Nitrogen limitation impairs plant control over the arbuscular mycorrhizal symbiosis in response to phosphorus and shading in two European sand dune species

The symbiosis of plants with arbuscular mycorrhizal fungi (AMF) may become parasitic if the cost:benefit ratio (carbon:phosphorus ratio) increases. In case of mycorrhizal parasitism, a plant may prevent growth depression through the reduction of root colonization as a form of control over the symbiosis. In this greenhouse study, we attempted to manipulate the cost:benefit ratio of the arbuscular mycorrhizal symbiosis by shading and/or phosphorus (P) fertilization in the differentially mycotrophic plant species Hieracium pilosella and Corynephorus canescens. By repeated sampling of soil cores, we assessed the temporal progress of plant investment towards mycorrhizal structures as a measure of plant control over the AMF. Unexpectedly, we found no obvious treatment effects on mycorrhizal growth dependency (MGD), most likely caused by constant N-limitation in AM plants being enhanced by P-fertilization and shade probably not exacerbating plant C-budget for AMF. This highlights the importance of N:P:C stoichiometry for the outcome of the symbiosis. Nevertheless, we found possible control mechanisms in shaded H. pilosella, with considerably higher resource investments into root than into hyphal growth, while root colonization was only marginally suppressed. This control only manifested after 4 weeks of growth under potentially detrimental conditions, emphasizing the importance of time in plant control over the arbuscular mycorrhizal symbiosis. In contrast, the less mycotrophic C. canescens did not exhibit obvious changes in mycorrhizal investments in reaction to shading and P-fertilization, possibly because the low mycotrophy and AMF colonization already imposes a functioning control mechanism in this species. Our study suggests that highly mycotrophic plants may have a stronger need to keep AMF in check than less mycotrophic plants, which may have implications for the role of mycotrophy in the outcome of symbiotic interactions in natural situations.     

Comparative proteomic analysis of leaf tissue from tomato plants colonized with <Emphasis Type="Italic">Rhizophagus irregularis</Emphasis>

A comparative proteomic approach was performed to analyze the differential accumulation of leaf proteins in response to the symbiosis between Solanum lycopersicum and the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis. Protein profiling was examined in leaves from tomato plants colonized with AMF (M), as well as non-colonized plants fertilized with low phosphate (20 μM P; NM-LP) and non-colonized plants fertilized with regular phosphate Hoagland’s solution (200 μM P; NM-RP). Comparisons were made between these groups, and 2D-SDS-PAGE revealed that 27 spots were differentially accumulated in M vs. NM-LP. Twenty-three out of the 27 spots were successfully identified by mass spectrometry. Two of these proteins, 2-methylene-furan-3-one reductase and auxin-binding protein ABP19a, were up-accumulated in M plants. The down-accumulated proteins in M plants were associated mainly with photosynthesis, redox, and other molecular functions. Superoxide dismutase, harpin binding protein, and thioredoxin peroxidase were down-accumulated in leaves of M tomato plants when compared to NM-LP and NM-RP, indicating that these proteins are responsive to AMF colonization independently of the phosphate regime under which they were grown. 14-3-3 protein was up-accumulated in NM-RP vs. NM-LP plants, whereas it was down-accumulated in M vs. NM-LP and M vs. NM-RP, regardless of their phosphate nutrition. This suggests a possible regulation by P nutrition and AMF colonization. Our results demonstrate AMF-induced systemic changes in the expression of tomato leaf proteins, including the down-accumulation of proteins related to photosynthesis and redox function.     

Tomato belowground–aboveground interactions: <Emphasis Type="Italic">Rhizophagus irregularis</Emphasis> affects foraging behavior and life history traits of the predator <Emphasis Type="Italic">Macrolophus pygmaeus</Emphasis> (Hemiptera: Miridae)

In recent years, studies on arbuscular mycorrhizal fungi (AMF) have been revealing that the belowground symbiosis can influence the performance of aboveground herbivores and their natural enemies through its effects on the host plant. In this study, we tested whether the colonization of tomato plants by the arbuscular mycorrhizal fungus Rhizophagus irregularis (Syn. Glomus intraradices Schenk and Smith) (Glomeromycota: Glomeraceae) affects the performance of the zoophytophagous mirid bug Macrolophus pygmaeus Rambur (Hemiptera: Miridae). Mycorrhizal colonization in tomato plants positively influenced the predator host-plant acceptance for feeding and oviposition, as well as nymphal survival and female weight. We hypothesize that AMF can modify mirid bug foraging behavior and performance.     

Seasonal differences in arbuscular mycorrhizal fungal communities in two woody species dominating semiarid caatinga forests

Tropical dry forests are strongly affected by seasonality, but its effects on belowground communities are poorly studied. Thus, the objective of this study was to reveal the effect of the season (dry versus wet) on the mycorrhizal status of roots and their potential colonization, and to determine the composition and abundance of spore-based communities of arbuscular mycorrhizal fungi (AMF) in rhizospheric soil of two dominant woody species in caatinga communities (tropical dry forest of the Brazilian Northeast). Soil and root samples were taken four times in each season (dry and wet). In the cases of the number of glomerospores and the number of infective propagules of AMF, there were significant differences between the hosts, with greater values observed in the rhizosphere of Commiphora leptophloeos than Mimosa tenuiflora. Mycorrhizal colonization and the number of infective propagules of AMF differed also between the seasons, being higher in the dry than the wet season. In total, fourteen AMF species were found in the rhizosphere of C. leptophloeos and twelve species were associated with M. tenuiflora. There was a predominance of the fungal genus Acaulospora, with seven species, followed by Gigaspora and Glomus. The species studied and the seasons differ in the composition and structure of the AMF community in the rhizosphere of the plants. The ecological significance of those differences needs to be examined further.     

Different levels of hyphal self-incompatibility modulate interconnectedness of mycorrhizal networks in three arbuscular mycorrhizal fungi within the Glomeraceae

Arbuscular mycorrhizal fungi (AMF) live in symbiosis with most plant species and produce underground extraradical hyphal networks functional in the uptake and translocation of mineral nutrients from the soil to host plants. This work investigated whether fungal genotype can affect patterns of interconnections and structural traits of extraradical mycelium (ERM), by comparing three Glomeraceae species growing in symbiosis with five plant hosts. An isolate of Funneliformis coronatus consistently showed low ability to form interconnected ERM and self-incompatibility that represented up to 21 % of hyphal contacts. The frequency of post-fusion self-incompatible interactions, never detected before in AMF extraradical networks, was 8.9 %. In F. coronatus ERM, the percentage of hyphal contacts leading to perfect hyphal fusions was 1.2–7.7, while it ranged from 25.8–48 to 35.6–53.6 in Rhizophagus intraradices and Funneliformis mosseae, respectively. Low interconnectedness of F. coronatus ERM resulted also from a very high number of non-interacting contacts (83.2 %). Such findings show that AMF genotypes in Glomeraceae can differ significantly in anastomosis behaviour and that ERM interconnectedness is modulated by the fungal symbiont, as F. coronatus consistently formed poorly interconnected networks when growing in symbiosis with five different host plants and in the asymbiotic stage. Structural traits, such as extent, density and hyphal self-compatibility/incompatibility, may represent key factors for the differential performance of AMF, by affecting fungal absorbing surface and foraging ability and thus nutrient flow from soil to host roots.     

Mycorrhizal benefits on native plants of the Caatinga,a Brazilian dry tropical forest

In the terrestrial ecosystems, soil is an important component, characterized by holding high diversity of microorganisms which play a key role for productivity and vegetal composition. The group of symbionts microorganisms stands out for contributing directly to the growth and plant nutrition, and among them, the arbuscular mycorrhizal fungi form one of the oldest and well established associations. In order to increase the knowledge and contribute for further research with AMF and plants of Caatinga, in this review we compile data from previous studies on the effects of symbiosis between arbuscular mycorrhizal fungi (AMF) and plants of the Caatinga, a type of dry tropical forest found in the northeast of Brazil. These studies collected data under various experimental conditions, emphasizing fungal efficiency and host responsiveness in soils with varied fertility. From our analysis we conclude that in general the symbiotic efficiency on these plants depends on many factors, such as the plant-fungi combination, fertility and soil type. Furthermore, in leguminosae the impact of a joint inoculation with nitrogen fixing bacteria must be taken into account. Claroideoglomus etunicatum was the most tested AMF species benefiting almost all plants tested. Approximately 30 plant species were studied regarding possible benefits provided by AMF and of these only Hymenea courbaril and Aspidosperma pyrifolium did not respond to mycorrhization. Higher efficiency of the mycorrhizal symbiosis can be obtained in soils with low P levels, emphasizing the essential role of these microorganisms in the growth and survival of plant species from the Caatinga biome.     

Antiviral activity of <Emphasis Type="Italic">Poncirus trifoliata</Emphasis> seed extract against oseltamivir-resistant influenza virus

The emergence of oseltamivir-resistant variants of influenza virus has highlighted the necessity for the development of more effective novel antiviral drugs. To date, numerous researchers have focused on developing antiviral drugs using natural resources, such as traditional herbal medicines. Poncirus trifoliata is widely used in oriental medicine as a remedy for gastritis, dysentery, inflammation and digestive ulcers. In this study, we investigated the potential antiviral effect of the Poncirus trifoliata orange seed extract against influenza virus. An ethanol extract of Poncirus trifoliata seeds (PTex) inhibited the activity of influenza viruses, in particular, oseltamivir- resistant strains, in Madin-Darby canine kidney cells. In contrast to oseltamivir, PTex exerted a significant inhibitory effect on the cellular penetration pathway of the virus rather than HA receptor binding. The potent antiviral effect and novel working mechanism of PTex support its further development as an effective natural antiviral drug with a wide spectrum of activity against influenza and oseltamivir-resistant viruses.     

Effect of AMF inoculum from field isolates on the yield of green pepper,parsley, carrot,and tomato

Inoculum of an indigenous mixture of arbuscular mycorrhizal fungi (AMF) containingGlomus mosseae, Glomus fasciculatum, Glomus etunicatum, Glomus intraradices andScutellospora sp. was applied to four of the most frequently used crop species in Slovenia: green pepper (Capsicum annuum), parsley (Petroselinum crispum), carrot (Daucus carrota) and tomato (Lycopersicon esculentum). A simple, feasible, and effective protocol for application of AMF biotechnology in horticulture was adopted.Mycorrhizal inoculation significantly increased the plant biomass parameters of pepper, and parsley and the root biomass of carrots. Statistically significant correlations between biomass parameters of pepper, parsley, and the root biomass of carrots with mycorrhizal colonization parameters (mycorrhizal frequency (F%), global mycorrhizal intensity (M%) and arbuscular richness (A%) were calculated. A significant increase in chlorophyll content was observed in mycorrhizal parsley and a significant increase in carotenoids was observed in mycorrhizal parsley, carrots, and tomato fruits. A significant increase in titratable acidity of fruits from inoculated tomato plants indicates prolonged fruiting period of mycorrhizal tomatoes. In addition, inoculation with an indigenous AMF mixture significantly increased the mycorrhizal potential of soil and thus the growth of non-inoculated plants in the second season. Thus, the results confirmed the potential of applying mycorrhizal biotechnology in sustainable horticulture.     

Mycorrhiza alters the profile of root hairs in trifoliate orange

Root hairs and arbuscular mycorrhiza (AM) coexist in root systems for nutrient and water absorption, but the relation between AM and root hairs is poorly known. A pot study was performed to evaluate the effects of four different AM fungi (AMF), namely, Claroideoglomus etunicatum, Diversispora versiformis, Funneliformis mosseae, and Rhizophagus intraradices on root hair development in trifoliate orange (Poncirus trifoliata) seedlings grown in sand. Mycorrhizal seedlings showed significantly higher root hair density than non-mycorrhizal seedlings, irrespective of AMF species. AMF inoculation generally significantly decreased root hair length in the first- and second-order lateral roots but increased it in the third- and fourth-order lateral roots. AMF colonization induced diverse responses in root hair diameter of different order lateral roots. Considerably greater concentrations of phosphorus (P), nitric oxide (NO), glucose, sucrose, indole-3-acetic acid (IAA), and methyl jasmonate (MeJA) were found in roots of AM seedlings than in non-AM seedlings. Levels of P, NO, carbohydrates, IAA, and MeJA in roots were correlated with AM formation and root hair development. These results suggest that AMF could alter the profile of root hairs in trifoliate orange through modulation of physiological activities. F. mosseae, which had the greatest positive effects, could represent an efficient AM fungus for increasing fruit yields or decreasing fertilizer inputs in citrus production.     

High-density linkage maps for <Emphasis Type="Italic">Citrus sunki</Emphasis> and <Emphasis Type="Italic">Poncirus trifoliata</Emphasis> using DArTseq markers

The construction of a high-resolution genetic map of citrus would be of great value to breeders and to associate genomic regions with characteristics of agronomic interest. Here, we describe a novel high-resolution map of citrus using a population derived from a controlled cross between Citrus sunki (female parent) and Poncirus trifoliata (male parent). The genetic linkage maps were constructed using DArTseq markers and a pseudo-testcross strategy; only markers showing the expected segregation ratio were considered. To investigate synteny, all markers from both linkage maps were aligned with the genome of Citrus sinensis. The C. sunki map has a total of 2778 molecular markers and a size of 2446.6 cM, distributed across ten linkage groups. The map of P. trifoliata was built with 3084 markers distributed in a total of nine linkage groups, with a total size of 2411.6 cM. These maps are the most saturated linkage maps available for C. sunki and P. trifoliata and have high genomic coverage. We also demonstrated that the maps reported here are closely related to the reference genome of C. sinensis.     

Effects of using different host plants and long-term fertilization systems on population sizes of infective arbuscular mycorrhizal fungi

The effect of cultivation of mycorrhizal and non-mycorrhizal plants and mineral fertilization on the arbuscular mycorrhizal fungal (AMF) community structure of maize (Zea mays L.) plants was studied. Soil samples were collected from two field experiments treated for 5 years with three fertilization systems (Control – no fertilization; Mineral – NPK fertilization; and Organic – Farmyard manure fertilization). Soil samples containing soil and root fragments of rapeseed (Brassica napus L., non-mycorrhizal plant) and wheat (Triticum aestivum L., mycorrhizal plant) collected from the field plots were used as native microbial inoculum sources to maize plants. Maize plants were sown in pots containing these inoculum sources for four months under glasshouse conditions. Colonization of wheat roots by AMF, AMF community structure, AMF diversity (Shannon’s index), AMF dominance (Simpson’s index) and growth of maize were investigated. Sixteen AMF species were identified from rhizosphere soil samples as different species of genera Acaulospora, Claroideoglomus, Dentiscutata, Funneliformis, Gigaspora, Quatunica, Racocetra, and Rhizoglomus. Maize plants grown in manure-fertilized soils had a distinct AMF community structure from plants either fertilized with mineral NPK-fertilizer or non-fertilized. The results also showed that inoculum from non-mycorrhizal plants combined with mineral fertilization decreased AMF diversity (Shannon’s index), AMF dominance (Simpson’s index) and growth of maize. Our findings suggest that non-mycorrhizal plants, such as B. napus, can negatively affect the presence and the effects of soil inoculation on maize growth. Also, our results highlight the importance of considering the long-term effect of rapeseed cultivation system on the reduction of population sizes of infective AMF, and its effect on succeeding annual crops.