non-monilioid hyphae, a feature that was not reported for the Sebacinales hitherto. Serendipita restingae promoted the germination of E. fulgens seeds, forming typical mycorrhizal pelotons within protocorm cells. It was also able to colonize the roots of Arabidopsis thaliana under in vitro conditions. Arabidopsis plants grown in association with S. restingae increased their biomass more than fourfold. Serendipita restingae is the first Serendipitaceae species described for the Americas.

Evaluation of genome size and quantitative features of the dolipore septum as taxonomic predictors for the Serendipita 'williamsii' species complex

Despite multiple taxonomic revisions, several uncertainties at the genus and species level remain to be resolved within the Serendipitaceae family (Sebacinales). This volatile classification is attributed to the limited number of available axenic cultures and the scarcity of useful morphological traits. In the current study, we attempted to discover alternative taxonomic markers not relying on DNA sequences to differentiate among the closely related members of our Congolese Serendipita isolate collection and the reference strains S. indica (syn. Piriformospora indica) and S. williamsii (syn. P. williamsii). We demonstrated that nuclear distribution across hyphal cells and genome size (determined by flow cytometry) did not have enough resolving power, but quantitative and qualitative variations in the ultrastructure of the dolipore septa investigated by transmission electron microscopy did provide useful markers. Multivariate analysis revealed that subtle differences in ultrastructural characteristics of the parenthesome and the attached endoplasmic reticulum are most relevant when studying this fungal group. Moreover, the observed clustering pattern showed that there might be more diversity amongst the Congolese isolates within the S. 'williamsii' species complex than previously anticipated based on molecular data. Altogether, our results provide novel perspectives on the use of integrative approaches to support sebacinoid and Serendipitaceae taxonomy.

Internal transcribed spacer primers and sequences for improved characterization of basidiomycetous orchid mycorrhizas

Despite advances owing to molecular approaches, several hurdles still obstruct the identification of fungi forming orchid mycorrhizas. The Tulasnellaceae exhibit accelerated evolution of the nuclear ribosomal operon, causing most standard primers to fail in polymerase chain reaction (PCR) trials. Insufficient sequences are available from well characterized isolates and fruitbodies. Lastly, taxon-specific PCR primers are needed in order to explore the ecology of the fungi outside of the orchid root. Here, progress in overcoming these hurdles is reported. Broad-spectrum basidiomycete internal transcribed spacer (ITS) primers that do not exclude most known Tulasnellaceae are presented. blast searches and empirical PCR tests support their wide utility within the Basidiomycota. Taxon-specific ITS primers are presented targeted to orchid-associated Tulasnella, and a core component of the Thelephora-Tomentella complex. The efficiency and selectivity of these primer sets are again supported by blast searches and empirical tests. Lastly, ITS DNA sequences are presented from several strains of Epulorhiza, Ceratorhiza, Ceratobasidium, Sistotrema, Thanatephorus and Tulasnella that were originally described in the landmark mycorrhizal studies of Currah and Warcup. Detailed phylogenetic analyses reveal some inconsistencies in species concepts in these taxonomically challenging resupinate basidiomycetes, but also help to place several sequences from environmental samples.

Diversity of mycorrhizal fungi of terrestrial orchids: compatibility webs, brief encounters, lasting relationships and alien invasions

The diversity of mycorrhizal fungi associated with an introduced weed-like South African orchid (Disa bracteata) and a disturbance-intolerant, widespread, native West Australian orchid (Pyrorchis nigricans) were compared by molecular identification of the fungi isolated from single pelotons. Molecular identification revealed both orchids were associated with fungi from diverse groups in the Rhizoctonia complex with worldwide distribution. Symbiotic germination assays confirmed the majority of fungi isolated from pelotons were mycorrhizal and a factorial experiment uncovered complex webs of compatibility between six terrestrial orchids and 12 fungi from Australia and South Africa. Two weed-like (disturbance-tolerant rapidly spreading) orchids - D. bracteata and the indigenous Australian Microtis media, had the broadest webs of mycorrhizal fungi. In contrast, other native orchids had relatively small webs of fungi (Diuris magnifica and Thelymitra crinita), or germinated exclusively with their own fungus (Caladenia falcata and Pterostylis sanguinea). Orchids, such as D. bracteata and M. media, which form relationships with diverse webs of fungi, had apparent specificity that decreased with time, as some fungi had brief encounters with orchids that supported protocorm formation but not subsequent seedling growth. The interactions between orchid mycorrhizal fungi and their hosts are discussed.

In situ and in vitro specificity between Rhizoctonia spp. and Spiranthes sinensis (Persoon) Ames, var. amoena (M. Bieberstein) Hara (Orchidaceae)

The relationships between the orchid Spiranthes sinensis (Persoon) Ames. var. amoena (M. Bieberstein) Hara and Rhizoctonia spp. were investigated in situ at germination and in adult plants, Seeds of the orchid placed in cotton gauze were buried at 210 sampling points in turf grassland, the orchid habitat (in situ germination). Eight weeks later, protocorm development of the orchid was confirmed at 67 of the 210 sampling points. Isolation of fungi from protocorms showed that in situ germination was induced mainly by Rhizoctonia rapens Bernard. Similarly, R. repens was the dominant mycorrhizal fungus isolated from roots of adult plants. The number of adult plants within a radius of either 30 or 50 cm of burial points did not influence seed germination. The distribution of Rhizoctonia spp. other than R. repens in the sample site was examined with a baiting method using buckwheat stems. Thirty-two isolates consisting of binucleate Rhizoctonia anastomosis group (AG)-A, AG-B, AG-G, and AG-1, R. solani Kuhn AG-4, Waitea circinata Warcup & Talbot, which anastomozed with WAG-O and WAG-Z, and a multinucteate Rhizoctonia sp. were isolated. Three AG-G isolates were obtained from the points at which protocorm development was induced by R. repens. Seeds of S. sinensis var. amoena were inoculated in vitro with these isolates to test for symbiotic germination. Most Rhizoctonia spp, not associated with the germination in situ induced seed germination in vitro. Seedlings which developed with these isolates in vitro were transferred to ex vitro conditions. New leaves developed and elongated as seedlings continued to grow for 3 months, The seed burial method enabled the clarification of the differences in orchid-fungal specificity in situ and in vitro. We concluded that the specificity between S. sinensis var. amoena and fungi in situ conditions was different to that in vitro.

The biology of myco-heterotrophic ('saprophytic') plants

More than 400 species of vascular plants, in 87 genera, are acholophyllous and heterotrophic, but not directly parasitic upon autotrophs. They are usually, but incorrectly, described as 'saprophytes'since they are in fact nourished by means of specialized mycorrhizal associations. Although distributed world-wide, they are most abundant and show the greatest species-richness in the Neotropics and Palaeotropical regions. Their aerial parts range in size from a few centimetres to extensive liane types up to 40 m long. With few exceptions, their habitats are dense moist forests in which there is a surface accumulation of leaf litter, often in situations which are too shaded for autotrophic growth. Although the achlorophyllous mycorrhizal mode of life has evolved independently many times and in widely disparate taxonomic groups, such plants show strong convergent evolution in particular adaptations to their peculiar mode of life. Most prominant amongst these are reductions in the size of seed and embryo, and the lack of differentiation of the embryo at maturity. The number of seeds produced by each flower is typically very large and the shape, structure and surface features of seeds involving adaptation for wind dispersal show remarkable parallels in many species. Specific adaptations for zoochory are rare but well developed in a small number of genera, some of which produce scents like fungal fruit bodies or floral parts which mimic fungal sporocarps. Vegetative parts are often even more conspicuously reduced. Most myco-heterotrophs are entirely subterranean for most of their lives and these stages exhibit adaptations consistent with a change in function from organs of absorption to organs of storage, shown by the almost universal loss of root hairs, decrease in surface area as exhibited in short cylindric'vermiform'and tuberous roots or, in extreme cases, the complete suppression of roots and the formation of a swollen tuber or rhizome. Increased width of the root cortex often accommodates mycorrhizal infection and stores of carbohydrates and other materials obtained from the fungal symbiont. Mycorrhizal infection is confined to the below-ground parts of the plants but may be found there in modified stems as well as in roots. In many genera, stems are exceptionally slender and thread-like and their vascular tissues are either reduced to a single narrow cylinder of bicollateral bundles or, minimally, to four or six narrow bundles in the cortex. Secondary thickening is poorly developed in all but a tiny minority of species, lignification being confined to annular or, rarely, a few scalariform xylem vessels. Phloem is present in very small amounts and then mainly as parenchyma with sieve tubes frequently recorded as narrow and possibly with abherent sieve plates. Leaves are typically reduced to widely spaced achlorophyllous scales on the inflorescence axis. Occasionally, they are present only on underground rhizomes or tubers. The vascular supply to the leaf-scales, normally reduced to a single trace, may be absent. Vestigial stomata are sometimes found on leaves and, in a few species which retain traces of chlorophyll, on shoots but, in most fully heterotrophic species, stomata are absent from aerial parts. Since their seeds are very small and contain minimal reserve carbohydrates, the germination of myco-heterotrophs in nature would appear to depend upon infection by an appropriate symbiotic fungus at an early stage. The nature of the carbohydrates transferred from the fungus to the plants has not been determined. Once acquired from The fungal partner, most plants store carbon in a variety of forms, the most common of which is starch, although other compounds including glucomannan, fructan and calcium oxalate art important in some specks. Asexual reproduction is frequently important with root tubers, tubercles and rhizomes providing the means of vegetative spread. Nonetheless, all the angiospermous species recorded to date also reproduce sexually. Floral structures show varying degrees of reduction concomitant with myco-heterotrophy. Inflorescences are typically small, often with a single terminal flower, and the floral parts often show extreme simplification, with the production of unilocular or, more rarely, bilocular and trilocular ovaries. In some of the most highly adapted species, there is reduction of integumentary layers surrounding each ovule from the normal bitegmic condition to unitegmic or, occasionally, ategmic. With the principal exception of the Monotropaceae, placentation is typically trimerous and parietal. Flowers normally appear to be cross-pollinated and are brightly coloured. nectiferous, occasionally scented, and can demonstrate extreme morphological adaptations which attract insects as in the production of lone caudate tepals or fungus-mimicking structures. Much is still to be learned about the adaptive features and especially about the physiology of these plants and of their early developmental stages during which the essential associations with fungi are established. Similarly, studies of the taxonomy and physiology of most of their fungal partners are still in their infancy. Contents Summary 171 I. Introduction 172 II. Taxonomic and phylogenetic relationships of myco-heterotrophic plants 174 III. Distribution patterns 180 IV. Habitats 183 V. Embryology 185 VI. Characteristics of seeds 186 VII. Mycorrhizal infection 192 VIII. Morphologies of roots 196 IX. Characteristics of shoots 199 X. Carbon assimilation and storage by mycoheterotrophic plants 202 XI. Reproduction 208 XII. Mutualism or parasitistn? 210 XIII. Future directions for research in mycoheterotrophic plants 210 XIV. Conclusions 211 Acknowledgements 211 References 211.

Ultrastructural evidence for the identity of some multinucleate rhizoctonias

The ultrastructure of the pore cap in ten multinucleate rhizoctonias, endophytes of non-orchids and of Microtis (Orchidaceae), was compared with that in isolates of four Sebacina species. The pore cap in all the rhizoctonias was an imperforate, disc-like structure, Slightly smaller in diameter than the flange forming the septal pore canal. The cap was formed by two Bat lamellae continuous with, but more electron-dense than, the membranes of the endoplasmic reticulum. The lamellae enclosed a lumen containing a deposit of electron-dense material. The pore cap in the rhizoctonias was indistinguishable from that in an isolate of Sebacina vermifera Oberwinkler. In contrast, the pore caps in isolates of S. calcea (Pers.: Fr.) Bres., S. umbrina Rogers and S. grisea (Pets.: Fr.) Bres., which were also imperforate and contained a band of electron dense material, were strongly dome-shaped and extended from one side of the flange to the other.