Mycorrhiza

Although the absence of normal leaves is often considered a sign of full heterotrophy, some plants remain at least partially autotrophic despite their leafless habit. Leafless orchids with green stems and capsules probably represent a late evolutionary stage toward full mycoheterotrophy and serve as valuable models for understanding the pathways leading to this nutritional strategy. In this study, based on molecular barcoding and isotopic analysis, we explored the physiological ecology of the leafless orchid Eulophia zollingeri, which displays green coloration, particularly during its fruiting phase. Although previous studies had shown that E. zollingeri, in its adult stage, is associated with Psathyrellaceae fungi and exhibits high 13C isotope signatures similar to fully mycoheterotrophic orchids, it remained uncertain whether this symbiotic relationship is consistent throughout the orchid’s entire life cycle and whether the orchid relies exclusively on mycoheterotrophy for its nutrition during the fruiting season. Our study has demonstrated that E. zollingeri maintains a specialized symbiotic relationship with Psathyrellaceae fungi throughout all life stages. However, isotopic analysis and chlorophyll data have shown that the orchid also engages in photosynthesis to meet its carbon needs, particularly during the fruiting stage. This research constitutes the first discovery of partial mycoheterotrophy in leafless orchids associated with saprotrophic non-rhizoctonia fungi.

The role of mycorrhizal infection in the resistance of Vaccinium macrocarpon to manganese was investigated in perlite culture containing nutrient solution amended with Mn at 0, 250, 500 or 1000 μg/ml. Shoot and root dry weights of the mycorrhizal plants were higher than nonmycorrhizal plants. The mycorrhizal plants produced significantly longer main roots than the nonmycorrhizal plants. Differences between shoot and root Mn concentrations of mycorrhizal and nonmycorrhizal plants arose by reduction of Mn in the leaves of mycorrhizal plants and a corresponding increase in root tissues.

Arbuscular mycorrhizal (AM) fungi and rhizobium are likely important drivers of plant coexistence and grassland productivity due to complementary roles in supplying limiting nutrients. However, the interactive effects of mycorrhizal and rhizobial associations on plant community productivity and competitive dynamics remain unclear. To address this, we conducted a greenhouse experiment to determine the influences of these key microbial functional groups on communities comprising three plant species by comparing plant communities grown with or without each symbiont. We also utilized N-fertilization and clipping treatments to explore potential shifts in mycorrhizal and rhizobial benefits across abiotic and biotic conditions. Our research suggests AM fungi and rhizobium co-inoculation was strongly facilitative for plant community productivity and legume (Medicago sativa) growth and nodulation. Plant competitiveness shifted in the presence of AM fungi and rhizobium, favoring M. sativa over a neighboring C4 grass (Andropogon gerardii) and C3 forb (Ratibida pinnata). This may be due to rhizobial symbiosis as well as the relatively greater mycorrhizal growth response of M. sativa, compared to the other model plants. Clipping and N-fertilization altered relative costs and benefits of both symbioses, presumably by altering host-plant nitrogen and carbon dynamics, leading to a relative decrease in mycorrhizal responsiveness and proportional biomass of M. sativa relative to the total biomass of the entire plant community, with a concomitant relative increase in A. gerardii and R. pinnata proportional biomass. Our results demonstrate a strong influence of both microbial symbioses on host-plant competitiveness and community dynamics across clipping and N-fertilization treatments, suggesting the symbiotic rhizosphere community is critical for legume establishment in grasslands.

The presence and quality of the belowground mycorrhizal fungal community could greatly influence plant community structure and host species response. This study tests whether mycorrhizal fungal communities in areas highly impacted by anthropogenic disturbance and urbanization are less species rich or exhibit lower host root colonization rates when compared to those of less disturbed systems. Using a soil bioassay, we sampled the ectomycorrhizal fungal (EMF) communities associating with Quercus rubra (northern red oak) seedlings in soil collected from seven sites: two mature forest reference sites and five urban sites of varying levels of disturbance. Morphological and polymerase chain reaction–restriction fragment length polymorphism analyses of fungi colonizing root tips revealed that colonization rates and fungal species richness were significantly lower on root systems of seedlings grown in disturbed site soils. Analysis of similarity showed that EMF community composition was not significantly different among several urban site soils but did differ significantly between mature forest sites and all but one urban site. We identified a suite of fungal species that occurred across several urban sites. Lack of a diverse community of belowground mutualists could be a constraint on urban plant community development, especially of late-successional woodlands. Analysis of urban EMF communities can add to our understanding of urban plant community structure and should be addressed during ecological assessment before pragmatic decisions to restore habitats are framed.

The root-knot nematode Meloidogyne incognita poses a worldwide threat to agriculture, with an increasing demand for alternative control options since most common nematicides are being withdrawn due to environmental concerns. The biocontrol potential of arbuscular mycorrhizal fungi (AMF) against plant-parasitic nematodes has been demonstrated, but the modes of action remain to be unraveled. In this study, M. incognita penetration of second-stage juveniles at 4, 8 and 12 days after inoculation was compared in tomato roots (Solanum lycopersicum cv. Marmande) pre-colonized or not by the AMF Glomus mosseae. Further life stage development of the juveniles was also observed in both control and mycorrhizal roots at 12 days, 3 weeks and 4 weeks after inoculation by means of acid fuchsin staining. Penetration was significantly lower in mycorrhizal roots, with a reduction up to 32%. Significantly lower numbers of third- and fourth-stage juveniles and females accumulated in mycorrhizal roots, at a slower rate than in control roots. The results show for the first time that G. mosseae continuously suppresses root-knot nematodes throughout their entire early infection phase of root penetration and subsequent life stage development.

For the regression of sigmoid-shaped responses with time t of colonization C of roots by arbuscular mycorrhizal fungi, C=C p/1+[e −k(t−ti) ] is the most useful form of the logistic equation. At the time of inflection t i the slope is maximal and directly proportional to the product of the colonization plateau C p and the abruptness k of the curve. Coefficient k has a high value when the curve rises abruptly following and preceding long shallow phases. The logistic equation has a curve that is symmetrical about t i such that C=C p/2 at inflection. Although the logistic equation can generate a good fit to many data sets for changes in colonization with time, there are cases that are not sigmoid and the logistic equation does not apply. For sigmoid curves, the lag in the development of colonization is directly related to both t i and k but not to the plateau and not to the value of the maximum slope. Higher values of k or t i reflect longer lag. When considered alone, t i and k do not fully summarize the lag in colonization, and so a numerical method to combine them is presented here which allows lag to be compared between curves. In this method, the lag is evaluated by calculating the time during early colonization when the slope equals half of the value of the maximum slope. In summary, use of the logistic equation for regression of sigmoid curves of colonization with time allows numerical comparison between curves of the lag, the period of steep ascent, and the plateau. The logistic equation does not model directly the fundamental processes at work in the development of the mycorrhizae. Instead, it can be used as described here to gain insight into the colonization process by comparing the dynamics of that colonization for different species under various conditions.

The genus Cistus comprises a group of about 20 shrub species found in wide areas throughout the whole Mediterranean region to the Caucasus. Being one of the main constituents of the Mediterranean-type maquis, this plant genus is peculiar in that it has developed a range of specific adaptations to resist summer drought and frequent disturbance events, such as fire and grazing. In addition, it can form both ectomycorrhizas and arbuscular mycorrhizas. In this paper, we review the information available on the ectomycorrhizal fungi of Cistus across its entire geographic range, as gathered and critically sifted from both published literature sources and personal observations. Although the resulting data matrix was based primarily on accounts of sporocarp inventories in the field, existing knowledge on the features of Cistus natural and synthesized ectomycorrhizas was also included and discussed. In total, more than 200 fungal species belonging to 40 genera have been reported so far to be associated with Cistus. An analysis of the pattern of ectomycorrhizal diversity and host specificity revealed that members of the Cortinariaceae and Russulaceae make the most of both Cistus-aspecific and Cistus-specific mycobionts. Further studies are needed to expand our preliminary knowledge of the mycorrhizal ecology and biology of Cistus and its fungal associates, focusing on topics such as mycobiont diversity, host specificity, fungal succession, mycorrhizal influence on stress tolerance, and impact of disturbances, while comparing the findings with those from other ecosystems.

Arbuscular mycorrhizal (AM) C-costs in grapevines were investigated. Dormant vines rely on stored C for initial growth. Therefore AM colonisation costs would compete with plant growth for available C reserves. One-year-old grapevines, colonised with Glomus etunicatum (Becker and Gerdemann), were cultivated under glasshouse conditions. The C-economy and P utilisation of the symbiosis were sequentially analysed. AM colonisation, during the 0–67 day growth period, used more stem C relative to root C, which resulted in lower shoot growth. The decline in AM colonisation during the period of 67–119 days coincided with stem C replenishment and higher shoot growth. Construction costs of AM plants and root C allocation increased with root P uptake. The efficiency of P utilisation was lower in AM roots. The reliance of AM colonisation on stem C declined with a decrease in colonisation, providing more C for the refilling of stem carbohydrate reserves and shoot growth. Once established, the AM symbiosis increased P uptake at the expense of refilling of root C reserves. Although higher root C allocation increased plant construction costs, AM roots were more efficient at P utilisation.