Impact of arbuscular mycorrhizal fungi on the allergenic potential of tomato

Effects of Arbuscular Mycorrhization on Fruit Quality in Industrialized Tomato Production

Industrialized tomato production faces a decrease in flavors and nutritional value due to conventional breeding. Moreover, tomato production heavily relies on nitrogen and phosphate fertilization. Phosphate uptake and improvement of fruit quality by arbuscular mycorrhizal (AM) fungi are well-studied. We addressed the question of whether commercially used tomato cultivars grown in a hydroponic system can be mycorrhizal, leading to improved fruit quality. Tomato plants inoculated with Rhizophagus irregularis were grown under different phosphate concentrations and in substrates used in industrial tomato production. Changes in fruit gene expression and metabolite levels were checked by RNAseq and metabolite determination, respectively. The tests revealed that reduction of phosphate to 80% and use of mixed substrate allow AM establishment without affecting yield. By comparing green fruits from non-mycorrhizal and mycorrhizal plants, differentially expressed genes (DEGs) were found to possibly be involved in processes regulating fruit maturation and nutrition. Red fruits from mycorrhizal plants showed a trend of higher BRIX values and increased levels of carotenoids in comparison to those from non-mycorrhizal plants. Free amino acids exhibited up to four times higher levels in red fruits due to AM, showing the potential of mycorrhization to increase the nutritional value of tomatoes in industrialized production.

Gr and hp-1 tomato mutants unveil unprecedented interactions between arbuscular mycorrhizal symbiosis and fruit ripening

Systemic responses to an arbuscular mycorrhizal fungus reveal opposite phenological patterns in two tomato ripening mutants depending whether ethylene or light reception is involved. The availability of tomato ripening mutants has revealed many aspects of the genetics behind fleshy fruit ripening, plant hormones and light signal reception. Since previous analyses revealed that arbuscular mycorrhizal symbiosis influences tomato berry ripening, we wanted to test the hypothesis that an interplay might occur between root symbiosis and fruit ripening. With this aim, we screened seven tomato mutants affected in the ripening process for their responsiveness to the arbuscular mycorrhizal fungus Funneliformis mosseae. Following their phenological responses we selected two mutants for a deeper analysis: Green ripe (Gr), deficient in fruit ethylene perception and high-pigment-1 (hp-1), displaying enhanced light signal perception throughout the plant. We investigated the putative interactions between ripening processes, mycorrhizal establishment and systemic effects using biochemical and gene expression tools. Our experiments showed that both mutants, notwithstanding a normal mycorrhizal phenotype at root level, exhibit altered arbuscule functionality. Furthermore, in contrast to wild type, mycorrhization did not lead to a higher phosphate concentration in berries of both mutants. These results suggest that the mutations considered interfere with arbuscular mycorrhiza inducing systemic changes in plant phenology and fruits metabolism. We hypothesize a cross talk mechanism between AM and ripening processes that involves genes related to ethylene and light signaling.

Impact of Wild Loci on the Allergenic Potential of Cultivated Tomato Fruits

Tomato (Solanum lycopersicum) is one of the most extensively consumed vegetables but, unfortunately, it is also able to induce allergic reactions. In the past, it has been shown that the choice of tomato cultivar significantly influenced the allergic reaction of tomato allergic subjects. In this study we investigated the allergenic potential of the cultivated tomato line M82 and of two selected lines carrying small chromosome regions from the wild species Solanum pennellii (i.e. IL7-3 and IL12-4). We evaluated the positive interactions of IgEs of allergic subjects in order to investigate the different allergenic potential of the lines under investigation. We used proteomic analyses in order to identify putative tomato allergens. In addition, bioinformatic and transcriptomic approaches were applied in order to analyse the structure and the expression profiles of the identified allergen-encoding genes. These analyses demonstrated that fruits harvested from the two selected introgression lines harbour a different allergenic potential as those from the cultivated genotype M82. The different allergenicity found within the three lines was mostly due to differences in the IgE recognition of a polygalacturonase enzyme (46 kDa), one of the major tomato allergens, and of a pectin methylesterase (34 kDa); both the proteins were more immunoreactive in IL7-3 compared to IL12-4 and M82. The observed differences in the allergenic potential were mostly due to line-dependent translational control or post-translational modifications of the allergens. We demonstrated, for the first time, that the introgression from a wild species (S. pennellii) in the genomic background of a cultivated tomato line influences the allergenic properties of the fruits. Our findings could support the isolation of favorable wild loci promoting low allergenic potential in tomato.

Arbuscular mycorrhizal fungi affect glucosinolate and mineral element composition in leaves of Moringa oleifera

Moringa is a mycorrhizal crop cultivated in the tropics and subtropics and appreciated for its nutritive and health-promoting value. As well as improving plant mineral nutrition, arbuscular mycorrhizal fungi (AMF) can affect plant synthesis of compounds bioactive against chronic diseases in humans. Rhizophagus intraradices and Funneliformis mosseae were used in a full factorial experiment to investigate the impact of AMF on the accumulation of glucosinolates, flavonoids, phenolic acids, carotenoids, and mineral elements in moringa leaves. Levels of glucosinolates were enhanced, flavonoids and phenolic acids were not affected, levels of carotenoids (including provitamin A) were species-specifically reduced, and mineral elements were affected differently, with only Cu and Zn being increased by the AMF. This study presents novel results on AMF effects on glucosinolates in leaves and supports conclusions that the impacts of these fungi on microelement concentrations in edible plants are species dependent. The nonspecific positive effects on glucosinolates and the species-specific negative effects on carotenoids encourage research on other AMF species to achieve general benefits on bioactive compounds in moringa.

Comparison of systemic and local interactions between the arbuscular mycorrhizal fungus Funneliformis mosseae and the root pathogen Aphanomyces euteiches in Medicago truncatula

It has been shown in a number of pathosystems that arbuscular mycorrhizal (AM) fungi confer resistance against root pathogens, including in interactions between Medicago truncatula and the root rot-causing oomycete Aphanomyces euteiches. For the current study of these interactions, a split root system was established for plant marker gene analysis in order to study systemic defense responses and to compare them with local interactions in conventional pot cultures. It turned out, however, that split root systems and pot cultures were in different physiological stages. Genes for pathogenesis-related proteins and for enzymes involved in flavonoid biosynthesis were generally more highly expressed in split root systems, accompanied by changes in RNA accumulation for genes encoding enzymes involved in phytohormone biosynthesis. Against expectations, the pathogen showed increased activity in these split root systems when the AM fungus Funneliformis mosseae was present separately in the distal part of the roots. Gene expression analysis revealed that this is associated in the pathogen-infected compartment with a systemic down-regulation of a gene coding for isochorismate synthase (ICS), a key enzyme of salicylic acid biosynthesis. At the same time, transcripts of genes encoding pathogenesis-related proteins and for enzymes involved in the biosynthesis of flavonoids accumulated to lower levels. In conventional pot cultures showing decreased A. euteiches activity in the presence of the AM fungus, the ICS gene was down regulated only if both the AM fungus and the pathogen were present in the root system. Such negative priming of salicylic acid biosynthesis could result in increased activities of jasmonate-regulated defense responses and could explain mycorrhiza-induced resistance. Altogether, this study shows that the split root system does not reflect a systemic interaction between F. mosseae and A. euteiches in M. truncatula and indicates the importance of testing such systems prior to the analysis of mycorrhiza-induced resistance.

Overview of plant chitinases identified as food allergens

Food allergies are induced by proteins belonging to a limited number of families. Unfortunately, relationships between protein structure and capacity to induce the immune response have not been completely clarified yet, which precludes possible improvements in the diagnosis, prevention, and therapy of allergies. Plant chitinases constitute a good example of food allergenic proteins for which structural analysis of allergenicity has only been carried out partially. In plants, there are at least five structural classes of chitinases plus a number of chitinase-related polypeptides. Their allergenicity has been mostly investigated for chitinases of class I, due to both their higher prevalence among plant chitinases and by the high structural similarity between their substrate-binding domain and hevein, a well-known allergen present in the latex of rubber trees. Even if allergenic molecules have been identified for at least three other classes of plant chitinases, the involvement of the different structural motifs in the allergenicity of molecules has been disregarded so far. In this review, we provide a structurally based catalog of plant chitinases investigated for allergenicity, which could be a useful base for further studies aimed at better clarifying the structure-allergenicity relationships for this protein family.

The reduced mycorrhizal colonisation (rmc) mutation of tomato disrupts five gene sequences including the CYCLOPS/IPD3 homologue

Arbuscular mycorrhizal (AM) symbiosis in vascular plant roots is an ancient mutualistic interaction that evolved with land plants. More recently evolved root mutualisms have recruited components of the AM signalling pathway as identified with molecular approaches in model legume research. Earlier we reported that the reduced mycorrhizal colonisation (rmc) mutation of tomato mapped to chromosome 8. Here we report additional functional characterisation of the rmc mutation using genotype grafts and proteomic and transcriptomic analyses. Our results led to identification of the precise genome location of the Rmc locus from which we identified the mutation by sequencing. The rmc phenotype results from a deletion that disrupts five predicted gene sequences, one of which has close sequence match to the CYCLOPS/IPD3 gene identified in legumes as an essential intracellular regulator of both AM and rhizobial symbioses. Identification of two other genes not located at the rmc locus but with altered expression in the rmc genotype is also described. Possible roles of the other four disrupted genes in the deleted region are discussed. Our results support the identification of CYCLOPS/IPD3 in legumes and rice as a key gene required for AM symbiosis. The extensive characterisation of rmc in comparison with its 'parent' 76R, which has a normal mycorrhizal phenotype, has validated these lines as an important comparative model for glasshouse and field studies of AM and non-mycorrhizal plants with respect to plant competition and microbial interactions with vascular plant roots.

Pepino mosaic virus infection of tomato affects allergen expression, but not the allergenic potential of fruits

The plant pathogen Pepino mosaic virus (PepMV) is a major disease of greenhouse tomato crops worldwide. Plant pathogens can induce expression of defence- or pathogenesis-related proteins, including identified allergens. Therefore we hypothesised that PepMV infection results in the expression of allergens leading to a higher allergenic potential of tomato fruits. Transcript level analyses showed differential expression of 17 known and putative tomato fruit allergen encoding genes at early and late time points after PepMV inoculation, but no general induction was detected. Immunoblot analyses were conducted and IgEs from a serum pool of tomato allergic subjects reacted with 20 proteins, of which ten have not yet been described. In parallel, skin prick tests with a group of tomato allergic subjects did not show a general difference between PepMV infected and non-infected tomato fruits and basophil activation tests confirmed these results. In summary, PepMV infection of tomato plants can lead to long-lasting up-regulation of particular allergens in fruits, but the hypothesis that this results in a higher allergenic potential of the fruits proved invalid.

The arbuscular mycorrhizal status has an impact on the transcriptome profile and amino acid composition of tomato fruit

BACKGROUND

Arbuscular mycorrhizal (AM) symbiosis is the most widespread association between plant roots and fungi in natural and agricultural ecosystems. This work investigated the influence of mycorrhization on the economically relevant part of the tomato plant, by analyzing its impact on the physiology of the fruit. To this aim, a combination of phenological observations, transcriptomics (Microarrays and qRT-PCR) and biochemical analyses was used to unravel the changes that occur on fruits from Micro-Tom tomato plants colonized by the AM fungus Glomus mosseae.

RESULTS

Mycorrhization accelerated the flowering and fruit development and increased the fruit yield. Eleven transcripts were differentially regulated in the fruit upon mycorrhization, and the mycorrhiza-responsive genes resulted to be involved in nitrogen and carbohydrate metabolism as well as in regulation and signal transduction. Mycorrhization has increased the amino acid abundance in the fruit from mycorrhizal plants, with glutamine and asparagine being the most responsive amino acids.

CONCLUSIONS

The obtained results offer novel data on the systemic changes that are induced by the establishment of AM symbiosis in the plant, and confirm the work hypothesis that AM fungi may extend their influence from the root to the fruit.