Arbuscular mycorrhizal (AM) symbioses are known to play a role in increased resistance of plants against soilborne pathogens. Mechanisms involved in this phenomenon are not yet well understood. This work investigates possible roles of endoproteolytic activities in bioprotection of Pisum sativum roots by Glomus mosseae against Aphanomyces euteiches. First, it is demonstrated that bioprotection occurs only in pre-mycorrhizal plants. Second, endoproteolytic activities were analysed qualitatively and quantitatively during AM symbiosis, in plants infected with either zoospores or mycelium of A. euteiches, and in mycorrhizal plants infected with the pathogen. In mycorrhizal symbiosis a progressive increase in endoproteolytic activities was observed following root colonization by G. mosseae. By contrast, in roots inoculated with A. euteiches, a drastic increase in endoproteolytic activities was observed which was correlated with the amount of pathogen occurring in roots. Qualitative differences were seen among the endoproteolytic activities detected in roots inoculated with zoospores or mycelium. The constitutive as well as mycorrhizal and pathogen-induced activities were further characterized as ‘trypsin-like’ serine endoproteases. Interestingly, in a situation of bioprotection, only low levels of the activities normally associated with the infection by A. euteiches were detected, suggesting that the synthesis of these proteins is directly linked to the growth or virulence of the pathogen.

Although mat-forming cyanobacteria dominate many freshwater ecosystems in the Arctic and Antarctic, their optimal temperature for growth (Topt) is usually much higher than the temperature range of their native habitat. The present study compared the temperature dependence of growth, pigment composition and absorbance, photosynthesis and photosynthate partitioning for two strains of cyanobacteria with contrasting Topt values; Phormidium subfuscum, isolated from McMurdo Ice Shelf, Antarctica, and Phormidium tenue, collected from the Kuparuk River in the tundra region of northern Alaska. Phormidium subfuscum grew between 5 and 20°C with a Topt of 15°C whereas P. tenue showed detectable growth from 10 to 40°C and a Topt of 30°C. Light utilization efficiency, photosynthetic capacity and the irradiance at the onset of light saturation increased with increasing temperature up to Topt in both strains. The cellular concentrations of chlorophyll a (Chl a) and carotenoid (CAR) and the in vivo absorbance maxima for Chl a, CAR, C-phycocyanin and allophycocyanin changed little for P. subfuscum but all these variables increased across the temperature range up to Topt for P. tenue. Neither P. subfuscum nor P. tenue showed changes in relative carbon allocation with varying temperature, suggesting that gross biochemical alterations are not a characteristic of temperature acclimation in these cyanobacteria. We conclude that the eurythermal cyanobacterium P. tenue optimizes growth over a wide range of temperatures by adjusting its light-capturing as well as carbon fixation characteristics, whereas stenothermal P. subfuscum relies on changes in carbon fixation without concomitant shifts in pigment content.

A glasshouse experiment was done to assess the development and phosphate metabolism of mycorrhizas formed by species of arbuscular mycorrhizal fungi (AMF) from two different genera, Gigaspora and Glomus on Desmodium ovalifolium plants at three concentrations of a phosphate source. The addition of phosphate (0–100 mg P kg−1) had no effect on the alkaline phosphatase activity, stained histochemically, in the intra-radical mycelium of Gigaspora rosea (BEG111), but decreased that of Glomus manihotis (BEG112) over a 10-wk period. The alkaline phosphatase activity of the extra-radical mycelium was unaffected by increasing phosphate addition (0–100 mg P kg−1) in both species of AMF over a 10-wk period. The extra-radical mycelium of Gi. rosea (BEG111) accumulated polyphosphate, determined by staining with 4′,6-diamidino-2-phenylindole, whereas polyphosphate was not detected in the extra-radical mycelium of G. manihotis (BEG112). This work indicates differences in the mechanisms of phosphate metabolism in the mycelium of AMF from different genera on a tropical host. This might be determined by the life-cycle strategies of these fungi, in particular the formation of auxiliary cells in Gigaspora. The possibility of a negative-feedback mechanism between alkaline phosphatase and polyphosphate in the extra-radical mycelium of Gi. rosea (BEG111) and the role of polyphosphate in the symbiosis are discussed.

Mycelial cord systems, up to 50 cm in diameter, of the basidiomycete Phanerochaete velutina, a common woodland saprotroph, were grown on unsterile soil in model laboratory microcosms from either 4 or 16 cm3 wood inocula. After 48 d, systems were supplied at 10-d intervals with pairs of new 4 cm3 beechwood resources placed behind the foraging colony margin, where possible on opposite sides of the system. Image analysis was used to quantify radial extension, hyphal cover, the mass (DBM) and surface fractal (DBS) dimensions of the mycelial systems and wood-resource bleaching activity. Mycelial systems developing from small inocula had significantly (P[les ]0.05) lower radial extension rates, hyphal cover and DBM[ratio ]DBS ratio than those from large inocula. Initially, systems developing from small inocula also displayed significantly (P<0.05) slower wood-resource bleaching activity than those from large inocula, suggesting that carbon limitation affected both foraging behaviour and resource utilization. A separate central compartment containing the inoculum was supplied with 32P orthophosphate and its partitioning amongst wood resources was monitored nondestructively for 44 d. Total 32P acquisition by wood resources was not significantly (P>0.05) affected by inoculum resource size. However, the proportion of total acquired P allocated to resources, which varied according to the length of time that resources had been in contact with the mycelium, was dependent upon inoculum size. The results support the hypothesis that phosphorus translocation is not a demand-driven process. We suggest that the time taken before the greatest rate of phosphorus withdrawal from a uniform translocation stream is dependent upon prior availability of carbon within colonized resources.

The extent of arbuscular mycorrhizal colonization was assessed in 10 field-collected plant species, representing three annual forbs, three perennial forbs, three perennial grasses and one annual grass. Each root system of each plant was split into four portions, and for each portion, mycorrhizal structures were revealed with epifluorescence microscopy (under which only arbuscules are generally visible) and three commonly used stains (Chlorazol Black E, Acid Fuchsin and Trypan Blue). The aim of the study was not to evaluate the efficacy of each method, but to compare results obtained by each under standard laboratory conditions. The recorded colonization levels of arbuscules, total arbuscular mycorrhizal fungal material and total fungal (arbuscular mycorrhizal+non-arbuscular mycorrhizal) material differed significantly between visualization methods in a number of species. However, there were also interactions between stain and plant species, indicating that the performance of a stain is dependent on the plant species being examined. In some cases (e.g. Plantago lanceolata), each visualization method produced the same colonization level, while in others (e.g. Dactylis glomerata), each method gave a different result. These data therefore suggest that the level of mycorrhizal colonization recorded in any particular plant species at a particular time is dependent on the technique employed.

Interactive effects of elevated atmospheric CO2 and arbuscular mycorrhizal (AM) fungi on biomass production and N2 fixation were investigated using black locust (Robinia pseudoacacia). Seedlings were grown in growth chambers maintained at either 350 μmol mol−1 or 710 μmol mol−1 CO2. Seedlings were inoculated with Rhizobium spp. and were grown with or without AM fungi. The 15N isotope dilution method was used to determine N source partitioning between N2 fixation and inorganic fertilizer uptake. Elevated atmospheric CO2 significantly increased the percentage of fine roots that were colonized by AM fungi. Mycorrhizal seedlings grown under elevated CO2 had the greatest overall plant biomass production, nodulation, N and P content, and root N absorption. Additionally, elevated CO2 levels enhanced nodule and root mass production, as well as N2 fixation rates, of non- mycorrhizal seedlings. However, the relative response of biomass production to CO2 enrichment was greater in non-mycorrhizal seedlings than in mycorrhizal seedlings. This study provides strong evidence that arbuscular mycorrhizal fungi play an important role in the extent to which plant nutrition of symbiotic N2-fixing tree species is affected by enriched atmospheric CO2.

Fungal endophytes associated with the palm, Trachycarpus fortunei, within and outside its natural geographic range were investigated. Endophytes were relatively common with colonization rates of 23–57% at the four sites sampled. The endophyte assemblages at the different sites were diverse with 75 fertile species of ascomycetes and mitosporic fungi. The assemblage composition at each site was similar and between seven and 13 species comprised 81–89% of the taxa present in relative frequencies of >1%. Glomerella cingulata and Phomopsis spp., were consistently dominant, and a large number of rare species were recorded. The diversity at each site was similar in number, but the abundance of isolates varied. The results obtained were comparable to those of previous studies of palm endophyte assemblages, but the assemblages showed more affinity with unrelated temperate hosts than with tropical palm hosts. Quantitative and qualitative differences in endophyte assemblages from old and young tissues were observed, and more isolates were recovered from old tissues independent of the age of the palm. The composition of the assemblage varied with several taxa being exclusively or more commonly isolated from old tissues (e.g. xylariaceous taxa, Oxydothis sp. nov.) or young tissues (e.g. Stagonospora spp., Phoma multirostrata). Some differences in the composition of the assemblage and in relative frequencies of various species were observed in trees and saplings. Significantly more isolates were recovered from the vein than intervein tissues, independent of leaf age or tree age. Tissue specificity was not exhibited by any taxa isolated from either leaf or petiole tissues, except for xylariaceous taxa in leaf tissues. Some other taxa showed a preference for leaf tissues or petioles, whereas others were equally distributed amongst all tissues. Endophyte assemblages of palms from continuous distributions were similar, but those from disjunct distributions (i.e. outside the natural geographic range of the palm, such as Australia and Switzerland), differed significantly from each other and from assemblages within continuous distributions. The relative importance values of endophyte species at the two sites in China indicates the site-specific nature of the endophyte assemblages. Most previous studies on the endophytes of palm hosts have concentrated on tropical palms. However, this study examines the endophytes of the warm temperate palm Trachycarpus fortunei, and investigates the relative importance of host and climate related processes.

A factorial analysis was conducted to investigate the effects of different levels of photosynthetic photon flux (PPF) and CO2 concentration on the interactions between the vesicular–arbuscular endomycorrhizal fungus Glomus intraradices and potato plantlets (Solanum tuberosum) cultured in an in vitro tripartite system. We observed that CO2 enrichment from 350 to 10000 ppm stimulated root colonization by the fungus, and that this stimulation was more pronounced under high PPF (300 μmol m−2 s−1) than low PPF (60 μmol m−2 s−1). Consistent with these observations, the effects of G. intraradices on dry matter production in potato plantlets were strongly dependent on the CO2 and PPF levels during cultivation. There was no significant effect of the mycorrhizal fungus on dry matter production at 350 ppm of CO2. However, under the high CO2 concentration, mycorrhiza had opposite effects on dry matter production depending on the PPF: a decrease (−21%) and a stimulation (+25%) of dry matter production after 2 wk of growth under low and high PPF, respectively, were observed in presence of G. intraradices relative to plantlets grown in its absence. Furthermore, in mycorrhizal plantlets grown under high levels of both PPF and CO2, the chlorophyll and carotenoid contents as well as the quantum yields of photosynthetic electron transport and the photochemical quenching qP of the chlorophyll-a fluorescence measured near the PPF during growth were all higher than in non-infected plantlets. Our results therefore indicate that mycorrhizal G. intraradices can alleviate the down regulation of photosynthesis related to sink limitation, and its effect on dry matter production is strongly dependent on the levels of CO2 and PPF during growth which determine the balance between the photosynthetic carbon uptake by the plantlets and the carbon cost by the fungus.

The effect of arbuscular mycorrhizas on fructan accumulation was studied in barley (Hordeum vulgare) infected with Glomus mosseae. Treatments with and without fertilizer were included in order to distinguish between mere fertilizer effects and the effects of the symbiosis, and plants were harvested at two different time points, 35 and 50 d after planting. Fructan was the major storage carboyhdrate in both leaves and roots. The amounts of fructan were markedly altered in the mycorrhizal plants. In roots of non-fertilized mycorrhizal plants, fructan pools were significantly greater than in the corresponding non-mycorrhizal plants. By contrast, fertilization caused a general decrease in amounts of fructan in roots. The increase of fructan in mycorrhizal roots was correlated with a decrease of invertase activity. In leaves, fructan pools decreased or remained unchanged upon mycorrhizal infection; fertilization had a similar effect. However, when individual leaves of a plant were compared, intriguing effects of the mycorrhizal symbiosis could be observed. Whereas in non-mycorrhizal plants, the youngest leaves had the highest fructan contents and the oldest leaves the lowest (as previously reported), this gradient was markedly altered in mycorrhizal plants, indicating systemic effects of mycorrhiza on assimilate partitioning in shoots.

Genomic DNA from spores of the endomycorrhizal fungus Gigaspora margarita was amplified with two sets of degenerate primers designed on conserved regions of chitin synthase polypeptides. Three chitin synthase (chs) gene portions were identified: Gimchs1, Gimchs2 and Gimchs3, encoding one class II and two class IV chitin synthases, respectively. Gimchs1 and Gimchs3 transcripts were detected by reverse transcription polymerase chain reaction (RT-PCR) during root colonization, whereas none of three mRNAs was revealed during spore germination. The potential role of the genes expressed is discussed in the light of morphogenetic and signalling events.

The effects of Cd and Zn on cross-colonization by Paxillus involutus of Scots pine seedlings was examined by using pairs of ectomycorrhizal (ECM) and non-mycorrhizal (NM) seedlings grown in the same vessel. This was done to assess, first, the ability of P. involutus to colonize NM Scots pine seedlings by growth from colonized roots of other Scots pine seedlings in the presence of Cd or Zn, and, second whether ECM colonization of Scots pine by P. involutus provided a competitive advantage over NM seedlings. Ectomycorrhizal colonization of Scots pine was shown to be more sensitive than Scots pine itself to Cd and Zn, but prior colonization did provide a competitive advantage with respect to biomass production. This beneficial effect over NM seedlings was, however, equal in the control, Cd and Zn treatments, and was due simply to growth stimulation in the presence of ECM colonization. Cross-colonization from an ECM to a NM seedling was reduced but not prevented by Cd and Zn. Cd had a more negative effect on cross-colonization than on initial colonization of seedlings, whereas Zn had an equally inhibitory effect on both parameters. These results have important implications for plant establishment on metal-contaminated sites. If cross-colonization between plants is reduced by toxic metals, plant establishment on contaminated sites might be retarded.

The size and distribution of Cortinarius rotundisporus genets at three sclerophyll forest field sites in New South Wales, Australia, were estimated by using microsatellite-primed PCR (MS-PCR) of DNA extracted from sporocarp tissue. MS-PCR fingerprints generated with the primers (GTG)5 and (GACA)4 indicated that two to five genets were present at each site, with each site being characterized by a single large genet (9–30 m in diameter). Analysis of internal transcribed spacer (ITS)-RFLP patterns from individual sporocarps used in the study suggested that three distinct RFLP types were present in the sampled C. rotundisporus population. ITS sequence data indicate that the three RFLP types had less than 88.4% sequence identity to each other, strongly suggesting that C. rotundisporus is a complex of three species.

The morphology and anatomy of ectomycorrhizas of Rhizopogon arctostaphyli, R. ellenae, R. flavofibrillosus, R. occidentalis, R. rubescens, R. smithii, R. subcaerulescens and R. truncatus synthesized on Ponderosa pine (Pinus ponderosa) in glasshouse conditions using spore slurries, are described and compared. All species produced a well-developed Hartig net, and a well-developed fungal mantle. The mantles of R. arctostaphyli, R. smithii and R. subcaerulescens ectomycorrhizas were two-layered with outer mantle hyphae of wider diameter than inner mantle hyphae. The mantle of R. subcaerulescens ectomycorrhizas also had distinctive peg-like structures (cystidia) along peripheral hyphae. Rhizopogon truncatus ectomycorrhizas were tuberculate in morphology and had a rind-like mantle enclosing adjacent roots. In addition, several species exhibited crystal inclusions in the outer mantle, presumably at the interface between mantle and soil.

Seedlings and old trees of Scots pine in self-regenerating, old, virgin boreal forest in northern Sweden were compared in terms of the species composition of their associated ectomycorrhizal (EM) fungi. The natural regeneration of seedlings was mimicked by annual seeding for 10 yr (1986–1995) in intact field vegetation at three sites. In 1995, all seedlings were harvested, and 171 were examined for mycorrhizas. Twenty-five soil cores were taken in order to study the mycorrhizas on the established trees. Most short roots were mycorrhizal. Using mycorrhizal morphology and restriction fragment length polymorphisms (RFLP) analysis, we were able to distinguish 43 ectomycorrhizal taxa. Fourteen of these taxa were identified using a sporocarp and mycelial culture based internal transcribed spacer (ITS)-RFLP reference database, and another three were identified by mycorrhizal morphology. Cenococcum geophilum, Piloderma croceum and Suillus variegatus were present on the seedlings, irrespective of age, as well as on the old trees. At one forest, 23 ectomycorrhizal taxa were found as mycorrhizas, of which 10 occurred on both seedlings and old trees. These 10 species accounted for 92 and 73% of the mycorrhizas on the seedlings and old trees, respectively. A 3-yr survey of ectomycorrhizal sporocarps at the same site revealed 62 EM taxa. The degree of similarity between the composition of EM species reflected by sporocarps and by mycorrhizas, was low. Ectomycorrhizal species present in our ITS-RFLP database constituted 7.5–19% of the mycorrhizas on seedlings and old trees, whereas they constituted 80–95% of the total production of epigeous sporocarps. The unidentified taxa comprised 70–87% of the mycorrhizas. Our findings support the view that the species composition of mycorrhizas colonizing naturally regenerated seedlings in forests is similar to that of mycorrhizas colonizing surrounding trees. We suggest that the concept of the mycelial network be expanded in order to embrace both the significance of interconnections between different trees as well as the continuity or perpetuation of EM fungal communities.

An experiment was performed to find out whether ectomycorrhizal (ECM) fungi alter the nitrogen (N) isotope composition, δ15N, of N during the transport of N from the soil through the fungus into the plant. Non- mycorrhizal seedlings of Pinus sylvestris were compared with seedlings inoculated with either of three ECM fungi, Paxillus involutus, Suillus bovinus and S. variegatus. Plants were raised in sand in pots supplied with a nutrient solution with N given as either NH4+ or NO3−. Fractionation against 15N was observed with both N sources; it decreased with increasing plant N uptake, and was larger when NH4+ was the source. At high ratios of Nuptake/Nsupplied there was no (NO3−), or little (NH4+), fractionation. There seemed to be no difference in fractionation between ECM and non-mycorrhizal plants, but fungal rhizomorphs were sometimes enriched in 15N (up to 5‰ at most) relative to plant material; they were also enriched relative to the N source. However, this enrichment of the fungal material was calculated to cause only a marginal decrease (−0.1‰ in P. involutus) in δ15N of the N passing from the substrate through the fungus to the host, which is explained by the small size of the fungal N pool relative to the total N of the plant, i.e. the high efficiency of transfer. We conclude that the relatively high 15N abundance observed in ECM fungal species should be a function of fungal physiology in the ECM symbiosis, rather than a reflection of the isotopic signature of the N source(s) used. This experiment also shows that the δ15N of plant N is a good approximation of δ15N of the available N source(s), provided that N is limiting growth.

In the December 1998 issue of New Phytologist, we published the research paper entitled ‘Natural occurrence of Ampelomyces intracellular mycoparasites in mycelia of powdery mildew fungi’ by Levente Kiss (New Phytol. (1998) 140, 709–714). Since its publication, an error in the published title has been brought to our attention: this refers to the ampelomyces, but the name should be Ampelomyces, meaning the genus.

We apologise to the author and to our readers for this mistake.

Previous work in the Colorado alpine ecosystem has shown that amino acids are a potentially important N source for the sedge, Kobresia myosuroides. This plant is the only known sedge to harbour associations with ectomycorrhizal fungi. The aim of the present work was to test the hypothesis that these ectomycorrhizas transfer N from amino acids in the soil solution to the host plant, and thereby have an important role in the N nutrition of this species. We used a two-chamber system (rhizoboxes) in which K. myosuroides plants were separated from a soil chamber by nylon mesh that allowed fungal hyphae, but not plant roots, to cross it. Injections of [15N, 2-13C]glycine were made into the soil chamber. The hyphal crossings on half of the rhizoboxes were regularly disrupted to control for leakage of label across the barrier. Plants in the intact rhizoboxes showed significantly higher 15N enrichment than those in controls, and mycorrhizal root tips were significantly more enriched than bulk roots. The mycorrhizas transferred an average of 1.3% of the added 15N label to plants, a figure comparable to those obtained in previous studies in which plant roots were directly exposed to label. We conclude that fungal associations have an important role in the N nutrition of K. myosuroides by transferring N from amino acids to their hosts.

The use of biofuels has been proposed as one possible substitute for fossil fuels, which contribute substantially to the increase in [CO2] in the atmosphere. However, increased harvesting of forest residues for biofuel might affect the availability of base cations, P and N, as well as the development, community dynamics and function of ectomycorrhizas. This in turn might influence nutrient uptake and tree growth. In this study we investigated the effects of repeated forest residue harvesting on ectomycorrhizal species colonizing spruce roots in the humus layer of a 35-yr-old forest. Harvesting significantly decreased the thickness of the humus layer as well as decreasing the numbers of ectomycorrhizal root tips both per metre root length and per unit humus volume. Changes in mycorrhizal community structure were studied by ITS typing with the use of PCR–RFLP analysis. In total, 19 different ITS types were found on two different sampling occasions (autumn and spring); 11 of these were common to both samplings. Nine of the ITS types were identified to at least the genus level by comparison with RFLP patterns of identified fruiting bodies or axenic cultures. Five species, Cortinarius sp. 2, Thelephora terrestris (Ehrenb.) Fr., Lactarius theiogalus (Bull.[ratio ]Fr.) S. F. Gray s.st. Neuhoff, Tylospora fibrillosa Donk and Tö-96-12, occurred on over 5% of the total sampled root tips. Together these five types colonized 63% of the mycorrhizas screened. A similarity index assessment showed no shift in mycorrhizal community structure as a result of harvesting. Our findings suggest that the repeated removal of forest residues might have a strong effect on the quantity and development of ectomycorrhizal roots in the organic horizon, but little effect on the species composition of the community.

In the January 1999 issue of New Phytologist, we published the research paper entitled ‘The effect of light on the growth and reproduction of Floerkea proserpinacoides’ by Margaret F. McKenna and Gilles Houle (New Phytol. (1999) 141, 99–108). Since its publication, the authors have identified two important errors in the text. First, values given for leaf area ratio (LAR) and specific leaf area (SLA) have been given erroneously in units of cm2 g−1; the correct units are cm2 mg−1. Second, the values for unit leaf rate (ULR) have been given incorrectly as tenfold lower than they should be; these should be multiplied by ten to give the correct values in mg m−2 d−1.

We apologise to our readers for these mistakes.

In the November 1998 issue of New Phytologist, we published the Tansley review ‘Gibberellins: regulating genes and germination’ by Sian Ritchie and Simon Gilroy (New Phytol. (1998) 140, 363–383). Since its publication, it has come to our attention that text associated with Fig. 4 was omitted during production. The correct figure is reprinted here in full.

We apologise to the author and to our readers for this mistake.

Figure 4. Promoter sequences of various genes expressed in the cereal aleurone and shown to be regulated by GA. The position of each sequence is indicated relative to the start codon. Regions identified as being involved in regulation of the genes are highlighted, as are similar regions in other genes. Sites at which protein has been shown to bind are also indicated. (a) Barley Amy 32b (Sutcliff et al., 1993; Whittier et al., 1987); wheat Amy 2/54 (Huttley et al., 1992; Rushton et al., 1992; Rushton et al., 1995); barley Amy 46 (Khursheed & Rogers, 1988); barley Amy 2/p155 (Knox et al., 1987); barley aleurain (Whittier et al., 1987); barley β-glucanase II (Wolf, 1992); wheat cathepsin B-like (Cejudo et al., 1992); rice ubiquitin-conjugating enzyme (Chen et al., 1995). (b). Wheat Amy 1/18 (Rushton et al., 1992); barley Amy pHV 19 (Jacobsen & Close, 1991; Gubler & Jacobsen, 1992)/ Amy 1/6-4 (Khursheed & Rogers, 1988; Rogers, Lanahan & Rogers 1994); rice OSamy-a/Amy 3c (Ou-Lee et al., 1988; Sutcliff et al., 1991; Yu et al., 1992; Goldman et al., 1994); rice Amy 3B (Sutcliffe et al., 1991); rice OSamy-c (Kim et al., 1992; Kim & Wu, 1992; Tanida et al., 1994); rice Amy 1A (Huang et al., 1990; Itoh et al., 1995).

Figure 4 (b). For legend see facing page.