Phosphate Suppression of Arbuscular Mycorrhizal Symbiosis Involves Gibberellic Acid Signaling

Symbiont plasticity as a driver of plant success

We discuss which plant species are likely to become winners, that is achieve the highest global abundance, in changing landscapes, and whether plant-associated microbes play a determining role. Reduction and fragmentation of natural habitats in historic landscapes have led to the emergence of patchy, hybrid landscapes, and novel landscapes where anthropogenic ecosystems prevail. In patchy landscapes, species with broad niches are favoured. Plasticity in the degree of association with symbiotic microbes may contribute to broader plant niches and optimization of symbiosis costs and benefits, by downregulating symbiosis when it is unnecessary and upregulating it when it is beneficial. Plasticity can also be expressed as the switch from one type of mutualism to another, for example from nutritive to defensive mutualism with increasing soil fertility and the associated increase in parasite load. Upon dispersal, wide mutualistic partner receptivity is another facet of symbiont plasticity that becomes beneficial, because plants are not limited by the availability of specialist partners when arriving at new locations. Thus, under conditions of global change, symbiont plasticity allows plants to optimize the activity of mutualistic relationships, potentially allowing them to become winners by maximizing geographic occupancy and local abundance.

Root plasticity versus elasticity - when are responses acclimative?

Spatiotemporal soil heterogeneity and the resulting edaphic stress cycles can be decisive for crop growth. However, our understanding of the acclimative value of root responses to heterogeneous soil conditions remains limited. We outline a framework to evaluate the acclimative value of root responses that distinguishes between stress responses that are persistent and reversible upon stress release, termed 'plasticity' and 'elasticity', respectively. Using energy balances, we provide theoretical evidence that the advantage of plasticity over elasticity increases with the number of edaphic stress cycles and if responses lead to comparatively high energy gains. Our framework provides a conceptual basis for assessing the acclimative value of root responses to soil heterogeneity and can catalyse research on crop adaptations to heterogeneous belowground environments.

Monoterpene glucosides in Eustoma grandiflorum roots promote hyphal branching in arbuscular mycorrhizal fungi

Host plant-derived strigolactones trigger hyphal branching in arbuscular mycorrhizal (AM) fungi, initiating a symbiotic interaction between land plants and AM fungi. However, our previous studies revealed that gibberellin-treated lisianthus (Eustoma grandiflorum, Gentianaceae) activates rhizospheric hyphal branching in AM fungi using unidentified molecules other than strigolactones. In this study, we analyzed independent transcriptomic data of E. grandiflorum and found that the biosynthesis of gentiopicroside (GPS) and swertiamarin (SWM), characteristic monoterpene glucosides in Gentianaceae, was upregulated in gibberellin-treated E. grandiflorum roots. Moreover, these metabolites considerably promoted hyphal branching in the Glomeraceae AM fungi Rhizophagus irregularis and Rhizophagus clarus. GPS treatment also enhanced R. irregularis colonization of the monocotyledonous crop chive (Allium schoenoprasum). Interestingly, these metabolites did not provoke the germination of the root parasitic plant common broomrape (Orobanche minor). Altogether, our study unveiled the role of GPS and SWM in activating the symbiotic relationship between AM fungi and E. grandiflorum.

Tomato geranylgeranyl diphosphate synthase isoform 1 is involved in the stress-triggered production of diterpenes in leaves and strigolactones in roots

Carotenoids are photoprotectant pigments and precursors of hormones such as strigolactones (SL). Carotenoids are produced in plastids from geranylgeranyl diphosphate (GGPP), which is diverted to the carotenoid pathway by phytoene synthase (PSY). In tomato (Solanum lycopersicum), three genes encode plastid-targeted GGPP synthases (SlG1 to SlG3) and three genes encode PSY isoforms (PSY1 to PSY3). Here, we investigated the function of SlG1 by generating loss-of-function lines and combining their metabolic and physiological phenotyping with gene co-expression and co-immunoprecipitation analyses. Leaves and fruits of slg1 lines showed a wild-type phenotype in terms of carotenoid accumulation, photosynthesis, and development under normal growth conditions. In response to bacterial infection, however, slg1 leaves produced lower levels of defensive GGPP-derived diterpenoids. In roots, SlG1 was co-expressed with PSY3 and other genes involved in SL production, and slg1 lines grown under phosphate starvation exuded less SLs. However, slg1 plants did not display the branched shoot phenotype observed in other SL-defective mutants. At the protein level, SlG1 physically interacted with the root-specific PSY3 isoform but not with PSY1 and PSY2. Our results confirm specific roles for SlG1 in producing GGPP for defensive diterpenoids in leaves and carotenoid-derived SLs (in combination with PSY3) in roots.

Harnessing the Potential of Symbiotic Associations of Plants in Phosphate-Deficient Soil for Sustainable Agriculture

Many plants associate with arbuscular mycorrhizal (AM) fungi for nutrient acquisition, and most legumes also associate with nitrogen-fixing rhizobial bacteria for nitrogen acquisition. The association of plants with AM fungi and rhizobia depends on the perception of lipo-chitooligosaccharides (LCOs) produced by these micro-symbionts. Recent studies reveal that cereals can perceive LCOs better in soil deprived of phosphate (Pi) and nitrogen to activate symbiosis signaling and form efficient AM symbiosis. Nevertheless, the Pi deficiency in the soil hinders the symbiotic association of legumes with rhizobia, ultimately reducing nitrogen fixation. Here, we discuss a mechanistic overview of the factors regulating root nodule symbiosis under Pi-deficient conditions and further emphasize the possible ways to overcome this hurdle. Ignoring the low Pi problem not only can compromise the functionality of the nitrogen cycle by nitrogen fixation through legumes but can also put food security at risk globally. This review aims to bring the scientific community's attention toward the detrimental response of legumes toward Pi-deficient soil for the formation of root nodule symbiosis and hence reduced nitrogen fixation. In this review, we have highlighted the recent studies that have advanced our understanding of these critical areas and discussed some future directions. Furthermore, this review highlights the importance of communicating science with farmers and the agriculture community to fully harness the potential of the symbiotic association of plants in nutrient-deficient soil for sustainable agriculture.

Thriving beneath olive trees: The influence of organic farming on microbial communities

Soil health and root-associated microbiome are interconnected factors involved in plant health. The use of manure amendment on agricultural fields exerts a direct benefit on soil nutrient content and water retention, among others. However, little is known about the impact of manure amendment on the root-associated microbiome, particularly in woody species. In this study, we aimed to evaluate the effects of ovine manure on the microbial communities of the olive rhizosphere and root endosphere. Two adjacent orchards subjected to conventional (CM) and organic (OM) management were selected. We used metabarcoding sequencing to assess the bacterial and fungal communities. Our results point out a clear effect of manure amendment on the microbial community. Fungal richness and diversity were increased in the rhizosphere. The fungal biomass in the rhizosphere was more than doubled, ranging from 1.72 × 10⁶ ± 1.62 × 10⁵ (CM) to 4.54 × 10⁶ ± 8.07 × 10⁵ (OM) copies of the 18 S rRNA gene g^-1 soil. Soil nutrient content was also enhanced in the OM orchard. Specifically, oxidable organic matter, total nitrogen, nitrate, phosphorous, potassium and sulfate concentrations were significantly increased in the OM orchard. Moreover, we predicted a higher abundance of bacteria in OM with metabolic functions involved in pollutant degradation and defence against pathogens. Lastly, microbial co-occurrence network showed more positive interactions, complexity and shorter geodesic distance in the OM orchard. According to our results, manure amendment on olive orchards represents a promising tool for positively modulating the microbial community in direct contact with the plant.

Genome-Wide Analysis of the PHT Gene Family and Its Response to Mycorrhizal Symbiosis in Tomatoes under Phosphate Starvation Conditions

Phosphate is one of the essential mineral nutrients. Phosphate transporter genes (PHTs) play an important role in Pi acquisition and homeostasis in tomato plants. However, basic biological information on PHT genes and their responses of symbiosis with arbuscular mycorrhizal in the genome remains largely unknown. We analyzed the physiological changes and PHT gene expression in tomatoes (Micro-Tom) inoculated with arbuscular mycorrhizal (AM) fungi (Funneliformis mosseae) under different phosphate conditions (P1: 0 µM, P2: 25 µM, and P3: 200 µM Pi). Twenty-three PHT genes were identified in the tomato genomics database. Protein sequence alignment further divided the 23 PHT genes into three groups, with similar classifications of exons and introns. Good colonization of plants was observed under low phosphate conditions (25 µM Pi), and Pi stress and AM fungi significantly affected P and N accumulation and root morphological plasticity. Moreover, gene expression data showed that genes in the SlPHT1 (SlPT3, SlPT4, and SlPT5) gene family were upregulated by Funneliformis mosseae under all conditions, which indicated that these gene levels were significantly increased with AM fungi inoculation. None of the analyzed SlPHT genes in the SlPH2, SlPHT3, SlPHT4, and SlPHO gene families were changed at any Pi concentration. Our results indicate that inoculation with AM fungi mainly altered the expression of the PHT1 gene family. These results will lay a foundation for better understanding the molecular mechanisms of inorganic phosphate transport under AM fungi inoculation.

Arbuscular mycorrhizal fungi-indicative blumenol-C-glucosides predict lipid accumulations and fitness in plants grown without competitors

Hydroxy- and carboxyblumenol C-glucosides specifically accumulate in roots and leaves of plants harboring arbuscular mycorrhizal fungi (AMF). To explore blumenol function in AMF relationships, we silenced an early key-gene in blumenol biosynthesis, CCD1 (carotenoid cleavage dioxygenase 1), in the ecological model plant, Nicotiana attenuata, and analyzed whole-plant performance in comparison with control and CCaMK-silenced plants, unable to form AMF associations. Root blumenol accumulations reflected a plant's Darwinian fitness, as estimated by capsule production, and were positively correlated with AMF-specific lipid accumulations in roots, with relationships that changed as plants matured when grown without competitors. When grown with wild-type competitors, transformed plants with decreased photosynthetic capacity or increased carbon flux to roots had blumenol accumulations that predicted plant fitness and genotype trends in AMF-specific lipids, but had similar levels of AMF-specific lipids between competing plants, likely reflecting AMF-networks. We propose that when grown in isolation, blumenol accumulations reflect AMF-specific lipid allocations and plant fitness. When grown with competitors, blumenol accumulations predict fitness outcomes, but not the more complicated AMF-specific lipid accumulations. RNA-seq analysis provided candidates for the final biosynthetic steps of these AMF-indicative blumenol C-glucosides; abrogation of these steps will provide valuable tools for understanding blumenol function in this context-dependent mutualism.

Plant phosphate status influences root biotic interactions

Phosphorus (P) deficiency stress in combination with biotic stress(es) severely impacts crop yield. Plant responses to P deficiency overlapping with that of other stresses exhibit a high degree of complexity involving different signaling pathways. On the one hand, plants engage with rhizosphere microbiome/arbuscular mycorrhizal fungi for improved phosphate (Pi) acquisition and plant stress response upon Pi deficiency; on the other hand, this association is gets disturbed under Pi sufficiency. This nutrient-dependent response is highly regulated by the phosphate starvation response (PSR) mediated by the master regulator, PHR1, and its homolog, PHL. It is interesting to note that Pi status (deficiency/sufficiency) has a varying response (positive/negative) to different biotic encounters (beneficial microbes/opportunistic pathogens/insect herbivory) through a coupled PSR-PHR1 immune system. This also involves crosstalk among multiple players including transcription factors, defense hormones, miRNAs, and Pi transporters, among others influencing the plant-biotic-phosphate interactions. We provide a comprehensive view of these key players involved in maintaining a delicate balance between Pi homeostasis and plant immunity. Finally, we propose strategies to utilize this information to improve crop resilience to Pi deficiency in combination with biotic stresses.

Arbuscular mycorrhizal fungi enhanced rice proline metabolism under low temperature with nitric oxide involvement

Arbuscular mycorrhizal fungi (AMF) are known to improve plant stress tolerance by regulating proline accumulation, and nitric oxide (NO) plays an important signaling role in proline metabolism. Environmental nitrogen (N) affects AMF colonization and its contribution to host plants resistance to stress conditions. However, the relationship between proline metabolism and NO in mycorrhizal rice and the effect of N application on symbiont proline metabolism under low temperature have not been established. Pot culture experiments with different temperature, N and exogenous NO donor treatments were conducted with non-mycorrhizal and mycorrhizal rice. The results showed that AMF enhanced rice proline accumulation under low-temperature stress and decreased glutamate (Glu) and ornithine (Orn) concentrations significantly. In comparison with non-mycorrhizal rice, AMF colonization significantly decreased the Glu concentration, but had little effect on the Orn concentration under low-temperature stress, accompanied by increasing expression of OsP5CS2, OsOAT and OsProDH1. Exogenous application of NO increased proline concentration both under normal and low temperature, which exhibited a higher increase in mycorrhizal rice. NO also triggered the expression of key genes in the Glu and Orn pathways of proline synthesis as well as proline degradation. Higher N application decreased the AMF colonization, and AMF showed greater promotion of proline metabolism at low N levels under low temperature stress by regulating the Glu synthetic pathway. Meanwhile, AMF increased rice nitrate reductase (NR) and nitric oxide synthase (NOS) activities and then enhanced NO accumulation under low N levels. Consequently, it could be hypothesized that one of the mechanisms by which AMF improves plant resistance to low-temperature stress is the accumulation of proline via enhancement of the Glu and Orn synthetic pathways, with the involvement of the signaling molecule NO. However, the contribution of AMF to rice proline accumulation under low-temperature stress was attenuated by high N application.

Physiological and transcriptomic response of Medicago truncatula to colonization by high- or low-benefit arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi form a root endosymbiosis with many agronomically important crop species. They enhance the ability of their host to obtain nutrients from the soil and increase the tolerance to biotic and abiotic stressors. However, AM fungal species can differ in the benefits they provide to their host plants. Here, we examined the putative molecular mechanisms involved in the regulation of the physiological response of Medicago truncatula to colonization by Rhizophagus irregularis or Glomus aggregatum, which have previously been characterized as high- and low-benefit AM fungal species, respectively. Colonization with R. irregularis led to greater growth and nutrient uptake than colonization with G. aggregatum. These benefits were linked to an elevated expression in the roots of strigolactone biosynthesis genes (NSP1, NSP2, CCD7, and MAX1a), mycorrhiza-induced phosphate (PT8), ammonium (AMT2;3), and nitrate (NPF4.12) transporters and the putative ammonium transporter NIP1;5. R. irregularis also stimulated the expression of photosynthesis-related genes in the shoot and the upregulation of the sugar transporters SWEET1.2, SWEET3.3, and SWEET 12 and the lipid biosynthesis gene RAM2 in the roots. In contrast, G. aggregatum induced the expression of biotic stress defense response genes in the shoots, and several genes associated with abiotic stress in the roots. This suggests that either the host perceives colonization by G. aggregatum as pathogen attack or that G. aggregatum can prime host defense responses. Our findings highlight molecular mechanisms that host plants may use to regulate their association with high- and low-benefit arbuscular mycorrhizal symbionts.

Molecular Regulation of Arbuscular Mycorrhizal Symbiosis

Plant-microorganism interactions at the rhizosphere level have a major impact on plant growth and plant tolerance and/or resistance to biotic and abiotic stresses. Of particular importance for forestry and agricultural systems is the cooperative and mutualistic interaction between plant roots and arbuscular mycorrhizal (AM) fungi from the phylum Glomeromycotina, since about 80% of terrestrial plant species can form AM symbiosis. The interaction is tightly regulated by both partners at the cellular, molecular and genetic levels, and it is highly dependent on environmental and biological variables. Recent studies have shown how fungal signals and their corresponding host plant receptor-mediated signalling regulate AM symbiosis. Host-generated symbiotic responses have been characterized and the molecular mechanisms enabling the regulation of fungal colonization and symbiosis functionality have been investigated. This review summarizes these and other recent relevant findings focusing on the molecular players and the signalling that regulate AM symbiosis. Future progress and knowledge about the underlying mechanisms for AM symbiosis regulation will be useful to facilitate agro-biotechnological procedures to improve AM colonization and/or efficiency.

Conserved and Diverse Transcriptional Reprogramming Triggered by the Establishment of Symbioses in Tomato Roots Forming Arum-Type and Paris-Type Arbuscular Mycorrhizae

Arbuscular mycorrhizal (AM) fungi allocate mineral nutrients to their host plants, and the hosts supply carbohydrates and lipids to the fungal symbionts in return. The morphotypes of intraradical hyphae are primarily determined on the plant side into Arum- and Paris-type AMs. As an exception, Solanum lycopersicum (tomato) forms both types of AMs depending on the fungal species. Previously, we have shown the existence of diverse regulatory mechanisms in Arum- and Paris-type AM symbioses in response to gibberellin (GA) among different host species. However, due to the design of the study, it remained possible that the use of different plant species influenced the results. Here, we used tomato plants to compare the transcriptional responses during Arum- and Paris-type AM symbioses in a single plant species. The tomato plants inoculated with Rhizophagus irregularis or Gigaspora margarita exhibited Arum- and Paris-type AMs, respectively, and demonstrated similar colonization rates and shoot biomass. Comparative transcriptomics showed shared expression patterns of AM-related genes in tomato roots upon each fungal infection. On the contrary, the defense response and GA biosynthetic process was transcriptionally upregulated during Paris-type AM symbiosis. Thus, both shared and different transcriptional reprogramming function in establishing Arum- and Paris-type AM symbioses in tomato plants.

A novel SCARECROW-LIKE3 transcription factor LjGRAS36 in Lotus japonicus regulates the development of arbuscular mycorrhizal symbiosis

The symbiosis with arbuscular mycorrhizal (AM) fungi improves plants' nutrient uptake. During this process, transcription factors have been highlighted to play crucial roles. Members of the GRAS transcription factor gene family have been reported involved in AM symbiosis, but little is known about SCARECROW-LIKE3 (SCL3) genes belonging to this family in Lotus japonicus. In this study, 67 LjGRAS genes were identified from the L. japonicus genome, seven of which were clustered in the SCL3 group. Three of the seven LjGRAS genes expression levels were upregulated by AM fungal inoculation, and our biochemical results showed that the expression of LjGRAS36 was specifically induced by AM colonization. Functional loss of LjGRAS36 in mutant ljgras36 plants exhibited a significantly reduced mycorrhizal colonization rate and arbuscular size. Transcriptome analysis showed a deficiency of LjGRAS36 led to the dysregulation of the gibberellic acid signal pathway associated with AM symbiosis. Together, this study provides important insights for understanding the important potential function of SCL3 genes in regulating AM symbiotic development.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01161-z.

PHOSPHATE STARVATION RESPONSE transcription factors enable arbuscular mycorrhiza symbiosis

Arbuscular mycorrhiza (AM) is a widespread symbiosis between roots of the majority of land plants and Glomeromycotina fungi. AM is important for ecosystem health and functioning as the fungi critically support plant performance by providing essential mineral nutrients, particularly the poorly accessible phosphate, in exchange for organic carbon. AM fungi colonize the inside of roots and this is promoted at low but inhibited at high plant phosphate status, while the mechanistic basis for this phosphate-dependence remained obscure. Here we demonstrate that a major transcriptional regulator of phosphate starvation responses in rice PHOSPHATE STARVATION RESPONSE 2 (PHR2) regulates AM. Root colonization of phr2 mutants is drastically reduced, and PHR2 is required for root colonization, mycorrhizal phosphate uptake, and yield increase in field soil. PHR2 promotes AM by targeting genes required for pre-contact signaling, root colonization, and AM function. Thus, this important symbiosis is directly wired to the PHR2-controlled plant phosphate starvation response.

PHOSPHATE STARVATION RESPONSE transcription factors enable arbuscular mycorrhiza symbiosis

Arbuscular mycorrhiza (AM) is a widespread symbiosis between roots of the majority of land plants and Glomeromycotina fungi. AM is important for ecosystem health and functioning as the fungi critically support plant performance by providing essential mineral nutrients, particularly the poorly accessible phosphate, in exchange for organic carbon. AM fungi colonize the inside of roots and this is promoted at low but inhibited at high plant phosphate status, while the mechanistic basis for this phosphate-dependence remained obscure. Here we demonstrate that a major transcriptional regulator of phosphate starvation responses in rice PHOSPHATE STARVATION RESPONSE 2 (PHR2) regulates AM. Root colonization of phr2 mutants is drastically reduced, and PHR2 is required for root colonization, mycorrhizal phosphate uptake, and yield increase in field soil. PHR2 promotes AM by targeting genes required for pre-contact signaling, root colonization, and AM function. Thus, this important symbiosis is directly wired to the PHR2-controlled plant phosphate starvation response.

Conservation and Diversity in Gibberellin-Mediated Transcriptional Responses Among Host Plants Forming Distinct Arbuscular Mycorrhizal Morphotypes

Morphotypes of arbuscular mycorrhizal (AM) symbiosis, Arum, Paris, and Intermediate types, are mainly determined by host plant lineages. It was reported that the phytohormone gibberellin (GA) inhibits the establishment of Arum-type AM symbiosis in legume plants. In contrast, we previously reported that GA promotes the establishment of Paris-type AM symbiosis in Eustoma grandiflorum, while suppressing Arum-type AM symbiosis in a legume model plant, Lotus japonicus. This raises a hitherto unexplored possibility that GA-mediated transcriptional reprogramming during AM symbiosis is different among plant lineages as the AM morphotypes are distinct. Here, our comparative transcriptomics revealed that several symbiosis-related genes were commonly upregulated upon AM fungal colonization in L. japonicus (Arum-type), Daucus carota (Intermediate-type), and E. grandiflorum (Paris-type). Despite of the similarities, the fungal colonization levels and the expression of symbiosis-related genes were suppressed in L. japonicus and D. carota but were promoted in E. grandiflorum in the presence of GA. Moreover, exogenous GA inhibited the expression of genes involved in biosynthetic process of the pre-symbiotic signal component, strigolactone, which resulted in the reduction of its endogenous accumulation in L. japonicus and E. grandiflorum. Additionally, differential regulation of genes involved in sugar metabolism suggested that disaccharides metabolized in AM roots would be different between L. japonicus and D. carota/E. grandiflorum. Therefore, this study uncovered the conserved transcriptional responses during mycorrhization regardless of the distinct AM morphotype. Meanwhile, we also found diverse responses to GA among phylogenetically distant AM host plants.

Phosphate-Dependent Regulation of Growth and Stresses Management in Plants

The importance of phosphorus in the regulation of plant growth function is well studied. However, the role of the inorganic phosphate (Pi) molecule in the mitigation of abiotic stresses such as drought, salinity, heavy metal, heat, and acid stresses are poorly understood. We revisited peer-reviewed articles on plant growth characteristics that are phosphorus (P)-dependently regulated under the sufficient-P and low/no-P starvation alone or either combined with one of the mentioned stress. We found that the photosynthesis rate and stomatal conductance decreased under Pi-starved conditions. The total chlorophyll contents were increased in the P-deficient plants, owing to the lack of Pi molecules to sustain the photosynthesis functioning, particularly, the Rubisco and fructose-1,6-bisphosphatase function. The dry biomass of shoots, roots, and P concentrations were significantly reduced under Pi starvation with marketable effects in the cereal than in the legumes. To mitigate P stress, plants activate alternative regulatory pathways, the Pi-dependent glycolysis, and mitochondrial respiration in the cytoplasm. Plants grown under well-Pi supplementation of drought stress exhibited higher dry biomass of shoots than the no-P treated ones. The Pi supply to plants grown under heavy metals stress reduced the metal concentrations in the leaves for the cadmium (Cd) and lead (Pb), but could not prevent them from absorbing heavy metals from soils. To detoxify from heavy metal stress, plants enhance the catalase and ascorbate peroxidase activity that prevents lipid peroxidation in the leaves. The HvPIP and PHO1 genes were over-expressed under both Pi starvation alone and Pi plus drought, or Pi plus salinity stress combination, implying their key roles to mediate the stress mitigations. Agronomy Pi-based interventions to increase Pi at the on-farm levels were discussed. Revisiting the roles of P in growth and its better management in agricultural lands or where P is supplemented as fertilizer could help the plants to survive under abiotic stresses.