Are liverworts imitating mycorrhizas?

The origin and evolution of mycorrhizal symbioses: from palaeomycology to phylogenomics

Contents Summary 1012 I. Introduction 1013 II. The mycorrhizal symbiosis at the dawn and rise of the land flora 1014 III. From early land plants to early trees: the origin of roots and true mycorrhizas 1016 IV. The diversification of the AM symbiosis 1019 V. The ECM symbiosis 1021 VI. The recently evolved ericoid and orchid mycorrhizas 1023 VII. Limits of paleontological vs genetic approaches and perspectives 1023 Acknowledgements 1025 References 1025 SUMMARY: The ability of fungi to form mycorrhizas with plants is one of the most remarkable and enduring adaptations to life on land. The occurrence of mycorrhizas is now well established in c. 85% of extant plants, yet the geological record of these associations is sparse. Fossils preserved under exceptional conditions provide tantalizing glimpses into the evolutionary history of mycorrhizas, showing the extent of their occurrence and aspects of their evolution in extinct plants. The fossil record has important roles to play in establishing a chronology of when key fungal associations evolved and in understanding their importance in ecosystems through time. Together with calibrated phylogenetic trees, these approaches extend our understanding of when and how groups evolved in the context of major environmental change on a global scale. Phylogenomics furthers this understanding into the evolution of different types of mycorrhizal associations, and genomic studies of both plants and fungi are shedding light on how the complex set of symbiotic traits evolved. Here we present a review of the main phases of the evolution of mycorrhizal interactions from palaeontological, phylogenetic and genomic perspectives, with the aim of highlighting the potential of fossil material and a geological perspective in a cross-disciplinary approach.

Ancient plants with ancient fungi: liverworts associate with early-diverging arbuscular mycorrhizal fungi

Arbuscular mycorrhizas are widespread in land plants including liverworts, some of the closest living relatives of the first plants to colonize land 500 million years ago (MYA). Previous investigations reported near-exclusive colonization of liverworts by the most recently evolved arbuscular mycorrhizal fungi, the Glomeraceae, indicating a recent acquisition from flowering plants at odds with the widely held notion that arbuscular mycorrhizal-like associations in liverworts represent the ancestral symbiotic condition in land plants. We performed an analysis of symbiotic fungi in 674 globally collected liverworts using molecular phylogenetics and electron microscopy. Here, we show every order of arbuscular mycorrhizal fungi colonizes early-diverging liverworts, with non-Glomeraceae being at least 10 times more common than in flowering plants. Arbuscular mycorrhizal fungi in liverworts and other ancient plant lineages (hornworts, lycopods, and ferns) were delimited into 58 taxa and 36 singletons, of which at least 43 are novel and specific to liverworts. The discovery that early plant lineages are colonized by early-diverging fungi supports the hypothesis that arbuscular mycorrhizas are an ancestral symbiosis for all land plants.

What have we learnt from studying the evolution of the arbuscular mycorrhizal symbiosis?

The arbuscular mycorrhizal (AM) symbiosis is a nearly ubiquitous association formed by most land plants. Numerous insights into the molecular mechanisms governing this symbiosis have been obtained in recent years leading to the identification of a core set of plant genes essential for successful formation of the AM symbiosis by angiosperm hosts. Recent phylogenetic analyses indicate that while the origin of some of these symbiotic genes predated the first land plants, the rest appeared through processes including de novo evolution and gene duplication that occurred specifically in the land plants. Purifying selection on this core gene set has been maintained over millions of years of plant evolution to conserve the AM symbiosis. However, several independent losses of this association have been recorded in numerous embryophyte lineages. In these lineages, potential compensatory mechanisms have been identified that could have helped these plants overcome the adversities imposed by the loss of the AM symbiosis. This review will focus on the processes governing the conservation of the AM symbiosis in the land plant lineage.

From rhizoids to roots? Experimental evidence of mutualism between liverworts and ascomycete fungi

Background and Aims

The rhizoids of leafy liverworts (Jungermanniales, Marchantiophyta) are commonly colonized by the ascomycete fungus Pezoloma ericae. These associations are hypothesized to be functionally analogous to the ericoid mycorrhizas (ErMs) formed by P. ericae with the roots of Ericaceae plants in terms of bi-directional phosphorus for carbon exchange; however, this remains unproven. Here, we test whether associations between the leafy liverwort Cephalozia bicuspidata and P. ericae are mutualistic.

Methods

We measured movement of phosphorus and carbon between C. bicuspidata and P. ericae using [33P]orthophosphate and 14CO2 isotope tracers in monoxenic cultures. We also measured leafy liverwort growth, with and without P. ericae.

Key Results

We present the first demonstration of nutritionally mutualistic symbiosis between a non-vascular plant and an ErM-forming fungus, showing transfer of fungal-acquired P to the liverwort and of liverwort-fixed C to the fungus alongside increased growth in fungus-colonized liverworts.

Conclusions

Thus, this ascomycete-liverwort symbiosis can now be described as mycorrhiza-like, providing further insights into ericoid mycorrhizal evolution and adding Ascomycota fungi to mycorrhizal fungal groups engaging in mutualisms with plants across the land plant phylogeny. As P. ericae also colonizes the rhizoids of Schistochilaceae liverworts, which originated in the Triassic and are sister to all other jungermannialean liverworts associated with fungi, our findings point toward an early origin of ascomycete-liverwort symbioses, possibly pre-dating their evolution in the Ericales by some 150 million years.

Sebacinales - one thousand and one interactions with land plants

20 I 21 II 21 III 23 IV 29 V 33 VI 35 36 36 References 36 SUMMARY: Root endophytism and mycorrhizal associations are complex derived traits in fungi that shape plant physiology. Sebacinales (Agaricomycetes, Basidiomycota) display highly diverse interactions with plants. Although early-diverging Sebacinales lineages are root endophytes and/or have saprotrophic abilities, several more derived clades harbour obligate biotrophs forming mycorrhizal associations. Sebacinales thus display transitions from saprotrophy to endophytism and to mycorrhizal nutrition within one fungal order. This review discusses the genomic traits possibly associated with these transitions. We also show how molecular ecology revealed the hyperdiversity of Sebacinales and their evolutionary diversification into two sister families: Sebacinaceae encompasses mainly ectomycorrhizal and early-diverging saprotrophic species; the second family includes endophytes and lineages that repeatedly evolved ericoid, orchid and ectomycorrhizal abilities. We propose the name Serendipitaceae for this family and, within it, we transfer to the genus Serendipita the endophytic cultivable species Piriformospora indica and P. williamsii. Such cultivable Serendipitaceae species provide excellent models for root endophytism, especially because of available genomes, genetic tractability, and broad host plant range including important crop plants and the model plant Arabidopsis thaliana. We review insights gained with endophytic Serendipitaceae species into the molecular mechanisms of endophytism and of beneficial effects on host plants, including enhanced resistance to abiotic and pathogen stress.

Mollicutes-related endobacteria thrive inside liverwort-associated arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) can host Gram-positive endobacteria (BLOs) in their cytoplasm. These have been identified as Mollicutes-related microbes based on an inventory of AMF spores from fungal collections. Bacteria-like organisms (BLOs) of unknown identity have also been reported in the cytoplasm of AMF associated with liverworts, the earliest-diverged extant lineage of land plants. A combination of morphological, molecular and phylogenetic analyses revealed that three samples of two liverwort species (Conocephalum conicum and Lunularia cruciata) growing spontaneously in a botanical garden harboured AMF belonging to Glomerales, and these, in turn, hosted coccoid BLOs. 16S rDNA sequences from these BLOs clustered with the Mollicutes sequences identified from the spore collections but revealed the presence of novel phylotypes. Electron microscopy and fluorescence in situ hybridization (FISH) confirmed the presence of BLOs inside the cytoplasm of AMF hyphae colonizing the liverwort thalli. The high genetic variability of BLOs in liverwort-AMF associations thriving in the same ecological niche raises questions about the mechanisms underlying such diversity.

Mutualistic mycorrhiza-like symbiosis in the most ancient group of land plants

Over 35 years ago, it was hypothesized that mutualistic symbiotic soil fungi assisted land plants in their initial colonization of terrestrial environments. This important idea has become increasingly established with palaeobotanical and molecular investigations dating the interactions between arbuscular mycorrhizal fungi (AMF) and land plants to at least 400 Ma, but the functioning of analogous partnerships in 'lower' land plants remains unknown. In this study, we show with multifactorial experiments that colonization of a complex thalloid liverwort, a member of the most ancient extant clade of land plants, with AMF significantly promotes photosynthetic carbon uptake, growth and asexual reproduction. Plant fitness increased through fungal-enhanced acquisition of phosphorus and nitrogen from soil, with each plant supporting 100-400 m of AMF mycelia. A simulated CO(2)-rich atmosphere, similar to that of the Palaeozoic when land plants originated, significantly amplified the net benefits of AMF and likely selection pressures for establishment of the symbiosis. Our analyses provide essential missing functional evidence supporting AMF symbionts as drivers of plant terrestrialization in early Palaeozoic land ecosystems.

Presence of three mycorrhizal genes in the common ancestor of land plants suggests a key role of mycorrhizas in the colonization of land by plants

*The colonization of land by plants fundamentally altered environmental conditions on earth. Plant-mycorrhizal fungus symbiosis likely played a key role in this process by assisting plants to absorb water and nutrients from soil. *Here, in a diverse set of land plants, we investigated the evolutionary histories and functional conservation of three genes required for mycorrhiza formation in legumes and rice (Oryza sativa), DMI1, DMI3 and IPD3. *The genes were isolated from nearly all major plant lineages. Phylogenetic analyses showed that they had been vertically inherited since the origin of land plants. Further, cross-species mutant rescue experiments demonstrated that DMI3 genes from liverworts and hornworts could rescue Medicago truncatula dmi3 mutants for mycorrhiza formation. Yeast two-hybrid assays also showed that bryophyte DMI3 proteins could bind to downstream-acting M. trunculata IPD3 protein. Finally, molecular evolutionary analyses revealed that these genes were under purifying selection for maintenance of their ancestral functions in all mycorrhizal plant lineages. *These results indicate that the mycorrhizal genes were present in the common ancestor of land plants, and that their functions were largely conserved during land plant evolution. The evidence presented here strongly suggests that plant-mycorrhizal fungus symbiosis was one of the key processes that contributed to the origin of land flora.

Plants and arbuscular mycorrhizal fungi: an evolutionary-developmental perspective

Arbuscular mycorrhizas (AMs) are widespread symbiotic associations that are commonly described as the result of co-evolution events between fungi and plants where both partners benefit from the reciprocal nutrient exchange. Here, we review data from fossil records, characterizations of AM fungi in basal plants and live cell imaging of angiosperm colonization processes from an evolutionary-developmental perspective. The uniformity of plant cell responses to AM colonization in haploid gametophytes and diploid sporophytes, in non-root organs, and throughout many seed plant clades highlights the ancient origin of the interaction and suggests the existence of common molecular and cellular processes. The possibility that pre-existing mechanisms involved in plant cell division were recruited by plants to accommodate AM fungi is discussed.

Widespread association between the ericoid mycorrhizal fungus Rhizoscyphus ericae and a leafy liverwort in the maritime and sub-Antarctic

A recent study identified a fungal isolate from the Antarctic leafy liverwort Cephaloziella varians as the ericoid mycorrhizal associate Rhizoscyphus ericae. However, nothing is known about the wider Antarctic distribution of R. ericae in C. varians, and inoculation experiments confirming the ability of the fungus to form coils in the liverwort are lacking. Using direct isolation and baiting with Vaccinium macrocarpon seedlings, fungi were isolated from C. varians sampled from eight sites across a 1875-km transect through sub- and maritime Antarctica. The ability of an isolate to form coils in aseptically grown C. varians was also tested. Fungi with 98-99% sequence identity to R. ericae internal transcribed spacer (ITS) region and partial large subunit ribosomal (r)DNA sequences were frequently isolated from C. varians at all sites sampled. The EF4/Fung5 primer set did not amplify small subunit rDNA from three of five R. ericae isolates, probably accounting for the reported absence of the fungus from C. varians in a previous study. Rhizoscyphus ericae was found to colonize aseptically-grown C. varians intracellularly, forming hyphal coils. This study shows that the association between R. ericae and C. varians is apparently widespread in Antarctica, and confirms that R. ericae is at least in part responsible for the formation of the coils observed in rhizoids of field-collected C. varians.

Lunularia cruciata, a potential in vitro host for Glomus proliferum and G. intraradices

A study was conducted to define culture conditions for in vitro growth arbuscular mycorrhizal fungi (AMF) with liverworts as hosts. Lunularia cruciata (L.) Dumortier ex. Lindberg developed in vitro monoxenic mycothalli with both Glomus proliferum Dalpé & Declerck (MUCL 41827) and Glomus intraradices Schenck & Smith (MUCL 43204). AMF inoculated plants were co-cultured in plastic Petri dishes with semi-solidified medium supplemented with sucrose and grown under filtered light. Mycothalli of L. cruciata produced external hyphae and spores in quantities equivalent to those obtained with Ri T-DNA transformed root systems.

A comparative cytological analysis of fungal endophytes in the sporophyte rhizomes and vascularized gametophytes of Tmesipteris and Psilotum

This article describes the results of a light and electron microscopic study of the fungal endophytes and vascular anatomy in the rhizomes and gametophytes of Tmesipteris and Psilotum. The parenchymatous cortical cells of the rhizomes and subterranean gametophytes of Tmesipteris and Psilotum contain intracellular aseptate glomeromycotean fungi resembling the “Paris-type” of arbuscular mycorrhizas found in seed plants. The fungi differentiate into multinucleate vesicles and hyphal coils, both containing bacteria-like structures and accumulating lipid masses and crystals as they age. After several cycles of infection in the same cell, degenerate hyphae form amorphous masses encased by host wall material. Nearly identical host–fungus cytology between the autotrophic sporophytes and the heterotrophic gametophytes suggests that these psilophyte associations are exploitative of the fungus in both generations. Following the description of tracheids nearly 60 years ago in the gametophytes of Psilotum, vascular elements are described for the first time in the haploid generation of Tmesipteris. Close similarities between the water- and food-conducting elements in both generations, viz. vessel elements with scalariform perforation plates and sieve cells with refractive spherules and lacking callose at all stages in their develoment, add support to the homologous theory of the alternations of generations. Mitochondrial aggregations, cross-linked by small electron-opaque rods, are common in the stelar cells of both generations and appear to be a unique feature of the psilophyte clade.