Structural features of mycorrhizal associations in two members of the Monotropoideae, Monotropa uniflora and Pterospora andromedea

A plant growth-promoting symbiosis between Mycena galopus and Vaccinium corymbosum seedlings

Typically, Mycena species are viewed as saprotrophic fungi. However, numerous detections of Mycena spp. in the roots of green plants suggest that a continuum from saprotrophy to biotrophy could exist. In particular, mycenoid species have repeatedly been found in Ericaceae plant roots. Our study asked whether (1) Mycena species are commonly found in the roots of green Ericaceae plants; (2) Mycena sequences are limited to a single group/lineage within the genus; and (3) a Mycena sp. can behave as a beneficial root associate with a typical ericoid mycorrhizal plant (Vaccinium corymbosum), regardless of how much external labile carbon is available. We detected Mycena sequences in roots of all sampled Ericaceae plants. Our Mycena sequences clustered in four different groups distributed across the Mycena genus. Only one group could be assigned with confidence to a named species (M. galopus). Our Mycena sequences clustered with other Mycena sequences detected in roots of ericoid mycorrhizal plant species collected throughout Europe, America, and Australia. An isolate of M. galopus promoted growth of V. corymbosum seedlings in vitro regardless of external carbon supply in the media. Seedlings inoculated with M. galopus grew as well as those inoculated with the ericoid mycorrhizal fungus Rhizoscyphus ericae. Surprisingly, this M. galopus isolate colonized Vaccinium roots and formed distinctive peg-like structures. Our results suggest that Mycena species might operate along a saprotroph-symbiotic continuum with a range of ericoid mycorrhizal plant species. We discuss our results in terms of fungal partner recruitment by Ericaceae plants.

Polymicrobial Multi-functional Approach for Enhancement of Crop Productivity

There is an increasing global need for enhancing the food production to meet the needs of the fast-growing human population. Traditional approach to increasing agricultural productivity through high inputs of chemical nitrogen and phosphate fertilizers and pesticides is not sustainable because of high costs and concerns about global warming, environmental pollution, and safety concerns. Therefore, the use of naturally occurring soil microbes for increasing productivity of food crops is an attractive eco-friendly, cost-effective, and sustainable alternative to the use of chemical fertilizers and pesticides. There is a vast body of published literature on microbial symbiotic and nonsymbiotic nitrogen fixation, multiple beneficial mechanisms used by plant growth-promoting rhizobacteria (PGPR), the nature and significance of mycorrhiza-plant symbiosis, and the growing technology on production of efficacious microbial inoculants. These areas are briefly reviewed here. The construction of an inoculant with a consortium of microbes with multiple beneficial functions such as N(2) fixation, biocontrol, phosphate solubilization, and other plant growth-promoting properties is a positive new development in this area in that a single inoculant can be used effectively for increasing the productivity of a broad spectrum of crops including legumes, cereals, vegetables, and grasses. Such a polymicrobial inoculant containing several microorganisms for each major function involved in promoting the plant growth and productivity gives it greater stability and wider applications for a range of major crops. Intensifying research in this area leading to further advances in our understanding of biochemical/molecular mechanisms involved in plant-microbe-soil interactions coupled with rapid advances in the genomics-proteomics of beneficial microbes should lead to the design and development of inoculants with greater efficacy for increasing the productivity of a wide range of crops.

Mycorrhizal fungi associated with Monotropastrum humile (Ericaceae) in central Japan

We explored the diversity of mycorrhizal fungi associated with Monotropastrum humile in the central part of Japan's main island. We collected 103 M. humile individuals from 12 sites with various forest types. We analyzed the DNA sequences of the internal transcribed spacer region from fungal and plant nuclear ribosomal DNAs to assess the genetic diversity of the fungi associated with M. humile roots and to position the plant with respect to known Monotropoideae groups, respectively. The plants formed a monophyletic clade with other members of M. humile but were separated from M. humile var. glaberrimum and other monotropes (97% bootstrap support). Of the 50 fungal phylotypes, 49 had best matches with the Russulales, and the other had highest similarity with the Thelephoraceae. Our phylogenetic analysis suggests that M. humile roots have a highly specialized association with fungal partners in the Russulaceae. Moreover, a few fungal phylotypes from the M. humile roots had positions neighboring those from Monotropa uniflora roots. These results indicated that the genetic diversity of mycorrhizal fungi of M. humile was highly specific to the Russulaceae, but with high diversity within that family, and that the fungi associated with M. humile differ from those associated with M. uniflora.

Structural characteristics of root-fungus associations in two mycoheterotrophic species, Allotropa virgata and Pleuricospora fimbriolata (Monotropoideae), from southwest Oregon, USA

All members of the Monotropoideae (Ericaceae), including the species, Allotropa virgata and Pleuricospora fimbriolata, are mycoheterotrophs dependent on associated symbiotic fungi and autotrophic plants for their carbon needs. Although the fungal symbionts have been identified for A. virgata and P. fimbriolata, structural details of the fungal-root interactions are lacking. The objective of this study was, therefore, to determine the structural features of these plant root-fungus associations. Root systems of these two species did not develop dense clusters of mycorrhizal roots typical of some monotropoid species, but rather, the underground system was composed of elongated rhizomes with first- and second-order mycorrhizal adventitious roots. Both species developed mantle features typical of monotropoid mycorrhizas, although for A. virgata, mantle development was intermittent along the length of each root. Hartig net hyphae were restricted to the host epidermal cell layer, and fungal pegs formed either along the tangential walls (P. fimbriolata) or radial walls (A. virgata) of epidermal cells. Plant-derived wall ingrowths were associated with each fungal peg, and these resembled transfer cells found in other systems. Although the diffuse nature of the roots of these two plants differs from some members in the Monotropoideae, the structural features place them along with other members of the Monotropoideae in the "monotropoid" category of mycorrhizas.

Pityopus californicus: structural characteristics of seed and seedling development in a myco-heterotrophic species

Pityopus californicus (Eastw.) H. F. Copel., a monotypic member of the Monotropoideae in the family Ericaceae, is a myco-heterotrophic species with distribution limited to the Pacific Northwest of the USA. Young embryos of P. californicus developed mycorrhizal associations in seed packets that had been buried for up to 681 days, suggesting that seeds of P. californicus may require the presence of a fungus to achieve germination. Samples of nongerminated seeds and early stages in embryo and root development were subsequently processed for light microscopy, histochemistry, and transmission electron microscopy (TEM). Nongerminated seeds possessed a thick testa, lacked a shoot and root meristem, and consisted of an embryo with large parenchymatous cells containing protein bodies and starch grains as storage reserves. In the earliest developmental stage (seed coat still attached), fungal hyphae were present on the testa surface and between the testa and embryo. This stage was followed by embryo elongation, the organization of a root apical meristem, and the development of a well-developed fungal mantle surrounding the elongated embryo. At least two morphotypes were identified based on structural characteristics of the mantle. One of these, with ascomycetous septa, had Cenococcum-like features. Late-stage embryo/early root development revealed a typical mantle and Hartig net, with fungal pegs penetrating the outer tangential walls of epidermal cells. Transfer cell-like deposits of wall material, similar to those described in Monotropa spp., enclosed fungal pegs. The development of a Hartig net and fungal pegs suggests that nutrient exchange interfaces are required for seedling development.

Specialized mycorrhizal colonization pattern in achlorophyllous Epirixanthes spp. (Polygalaceae)

Roots of the achlorophyllous Epirixanthes papunana and E. elongata were sectioned in complete series in order to reconstruct the three-dimensional mycorrhizal colonization pattern within their tissues. Hyphal morphology, vesicles, as well as the exclusively intracellular mode of colonization indicate a PARIS-type of arbuscular mycorrhiza showing a hitherto unknown colonization pattern: (1) the outer cortex is colonized by persistent straight-growing hyphae which branch in a cascading manner, (2) a specific layer (called layer 2) is inhabited by persistent hyphal coils, (3) in the cells of the anatomically distinct inner cortex parenchyma layer (called layer 1) the hyphae immediately degenerate, and (4) the layer outside to layer 2 (called layer 3) is either transitional layer 2 when penetrated from the outer cortex or the fungal material degenerates when colonized from the layer 2. This complex colonization pattern is a reasonable adaptation to the particular demands of Epirixanthes as a myco-heterotrophic plant. It not only allows a sustained benefit from the fungal symbiont but also provides a two-level distribution system of hyphae within the roots. The outer cortex hyphae function as a permanent intraradical resource of living fungi providing connection to the external mycelium as well as a coarse distribution of hyphae within the root. Layer 2 represents the fine scale distribution of hyphae, having access to all potentially digesting cells of the layers 1 and 3. Common structural features of mycorrhizae in myco-heterotrophic plants are pointed out in order to find putative prerequisites for their heterotrophic mode of life.

The evolutionary ecology of myco-heterotrophy

Nonphotosynthetic mycorrhizal plants have long attracted the curiosity of botanists and mycologists, and they have been a target for unabated controversy and speculation. In fact, these puzzling plants dominated the very beginnings of the field of mycorrhizal biology. However, only recently has the mycorrhizal biology of this diverse group of plants begun to be systematically unravelled, largely following a landmark Tansley review a decade ago and crucial developments in the field of molecular ecology. Here I explore our knowledge of these evolutionarily and ecologically diverse plant-fungal symbioses, highlighting areas where there has been significant progress. The focus is on what is arguably the best understood example, the monotropoid mycorrhizal symbiosis, and the overarching goal is to lay out the questions that remain to be answered about the biology of myco-heterotrophy and epiparasitism.