An accurate understanding of the diversity and distribution of fungal symbioses in land plants is essential for mycorrhizal research. Here we update the seminal work of Wang and Qiu (Mycorrhiza ...16:299-363,
2006
) with a long-overdue focus on early-diverging land plant lineages, which were considerably under-represented in their survey, by examining the published literature to compile data on the status of fungal symbioses in liverworts, hornworts and lycophytes. Our survey combines data from 84 publications, including recent, post-2006, reports of Mucoromycotina associations in these lineages, to produce a list of at least 591 species with known fungal symbiosis status, 180 of which were included in Wang and Qiu (Mycorrhiza 16:299-363,
2006
). Using this up-to-date compilation, we estimate that fewer than 30% of liverwort species engage in symbiosis with fungi belonging to all three mycorrhizal phyla, Mucoromycota, Basidiomycota and Ascomycota, with the last being the most widespread (17%). Fungal symbioses in hornworts (78%) and lycophytes (up to 100%) appear to be more common but involve only members of the two Mucoromycota subphyla Mucoromycotina and Glomeromycotina, with Glomeromycotina prevailing in both plant groups. Our fungal symbiosis occurrence estimates are considerably more conservative than those published previously, but they too may represent overestimates due to currently unavoidable assumptions.
This light and electron microscope study reveals considerable uniformity in hornwort stomata morphology and density in contrast to common spatial and developmental abnormalities in tracheophytes and ...mosses. Stomata arise from a median longitudinal division of sporophyte epidermal cells morphologically indistinguishable from their neighbours apart from the retention of a single chloroplast whilst those in the other epidermal cells fragment. Chloroplast division and side-by-side repositioning of the two daughter chloroplasts determines the division plane in the stomatal mother cell. The nascent guard cells contain giant, starch-filled chloroplasts which subsequently divide and, post aperture opening, regain their spherical shape. Accumulation of wall material over the guard cells and of wax rodlets lining the pores follows opening. While the majority of stomata are bilaterally symmetrical those lining the dehiscence furrows display dextral and sinistral asymmetry due to differential expansion of the adjacent epidermal cells.
The ubiquity of open stomata suggests that these never close with the maturational wall changes rendering movement extremely unlikely. These structural limitations, a liquid-filled stage in the ontogeny of the intercellular spaces, and spores already at the tetrad stage when stomata open, suggest that their primary role is facilitating sporophyte desiccation leading to dehiscence and spore dispersal rather than gaseous exchange. Stomata ontogeny and very low densities, like those in Devonian fossils, suggest either ancient origins at a time when atmospheric carbon dioxide levels were much greater than today or a function other than gaseous exchange regulation. We found no evidence for stomatal homology between hornworts, mosses and tracheophytes.
Current understanding of the nature and function of fungal associations in pteridophytes is surprisingly patchy given their key evolutionary position, current research foci on other early-branching ...plant clades, and major efforts at unravelling mycorrhizal evolution and the mechanisms underlying this key interaction between plants and fungi. Here we provide a critical review of current knowledge of fungal associations across pteridophytes and consider future directions making recommendations along the way. From a comprehensive survey of the literature, a confused picture emerges： suggestions that members of the Lycopsida harbour Basidiomycota fungi contrast sharply with extensive cytological and recent molecular evidence pointing to exclusively Glomeromycota and/or Mucoromycotina associations in this group. Similarly, reports of dark septate, assumingly ascomycetous, hyphae in a range of pteridophytes, advocating a mutualistic relationship, are not backed by functional evidence and the fact that the fungus invariably occupies dead host tissue points to saprotrophy and not mutualism. The best conclusion that can be reached based on current evidence is that the fungal symbionts of pteridophytes belong to the two fungal lineages Mucoromycotina and Glomeromycota. Do symbiotic fungi and host pteridophytes engage in mutually beneficial partnerships？ To date, only two pioneering studies have addressed this key question demonstrating reciprocal exchange of nutrients between the sporophytes of Ophioglossum vulgatum and Osmunda regalis and their fungal symbionts. There is a pressing need for more functional investigations also extending to the gametophyte generation and coupled with in vitro isolation and resynthesis studies to unravel the effect of the fungi on their host.
Sequencing the plastid genomes of land plants provides crucial improvements to our understanding of the plastome evolution of land plants. Although the number of available complete plastid genome ...sequences has rapidly increased in the recent years, only a few sequences have been yet released for the three bryophyte lineages, namely hornworts, liverworts, and mosses. Here, we explore the disparity of the plastome structure of liverworts by increasing the number of sequenced liverwort plastomes from five to 18. The expanded sampling included representatives of all major lineages of liverworts including the genus Haplomitrium. The disparity of the liverwort genomes was compared with other 2386 land plant plastomes with emphasis on genome size and GC‐content. We found evidence for structural conservatism of the plastid genomes in liverworts and a trend towards reduced plastome sequence length in liverworts and derived mosses compared to other land plants, including hornworts and basal lineages of mosses. Furthermore, Aneura and Haplomitrium were distinct from other liverworts by an increased GC content, with the one found in Haplomitrium only second to the lycophyte Selaginella. The results suggest the hypothesis that liverworts and other land plants inherited and conserved the plastome structure of their most recent algal ancestors.
The nineteenth century air-gun explanation for explosive spore discharge in Sphagnum has never been tested experimentally. Similarly, the function of the numerous stomata ubiquitous in the capsule ...walls has never been investigated. Both intact and pricked Sphagnum capsules, that were allowed to dry out, all dehisced over an 8-12 h period during which time the stomatal guard cells gradually collapsed and their potassium content, measured by X-ray microanalysis in a cryoscanning electron microscope, gradually increased. By contrast, guard cell potassium fell in water-stressed Arabidopsis. The pricking experiments demonstrate that the air-gun notion for explosive spore discharge in Sphagnum is inaccurate; differential shrinkage of the capsule walls causes popping off the rigid operculum. The absence of evidence for a potassium-regulating mechanism in the stomatal guard cells and their gradual collapse before spore discharge indicates that their sole role is facilitation of sporophyte desiccation that ultimately leads to capsule dehiscence. Our novel functional data on Sphagnum, when considered in relation to bryophyte phylogeny, suggest the possibility that stomata first appeared in land plants as structures that facilitated sporophyte drying out before spore discharge and only subsequently acquired their role in the regulation of gaseous exchange.
• Background and Aims Moss food-conducting cells (leptoids and specialized parenchyma cells) have a highly distinctive cytology characterized by a polarized cytoplasmic organization and longitudinal ...alignment of plastids, mitochondria, endoplasmic reticulum and vesicles along endoplasmic microtubules. Previous studies on the desiccation biology of mosses have focused almost exclusively on photosynthetic tissues; the effects of desiccation on food-conducting cells are unknown. Reported here is a cytological study of the effects of de- and rehydration on food-conducting cells in the desiccation-tolerant moss Polytrichum formosum aimed at exploring whether the remarkable subcellular organization of these cells is related to the ability of mosses to survive desiccation. • Methods Shoots of Polytrichum formosum were dehydrated under natural conditions and prepared for transmission and scanning electron microscopy using both standard and anhydrous chemical fixation protocols. Replicate samples were then fixed at intervals over a 24-h period following rehydration in either water or in a 10 µm solution of the microtubule-disrupting drug oryzalin. • Key Results Desiccation causes dramatic changes; the endoplasmic microtubules disappear; the nucleus, mitochondria and plastids become rounded and the longitudinal alignment of the organelles is lost, though cytoplasmic polarity is in part retained. Prominent stacks of endoplasmic reticulum, typical of the hydrated condition, are replaced with membranous tubules arranged at right angles to the main cellular axis. The internal cytoplasm becomes filled with small vacuoles and the plasmalemma forms labyrinthine tubular extensions outlining newly deposited ingrowths of cell wall material. Whereas plasmodesmata in meristematic cells at the shoot apex and in stem parenchyma cells appear to be unaffected by dehydration, those in leptoids become plugged with electron-opaque material. Starch deposits in parenchyma cells adjoining leptoids are depleted in desiccated plants. Rehydration sees complete reestablishment over a 12- to 24-h period of the cytology seen in the control plants. Oryzalin effectively prevents leptoid recovery. • Conclusions The results point to a key role of the microtubular cytoskeleton in the rapid re-establishment of the elaborate cytoplasmic architecture of leptoids during rehydration. The reassembly of the endoplasmic microtubule system appears to dictate the time frame for the recovery process. The failure of leptoids to recover normal cytology in the presence of oryzalin further underlines the key role of the microtubules in the control of leptoid cytological organization.
BACKGROUND AND AIMS: Previous studies of protonemal morphogenesis in mosses have focused on the cytoskeletal basis of tip growth and the production of asexual propagules. This study provides the ...first comprehensive description of the differentiation of caulonemata and rhizoids, which share the same cytology, and the roles of the cytoskeleton in organelle shaping and spatial arrangement. METHODS: Light and electron microscope observations were carried out on in vitro cultured and wild protonemata from over 200 moss species. Oryzalin and cytochalasin D were used to investigate the role of the cytoskeleton in the cytological organization of fully differentiated protonemal cells; time-lapse photography was employed to monitor organelle positions. KEY RESULTS: The onset of differentiation in initially highly vacuolate subapical cells is marked by the appearance of tubular endoplasmic reticulum (ER) profiles with crystalline inclusions, closely followed by an increase in rough endoplasmic reticulum (RER). The tonoplast disintegrates and the original vacuole is replaced by a population of vesicles and small vacuoles originating de novo from RER. The cytoplasm then becomes distributed throughout the cell lumen, an event closely followed by the appearance of endoplasmic microtubules (MTs) in association with sheets of ER, stacks of vesicles that subsequently disperse, elongate mitochondria and chloroplasts and long tubular extensions at both poles of the nucleus. The production of large vesicles by previously inactive dictysomes coincides with the deposition of additional cell wall layers. At maturity, the numbers of endoplasmic microtubules decline, dictyosomes become inactive and the ER is predominantly smooth. Fully developed cells remain largely unaffected by cytochalasin; oryzalin elicits profound cytological changes. Both inhibitors elicit the formation of giant plastids. The plastids and other organelles in fully developed cells are largely stationary. CONCLUSIONS: Differentiation of caulonemata and rhizoids involves a remarkable series of cytological changes, some of which closely recall major events in sieve element ontogeny in tracheophytes. The cytology of fully differentiated cells is remarkably similar to that of moss food-conducting cells and, in both, is dependent on an intact microtubule cytoskeleton. The disappearance of the major vacuolar apparatus is probably related to the function of caulonema and rhizoids in solute transport. Failure of fully differentiated caulonema and rhizoid cells to regenerate is attributed to a combination of endo-reduplication and irreversible tonoplast fragmentation. The formation of giant plastids, most likely by fusion, following both oryzalin and cytochalasin treatments, suggests key roles for both microtubules and microfilaments in the spatial arrangement and replication of plastids.
The water relations of Funaria hygrometrica Hedw. sporophytes have never been investigated, although unchanged morphology through periods of drought suggests that they may be homoiohydric. Knowledge ...of the sporophyte maturation cycle is also incomplete and based on glasshouse plants.
We followed sporophyte development in wild populations of Funaria, recording fresh weights of every phenophase. Rates of water loss under laboratory conditions from sporophytes of different ages were recorded, and the ontogeny of the intercellular spaces and the maturational deposition of extra wall materials and waxes were investigated by cryo-SEM.
The sporophyte maturation cycle in wild Funaria lasted from December until July (> 200 days), nearly three times that recorded in glasshouses. Fresh weights of green capsules increased until after sporogenesis. Mature brown capsules were highly dehydrated. Prevailing weather conditions or addition of water had no effect on weights. Low rates of water loss, comparable with those from vascular plant leaves, decreased throughout sporophyte maturation under laboratory conditions. These data indicate that Funaria sporophytes mirror homoiohydric vascular plants. Deposition of additional wall materials around the stomatal apertures prevents closure soon after their opening towards the end of post-meiosis capsule expansion.
With phenophases similar to those of perennial species, Funaria may not be as much a fugitive species as previously assumed. The very brief window in nascent stomata ontogeny when reversible aperture changes might be possible indicates that the likely principal role of Funaria stomata is facilitation of capsule dehydration and not active regulation of gaseous exchange.
Liverworts form diverse associations with endophytic fungi similar to mycorrhizas in vascular plants. Whereas the widespread occurrence of glomeromycotes in the basal liverwort lineages is well ...documented, knowledge of the distribution of ascomycetes and basidiomycetes in derived thalloid and leafy clades is more fragmented. Our discovery that the ramified and septate rhizoids of the Schistochilaceae, the sister group to all other ascomycete-containing liverworts, are packed with fungal hyphae prompted this study on the effects of the fungi on rhizoid morphology, host specificity, the cytology of the association, and a molecular analysis of the endophytes. Two species of Pachyschistochila and their fungi were grown axenically. Axenic rhizoids were unbranched and nonseptate. Reinfected with their own fungus and that from the other species, both Pachyschistochila species produced branched and septate rhizoids identical to those in nature. Woronin bodies and simple septa identified the fungus as an ascomycete referable, according to phylogenetic analyses of ITS sequences, to the Rhizoscyphus (Hymenoscyphus) ericae aggregate, also found in other liverwort-ascomycete associations and in mycorrhizas in the Ericales. Healthy hyphae and host cytoplasm suggest that the Schistochila-fungus association reflects a balanced mutualistic relationship. The recent dating of the divergence of the Jungermanniales from the fungus-free Porellales in the Permian and the origins of the Schistochilaceae in the Triassic indicate that these associations in liverworts predate the appearance of the Ericales.
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