Competitive interactions are mediated in a sex‐specific manner by arbuscular mycorrhiza in Antennaria dioica

• Plants usually interact with other plants, and the outcome of such interaction ranges from facilitation to competition depending on the identity of the plants, including their sexual expression. Arbuscular mycorrhizal (AM) fungi have been shown to modify competitive interactions in plants. However, few studies have evaluated how AM fungi influence plant intraspecific and interspecific interactions in dioecious species.
• The competitive abilities of female and male plants of Antennaria dioica were examined in a greenhouse experiment. Females and males were grown in the following competitive settings: (i) without competition, (ii) with intrasexual competition, (iii) with intersexual competition, and (iv) with interspecific competition by Hieracium pilosella – a plant with similar characteristics to A. dioica. Half of the pots were grown with Claroideoglomus claroideum, an AM fungus isolated from the same habitat as the plant material. We evaluated plant survival, growth, flowering phenology, and production of AM fungal structures.
• Plant survival was unaffected by competition or AM fungi. Competition and the presence of AM fungi reduced plant biomass. However, the sexes responded differently to the interaction between fungal and competition treatments. Both intra‐ and interspecific competition results were sex‐specific, and in general, female performance was reduced by AM colonization. Plant competition or sex did not affect the intraradical structures, extraradical hyphae, or spore production of the AM fungus.
• These findings suggest that plant sexual differences affect fundamental processes such as competitive ability and symbiotic relationships with AM fungi.

     

The microRNA miR171h modulates arbuscular mycorrhizal colonization of Medicago truncatula by targeting NSP2

Most land plants live symbiotically with arbuscular mycorrhizal fungi. Establishment of this symbiosis requires signals produced by both partners: strigolactones in root exudates stimulate pre‐symbiotic growth of the fungus, which releases lipochito‐oligosaccharides (Myc‐LCOs) that prepare the plant for symbiosis. Here, we have investigated the events downstream of this early signaling in the roots. We report that expression of miR171h, a microRNA that targets NSP2, is up‐regulated in the elongation zone of the root during colonization by Rhizophagus irregularis (formerly Glomus intraradices) and in response to Myc‐LCOs. Fungal colonization was much reduced by over‐expressing miR171h in roots, mimicking the phenotype of nsp2 mutants. Conversely, in plants expressing an NSP2 mRNA resistant to miR171h cleavage, fungal colonization was much increased and extended into the elongation zone of the roots. Finally, phylogenetic analyses revealed that miR171h regulation of NSP2 is probably conserved among mycotrophic plants. Our findings suggest a regulatory mechanism, triggered by Myc‐LCOs, that prevents over‐colonization of roots by arbuscular mycorrhizal fungi by a mechanism involving miRNA‐mediated negative regulation of NSP2.     

The sucrose transporter SlSUT2 from tomato interacts with brassinosteroid functioning and affects arbuscular mycorrhiza formation

Mycorrhizal plants benefit from the fungal partners by getting better access to soil nutrients. In exchange, the plant supplies carbohydrates to the fungus. The additional carbohydrate demand in mycorrhizal plants was shown to be balanced partially by higher CO₂ assimilation and increased C metabolism in shoots and roots. In order to test the role of sucrose transport for fungal development in arbuscular mycorrhizal (AM) tomato, transgenic plants with down‐regulated expression of three sucrose transporter genes were analysed. Plants that carried an antisense construct of SlSUT2 (SlSUT2as) repeatedly exhibited increased mycorrhizal colonization and the positive effect of plants to mycorrhiza was abolished. Grafting experiments between transgenic and wild‐type rootstocks and scions indicated that mainly the root‐specific function of SlSUT2 has an impact on colonization of tomato roots with the AM fungus. Localization of SISUT2 to the periarbuscular membrane indicates a role in back transport of sucrose from the periarbuscular matrix into the plant cell thereby affecting hyphal development. Screening of an expression library for SlSUT2‐interacting proteins revealed interactions with candidates involved in brassinosteroid (BR) signaling or biosynthesis. Interaction of these candidates with SlSUT2 was confirmed by bimolecular fluorescence complementation. Tomato mutants defective in BR biosynthesis were analysed with respect to mycorrhizal symbiosis and showed indeed decreased mycorrhization. This finding suggests that BRs affect mycorrhizal infection and colonization. If the inhibitory effect of SlSUT2 on mycorrhizal growth involves components of BR synthesis and of the BR signaling pathway is discussed.     

Accumulation patterns of endogenous β‐aminobutyric acid during plant development and defence in Arabidopsis thaliana

• We recently discovered that β‐aminobutyric acid (BABA), a molecule known for its ability to prime defences in plants, is a natural plant metabolite. However, the role played by endogenous BABA in plants is currently unknown. In this study we investigated the systemic accumulation of BABA during pathogen infection, levels of BABA during plant growth and development and analysed mutants possibly involved in BABA transport or regulation.
• BABA was quantified by LC‐MS using an improved method adapted from a previously published protocol. Systemic accumulation of BABA was determined by analysing non‐infected leaves and roots after localised infections with Plectosphaerella cucumerina or Pseudomonas syringae pv. tomato (Pst) DC3000 avrRpt2. The levels of BABA were also quantified in different plant tissues and organs during normal plant growth, and in leaves during senescence. Mutants affecting amino acid transport (aap6, aap3, prot1 and gat1), γ‐aminobutyric acid levels (pop2) and senescence/defence (cpr5‐2) were analysed.
• BABA was found to accumulate only locally after bacterial or fungal infection, with no detectable increase in non‐infected systemic plant parts. In leaves, BABA content increased during natural and induced senescence. Reproductive organs had the highest levels of BABA, and the mutant cpr5‐2 produced constitutively high levels of BABA.
• Synthetic BABA is highly mobile in the receiving plant, whereas endogenous BABA appears to be produced and accumulated locally in a tissue‐specific way. We discuss a possible role for BABA in age‐related resistance and propose a comprehensive model for endogenous and synthetic BABA.

     

Effects of arbuscular mycorrhiza (AM) on health ofLinum usitatissimum L. infected by fungal pathogens

Effects of arbuscular mycorrhizal (AM) symbiosis on health ofLinum usitatissimum infected by fungal pathogens were investigated exemplarily. Physiological and biochemical analyses were done to explain the mechanisms underlying the AM effects. AM plants showed increased resistance against the wilt pathogen (Fusarium oxysporum f. sp.lini), the level of this effects depended on the plant cultivars which all showed the same level of root colonization by arbuscular mycorrhizal fungi (AMF). In contrary to that, AM plants were highly susceptible against the shoot pathogenOidium lini, but they suffered less than non-AM plants in terms of shoot fresh weight, CO₂ assimilation and content of sucrose in shoot apex. This indicates that AM not only activates resistance mechanisms but also can induce tolerance against pathogens. The concentration of phytohormones such as auxin- and gibberellin-like substances were increased in shoots of AM plants. In roots the ethylene production was increased, too. Furthermore the content and composition of free sterols were highly altered in leaves of AM plants. Root infection by AMF caused an increased respiratory activity and a reduced degree of DNA methylation, but both modifications only occurred in infected root parts indicating an increasing gene activity. The presented results suggest that nearly all parts of a plant are influenced by AM but not in the same manner. In the case of mildewed linseed the effect of AM on plant health was impressing, it indicates that AM has an ability to induce tolerance.     

Host‐ and stage‐dependent secretome of the arbuscular mycorrhizal fungus Rhizophagus irregularis

Arbuscular mycorrhizal fungi form the most wide‐spread endosymbiosis with plants. There is very little host specificity in this interaction, however host preferences as well as varying symbiotic efficiencies have been observed. We hypothesize that secreted proteins (SPs) may act as fungal effectors to control symbiotic efficiency in a host‐dependent manner. Therefore, we studied whether arbuscular mycorrhizal (AM) fungi adjust their secretome in a host‐ and stage‐dependent manner to contribute to their extremely wide host range. We investigated the expression of SP‐encoding genes of Rhizophagus irregularis in three evolutionary distantly related plant species, Medicago truncatula, Nicotiana benthamiana and Allium schoenoprasum. In addition we used laser microdissection in combination with RNA‐seq to study SP expression at different stages of the interaction in Medicago. Our data indicate that most expressed SPs show roughly equal expression levels in the interaction with all three host plants. In addition, a subset shows significant differential expression depending on the host plant. Furthermore, SP expression is controlled locally in the hyphal network in response to host‐dependent cues. Overall, this study presents a comprehensive analysis of the R. irregularis secretome, which now offers a solid basis to direct functional studies on the role of fungal SPs in AM symbiosis.     

Effects of parental light environment on growth and morphological responses of clonal offspring

• Environments experienced by parent ramets of clonal plants can potentially influence fitness of clonal offspring ramets. Such clonal parental effects may result from heritable epigenetic changes, such as DNA methylation, which can be removed by application of DNA de‐methylation agents such as 5‐azacytidine.
• To test whether parental shading effects occur via clonal generation and whether DNA methylation plays a role in such effects, parent plants of the clonal herb Alternanthera philoxeroides were first subjected to two levels of light intensity (high versus low) crossed with two levels of DNA de‐methylation (no or with de‐methylation by application of 5‐azacytidine), and then clonal offspring taken from each of these four types of parent plant were subjected to the same two light levels.
• Parental shading effects transmitted via clonal generation decreased growth and modified morphology of clonal offspring. Offspring responses were also influenced by DNA methylation level of parent plants. For clonal offspring growing under low light, parental shading effects on growth and morphology were always negative, irrespective of the parental de‐methylation treatment. For clonal offspring growing under high light, parental shading effects on offspring growth and morphology were negative when the parents were not treated with 5‐azacytidine, but neutral when they were treated with 5‐azacytidine.
• Overall, parental shading effects on clonal offspring performance of A. philoxeroides were found, and DNA methylation is likely to be involved in such effects. However, parental shading effects contributed little to the tolerance of clonal offspring to shading.

     

Mycorrhiza modulates aboveground tri‐trophic interactions to the fitness benefit of its host plant

1. Arbuscular mycorrhiza (AM), the association of AM fungi and plant roots, may alter morphological and physiological attributes of aboveground plant parts and thereby influence plant‐associated organisms such as herbivores and their natural enemies, predators and parasitoids. 2. The interactions between AM and the players of aboveground tri‐trophic systems have mainly been considered in isolation from each other. The effects of AM on aboveground herbivore–carnivore population dynamics and the consequences to plant fitness are unknown. 3. We explored AM‐induced compensatory mechanisms for AM‐promoted proliferation of the herbivorous spider mite, Tetranychus urticae Koch, on whole bean plants, Phaseolus vulgaris L. Vegetative and reproductive plant growth, AM fungal colonisation levels, and mite densities were assessed on spider mite‐infested plants colonised or not by the AM fungus Glomus mosseae Nicol. & Gerd, and harbouring the natural enemy of the spider mites, the predatory mite Phytoseiulus persimilis Anthias‐Henriot or not. 4. AM symbiosis modulated the aboveground tri‐trophic system to the fitness benefit of the plant. AM‐increased plant productivity outweighed the fitness decrease due to AM‐promoted herbivory: at similar vegetative growth, mycorrhizal plants produced more seeds than non‐mycorrhizal plants. 5. AM‐increased spider mite population levels were compensated for by enhanced population growth of the predators and increased plant tolerance to herbivory. 6. AM‐enhanced predator performance looped back to the AM fungus and stabilised its root colonisation levels, providing the first experimental evidence of a mutually beneficial interaction between AM and an aboveground third trophic level natural enemy.     

Sucrose synthase levels do not limit or regulate carbon transfer in the arbuscular mycorrhizal symbiosis

Abstract

Sucrose synthase (SuSy) is the main sucrose breakdown enzyme in plant sink tissues, including nodules, and is a possible candidate for the diversion of plant carbon to arbuscular mycorrhizal (AM) fungi in roots. We tested the involvement of SuSy in AM symbiosis of Glomus intraradices and Pisum sativum (pea). We observed that peas deficient in the predominant root isoform of SuSy were colonized successfully by AM fungi similar to wild-type roots. SuSy protein levels did not increase in roots as AM symbiosis developed, although SuSy protein levels did increase in nodules as the rhizobium symbiosis developed. Our results lead us to conclude that, unlike nodule symbiosis, SuSy protein does not limit or regulate carbon transfer in the AM symbiosis.     

Patterns of diversity and adaptation in Glomeromycota from three prairie grasslands

Arbuscular mycorrhizal (AM) fungi are widespread root symbionts that often improve the fitness of their plant hosts. We tested whether local adaptation in mycorrhizal symbioses would shape the community structure of these root symbionts in a way that maximizes their symbiotic functioning. We grew a native prairie grass (Andropogon gerardii) with all possible combinations of soils and AM fungal inocula from three different prairies that varied in soil characteristics and disturbance history (two native prairie remnants and one recently restored). We identified the AM fungi colonizing A. gerardii roots using PCR amplification and cloning of the small subunit rRNA gene. We observed 13 operational taxonomic units (OTUs) belonging to six genera in three families. Taxonomic richness was higher in the restored than the native prairies with one member of the Gigaspora dominating the roots of plants grown with inocula from native prairies. Inoculum source and the soil environment influenced the composition of AM fungi that colonized plant roots. Correspondingly, host plants and AM fungi responded significantly to the soil–inoculum combinations such that home fungi often had the highest fitness and provided the greatest benefit to A. gerardii. Similar patterns were observed within the soil–inoculum combinations originating from two native prairies, where five sequence types of a single Gigaspora OTU were virtually the only root colonizers. Our results indicate that indigenous assemblages of AM fungi were adapted to the local soil environment and that this process occurred both at a community scale and at the scale of fungal sequence types within a dominant OTU.     

Effects of prometryn and acetochlor on arbuscular mycorrhizal fungi and symbiotic system

Prometryn and acetochlor are common herbicides widely used to control weeds in agricultural systems. The impacts of the two herbicides on spore germination, hyphal elongation, the biomass and malondialdehyde content of carrot hairy roots were investigated using a strict in vitro cultivation system associating the Ri T‐DNA‐transferred carrot hairy roots with Glomus etunicatum. Alternatively, root colonization, daughter spore production and the proportion of hyphae with succinate dehydrogenase (SDH) and alkaline phosphatase (ALP) activities were also investigated. No significant impact on spore germination was noted in the presence of acetochlor at all three concentrations tested, while a significant decrease was observed with prometryn only at the highest concentration. Moreover, an inverse correlation was identified between herbicides concentrations and G. etunicatum root colonization and spore production as well as hyphal SDH and ALP activity, with a positive correlation identified among these four factors. Both herbicides exerted negative effects on the arbuscular mycorrhizal (AM) fungus and symbiosis at increasing concentrations, with prometryn apparently more toxic than acetochlor. Furthermore, the AM symbiotic system was shown to improve biomass, reduce malondialdehyde accumulation and ease lipid peroxidation in carrot hairy roots and decrease damage in host plants, thus enhancing plant tolerance to adverse conditions.

Significance and Impact of the Study

In this study, the effect of prometryn and acetochlor on the physiology and metabolic activities of the AM fungus Glomus etunicatum were investigated. Our findings demonstrate for the first time, the impact of the two herbicides at three concentrations (0·1, 1 and 10 mg l^?1) on transformed carrot hairy roots/AM fungus association under strict in vitro culture conditions, which may guide the application of the two herbicides in modern agriculture.     

Reflections on the role of environmentally-governed reproductive versatility in the adaptation of plant populations

Summary

Mycorrhizal associations vary widely in structure and function, but the commonest interaction is the Arbuscular Mycorrhizal (AM) symbiosis which forms between the roots of over 80% of all terrestrial plant species and Zygomycete fungi of the Order Glomales. These are obligate symbionts which colonise plant root cells. This symbiosis confers benefits directly to the host plants through the acquisition of phosphate and other mineral nutrients from the soil by the fungus while the fungus receives a carbon source from the host. In addition, the symbiosis may also enhance the plants resistance to biotic and abiotic stresses. The beneficial effects of AM symbioses occur as a result of a complex molecular dialogue between the two symbiotic partners. Identifying the molecules and genes involved in the dialogue is necessary for a greater understanding of the symbiosis. This paper reviews the process of AM fungal colonisation of plant roots and the underlying molecular mechanisms associated with the formation and functioning of an AM symbiosis.     

Compensation to simulated insect leaf herbivory in wild cotton (Gossypium hirsutum): responses to multiple levels of damage and associated traits

• Identifying the mechanisms of compensation to insect herbivory remains a major challenge in plant biology and evolutionary ecology. Most previous studies have addressed plant compensatory responses to one or two levels of insect herbivory, and the underlying traits mediating such responses remain elusive in many cases.
• We evaluated responses associated with compensation to multiple intensities of leaf damage (0% control, 10%, 25%, 50%, 75% of leaf area removed) by means of mechanical removal of foliar tissue and application of a caterpillar (Spodoptera exigua) oral secretions in 3‐month‐old wild cotton plants (Gossypium hirsutum). Four weeks post‐treatment, we measured plant growth and multiple traits associated with compensation, namely: changes in above‐ and belowground, biomass and the concentration of nutrients (nitrogen and phosphorus) and non‐structural carbon reserves (starch and soluble sugars) in roots, stems and leaves.
• We found that wild cotton fully compensated in terms of growth and biomass allocation when leaf damage was low (10%), whereas moderate (25%) to high leaf damage in some cases led to under‐compensation. Nonetheless, high levels of leaf removal (50% and 75%) in most cases did not cause further reductions in height and allocation to leaf and stem biomass relative to low and moderate damage. There were significant positive effects of leaf damage on P concentration in leaves and stems, but not roots, as well as a negative effect on soluble sugars in roots.
• These results indicate that wild cotton fully compensated for a low level of leaf damage but under‐compensated under moderate to high leaf damage, but can nonetheless sustain growth despite increasing losses to herbivory. Such responses were possibly mediated by a re‐allocation of carbohydrate reserves from roots to shoots.

     

Influence of arbuscular mycorrhiza on lipid peroxidation and antioxidant enzyme activity of maize plants under temperature stress

The influence of the arbuscular mycorrhizal (AM) fungus, Glomus etunicatum, on characteristics of growth, membrane lipid peroxidation, osmotic adjustment, and activity of antioxidant enzymes in leaves and roots of maize (Zea mays L.) plants was studied in pot culture under temperature stress. The maize plants were placed in a sand and soil mixture under normal temperature for 6 weeks and then exposed to five different temperature treatments (5oC, 15oC, 25oC, 35oC, and 40oC) for 1 week. AM symbiosis decreased membrane relative permeability and malondialdehyde content in leaves and roots. The contents of soluble sugar content and proline in roots were higher, but leaf proline content was lower in mycorrhizal than nonmycorrhizal plants. AM colonization increased the activities of superoxide dismutase, catalase, and peroxidase in leaves and roots. The results indicate that the AM fungus is capable of alleviating the damage caused by temperature stress on maize plants by reducing membrane lipid peroxidation and membrane permeability and increasing the accumulation of osmotic adjustment compounds and antioxidant enzyme activity. Consequently, arbuscular mycorrhiza formation highly enhanced the extreme temperature tolerance of maize plant, which increased host biomass and promoted plant growth.     

Arbuscular mycorrhizal fungi reduce growth and infect roots of the non‐host plant Arabidopsis thaliana

The arbuscular mycorrhizal (AM) symbiosis is widespread throughout the plant kingdom and important for plant nutrition and ecosystem functioning. Nonetheless, most terrestrial ecosystems also contain a considerable number of non‐mycorrhizal plants. The interaction of such non‐host plants with AM fungi (AMF) is still poorly understood. Here, in three complementary experiments, we investigated whether the non‐mycorrhizal plant Arabidopsis thaliana, the model organism for plant molecular biology and genetics, interacts with AMF. We grew A. thaliana alone or together with a mycorrhizal host species (either Trifolium pratense or Lolium multiflorum) in the presence or absence of the AMF Rhizophagus irregularis. Plants were grown in a dual‐compartment system with a hyphal mesh separating roots of A. thaliana from roots of the host species, avoiding direct root competition. The host plants in the system ensured the presence of an active AM fungal network. AM fungal networks caused growth depressions in A. thaliana of more than 50% which were not observed in the absence of host plants. Microscopy analyses revealed that R. irregularis supported by a host plant was capable of infecting A. thaliana root tissues (up to 43% of root length colonized), but no arbuscules were observed. The results reveal high susceptibility of A. thaliana to R. irregularis, suggesting that A. thaliana is a suitable model plant to study non‐host/AMF interactions and the biological basis of AM incompatibility.     

Plant Responses to Drought Stress and Exogenous ABA Application are Modulated Differently by Mycorrhization in Tomato and an ABA-deficient Mutant (<Emphasis Type="Italic">Sitiens</Emphasis>)

The aims of the present study are to find out whether the effects of arbuscular mycorrhizal (AM) symbiosis on plant resistance to water deficit are mediated by the endogenous abscisic acid (ABA) content of the host plant and whether the exogenous ABA application modifies such effects. The ABA-deficient tomato mutant sitiens and its near-isogenic wild-type parental line were used. Plant development, physiology, and expression of plant genes expected to be modulated by AM symbiosis, drought, and ABA were studied. Results showed that only wild-type tomato plants responded positively to mycorrhizal inoculation, while AM symbiosis was not observed to have any effect on plant development in sitiens plants grown under well-watered conditions. The application of ABA to sitiens plants enhanced plant growth both under well-watered and drought stress conditions. In respect to sitiens plants subjected to drought stress, the addition of ABA had a cumulative effect in relation to that of inoculation with G. intraradices. Most of the genes analyzed in this study showed different regulation patterns in wild-type and sitiens plants, suggesting that their gene expression is modulated by the plant ABA phenotype. In the same way, the colonization of roots with the AM fungus G. intraradices differently regulated the expression of these genes in wild-type and in sitiens plants, which could explain the distinctive effect of the symbiosis on each plant ABA phenotype. This also suggests that the effects of the AM symbiosis on plant responses and resistance to water deficit are mediated by the plant ABA phenotype.