At the core of the endomycorrhizal symbioses: intracellular fungal structures in orchid and arbuscular mycorrhiza.

Arbuscular (AM) and orchid (OrM) mycorrhiza are the most widespread mycorrhizal symbioses among flowering plants, formed by distinct fungal and plant species. They are both endosymbioses because the fungal hyphae can enter inside the plant cell to develop intracellular fungal structures that are surrounded by the plant membrane. The symbiotic plant-fungus interface is considered to be the major site of nutrient transfer to the host plant. We summarize recent data on nutrient transfer in OrM and compare the development and function of the arbuscules formed in AM and the pelotons formed in OrM in order to outline differences and conserved traits. We further describe the unexpected similarities in the form and function of the intracellular mycorrhizal fungal structures observed in orchids and in the roots of mycoheterotrophic plants forming AM. We speculate that these similarities may be the result of convergent evolution of mycorrhizal types in mycoheterotrophic plants and highlight knowledge gaps and new research directions to explore this scenario.

Partial mycoheterotrophy is common among chlorophyllous plants with Paris-type arbuscular mycorrhiza.

BACKGROUND AND AIMS: An arbuscular mycorrhiza is a mutualistic symbiosis with plants as carbon providers for fungi. However, achlorophyllous arbuscular mycorrhizal species are known to obtain carbon from fungi, i.e. they are mycoheterotrophic. These species all have the Paris type of arbuscular mycorrhiza. Recently, two chlorophyllous Paris-type species proved to be partially mycoheterotrophic. In this study, we explore the frequency of this condition and its association with Paris-type arbuscular mycorrhiza. METHODS: We searched for evidence of mycoheterotrophy in all currently published 13C, 2H and 15N stable isotope abundance patterns suited for calculations of enrichment factors, i.e. isotopic differences between neighbouring Paris- and Arum-type species. We found suitable data for 135 plant species classified into the two arbuscular mycorrhizal morphotypes. KEY RESULTS: About half of the chlorophyllous Paris-type species tested were significantly enriched in 13C and often also enriched in 2H and 15N, compared with co-occurring Arum-type species. Based on a two-source linear mixing model, the carbon gain from the fungal source ranged between 7 and 93 % with ferns > horsetails > seed plants. The seed plants represented 13 families, many without a previous record of mycoheterotrophy. The 13C-enriched chlorophyllous Paris-type species were exclusively herbaceous perennials, with a majority of them thriving on shady forest ground. CONCLUSIONS: Significant carbon acquisition from fungi appears quite common and widespread among Paris-type species, this arbuscular mycorrhizal morphotype probably being a pre-condition for developing varying degrees of mycoheterotrophy.

Mycoheterotrophic seedling growth of Gentiana zollingeri, a photosynthetic Gentianaceae plant species, in symbioses with arbuscular mycorrhizal fungi.

We found mycoheterotrophic seedling growth (initial mycoheterotrophy) of Gentiana zollingeri, a spring-flowering photosynthetic species of Gentianaceae family. Small seeds (about 300 µm in length) were buried in a habitat by using seed packets, and development of the subterranean seedlings to form shoots, more than 3 cm in length, was observed in symbiosis with arbuscular mycorrhizal (AM) fungi in the dark (i.e., underground of a field). Hyphal coils and their degenerations were observed in the root cortical cells of the subterranean seedlings as well as those of adult plants. Among the mycobionts identified on the basis of partial small subunit rDNA sequences, it was found that AM fungi of a lineage in Glomeraceae dominantly colonized, and the AM fungi were also dominant in adult individuals of G. zollingeri in three habitats separated one another by 17.2, 34.7, and 49.6 km. Though initial mycoheterotrophy in symbioses with AM fungi has been observed in some pteridophytes, this is the first study to demonstrate this type of symbiosis in a photosynthetic seed plant. The mycoheterotrophy means that an energy distribution occurs through the hyphal bridges of AM fungi among different photosynthetic seed plants, which may be important in constructing plant species diversity in some ecosystems.

Gibberellin Promotes Fungal Entry and Colonization during Paris-Type Arbuscular Mycorrhizal Symbiosis in Eustoma grandiflorum.

Arbuscular mycorrhizas (AMs) are divided into two types according to morphology: Arum- and Paris-type AMs. Gibberellins (GAs) mainly inhibit the establishment of Arum-type AM symbiosis in most model plants, whereas the effects of GAs on Paris-type AM symbiosis are unclear. To provide insight into the mechanism underlying this type of symbiosis, the roles of GAs were investigated in Eustoma grandiflorum when used as the host plant for Paris-type AM establishment. Eustoma grandiflorum seedlings were inoculated with the model AM fungus, Rhizophagus irregularis, and the effects of GA and the GA biosynthesis inhibitor uniconazole-P on the symbiosis were quantitatively evaluated. Exogenous GA significantly increased hyphopodium formation at the epidermis, thus leading to the promotion of fungal colonization and arbuscule formation in the root cortex. By contrast, the suppression of GA biosynthesis and signaling attenuated fungal entry to E. grandiflorum roots. Moreover, the exudates from GA-treated roots strongly induced the hyphal branching of R. irregularis. Our results show that GA has an contrasting effect on Paris-type AM symbiosis in E. grandiflorum compared with Arum-type AM symbiosis. This finding could be explained by the differential regulation of the early colonization stage, where fungal hyphae make contact with and penetrate the epidermis.

Conserved and Diverse Transcriptional Reprogramming Triggered by the Establishment of Symbioses in Tomato Roots Forming Arum-Type and Paris-Type Arbuscular Mycorrhizae.

Arbuscular mycorrhizal (AM) fungi allocate mineral nutrients to their host plants, and the hosts supply carbohydrates and lipids to the fungal symbionts in return. The morphotypes of intraradical hyphae are primarily determined on the plant side into Arum- and Paris-type AMs. As an exception, Solanum lycopersicum (tomato) forms both types of AMs depending on the fungal species. Previously, we have shown the existence of diverse regulatory mechanisms in Arum- and Paris-type AM symbioses in response to gibberellin (GA) among different host species. However, due to the design of the study, it remained possible that the use of different plant species influenced the results. Here, we used tomato plants to compare the transcriptional responses during Arum- and Paris-type AM symbioses in a single plant species. The tomato plants inoculated with Rhizophagus irregularis or Gigaspora margarita exhibited Arum- and Paris-type AMs, respectively, and demonstrated similar colonization rates and shoot biomass. Comparative transcriptomics showed shared expression patterns of AM-related genes in tomato roots upon each fungal infection. On the contrary, the defense response and GA biosynthetic process was transcriptionally upregulated during Paris-type AM symbiosis. Thus, both shared and different transcriptional reprogramming function in establishing Arum- and Paris-type AM symbioses in tomato plants.

The effects of gibberellin on the expression of symbiosis-related genes in Paris-type arbuscular mycorrhizal symbiosis in Eustoma grandiflorum.

Arbuscular mycorrhiza (AM) is a symbiotic interaction in terrestrial plants that is colonized by fungi in the Glomeromycotina. The morphological types of AM, including the Arum-type and Paris-type, are distinct, depending on the host plant species. A part of the regulatory pathways in Arum-type AM symbiosis has been revealed because most model plants form the Arum-type AM with a model AM fungus, Rhizophagus irregularis. Moreover, gibberellin (GA) is known to severely inhibit AM fungal colonization in Arum-type AM symbiosis. Recently, we showed that exogenous GA treatment significantly promoted AM fungal colonization in Paris-type AM symbiosis in Eustoma grandiflorum. In this study, we focused on the transcriptional changes in AM symbiosis-related genes in GA-treated E. grandiflorum. The expression levels of all examined E. grandiflorum genes were maintained or increased by GA treatment compared with those of the control treatment. Our new results suggest that signaling pathway(s) required for establishing AM symbiosis in E. grandiflorum may be distinct from the well-characterized pathway for that in model plants.

Growth and phosphorus nutrition of a Paris-type arbuscular mycorrhizal symbiosis.

• Paris -type arbuscular mycorrhizas (AM) are reportedly the most common morphological type of AM; however, most research has focused on the Arum -type. Asphodelus fistulosus , a common weed in southern Australia, forms Paris -type AM when colonised by Glomus coronatum . It is often found in sites with low nutrient levels, and may therefore be dependent on its AM associations for growth and phosphorus (P) nutrition. • A. fistulosus was inoculated with G. coronatum and grown in pots containing a soil/sand mixture with P added to give five soil P concentrations. The plants were grown in a glasshouse and harvested 6 and 9 wk after planting, at which times growth, P nutrition and colonisation were measured. • At low soil P, A. fistulosus showed very marked positive responses to colonisation both in P uptake and growth; both responses decreased with increasing P supply. Colonisation was not greatly reduced by increasing P supply. • This study appears to be one of the first detailed investigations of P responses in a Paris -type AM, providing insight into what is reportedly the more common but less well studied morphological type of AM.

Contrasting responses to mycorrhizal inoculation and phosphorus availability in seedlings of two tropical rainforest tree species.

• This work aimed at understanding the role of mycorrhizal status in phosphorus efficiency of tree seedlings in the tropical rainforest of French Guyana. • Mycorrhizal colonization, growth, phosphorus content, net photosynthesis and root respiration were determined on three occasions during a 9-month growth period for seedlings of two co-occurring species (Dicorynia guianensis and Eperua falcata) grown at three soil phosphorus concentrations, with or without inoculation with arbuscular mycorrhizas. • Seedlings of both species were unable to absorb phosphorus in the absence of mycorrhizal association. Mycorrhizal seedlings exhibited coils that are specific of Paris-type mycorrhizae. Both species benefited from the mycorrhizal symbiosis in terms of phosphorus acquisition but the growth of E. falcata seedlings was unresponsive to this mycorrhizal improvement of phosphorus status, probably because of the combination of high seed mass and P reserves, with low growth rate. • The two species belong to two different functional groups regarding phosphorus acquisition, D. guianensis being an obligate mycotrophic species.

Arbuscular mycorrhizal colonization in Asteraceae from white sand savannas, in Pinar del Rio, Cuba / Colonizacion micorrizica arbuscular en Asteraceae de las sabanas de arenas blancas, de Pinar del Rio, Cuba

Asteraceae family shows a great diversity in Cuba and presents threatened taxons in high number. Pinar del Río has 19 threatened species belonging to this family and three of them are characteristic of a sandy (quartzitic) soil at the Reserva Florística Manejada San Ubaldo-Sabanalamar (Floristic Reserve San Ubaldo-Sabanalamar). The arbuscular mycorrhizal status and colonization rate in Aster grisebachii, Erigeron bellidiastroides, Erigeron cuneifolius, Pectis juniperina and Sachsia polycephala (Asteraceae) from this Reserve was determined in October 2009 and April 2010, during hydrologic rainy and dry period, respectively. All species were mycotrophic in both periods with a high percentage of colonization rates, between 66 and 100% in rainy period and between 84 and 100% in dry period. Vesicles, arbuscules, spores, hyphal and arbuscular coils of arbuscular mycorrhizae were observed. Paris -type and Arum -type morphologies (both) were observed indistinctly in all the species, and Intermediate type was observed only in Aster grisebachii and Erigeron bellidiastroides, which could be a hint that these species are colonized by more than one fungal species, and therefore enhancing the relevance of fungal identity as a considerable character in the morphology association. Also, septate hyphae and microsclerotium of dark septate endophytes were identifyed inside the roots of all the species.

La familia Asteraceae muestra una gran diversidad en Cuba y presenta un elevado número de taxones amenazados. Pinar del Río cuenta con 19 especies amenazadas pertenecientes a esta familia, de las cuales tres son características de los suelos arenosos cuarcíticos de la Reserva Florística Manejada San Ubaldo-Sabanalamar. Se determinó el estatus micorrízico arbuscular y la tasa de colonización de Aster grisebachii, Erigeron bellidiastroides, Erigeron cuneifolius, Pectis juniperina y Sachsia polycephala (Asteraceae) presentes en esta Reserva, en octubre del 2009 y abril del 2010, durante los períodos hidrológicos húmedo y seco, respectivamente. Todas las especies estaban micorrizadas en ambos períodos con elevados porcentaje de colonización micorrízica, entre 66 y 100% en el período lluvioso y entre 84 y 100% en el período seco. Se identificaron vesículas, arbúsculos, esporas, enrollados hifales y arbusculares característicos de las micorrizas arbusculares. Los tipos morfológicos Paris y Arum (ambos) fueron observados indistintamente en todas las especies, y el tipo Intermedio solo se observó en Aster grisebachii y Erigeron bellidiastroides, lo cual sugiere la colonización de estas especies por más de una especie fúngica y con ello la importancia de la identidad fúngica en la determinación de la morfología de la asociación. Se identificaron además, hifas septadas y microesclerosios de endófitos oscuros septados en el interior de las raíces de todas las especies estudiadas.