Protein actors sustaining arbuscular mycorrhizal symbiosis: underground artists break the silence

Successional adaptive strategies revealed by correlating arbuscular mycorrhizal fungal abundance with host plant gene expression

The shifts in adaptive strategies revealed by ecological succession and the mechanisms that facilitate these shifts are fundamental to ecology. These adaptive strategies could be particularly important in communities of arbuscular mycorrhizal fungi (AMF) mutualistic with sorghum where strong AMF succession replaces initially ruderal species with competitive ones and where the strongest plant response to drought is to manage these AMF. Although most studies of agriculturally important fungi focus on parasites, the mutualistic symbionts, AMF, constitute a research system of human-associated fungi whose relative simplicity and synchrony are conducive to experimental ecology. First, we hypothesize that, when irrigation is stopped to mimic drought, competitive AMF species should be replaced by AMF species tolerant to drought stress. We then, for the first time, correlate AMF abundance and host plant transcription to test two novel hypotheses about the mechanisms behind the shift from ruderal to competitive AMF. Surprisingly, despite imposing drought stress, we found no stress tolerant AMF, likely due to our agricultural system having been irrigated for nearly six decades. Remarkably, we found strong and differential correlation between the successional shift from ruderal to competitive AMF and sorghum genes whose products (i) produce and release strigolactone signals, (ii) perceive mycorrhizal-lipochitinoligosaccharide (Myc-LCO) signals, (iii) provide plant lipid and sugar to AMF and, (iv) import minerals and water provided by AMF. These novel insights frame new hypotheses about AMF adaptive evolution and suggest a rationale for selecting AMF to reduce inputs and maximize yields in commercial agriculture.

Important innate differences in determining symbiotic responsiveness in host and non-hosts of arbuscular mycorrhiza

Genetic components that regulate arbuscular mycorrhizal (AM) interactions in hosts and non-hosts are not completely known. Comparative transcriptomic analysis was combined with phylogenetic studies to identify the factors that distinguish AM host from non-host. Mycorrhized host, non-mycorrhized host and non-host cultivars of tomato (Solanum lycopersicum) were subjected to RNA seq analysis. The top 10 differentially expressed genes were subjected to extensive in silico phylogenetic analysis along with 10 more candidate genes that have been previously reported for AM-plant interactions. Seven distantly related hosts and four non-hosts were selected to identify structural differences in selected gene/protein candidates. The screened genes/proteins were subjected to MEME, CODEML and DIVERGE analysis to identify evolutionary patterns that differentiate hosts from non-hosts. Based on the results, candidate genes were categorized as highly influenced (SYMRK and CCaMK), moderately influenced and minimally influenced by evolutionary constraints. We propose that the amino acid and nucleotide changes specific to non-hosts are likely to correspond to aberrations in functionality towards AM symbiosis. This study paves way for future research aimed at understanding innate differences in genetic make-up of AM hosts and non-hosts, in addition to the theory of gene losses from the "AM-symbiotic toolkit".

Arbuscular Mycorrhizal Fungi Colonization Promotes Changes in the Volatile Compounds and Enzymatic Activity of Lipoxygenase and Phenylalanine Ammonia Lyase in Piper nigrum L. 'Bragantina'

Arbuscular mycorrhizal fungi (AMF) have been used to promote numerous benefits to plants. In this study, we evaluated the symbiosis between AMF species (Rhizophagus clarus, Claroideoglomus etunicatum) and Piper nigrum L. 'Bragantina'. Volatile compounds, lipoxygenase (LOX) and phenylalanine ammonia-lyase (PAL) activities, and total phenolic content were monitored from 1 to 60 days post-inoculation (dpi). Hyphae, arbuscles, and vesicles were observed during the root colonization. In the leaves, AMF induced an increase of sesquiterpene hydrocarbons (54.0%-79.0%) and a decrease of oxygenated sesquiterpenes (41.3%-14.5%) at 7 dpi and 60 dpi (41.8%-21.5%), respectively. Cubenol, the main volatile compound of leaves, showed a significant decrease at 7 dpi (21.5%-0.28%) and 45 dpi (20.4%-18.42%). β-caryophyllene, the major volatile compound of the roots, displayed a significant reduction at 45 dpi (30.0%-20.0%). LOX increased in the roots at 21, 30, and 60 dpi. PAL was higher in leaves during all periods, except at 60 dpi, and increased at 21 and 45 dpi in the roots. The total phenolic content showed a significant increase only in the roots at 30 dpi. The results suggested that AMF provided changes in the secondary metabolism of P. nigrum, inducing its resistance.

Metabolome changes are induced in the arbuscular mycorrhizal fungus Gigaspora margarita by germination and by its bacterial endosymbiont

Metabolomic profiling is becoming an increasingly important technique in the larger field of systems biology by allowing the simultaneous measurement of thousands of small molecules participating in and resulting from cellular reactions. In this way, metabolomics presents an opportunity to observe the physiological state of a system, which may provide the ability to monitor the whole of cellular metabolism as the technology progresses. The arbuscular mycorrhizal fungus Gigaspora margarita has not previously been explored with regard to metabolite composition. To develop a better understanding of G. margarita and the influences of its endosymbiont Candidatus Glomeribacter gigasporarum, a metabolomic analysis was applied to quiescent and germinated spores with and without endobacteria. Over 100 metabolites were identified and greater than 2600 unique unidentified spectral features were observed. Multivariate analysis of the metabolomes was performed, and a differentiation between all metabolic states of spores and spores hosting the endobacteria was observed. The known metabolites were recruited to many biochemical pathways, with many being involved in maintenance of the antioxidant potential, tyrosine metabolism, and melanin production. Each of the pathways had higher metabolite abundances in the presence of the endosymbiont. These metabolomics data also agree with previously reported transcriptomics results demonstrating the capability of this technique to confirm hypotheses and showing the feasibility of multi-omic approaches for the study of arbuscular mycorrhizal fungi and their endobacterial communities. Challenges still exist in metabolomic analysis, e.g., the identification of compounds is demanding due to incomplete libraries. A metabolomics technique to probe the effects of bacterial endosymbionts on fungal physiology is presented herein, and this method is useful for hypothesis generation as well as testing as noted above.

Transcriptome changes induced by arbuscular mycorrhizal fungi in sunflower (Helianthus annuus L.) roots

Arbuscular mycorrhizal (AM) fungi are essential elements of soil fertility, plant nutrition and productivity, facilitating soil mineral nutrient uptake. Helianthus annuus is a non-model, widely cultivated species. Here we used an RNA-seq approach for evaluating gene expression variation at early and late stages of mycorrhizal establishment in sunflower roots colonized by the arbuscular fungus Rhizoglomus irregulare. mRNA was isolated from roots of plantlets at 4 and 16 days after inoculation with the fungus. cDNA libraries were built and sequenced with Illumina technology. Differential expression analysis was performed between control and inoculated plants. Overall 726 differentially expressed genes (DEGs) between inoculated and control plants were retrieved. The number of up-regulated DEGs greatly exceeded the number of down-regulated DEGs and this difference increased in later stages of colonization. Several DEGs were specifically involved in known mycorrhizal processes, such as membrane transport, cell wall shaping, and other. We also found previously unidentified mycorrhizal-induced transcripts. The most important DEGs were carefully described in order to hypothesize their roles in AM symbiosis. Our data add a valuable contribution for deciphering biological processes related to beneficial fungi and plant symbiosis, adding an Asteraceae, non-model species for future comparative functional genomics studies.

Strigolactone Levels in Dicot Roots Are Determined by an Ancestral Symbiosis-Regulated Clade of the PHYTOENE SYNTHASE Gene Family

Strigolactones (SLs) are apocarotenoid phytohormones synthesized from carotenoid precursors. They are produced most abundantly in roots for exudation into the rhizosphere to cope with mineral nutrient starvation through support of root symbionts. Abscisic acid (ABA) is another apocarotenoid phytohormone synthesized in roots, which is involved in responses to abiotic stress. Typically low carotenoid levels in roots raise the issue of precursor supply for the biosynthesis of these two apocarotenoids in this organ. Increased ABA levels upon abiotic stress in Poaceae roots are known to be supported by a particular isoform of phytoene synthase (PSY), catalyzing the rate-limiting step in carotenogenesis. Here we report on novel PSY3 isogenes from Medicago truncatula (MtPSY3) and Solanum lycopersicum (SlPSY3) strongly expressed exclusively upon root interaction with symbiotic arbuscular mycorrhizal (AM) fungi and moderately in response to phosphate starvation. They belong to a widespread clade of conserved PSYs restricted to dicots (dPSY3) distinct from the Poaceae-PSY3s involved in ABA formation. An ancient origin of dPSY3s and a potential co-evolution with the AM symbiosis is discussed in the context of PSY evolution. Knockdown of MtPSY3 in hairy roots of M. truncatula strongly reduced SL and AM-induced C₁₃ α-ionol/C₁₄ mycorradicin apocarotenoids. Inhibition of the reaction subsequent to phytoene synthesis revealed strongly elevated levels of phytoene indicating induced flux through the carotenoid pathway in roots upon mycorrhization. dPSY3 isogenes are coregulated with upstream isogenes and downstream carotenoid cleavage steps toward SLs (D27, CCD7, CCD8) suggesting a combined carotenoid/apocarotenoid pathway, which provides "just in time"-delivery of precursors for apocarotenoid formation.

An interdomain network: the endobacterium of a mycorrhizal fungus promotes antioxidative responses in both fungal and plant hosts

Arbuscular mycorrhizal fungi (AMF) are obligate plant biotrophs that may contain endobacteria in their cytoplasm. Genome sequencing of Candidatus Glomeribacter gigasporarum revealed a reduced genome and dependence on the fungal host. RNA-seq analysis of the AMF Gigaspora margarita in the presence and absence of the endobacterium indicated that endobacteria have an important role in the fungal pre-symbiotic phase by enhancing fungal bioenergetic capacity. To improve the understanding of fungal-endobacterial interactions, iTRAQ (isobaric tags for relative and absolute quantification) quantitative proteomics was used to identify differentially expressed proteins in G. margarita germinating spores with endobacteria (B+), without endobacteria in the cured line (B-) and after application of the synthetic strigolactone GR24. Proteomic, transcriptomic and biochemical data identified several fungal and bacterial proteins involved in interspecies interactions. Endobacteria influenced fungal growth, calcium signalling and metabolism. The greatest effects were on fungal primary metabolism and respiration, which was 50% higher in B+ than in B-. A shift towards pentose phosphate metabolism was detected in B-. Quantification of carbonylated proteins indicated that the B- line had higher oxidative stress levels, which were also observed in two host plants. This study shows that endobacteria generate a complex interdomain network that affects AMF and fungal-plant interactions.

Strigolactone-Induced Putative Secreted Protein 1 Is Required for the Establishment of Symbiosis by the Arbuscular Mycorrhizal Fungus Rhizophagus irregularis

Arbuscular mycorrhizal (AM) symbiosis is the most widespread association between plants and fungi. To provide novel insights into the molecular mechanisms of AM symbiosis, we screened and investigated genes of the AM fungus Rhizophagus irregularis that contribute to the infection of host plants. R. irregularis genes involved in the infection were explored by RNA-sequencing (RNA-seq) analysis. One of the identified genes was then characterized by a reverse genetic approach using host-induced gene silencing (HIGS), which causes RNA interference in the fungus via the host plant. The RNA-seq analysis revealed that 19 genes are up-regulated by both treatment with strigolactone (SL) (a plant symbiotic signal) and symbiosis. Eleven of the 19 genes were predicted to encode secreted proteins and, of these, SL-induced putative secreted protein 1 (SIS1) showed the largest induction under both conditions. In hairy roots of Medicago truncatula, SIS1 expression is knocked down by HIGS, resulting in significant suppression of colonization and formation of stunted arbuscules. These results suggest that SIS1 is a putative secreted protein that is induced in a wide spatiotemporal range including both the presymbiotic and symbiotic stages and that SIS1 positively regulates colonization of host plants by R. irregularis.

Apocarotenoids: A New Carotenoid-Derived Pathway

Carotenoids are precursors of carotenoid derived molecules termed apocarotenoids, which include isoprenoids with important functions in plant-environment interactions such as the attraction of pollinators and the defense against pathogens and herbivores. Apocarotenoids also include volatile aromatic compounds that act as repellents, chemoattractants, growth simulators and inhibitors, as well as the phytohormones abscisic acid and strigolactones. In plants, apocarotenoids can be found in several types of plastids (etioplast, leucoplast and chromoplast) and among different plant tissues such as flowers and roots. The structural similarity of some flower and spice isoprenoid volatile organic compounds (β-ionone and safranal) to carotenoids has led to the recent discovery of carotenoid-specific cleavage oxygenases, including carotenoid cleavage dioxygenases and 9-cis-epoxydioxygenases, which tailor and transform carotenoids into apocarotenoids. The great diversity of apocarotenoids is a consequence of the huge amount of carotenoid precursors, the variations in specific cleavage sites and the modifications after cleavage. Lycopene, β-carotene and zeaxanthin are the precursors of the main apocarotenoids described to date, which include bixin, crocin, picrocrocin, abscisic acid, strigolactone and mycorradicin.The current chapter will give rise to an overview of the biosynthesis and function of the most important apocarotenoids in plants, as well as the current knowledge about the carotenoid cleavage oxygenase enzymes involved in these biosynthetic pathways.

Rice arbuscular mycorrhiza as a tool to study the molecular mechanisms of fungal symbiosis and a potential target to increase productivity

Rice (Oryza sativa L.) is a monocot model crop for cereal molecular biology. Following the emergence of molecular genetics of arbuscular mycorrhizal (AM) symbiosis in model legumes in the 1990s, studies on rice genetic resources have considerably contributed to our understanding of the molecular mechanisms and evolution of root intracellular symbioses.In this review, we trace the history of these studies and suggest the potential utility of AM symbiosis for improvement in rice productivity.

Evolution of root-specific carotenoid precursor pathways for apocarotenoid signal biogenesis

Various cleavage products of C40 carotenoid substrates are formed preferentially or exclusively in roots. Such apocarotenoid signaling or regulatory compounds differentially induced in roots during environmental stress responses including root colonization by arbuscular mycorrhizal fungi include ABA, strigolactones and C13 α-ionol/C14 mycorradicin derivatives. The low carotenoid levels in roots raise the question of whether there is a regulated precursor supply channeled into apocarotenoid formation distinct from default carotenoid pathways. This review describes root-specific isogene components of carotenoid pathways toward apocarotenoid formation, highlighting a new PSY3 class of phytoene synthase genes in dicots. It is clearly distinct from the monocot PSY3 class co-regulated with ABA formation. At least two members of the exclusive dicot PSY3s are regulated by nutrient stress and mycorrhization. This newly recognized dicot PSY3 (dPSY3 vs. mPSY3 from monocots) class probably represents an ancestral branch in the evolution of the plant phytoene synthase family. The evolutionary history of PSY genes is compared with the evolution of MEP pathway isogenes encoding 1-deoxy-d-xylulose 5-phosphate synthases (DXS), particularly DXS2, which is co-regulated with dPSY3s in mycorrhizal roots. Such stress-inducible isoforms for rate-limiting steps in root carotenogenesis might be components of multi-enzyme complexes committed to apocarotenoid rather than to carotenoid formation.

Membrane transporters and drought resistance - a complex issue

Land plants have evolved complex adaptation strategies to survive changes in water status in the environment. Understanding the molecular nature of such adaptive changes allows the development of rapid innovations to improve crop performance. Plant membrane transport systems play a significant role when adjusting to water scarcity. Here we put proteins participating in transmembrane allocations of various molecules in the context of stomatal, cuticular, and root responses, representing a part of the drought resistance strategy. Their role in the transport of signaling molecules, ions or osmolytes is summarized and the challenge of the forthcoming research, resulting from the recent discoveries, is highlighted.