Ecto- and arbuscular mycorrhizal symbiosis can induce tolerance to toxic pulses of phosphorus in jarrah (Eucalyptus marginata) seedlings

In common with many plants native to low P soils, jarrah (Eucalyptus marginata) develops toxicity symptoms upon exposure to elevated phosphorus (P). Jarrah plants can establish arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) associations, along with a non-colonizing symbiosis described recently. AM colonization is known to influence the pattern of expression of genes required for P uptake of host plants and our aim was to investigate this phenomenon in relation to P sensitivity. Therefore, we examined the effect on hosts of the presence of AM and ECM fungi in combination with toxic pulses of P and assessed possible correlations between the induced tolerance and the shoot P concentration. The P transport dynamics of AM (Rhizophagus irregularis and Scutellospora calospora), ECM (Scleroderma sp.), non-colonizing symbiosis (Austroboletus occidentalis), dual mycorrhizal (R. irregularis and Scleroderma sp.), and non-mycorrhizal (NM) seedlings were monitored following two pulses of P. The ECM and A. occidentalis associations significantly enhanced the shoot P content of jarrah plants growing under P-deficient conditions. In addition, S. calospora, A. occidentalis, and Scleroderma sp. all stimulated plant growth significantly. All inoculated plants had significantly lower phytotoxicity symptoms compared to NM controls 7 days after addition of an elevated P dose (30 mg P kg^?1 soil). Following exposure to toxicity-inducing levels of P, the shoot P concentration was significantly lower in R. irregularis-inoculated and dually inoculated plants compared to NM controls. Although all inoculated plants had reduced toxicity symptoms and there was a positive linear relationship between rank and shoot P concentration, the protective effect was not necessarily explained by the type of fungal association or the extent of mycorrhizal colonization.     

<Emphasis Type="Italic">Pyrola japonica</Emphasis>, a partially mycoheterotrophic Ericaceae,has mycorrhizal preference for russulacean fungi in central Japan

Mycorrhizal symbiosis often displays low specificity, except for mycoheterotrophic plants that obtain carbon from their mycorrhizal fungi and often have higher specificity to certain fungal taxa. Partially mycoheterotrophic (or mixotrophic, MX) plant species tend to have a larger diversity of fungal partners, e.g., in the genus Pyrola (Monotropoideae, Ericaceae). Preliminary evidence however showed that the Japanese Pyrola japonica has preference for russulacean fungi based on direct sequencing of the fungal internal transcribed spacer (ITS) region from a single site. The present study challenges this conclusion using (1) sampling of P. japonica in different Japanese regions and forest types and (2) fungal identification by ITS cloning. Plants were sampled from eight sites in three regions, in one of which the fungal community on tree ectomycorrhizal (ECM) tips surrounding P. japonica was also analyzed. In all, 1512 clone sequences were obtained successfully from 35 P. japonica plants and 137 sequences from ECM communities. These sequences were collectively divided into 74 molecular operational taxonomic units (MOTUs) (51 and 33 MOTUs, respectively). MOTUs from P. japonica involved 36 ECM taxa (96 % of all clones), and 17 of these were Russula spp. (76.2 % of all clones), which colonized 33 of the 35 sampled plants. The MOTU composition significantly differed between P. japonica and ECM tips, although shared species represented 26.3 % of the ECM tips community in abundance. This suggests that P. japonica has a preference for russulacean fungi.     

Ectomycorrhizal Pisolithus albus inoculation of Acacia spirorbis and Eucalyptus globulus grown in ultramafic topsoil enhances plant growth and mineral nutrition while limits metal uptake

Ectomycorrhizal fungi (ECM) isolates of Pisolithus albus (Cooke and Massee) from nickel-rich ultramafic topsoils in New Caledonia were inoculated onto Acacia spirorbis Labill. (an endemic Fabaceae) and Eucalyptus globulus Labill. (used as a Myrtaceae plant host model). The aim of the study was to analyze the growth of symbiotic ECM plants growing on the ultramafic substrate that is characterized by high and toxic metal concentrations i.e. Co, Cr, Fe, Mn and Ni, deficient concentrations of plant essential nutrients such as N, P, K, and that presents an unbalanced Ca/Mg ratio (1/19). ECM inoculation was successful with a plant level of root mycorrhization up to 6.7%. ECM symbiosis enhanced plant growth as indicated by significant increases in shoot and root biomass. Presence of ECM enhanced uptake of major elements that are deficient in ultramafic substrates; in particular P, K and Ca. On the contrary, the ECM symbioses strongly reduced transfer to plants of element in excess in soils; in particular all metals. ECM-inoculated plants released metal complexing molecules as free thiols and oxalic acid mostly at lower concentrations than in controls. Data showed that ECM symbiosis helped plant growth by supplying uptake of deficient elements while acting as a protective barrier to toxic metals, in particular for plants growing on ultramafic substrate with extreme soil conditions. Isolation of indigenous and stress-adapted beneficial ECM fungi could serve as a potential tool for inoculation of ECM endemic plants for the successful restoration of ultramafic ecosystems degraded by mining activities.     

Cryopreservation of ectomycorrhizal fungi has minor effects on root colonization of Pinus sylvestris plantlets and their subsequent nutrient uptake capacity

The use of ectomycorrhizal (ECM) fungi for afforestation, bioremediation, and timber production requires their maintenance over long periods under conditions that preserve their genetic, phenotypic, and physiological stability. Cryopreservation is nowadays considered as the most suitable method to maintain the phenotypic and genetic stability of a large number of filamentous fungi including the ECM fungi. Here, we compared the ability of eight ECM fungal isolates to colonize Pinus sylvestris roots and to transport inorganic phosphate (Pi) and NH₄ ⁺ from the substrate to the plant after cryopreservation for 6 months at ?130 °C or after storage at 4 °C. Overall, the mode of preservation had no significant effect on the colonization rates of P. sylvestris, the concentrations of ergosterol in the roots and substrate, and the uptake of Pi and NH₄ ⁺. Comparing the isolates, differences were sometimes observed with one or the other method of preservation. Suillus bovinus exhibited a reduced ability to form mycorrhizas and to take up Pi following cryopreservation, while one Suillus luteus isolate exhibited a decreased ability to take up NH₄ ⁺. Conversely, Hebeloma crustuliniforme, Laccaria bicolor, Paxillus involutus, and Pisolithus tinctorius exhibited a reduced ability to form mycorrhizas after storage at 4 °C, although this did not result in a reduced uptake of Pi and NH₄ ⁺. Cryopreservation appeared as a reliable method to maintain important phenotypic characteristics (i.e., root colonization and nutrient acquisition) of most of the ECM fungal isolates studied. For 50 % of the ECM fungal isolates, the colonization rate was even higher with the cultures cryopreserved at ?130 °C as compared to those stored at 4 °C.     

Responsiveness of Carob (Ceratonia siliqua L.) Plants to Arbuscular Mycorrhizal Symbiosis Under Different Phosphate Fertilization Levels

This experiment was carried out in pots in a greenhouse to evaluate the effects of arbuscular mycorrhizal fungi (Funneliformis mosseae, Rhizophagus intraradices and Rhizophagus fasciculatus) on carob plant performance under different levels of phosphate fertilization. Non-mycorrhizal (NMyc) and mycorrhizal (Myc) carob plants were subjected to three levels of phosphate fertilization, L1 (0 mg P kg⁻¹ soil), L2 (25 mg P kg⁻¹ soil) and L3 (100 mg P kg⁻¹ soil). Results showed that under L1 and L2 P-fertilization levels, arbuscular mycorrhizal symbiosis significantly improved growth and biomass production of carob plants. Moreover, mineral nutrient (P, K, Na and Ca) acquisition, photosynthetic activity (F_v/F_m), stomatal conductance, total chlorophyll content, and soluble sugar accumulation were also strongly improved in Myc plants in comparison with NMyc ones. Under L1 P-fertilization level, Myc plants showed strongly increased acid phosphatase activity in roots and in the rhizospheric soil than NMyc plants. Furthermore, Myc plants maintained high membrane integrity (over 80%) and low hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents, associated with increased activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (G-POD), and catalase (CAT) compared to NMyc plants. However, high phosphorus input (L3) negatively affected root colonization and mycorrhizal plant performance. Thus, carob plants associated with Funneliformis mosseae performed best under phosphorus deficiency and were the least sensitive to the variations of phosphorus input levels.

     

Resistance Responses of Potato to Vesicular-Arbuscular Mycorrhizal Fungi under Varying Abiotic Phosphorus Levels

In mycorrhizal symbioses, susceptibility of a host plant to infection by fungi is influenced by environmental factors, especially the availability of soil phosphorus. This study describes morphological and biochemical details of interactions between a vesicular-arbuscular mycorrhizal (VAM) fungus and potato (Solanum tuberosum L. cv Russet Burbank) plants, with a particular focus on the physiological basis for P-induced resistance of roots to infection. Root infection by the VAM fungus Glomus fasciculatum ([Thaxt. sensu Gerdemann] Gerdemann and Trappe) was extensive for plants grown with low abiotic P supply, and plant biomass accumulation was enhanced by the symbiosis. The capacity of excised roots from P-deficient plants to produce ethylene in the presence or absence of exogenous 1-amino cyclopropane-1-carboxylic acid (ACC) was markedly reduced by VAM infection. This apparent inhibition of ACC oxidase (ACC_ox) activity was localized to areas containing infected roots, as demonstrated in split-root studies. Furthermore, leachate from VAM roots contained a potent water-soluble inhibitor of ethylene generation from exogenous ACC by nonmycorrhizal (NM) roots. The leachate from VAM-infected roots had a higher concentration of phenolics, relative to that from NM roots. Moreover, the rates of ethylene formation and phenolic concentration in leachates from VAM roots were inversely correlated, suggesting that this inhibitor may be of a phenolic nature. The specific activity of extracellular peroxidase recovered in root leachates was not stimulated by VAM infection, although activity on a fresh weight basis was significantly enhanced, reflecting the fact that VAM roots had higher protein content than NM roots. Polyphenol oxidase activity of roots did not differ between NM and VAM roots. These results characterize the low resistance response of P-deficient plants to VAM infection. When plants were grown with higher abiotic P supply, the relative benefit of the VAM symbiosis to plant growth decreased and root infection was lower. The in vivo ACC_ox activity was also greater in roots of plants grown on high levels of P compared with those grown on low levels, although the influence of VAM infection was partially to counteract the nutritional effect of P on ACC_ox activity. Similar to ACC_ox activity, extracellular peroxidase activity of roots increased linearly with increasing abiotic P supply, thus indicating a greater potential for resistance to VAM infection. These findings suggest that VAM fungi may alter phenolic metabolism of roots so as to hinder ethylene production and the root's ability to invoke a defense response. Raising the abiotic P supply to plants at least partially restores the capacity of roots to produce ethylene and may, in this way, increase the root's resistance to VAM infection.     

Shifts in symbiotic associations in plants capable of forming multiple root symbioses across a long‐term soil chronosequence

Changes in soil nutrient availability during long‐term ecosystem development influence the relative abundances of plant species with different nutrient‐acquisition strategies. These changes in strategies are observed at the community level, but whether they also occur within individual species remains unknown. Plant species forming multiple root symbioses with arbuscular mycorrhizal (AM) fungi, ectomycorrhizal (ECM) fungi, and nitrogen‐(N) fixing microorganisms provide valuable model systems to examine edaphic controls on symbioses related to nutrient acquisition, while simultaneously controlling for plant host identity. We grew two co‐occurring species, Acacia rostellifera (N₂‐fixing and dual AM and ECM symbioses) and Melaleuca systena (AM and ECM dual symbioses), in three soils of contrasting ages (c. 0.1, 1, and 120 ka) collected along a long‐term dune chronosequence in southwestern Australia. The soils differ in the type and strength of nutrient limitation, with primary productivity being limited by N (0.1 ka), co‐limited by N and phosphorus (P) (1 ka), and by P (120 ka). We hypothesized that (i) within‐species root colonization shifts from AM to ECM with increasing soil age, and that (ii) nodulation declines with increasing soil age, reflecting the shift from N to P limitation along the chronosequence. In both species, we observed a shift from AM to ECM root colonization with increasing soil age. In addition, nodulation in A. rostellifera declined with increasing soil age, consistent with a shift from N to P limitation. Shifts from AM to ECM root colonization reflect strengthening P limitation and an increasing proportion of total soil P in organic forms in older soils. This might occur because ECM fungi can access organic P via extracellular phosphatases, while AM fungi do not use organic P. Our results show that plants can shift their resource allocation to different root symbionts depending on nutrient availability during ecosystem development.     

Management of nursery practices for efficient ectomycorrhizal fungi application in the production of Quercus ilex

The application of ectomycorrhizal (ECM) fungi on forest nursery production is regarded as part of good management practice. However, before employing large scale inoculations in a nursery the interaction between ECM symbionts, growth substrate and fertilisation input should be studied to select the most suitable nursery practices for promoting plant growth and ECM colonisation. In this study, seedlings of Quercus ilex were inoculated with Paxillus involutus, Hebeloma mesophaeum or Cenococcum geophilum and grown in three different substrates commonly used in forest nurseries: peat-based compost, forest soil or composted pine bark. The effect of various fertilisation regimes was also studied. The choice of substrate had a significant effect on plant growth and ECM colonisation. The most appropriate combination of substrate and ECM fungus for Q. ilex growth and nutrition was peat and H. mesophaeum. Plants grown on a peat-based compost and inoculated with H. mesophaeum had a significantly greater biomass and leaf phosphorus concentration without fertilisation. Composted pine bark was found not to be suitable for growth or for mycorrhization. If the appropriate growth substrate is selected, it is possible to replace the use of chemical fertilisers by inoculation with selected ECM fungi. This results in a significant increase in plant development, and thus ECM fungi can be recommended as a more environmental friendly biotechnological approach to plant management in the nursery.     

Ectomycorrhizal fungi: the symbiotic route to the root for phosphorus in forest soils

Many forest trees have evolved mutualistic symbioses with ectomycorrhizal (ECM) fungi that contribute to their phosphorus (P) nutrition. Forest productivity is frequently limited by P, a phenomenon that is likely to become more widespread under future conditions of elevated atmospheric CO₂ concentration [CO₂]. It is thus timely that this review considers current understanding of the key processes (absorption, translocation and transfer to the plant host) in ECM fungus-mediated P nutrition of forest trees. Solubilisation of inorganic P (P_i) and hydrolysis of organic P by ECM fungi in soil occurs largely at the growing mycelial front, where P_i absorption is facilitated by high affinity transporters. While large gaps remain in our understanding of the physiological and molecular mechanisms that underpin movement of P in ECM mycelia in soil and P transfer to the plant, host P demand seems likely to be a key driver of these processes. ECM fungi may make considerable contributions to meeting the likely increased P demand of trees under elevated [CO₂] via increased colonization levels, shifts in ECM fungal community structure and changed patterns of EMM production. Further research into the spatial scale of ECM-mediated P movements in soil, along with the interplay between ECM fungi and other soil microflora is advocated.     

Diversity and host preference of fungi co-inhabiting Cenococcum mycorrhizae

Diverse fungal assemblages colonize the fine feeder roots of woody plants, including mycorrhizal fungi, fungal root endophytes and soil saprotrophs. The fungi co-inhabiting Cenococcum geophilum ectomycorrhizae (ECM) of Abies balsamea, Betula papyrifera and Picea glauca were studied at two boreal forest sites in Eastern Canada by direct PCR of ITS rDNA. 50 non-Cenococcum fungal sequence types were detected, including several potentially mycorrhizal species as well as fungal root endophytes. Non-melanized ascomycetes dominated, in contrast to the dark septate endophytes (DSE) reported in most culture dependent studies. The results demonstrate significant differences in root associated fungal assemblages among the host species studied. Fungal diversity was also host dependent, with P. glauca roots supporting a more diverse community than A. balsamea. Differences in root associated fungal communities may well influence ecological interactions among host plant species.     

Determination of Diversity,Distribution and Host Specificity of Korean Laccaria Using Four Approaches

The genus Laccaria (Hydnangiaceae, Agaricales) plays an important role in forest ecosystems as an ectomycorrhizal fungus, contributing to nutrient cycles through symbiosis with many types of trees. Though understanding Laccaria diversity and distribution patterns, as well as its association with host plants, is fundamental to constructing a balanced plant diversity and conducting effective forest management, previous studies have not been effective in accurately investigating, as they relied heavily on specimen collection alone. To investigate the true diversity and distribution pattern of Laccaria species and determine their host types, we used four different approaches: specimen-based analysis, open database search (ODS), NGS analysis, and species-specific PCR (SSP). As a result, 14 Laccaria species have been confirmed in Korea. Results regarding the species distribution pattern were different between specimen-based analysis and SSP. However, when both were integrated, the exact distribution pattern of each Laccaria species was determined. In addition, the SSP revealed that many Laccaria species have a wide range of host types. This study shows that using these four different approaches is useful in determining the diversity, distribution, and host of ECM fungi. Furthermore, results obtained for Laccaria will serve as a baseline to help understand the role of ECM fungi in forest management in response to climate change.     

Changes in the activity of enzymes involved with primary nitrogen metabolism due to ectomycorrhizal symbiosis on jack pine seedlings

Jack pine (Pinus banksiana Lamb.) seedlings were inoculated with either one of the ectomycorrhizal (ECM) fungi, Laccaria bicolor (Maire) Orton or Pisolithus tinctorius (Pers.) Coker and Couch, and grown for 16 weeks in a growth chamber along with non-ECM controls. Five enzymes involved with the assimilation of nitrogen or the synthesis of amino acids were measured in the 3 jack pine root systems as well as in the pure fungal cultures. Pisolithus tinctorius in pure culture had no detectable activity of nitrate reductase (NR. EC 1.6.6.1), glutamate dehydrogenase (GDH. EC 1.4.1.2), glutamate decarboxylase (GDCO. EC 4.1.1.15) or glutamate oxoglutarate aminotransferase (GOGAT, EC 1.4.1.13) but did have some glutamine synthetase (GS, EC 6.3.1.2) activity. Laccaria bicolor in pure culture had no NR activity, small levels of GDCO activity, and high GS, GDH and GOGAT activity. The high levels of enzymatic activity present in L. bicolor indicate that it may play a greater role in the nitrogen metabolism of its host plant than P. tinctorius. ECM infection clearly altered the enzymatic activity in jack pine roots but the nature of these changes depended on the fungal associate. Non-ECM root systems had higher specific activities than ECM root systems for NR, GS, GDH and GDCO but GOGAT activites were the same for both the ECM and non-ECM roots. Root systems infected with L. bicolor had significantly greater NR and GDCO activity than those infected with P. tinctorius. Differences in the GS activity of the two fungi in pure culture corresponded to the GS activity of jack pine roots in symbiotic association with these fungi. While the free amino acid profiles in roots were significantly affected by ECM infection, the profile of free amino acids exported to the stem was the same for all treatments. High asparagine and low glutamine in roots infected with P. tinctorius indicates that asparagine synthetase (EC x.x.x.x) activity should be higher within this symbiotic association than in the L. bicolor association or in the non-mycorrhizal roots.     

The expression of GintPT, the phosphate transporter of Rhizophagus irregularis, depends on the symbiotic status and phosphate availability

The development of mutualistic interactions with arbuscular mycorrhizal (AM) fungi is one of the most important adaptation of terrestrial plants to face mineral nutrition requirements. As an essential plant nutrient, phosphorus uptake is acknowledged as a major benefit of the AM symbiosis, but the molecular mechanisms of its transport as inorganic phosphate (Pi) from the soil to root cells via AM fungi remain poorly known. Here we monitored the expression profile of the high-affinity phosphate transporter (PT) gene (GintPT) of Rhizophagus irregularis (DAOM 197198) in fungal structures (spores, extraradical mycelium and arbuscules), under different Pi availability, and in respect to plant connection. GintPT resulted constitutively expressed along the major steps of the fungal life cycle and the connection with the host plant was crucial to warrant GintPT high expression levels in the extraradical mycelium. The influence of Pi availability on gene expression of the fungal GintPT and the Medicago truncatula symbiosis-specific Pi transporter (MtPT4) was examined by qRT-PCR assay on microdissected arbusculated cells. The expression profiles of both genes revealed that these transporters are sensitive to changing Pi conditions: we observed that MtPT4 mRNA abundance is higher at 320 than at 32 μM suggesting that the flow towards the plant requires high concentrations. Taken on the whole, the findings highlight novel traits for the functioning of the GintPT gene and offer a molecular scenario to the models describing nutrient transfers as a cooperation between the mycorrhizal partners.     

Arbuscular mycorrhizal fungi alter phosphorus relations of broomsedge (Andropogon virginicus L.) plants

Broomsedge (Andropogon virginicus L.) is a dominant grass revegetating many abandoned coal-mined lands in West Virginia, USA. Residual soils on such sites are often characterized by low pH, low nutrients, and high aluminium. Experiments were conducted to assess the resistance of broomsedge to limited phosphorus (Pi) availability and to investigate the role that arbuscular mycorrhizal (AM) fungi play in aiding plant growth under low Pi conditions. Pregerminated mycorrhizal and non-mycorrhizal seedlings were grown in a sand-culture system with nutrient solutions containing Pi concentrations ranging from 10 to 100 microM for 8 weeks. Non-mycorrhizal plants exhibited severe inhibition of growth under Pi limitation (<60 microM). Colonization by AM fungi (combined Glomus clarum Nicolson & Schenck and Gigaspora gigantea (Nicol. & Gerd.) Gerd. & Trappe) greatly enhanced host plant growth at low Pi concentrations, but did not benefit growth when Pi was readily available (100 microM). In comparison to non-mycorrhizal plants, mycorrhizal plants had higher phosphorus use efficiency at low Pi concentrations and maintained nearly constant tissue nutrient concentrations across the gradient of Pi concentrations investigated. Manganese (Mn) and sodium (Na) accumulated in shoots of non-mycorrhizal plants under Pi limitation. Mycorrhizal plants exhibited lower instantaneous Pi uptake rates and significantly lower C(min) values compared to non-mycorrhizal plants. These patterns suggest that the symbiotic association between broomsedge roots and AM fungi effectively maintains nutrient homeostasis through changes in physiological properties, including nutrient uptake, allocation and use. The mycorrhizal association is thus a major adaptation that allows broomsedge to become established on infertile mined lands.     

Wheat Responses to Arbuscular Mycorrhizal Fungi in a Highly Calcareous Soil Differ from those of Clover, and Change with Plant Development and P supply

We evaluated the roles of arbuscular mycorrhizal (AM) fungi in growth and phosphorus (P) nutrition of wheat (Triticum aestivum L.) in a highly calcareous soil and compared the responses of wheat with those of clover (Trifolium subterraneum L). In the first experiment wheat (cv. Brookton) was harvested at 6 wk. Colonisation by four AM fungi was low (<20%). Clover was harvested at 8 wk. Colonisation varied with different fungi, with the highest value (52%) obtained with Glomus intraradices. Although suffering from P deficiency, non-mycorrhizal (NM) wheat grew relatively well with no added P (P0) and application of P at 100 mg kg⁻¹ (P100) increased the dry weight (DW). Shoot P concentrations increased with P application and there were positive effects of all AM fungi at P100. In contrast, NM clover grew very poorly at P0 and did not respond to P application. Clover responded positively to all AM fungi at both P levels, associated with increases in P uptake. In the second experiment colonisation by a single AM fungus (G. intraradices) of two wheat cultivars (Brookton and Krichauff) was well established at 6 wk (~50% in P0 plants) and continued to increase up to maturity (~70%), but decreased greatly at both harvests as P supply was increased (up to 150 mg P kg⁻¹: P150). Addition of P significantly increased plant growth, grain yield and P uptake irrespective of cultivar and harvest time, and the optimum soil P for grain yield was P100. In both cultivars, a growth depression in AM plants occurred at 6 wk at all P levels, but disappeared at 19 wk with added P. At P0, AM plants also produced lower grain yield (weight) per plant, but with higher P, AM plants produced higher grain yields than NM plants. There was a significant positive effect of AM on grain P concentration at P0, but not at other P levels. Brookton was somewhat more P efficient than Krichauff, and the latter responded more to AM fungi. This study showed that responses of wheat to AM inoculation and P supply were quite different from those of clover, and changed during development. Results are discussed in relation to the underlying soil properties.     

An ectomycorrhizal fungus alters sensitivity to jasmonate,salicylate, gibberellin,and ethylene in host roots

The phytohormones jasmonate, gibberellin, salicylate, and ethylene regulate an interconnected reprogramming network integrating root development with plant responses against microbes. The establishment of mutualistic ectomycorrhizal symbiosis requires the suppression of plant defense responses against fungi as well as the modification of root architecture and cortical cell wall properties. Here, we investigated the contribution of phytohormones and their crosstalk to the ontogenesis of ectomycorrhizae (ECM) between grey poplar (Populus tremula x alba) roots and the fungus Laccaria bicolor. To obtain the hormonal blueprint of developing ECM, we quantified the concentrations of jasmonates, gibberellins, and salicylate via liquid chromatography–tandem mass spectrometry. Subsequently, we assessed root architecture, mycorrhizal morphology, and gene expression levels (RNA sequencing) in phytohormone-treated poplar lateral roots in the presence or absence of L. bicolor. Salicylic acid accumulated in mid-stage ECM. Exogenous phytohormone treatment affected the fungal colonization rate and/or frequency of Hartig net formation. Colonized lateral roots displayed diminished responsiveness to jasmonate but regulated some genes, implicated in defense and cell wall remodelling, that were specifically differentially expressed after jasmonate treatment. Responses to salicylate, gibberellin, and ethylene were enhanced in ECM. The dynamics of phytohormone accumulation and response suggest that jasmonate, gibberellin, salicylate, and ethylene signalling play multifaceted roles in poplar L. bicolor ectomycorrhizal development.     

Nutrient uptake in mycorrhizal symbiosis

The role of mycorrhizal fungi in acquisition of mineral nutrients by host plants is examined for three groups of mycorrhizas. These are; the ectomycorrhizas (ECM), the ericoid mycorrhizas (EM), and the vesicular-arbuscular mycorrhizas (VAM). Mycorrhizal infection may affect the mineral nutrition of the host plant directly by enhancing plant growth through nutrient acquisition by the fungus, or indirectly by modifying transpiration rates and the composition of rhizosphere microflora. A capacity for the external hyphae to take up and deliver nutrients to the plant has been demonstrated for the following nutrients and mycorrhizas; P (VAM, EM, ECM), NH₄ ⁺ (VAM, EM, ECM), NO₃ ^- (ECM), K (VAM, ECM), Ca (VAM, EM), SO₄ ^2- (VAM), Cu (VAM), Zn (VAM) and Fe (EM). In experimental chambers, the external hyphae of VAM can deliver up to 80% of plant P, 25% of plant N, 10% of plant K, 25% of plant Zn and 60% of plant Cu. Knowledge of the role of mycorrhiza in the uptake of nutrients other than P and N is limited because definitive studies are few, especially for the ECM. Although further quantification is required, it is feasible that the external hyphae may provide a significant delivery system for N, K, Cu and Zn in addition to P in many soils. Proposals that ECM and VAM fungi contribute substantially to the Mg, B and Fe nutrition of the host plant have not been substantiated. ECM and EM fungi produce ectoenzymes which provide host plants with the potential to access organic N and P forms that are normally unavailable to VAM fungi or to non mycorrhizal roots. The relative contribution of these nutrient sources requires quantification in the field. Further basic research, including the quantification of nutrient uptake and transport by fungal hyphae in soil and regulation at the fungal-plant interface, is essential to support the selection and utilization of mycorrhizal fungi on a commercial scale.