Fertilization and forb:graminoid ratio affect arbuscular mycorrhiza in seedlings but not adult plants of Plantago lanceolata

Aims

Nutrients play a key role in arbuscular mycorrhizal (AM) symbiosis. We quantified the response of AM symbiosis of seedlings and adult plants of Plantago lanceolata to fertilization under field conditions in managed grasslands differing in nutrient availability and soil moisture.

Methods

The AM symbiosis was measured as the total extent of AM fungal colonization and frequency of arbuscules or vesicles, and as the relative proportions of morphotypes. We further examined the effects of the surrounding vegetation upon AM symbiosis.

Results

Fertilization decreased total AM colonization and relative arbuscular frequency of the whole mycorrhizal community and of Acaulospora and “fine endophyte” morphotypes in seedling roots, but it had no effect upon the mycorrhiza in adult plants. The decline in arbuscular frequency in seedling roots due to fertilization was greater at the sites with higher nutrient availability and lower N:P ratio. Seedlings surrounded by more forbs had a greater total AM colonization and higher vesicular frequency.

Conclusions

Increased nutrient availability in the initial stages of seedling development has a prominent effect upon AM symbiosis development, but these effects seem to diminish over the long term, as evidenced by the results obtained for adult plants and from the limited effects of parameters characterizing long-term nutrient availability.     

Signalling mechanisms involved in the response of two varieties of Humulus lupulus L. to soil drying: I. changes in xylem sap pH and the concentrations of abscisic acid and anions

Background and aims

Soil drying leads to the generation of chemical signals in plants that regulate water use via control of the stomatal aperture. The aim of our work was to identify the presence and identity of potential chemical signals, their dynamics, and their relationship with transpiration rate during soil drying in hop (Humulus lupulus (L.)) plants.

Methods

We used pressure chamber technique for measurement of shoot water potential and collection of shoot xylem sap. We analyzed concentrations of abscisic acid (ABA), nitrate, phosphate, sulphate and malate in sap and also the rate of whole plant transpiration.

Results

Transpiration rate decreased prior to changes in shoot water potential. The concentration of ABA in xylem sap continuously increased from early to later stages of water stress, whereas in leaves it increased only at later stages. Shoot sap pH increased simultaneously with the decrease of transpiration rate. Xylem sap alkalization was in some cases accompanied by a decrease in nitrate concentration and an increase in malate concentration. Concentration of sulphate increased in xylem sap during drying and sulphate in combination with a higher ABA concentration enhanced stomatal closure.

Conclusions

Several early chemical signals appear in sap of hop plants during soil drying and their impact on transpiration may vary according to the stage of soil drying.     

Mineralization and herbage recovery of animal manure nitrogen after application to various soil types

Aims

The study aims (1) to evaluate the effect of Mesorhizobium tianshanense on plant proline and polyamine levels of Lotus tenuis and its modulatory effect during plant response to short-term salt stress and (2) to compare these effects with those caused by mycorrhizal symbiosis.

Methods

Experiments consisted of a randomized factorial design of two factors: salinity (two levels, 0 and 150 mM NaCl) and symbiosis (three levels, uninoculated, Glomus intraradices, and M. tianshanense).

Results

Salinization led to increased proline levels regardless of plant organ and symbiotic status, excepting mycorrhizal L. tenuis roots. Salinity diminished the total polyamine level of control and rhizobial plants but not in mycorrhizal ones. Variations in the pattern response of the three individual polyamines (putrescine, spermidine, and spermine) differed in accordance with the symbiotic status of the plant, highlighting a divergence on proline and polyamine metabolisms between rhizobial and mycorrhizal symbiosis.

Conclusions

Spermidine and spermine contributed the most with the salt-induced root polyamine increment observed upon salinization in roots of nodulated plants, suggesting that these polyamines might mediate an adaptive role of the plant–M. tianshanense symbiosis in L. tenuis plants growing in a saline environment.     

Seasonal carbon allocation to arbuscular mycorrhizal fungi assessed by microscopic examination, stable isotope probing and fatty acid analysis

Background and Aim

Climate change models are limited by lack of baseline data, in particular carbon (C) allocation to – and dynamics within – soil microbial communities. We quantified seasonal C-assimilation and allocation by plants, and assessed how well this corresponds with intraradical arbuscular mycorrhizal fungal (AMF) storage and structural lipids (16:1ω5 NLFA and PLFA, respectively), as well as microscopic assessments of AMF root colonization.

Methods

Coastal Hypochoeris radicata plants were labeled with ¹³CO₂ in February, July and October, and ¹³C-allocation to fine roots and NLFA 16:1ω5, as well as overall lipid contents and AM colonization were quantified.

Results

C-allocation to fine roots and AMF storage lipids differed seasonally and mirrored plant C-assimilation, whereas AMF structural lipids and AM colonization showed no seasonal variation, and root colonization exceeded 80 % throughout the year. Molecular analyzes of the large subunit rDNA gene indicated no seasonal AMF community shifts.

Conclusions

Plants allocated C to AMF even at temperatures close to freezing, and fungal structures persisted in roots during times of low C-allocation. The lack of seasonal differences in PLFA and AM colonization indicates that NLFA analyses should be used to estimate fungal C-status. The implication of our findings for AM function is discussed.     

Identification of three wheat globulin genes by screening a Triticum aestivum BAC genomic library with cDNA from a diabetes-associated globulin

Background

Plant systemic signaling characterized by the long distance transport of molecules across plant organs involves the xylem and phloem conduits. Root-microbe interactions generate systemic signals that are transported to aerial organs via the xylem sap. We analyzed the xylem sap proteome of soybean seedlings in response to pathogenic and symbiotic interactions to identify systemic signaling proteins and other differentially expressed proteins.

Results

We observed the increase of a serine protease and peroxidase in the xylem sap in response to Phytophthora sojae elicitor treatment. The high molecular weight fraction of soybean xylem sap was found to promote the growth of Neurospora crassa in vitro at lower concentrations and inhibit growth at higher concentrations. Sap from soybean plants treated with a P. sojae elicitor had a significantly higher inhibitory effect than sap from control soybean plants. When soybean seedlings were inoculated with the symbiont Bradyrhizobium japonicum, the abundance of a xyloglucan transendoglycosyl transferase protein increased in the xylem sap. However, RNAi-mediated silencing of the corresponding gene did not significantly affect nodulation in soybean hairy root composite plants.

Conclusion

Our study identified a number of sap proteins from soybean that are differentially induced in response to B. japonicum and P. sojae elicitor treatments and a majority of them were secreted proteins.     

Arbuscular mycorrhizal symbiosis alleviates drought stress imposed on Knautia arvensis plants in serpentine soil

Background and Aims

Plants growing on serpentine bedrock have to cope with the unique soil chemistry and often also low water-holding capacity. As plant-soil interactions are substantially modified by arbuscular mycorrhizal (AM) symbiosis, we hypothesise that drought tolerance of serpentine plants is enhanced by AM fungi (AMF).

Methods

We conducted a pot experiment combining four levels of drought stress and three AMF inoculation treatments, using serpentine Knautia arvensis (Dipsacaceae) plants as a model.

Results

AMF inoculation improved plant growth and increased phosphorus uptake. The diminishing water supply caused a gradual decrease in plant growth, accompanied by increasing concentrations of drought stress markers (proline, abscisic acid) in root tissues. Mycorrhizal growth dependence and phosphorus uptake benefit increased with drought intensity, and the alleviating effect of AMF on plant drought stress was also indicated by lower proline accumulation.

Conclusions

We documented the role of AM symbiosis in plant drought tolerance under serpentine conditions. However, the potential of AMF to alleviate drought stress was limited beyond a certain threshold, as indicated by a steep decline in mycorrhizal growth dependence and phosphorus uptake benefit and a concomitant rise in proline concentrations in the roots of mycorrhizal plants at the highest drought intensity.     

A native Glomus intraradices strain from a Mediterranean saline area exhibits salt tolerance and enhanced symbiotic efficiency with maize plants under salt stress conditions

Aims

Arbuscular mycorrhizal (AM) fungi have been shown to occur naturally in saline environments and it has been suggested that differences in fungal behaviour and efficiency can be due to the origin and adaptation of the AM fungus. These findings invite to look out for AM fungal species isolated in saline environments and compare their salt-tolerance mechanisms with those of species living in non-saline areas.

Methods

A fungal strain of G. intraradices (Gi CdG) isolated from a region with serious problems of salinity and affected by desertification, has been compared with a collection strain of the same species, used as a model fungus. An in vitro experiment tested the ability of both AM fungi to grow under increasing salinity and an in vivo experiment compared their symbiotic efficiency with maize plants grown under salt stress conditions.

Results

The isolate Gi CdG developed better under saline conditions and induced considerably the expression of GintBIP, Gint14-3-3 and GintAQP1 genes, while it showed a lower induction of GintSOD1 gene than the collection G. intraradices strain. The isolate Gi CdG also stimulated the growth of maize plants under two levels of salinity more than the collection strain. The higher symbiotic efficiency of Gi CdG was corroborated by the enhanced efficiency of photosystem II and stomatal conductance and the lower electrolyte leakage exhibited by maize plants under the different conditions assayed.

Conclusions

The higher tolerance to salinity and symbiotic efficiency exhibited by strain Gi CdG as compared to the collection G. intraradices strain may be due to a fungal adaptation to saline environments. Such adaptation may be related to the significant up-regulation of genes encoding chaperones or genes encoding aquaporins. The present study remarks that AM fungi isolated from areas affected by salinity can be a powerful tool to enhance the tolerance of crops to saline stress conditions.     

Ninety-year-, but not single, application of phosphorus fertilizer has a major impact on arbuscular mycorrhizal fungal communities

Background and aims

Arbuscular mycorrhizal (AM) fungi play a significant role in P nutrition of crops in agriculture, but P accumulation in the soil, e.g., application of P-fertilizer, generally reduces AM fungal colonization. The impact of long-term application of chemical fertilizer on AM fungal communities was investigated with respect to the time scale.

Methods

Soils were collected from four plots with different fertilizer management in the long-term experimental field established in 1914. Lotus japonicus was grown in the soils in a greenhouse, while Glycine max was grown in the plots in the field. DNA was extracted from their roots, and the diversity and community compositions were analyzed based on occurrence of the AM fungal phylotypes defined by sequence similarity in the LSU rDNA.

Results

The 90-year-application of N and K in the absence of P increased AM fungal diversity and resulted in formation of a distinctive fungal community compared with those in the other treatments. This effect was not cancelled by single application of P. Whereas the impact of balanced application of N, P, and K was ambiguous.

Conclusion

These observations suggest that the presence/absence of P-fertilizer has a major impact on AM fungal communities, but the action may appear only on a long time scale.     

Interactions between root hair length and arbuscular mycorrhizal colonisation in phosphorus deficient barley (Hordeum vulgare)

Aims

Phosphorus (P) limits crop yield and P-fertilisers are frequently applied to agricultural soils. However, supplies of quality rock phosphate are diminishing. Plants have evolved mechanisms to improve P-acquisition and understanding these could improve the long-term sustainability of agriculture. Here we examined interactions between root hairs and arbuscular mycorrhizal (AM) colonisation in barley (Hordeum vulgare L.).

Methods

Barley mutants exhibiting different root hair phenotypes, wild type barley and narrowleaf plantain (Plantago lanceolata L.) were grown in the glasshouse in P-sufficient and P-deficient treatments and allowed to develop AM colonization from the natural soil community. Plants were harvested after 6 weeks growth and root hair length, AM-fungal colonisation, shoot biomass and P-accumulation measured.

Results

Under P-deficient conditions, root hair length and AM colonisation were negatively related suggesting that resources are allocated to root hairs rather than to AM fungi in response to P-deficiency. There was evidence that barley and narrowleaf plantain employed different strategies to increase P-acquisition under identical conditions, but root hairs were more effective.

Conclusions

This research suggests future barley breeding programmes should focus on maintaining or improving root hair phenotypes and that pursuing enhancements to AM associations under the prevalent agricultural conditions tested here would be ineffectual.     

As,Pb, Sb,and Zn transfer from soil to root of wild rosemary: do native symbionts matter?

Background and aims

This is an in natura study aimed to determine the potential of Rosmarinus officinalis for phytostabilization of trace metal and metalloid (TMM)-contaminated soils in the Calanques National Park (Marseille, southeast of France). The link between rosemary tolerance/accumulation of As, Pb, Sb, and Zn and root symbioses with arbuscular mycorrhizal (AM) fungi and/or dark septate endophytes (DSE) was examined.

Methods

Eight sites along a gradient of contamination were selected for soil and root collections. TMM concentrations were analyzed in all the samples and root symbioses were observed. Moreover, in the roots of various diameters collected in the most contaminated site, X-ray microfluorescence methods were used to determine TMM localization in tissues.

Results

Rosemary accumulated, in its roots, the most labile TMM fraction in the soil. The positive linear correlation between TMM concentrations in soil and endophyte root colonization rates suggests the involvement of AM fungi and DSE in rosemary tolerance to TMM. Moreover, a typical TMM localization in root peripheral tissues of thin roots containing endophytes forming AM and DSE development was observed using X-ray microfluorescence.

Conclusions

Rosemary and its root symbioses appeared as a potential candidate for a phytostabilization process of metal-contaminated soils in Mediterranean area.     

Arbuscular mycorrhizal symbiosis and methyl jasmonate avoid the inhibition of root hydraulic conductivity caused by drought

Hormonal regulation and symbiotic relationships provide benefits for plants to overcome stress conditions. The aim of this study was to elucidate the effects of exogenous methyl jasmonate (MeJA) application on root hydraulic conductivity (L) of Phaseolus vulgaris plants which established arbuscular mycorrhizal (AM) symbiosis under two water regimes (well-watered and drought conditions). The variation in endogenous contents of several hormones (MeJA, JA, abscisic acid (ABA), indol-3-acetic acid (IAA), salicylic acid (SA)) and the changes in aquaporin gene expression, protein abundance and phosphorylation state were analyzed. AM symbiosis decreased L under well-watered conditions, which was partially reverted by the MeJA treatment, apparently by a drop in root IAA contents. Also, AM symbiosis and MeJA prevented inhibition of L under drought conditions, most probably by a reduction in root SA contents. Additionally, the gene expression of two fungal aquaporins was upregulated under drought conditions, independently of the MeJA treatment. Plant aquaporin gene expression could not explain the behaviour of L. Conversely, evidence was found for the control of L by phosphorylation of aquaporins. Hence, MeJA addition modified the response of L to both AM symbiosis and drought, presumably by regulating the root contents of IAA and SA and the phosphorylation state of aquaporins.     

Enhanced nodulation and symbiotic effectiveness of Medicago truncatula when co-inoculated with Pseudomonas fluorescens WSM3457 and Ensifer (Sinorhizobium) medicae WSM419

Aims

Low numbers of rhizobia in soil or inoculants delay nodulation and decrease symbiotic legume productivity. This study investigated the effect of co-inoculation with a helper bacterium, Pseudomonas fluorescens WSM3457 on the Medicago truncatula - Ensifer (Sinorhizobium) medicae WSM419 symbiosis challenged by a low inoculum dose.

Methods

In a glasshouse experiment the effect of co-inoculation with WSM3457 on the kinetics of nodule initiation and development was assessed 5, 7, 10, 14, 17, 21, and 42 days after inoculation of M. truncatula cv. Caliph with 10³ cells/plant of E. medicae WSM419.

Results

Co-inoculated plants had enhanced rate of nodule initiation and development, greater numbers of larger crown nodules, and by day 42 accumulated more N than plants inoculated with E. medicae WSM419 alone. Nodule development was altered by co-inoculation. Approximately 25% of nodule initials on co-inoculated plants formed in closely associated pairs, young nodules were larger with multiple meristems and developed into cluster-like multi-lobed nodules compared to those on WSM419 inoculated plants. Molecular typing showed WSM3457 occupied a significant proportion of root nodules on co-inoculated plants.

Conclusion

Co-inoculation with P. fluorescens WSM3457 enhanced symbiotic effectiveness of M. truncatula when inoculated with a low inoculum dose of E. medicae WSM419.     

Effects of single and mixed inoculation with two arbuscular mycorrhizal fungi in two different levels of phosphorus supply on β-carotene concentrations in sweet potato (Ipomoea batatas L.) tubers

Aims

This study aimed to determine the effect of arbuscular mycorrhizal (AM) fungi and phosphorus (P) supply levels on β-carotene concentrations in sweet potato (Ipomoea batatas L.) tubers.

Methods

Two commercial AM fungal isolates of Glomus intraradices (IFP Glintra) and Glomus mosseae (IFP Glm) which differ in their life cycles were used. Sweet potato plants were grown in a horizontal split-root system that consisted of two root compartments. A root-free fungal compartment that allowed the quantification of mycelial development was inserted into each root compartment. The two root compartments were inoculated either with the same or with different AM isolates, or remained free of mycorrhizal propagules. Each fungal treatment was carried out in two P supply levels.

Results

In the low P supply level, mycorrhizal colonization significantly increased β-carotene concentrations in sweet potato tubers compared with the non-mycorrhizal plants. Glomus intraradices appeared to be more efficient in increasing β-carotene concentrations than G. mosseae. Dual inoculation of the root system with the two mycorrhizal fungi did not result in a higher increase in tuber β-carotene concentrations than inoculation with the single isolates. Improved P nutrition led to higher plant tuber biomass but was not associated with increased β-carotene concentrations.

Conclusions

The results indicate a remarkable potential of mycorrhizal fungi to improve β-carotene concentrations in sweet potato tubers in low P fertilized soils. These results also suggest that β-carotene metabolism in sweet potato tubers might be specifically activated by root mycorrhizal colonization.     

Growth response of crops to soil microbial communities from conventional monocropping and tree-based intercropping systems

Background and aims

Recent studies have shown that tree-based intercropping (TBI) systems support a more diverse soil microbial community compared to conventional agricultural systems. However, it is unclear whether differences in soil microbial diversity between these two agricultural systems have a functional effect on crop growth.

Methods

In this study, we used a series of greenhouse experiments to test whether crops respond differently to the total soil microbial community (Experiment 1) and to arbuscular mycorrhizal (AM) fungal communities alone (Experiment 2) from conventionally monocropped (CM) and TBI systems.

Results

The crops had a similar growth response to the total soil microbial communities from both cropping systems. However, when compared to sterilized controls, barley (Hordeum vulgare) and canola (Brassica napus) exhibited a negative growth response to the total soil microbial communities, while soybean (Glycine max) was unaffected. During the AM fungal establishment phase of the second experiment, ‘nurse’ plants had a strong positive growth response to AM fungal inoculation, and significantly higher biomass when inoculated with AM fungi from the CM system compared to the TBI system. Soybean was the only crop species to exhibit a significant positive growth response to AM fungal inoculation. Similar to the total soil microbial communities, AM fungi from the two cropping systems did not differ in their effect on crop growth.

Conclusion

Overall, AM fungi from both cropping systems had a positive effect on the growth of plants that formed a functional symbiosis. However, the results from these experiments suggest that negative effects of non-AM fungal microbes are stronger than the beneficial effects of AM fungi from these cropping systems.     

Changes in stem water content influence sap flux density measurements with thermal dissipation probes

Key message

Stem WC may decline during the day. Zero-flow dT _m increases when WC decreases. Use of nighttime dT _m in the calculation of sap flux density during the day might introduce errors.

Abstract

There is increasing evidence of diel variation in water content of stems of living trees as a result of changes in internal water reserves. The interplay between dynamic water storage and sap flow is of current interest, but the accuracy of measurement of both variables has come into question. Fluctuations in stem water content may induce inaccuracy in thermal-based measurements of sap flux density because wood thermal properties are dependent on water content. The most widely used thermal method for measuring sap flux density is the thermal dissipation probe (TDP) with continuous heating, which measures the influence of moving sap on the temperature difference between a heated needle and a reference needle vertically separated in the flow stream. The objective of our study was to investigate how diel fluctuations in water content could influence TDP measurements of sap flux density. We analysed the influence of water content on the zero-flow maximum temperature difference, dT _m, which is used as the reference for calculating sap flux density, and present results of a dehydration experiment on cut branch segments of American sycamore (Platanus occidentalis L.). We demonstrate both theoretically and experimentally that dT _m increases when stem water content declines. Because dT _m is measured at night when water content is high, this phenomenon could result in underestimations of sap flux density during the day when water content is lower. We conclude that diel dynamics in water content should be considered when TDP is used to measure sap flow.     

Zinc accumulation patterns in four Anthyllis vulneraria subspecies supplemented with mineral nitrogen or grown in the presence of their symbiotic bacteria

Aims

This work examines Zn accumulation in four Anthyllis vulneraria subspecies supplemented with mineral nitrogen or grown in the presence of their symbiotic bacteria.

Methods

Anthyllis vulneraria subspecies were grown hydroponically in the presence of high levels of ZnSO₄. The plants were either grown in symbiosis with one of two non-metallicolous or metallicolous Mesorhizobium inoculants or in the presence of KNO₃.

Results

When exposed to 1,000 μM Zn, shoot and root biomass of three out of our four Anthyllis subspecies cultivated with NO₃ dropped significantly by about 24–28 %; carpatica, the fourth subspecies, was not affected. Subspecies carpatica Zn tolerance was confirmed when in symbiosis with the metallicolous strain. In the presence of 1,000 μM Zn, the different Anthyllis subspecies concentrated more Zn in their roots than in their shoots and only subsp. carpatica accumulated a significant amount of Zn in its shoots. The most remarkable feature was the drastic decrease in Zn concentration in both roots (up to 2.5–3 fold) and shoots (2.6-fold) of subsp. carpatica exposed to 1,000 μM Zn and nodulated whatever the Mesorhizobium strain used, compared to the N-grown plants.

Conclusions

Our results bring new perspectives as regards phytostabilization, with the potential use of a rhizobium-inoculated leguminous subspecies displaying unusual Zn tolerance.