Medicago SPX1 and SPX3 regulate phosphate homeostasis, mycorrhizal colonization, and arbuscule degradation

Cross-kingdom nutrient exchange in the plant-arbuscular mycorrhizal fungus-bacterium continuum

The association between plants and arbuscular mycorrhizal fungi (AMF) affects plant performance and ecosystem functioning. Recent studies have identified AMF-associated bacteria as cooperative partners that participate in AMF-plant symbiosis: specific endobacteria live inside AMF, and hyphospheric bacteria colonize the soil that surrounds the extraradical hyphae. In this Review, we describe the concept of a plant-AMF-bacterium continuum, summarize current advances and provide perspectives on soil microbiology. First, we review the top-down carbon flow and the bottom-up mineral flow (especially phosphorus and nitrogen) in this continuum, as well as how AMF-bacteria interactions influence the biogeochemical cycling of nutrients (for example, carbon, phosphorus and nitrogen). Second, we discuss how AMF interact with hyphospheric bacteria or endobacteria to regulate nutrient exchange between plants and AMF, and the possible molecular mechanisms that underpin this continuum. Finally, we explore future prospects for studies on the hyphosphere to facilitate the utilization of AMF and hyphospheric bacteria in sustainable agriculture.

Mucoromycotina 'fine root endophytes': a new molecular model for plant-fungal mutualisms?

The most studied plant-fungal symbioses to date are the interactions between plants and arbuscular mycorrhizal (AM) fungi of the Glomeromycotina clade. Advancements in phylogenetics and microbial community profiling have distinguished a group of symbiosis-forming fungi that resemble AM fungi as belonging instead to the Mucoromycotina. These enigmatic fungi are now known as Mucoromycotina 'fine root endophytes' and could provide a means to understand the origins of plant-fungal symbioses. Most of our knowledge of the mechanisms of fungal symbiosis comes from investigations using AM fungi. Here, we argue that inclusion of Mucoromycotina fine root endophytes in future studies will expand our understanding of the mechanisms, evolution, and ecology of plant-fungal symbioses.

The phosphate starvation response regulator PHR2 antagonizes arbuscule maintenance in Medicago

Phosphate starvation response (PHR) transcription factors play essential roles in regulating phosphate uptake in plants through binding to the P1BS cis-element in the promoter of phosphate starvation response genes. Recently, PHRs were also shown to positively regulate arbuscular mycorrhizal colonization in rice and lotus by controlling the expression of many symbiotic genes. However, their role in arbuscule development has remained unclear. In Medicago, we previously showed that arbuscule degradation is controlled by two SPX proteins that are highly expressed in arbuscule-containing cells. Since SPX proteins bind to PHRs and repress their activity in a phosphate-dependent manner, we investigated whether arbuscule maintenance is also regulated by PHR. Here, we show that PHR2 is a major regulator of the phosphate starvation response in Medicago. Knockout of phr2 showed reduced phosphate starvation response, symbiotic gene expression, and fungal colonization levels. However, the arbuscules that formed showed less degradation, suggesting a negative role for PHR2 in arbuscule maintenance. This was supported by the observation that overexpression of PHR2 led to enhanced degradation of arbuscules. Although many arbuscule-induced genes contain P1BS elements in their promoters, we found that the P1BS cis-elements in the promoter of the symbiotic phosphate transporter PT4 are not required for arbuscule-containing cell expression. Since both PHR2 and SPX1/3 negatively affect arbuscule maintenance, our results indicate that they control arbuscule maintenance partly via different mechanisms. While PHR2 potentiates symbiotic gene expression and colonization, its activity in arbuscule-containing cells needs to be tightly controlled to maintain a successful symbiosis in Medicago.

Microbe-dependent and independent nitrogen and phosphate acquisition and regulation in plants

Nitrogen (N) and phosphorus (P) are the most important macronutrients required for plant growth and development. To cope with the limited and uneven distribution of N and P in complicated soil environments, plants have evolved intricate molecular strategies to improve nutrient acquisition that involve adaptive root development, production of root exudates, and the assistance of microbes. Recently, great advances have been made in understanding the regulation of N and P uptake and utilization and how plants balance the direct uptake of nutrients from the soil with the nutrient acquisition from beneficial microbes such as arbuscular mycorrhiza. Here, we summarize the major advances in these areas and highlight plant responses to changes in nutrient availability in the external environment through local and systemic signals.

Milestones in understanding transport, sensing, and signaling of the plant nutrient phosphorus

As an essential nutrient element, phosphorus (P) is primarily acquired and translocated as inorganic phosphate (Pi) by plant roots. Pi is often sequestered in the soil and becomes limited for plant growth. Plants have developed a sophisticated array of adaptive responses, termed P starvation responses, to cope with P deficiency by improving its external acquisition and internal utilization. Over the past 2 to 3 decades, remarkable progress has been made toward understanding how plants sense and respond to changing environmental P. This review provides an overview of the molecular mechanisms that regulate or coordinate P starvation responses, emphasizing P transport, sensing, and signaling. We present the major players and regulators responsible for Pi uptake and translocation. We then introduce how P is perceived at the root tip, how systemic P signaling is operated, and the mechanisms by which the intracellular P status is sensed and conveyed. Additionally, the recent exciting findings about the influence of P on plant-microbe interactions are highlighted. Finally, the challenges and prospects concerning the interplay between P and other nutrients and strategies to enhance P utilization efficiency are discussed. Insights obtained from this knowledge may guide future research endeavors in sustainable agriculture.

Recent advances in research on phosphate starvation signaling in plants

Phosphorus is indispensable for plant growth and development, with its status crucial for determining crop productivity. Plants have evolved various biochemical, morphological, and developmental responses to thrive under conditions of low P availability, as inorganic phosphate (Pi), the primary form of P uptake, is often insoluble in soils. Over the past 25 years, extensive research has focused on understanding these responses, collectively forming the Pi starvation response system. This effort has not only expanded our knowledge of strategies to cope with Pi starvation (PS) but also confirmed their adaptive significance. Moreover, it has identified and characterized numerous components of the intricate regulatory network governing P homeostasis. This review emphasizes recent advances in PS signaling, particularly highlighting the physiological importance of local PS signaling in inhibiting primary root growth and uncovering the role of TORC1 signaling in this process. Additionally, advancements in understanding shoot-root Pi allocation and a novel technique for studying Pi distribution in plants are discussed. Furthermore, emerging data on the regulation of plant-microorganism interactions by the PS regulatory system, crosstalk between the signaling pathways of phosphate starvation, phytohormones and immunity, and recent studies on natural variation in Pi homeostasis are addressed.

Modulation of plant immunity and biotic interactions under phosphate deficiency

Phosphorus (P) is an essential macronutrient for plant life and growth. P is primarily acquired in the form of inorganic phosphate (Pi) from soil. To cope with Pi deficiency, plants have evolved an elaborate system to improve Pi acquisition and utilization through an array of developmental and physiological changes, termed Pi starvation response (PSR). Plants also assemble and manage mutualistic microbes to enhance Pi uptake, through integrating PSR and immunity signaling. A trade-off between plant growth and defense favors the notion that plants lower a cellular state of immunity to accommodate host-beneficial microbes for nutrition and growth at the cost of infection risk. However, the existing data indicate that plants selectively activate defense responses against pathogens, but do not or less against non-pathogens, even under nutrient deficiency. In this review, we highlight recent advances in the principles and mechanisms with which plants balance immunity and growth-related processes to optimize their adaptation to Pi deficiency.

Spatial co-transcriptomics reveals discrete stages of the arbuscular mycorrhizal symbiosis

The symbiotic interaction of plants with arbuscular mycorrhizal (AM) fungi is ancient and widespread. Plants provide AM fungi with carbon in exchange for nutrients and water, making this interaction a prime target for crop improvement. However, plant-fungal interactions are restricted to a small subset of root cells, precluding the application of most conventional functional genomic techniques to study the molecular bases of these interactions. Here we used single-nucleus and spatial RNA sequencing to explore both Medicago truncatula and Rhizophagus irregularis transcriptomes in AM symbiosis at cellular and spatial resolution. Integrated, spatially registered single-cell maps revealed infected and uninfected plant root cell types. We observed that cortex cells exhibit distinct transcriptome profiles during different stages of colonization by AM fungi, indicating dynamic interplay between both organisms during establishment of the cellular interface enabling successful symbiosis. Our study provides insight into a symbiotic relationship of major agricultural and environmental importance and demonstrates a paradigm combining single-cell and spatial transcriptomics for the analysis of complex organismal interactions.

The Role of CLE Peptides in the Suppression of Mycorrhizal Colonization of Tomato

Symbioses with beneficial microbes are widespread in plants, but these relationships must balance the energy invested by the plants with the nutrients acquired. Symbiosis with arbuscular mycorrhizal (AM) fungi occurs throughout land plants, but our understanding of the genes and signals that regulate colonization levels is limited, especially in non-legumes. Here, we demonstrate that in tomato, two CLV3/EMBRYO-SURROUNDING REGION (CLE) peptides, SlCLE10 and SlCLE11, act to suppress AM colonization of roots. Mutant studies and overexpression via hairy transformation indicate that SlCLE11 acts locally in the root to limit AM colonization. Indeed, SlCLE11 expression is strongly induced in AM-colonized roots, but SlCLE11 is not required for phosphate suppression of AM colonization. SlCLE11 requires the FIN gene that encodes an enzyme required for CLE peptide arabinosylation to suppress mycorrhizal colonization. However, SlCLE11 suppression of AM does not require two CLE receptors with roles in regulating AM colonization, SlFAB (CLAVATA1 ortholog) or SlCLV2. Indeed, multiple parallel pathways appear to suppress mycorrhizal colonization in tomato, as double mutant studies indicate that SlCLV2 and FIN have an additive influence on mycorrhizal colonization. SlCLE10 appears to play a more minor or redundant role, as cle10 mutants did not influence intraradical AM colonization. However, the fact that cle10 mutants had an elevated number of hyphopodia and that ectopic overexpression of SlCLE10 did suppress mycorrhizal colonization suggests that SlCLE10 may also play a role in suppressing AM colonization. Our findings show that CLE peptides regulate AM colonization in tomato and at least SlCLE11 likely requires arabinosylation for activity.

Differential Responses of Medicago truncatula NLA Homologs to Nutrient Deficiency and Arbuscular Mycorrhizal Symbiosis

NITROGEN LIMITATION ADAPTATION (NLA), a plasma-membrane-associated ubiquitin E3 ligase, plays a negative role in the control of the phosphate transporter family 1 (PHT1) members in Arabidopsis and rice. There are three NLA homologs in the Medicago truncatula genome, but it has been unclear whether the function of these homologs is conserved in legumes. Here we investigated the subcellular localization and the responses of MtNLAs to external phosphate and nitrate status. Similar to AtNLA1, MtNLA1/MtNLA2 was localized in the plasma membrane and nucleus. MtNLA3 has three alternative splicing variants, and intriguingly, MtNLA3.1, the dominant variant, was not able to target the plasma membrane, whereas MtNLA3.2 and MtNLA3.3 were capable of associating with the plasma membrane. In contrast with AtNLA1, we found that MtNLAs were not affected or even upregulated by low-phosphate treatment. We also found that MtNLA3 was upregulated by arbuscular mycorrhizal (AM) symbiosis, and overexpressing MtNLA3.1 in Medicago roots resulted in a decrease in the transcription levels of STR, an essential gene for arbuscule development. Taken together, our results highlight the difference between MtNLA homologs and AtNLA1. Further characterization will be required to reveal the regulation of these genes and their roles in the responses to external nutrient status and AM symbiosis.

The Effects of Rhizophagus irregularis Inoculation on Transcriptome of Medicago lupulina Leaves at Early Vegetative and Flowering Stages of Plant Development

The study is aimed at revealing the effects of Rhizophagus irregularis inoculation on the transcriptome of Medicago lupulina leaves at the early (second leaf formation) and later (flowering) stages of plant development. A pot experiment was conducted under conditions of low phosphorus (P) level in the substrate. M. lupulina plants were characterized by high mycorrhizal growth response and mycorrhization parameters. Library sequencing was performed on the Illumina HiseqXTen platform. Significant changes in the expression of 4863 (padj < 0.01) genes from 34049 functionally annotated genes were shown by Massive Analysis of cDNA Ends (MACE-Seq). GO enrichment analysis using the Kolmogorov-Smirnov test was performed, and 244 functional GO groups were identified, including genes contributing to the development of effective AM symbiosis. The Mercator online tool was used to assign functional classes of differentially expressed genes (DEGs). The early stage was characterized by the presence of six functional classes that included only upregulated GO groups, such as genes of carbohydrate metabolism, cellular respiration, nutrient uptake, photosynthesis, protein biosynthesis, and solute transport. At the later stage (flowering), the number of stimulated GO groups was reduced to photosynthesis and protein biosynthesis. All DEGs of the GO:0016036 group were downregulated because AM plants had higher resistance to phosphate starvation. For the first time, the upregulation of genes encoding thioredoxin in AM plant leaves was shown. It was supposed to reduce ROS level and thus, consequently, enhance the mechanisms of antioxidant protection in M. lupulina plants under conditions of low phosphorus level. Taken together, the obtained results indicate genes that are the most important for the effective symbiosis with M. lupulina and might be engaged in other plant species.

Control of arbuscule development by a transcriptional negative feedback loop in Medicago

Most terrestrial plants establish a symbiosis with arbuscular mycorrhizal fungi (AMF), which provide them with lipids and sugars in exchange for phosphorus and nitrogen. Nutrient exchange must be dynamically controlled to maintain a mutually beneficial relationship between the two symbiotic partners. The WRI5a and its homologues play a conserved role in lipid supply to AMF. Here, we demonstrate that the AP2/ERF transcription factor MtERM1 binds directly to AW-box and AW-box-like cis-elements in the promoters of MtSTR2 and MtSTR, which are required for host lipid efflux and arbuscule development. The EAR domain-containing transcription factor MtERF12 is also directly activated by MtERM1/MtWRI5a to negatively regulate arbuscule development, and the TOPLESS co-repressor is further recruited by MtERF12 through EAR motif to oppose MtERM1/MtWRI5a function, thereby suppressing arbuscule development. We therefore reveal an ERM1/WRI5a-ERF12-TOPLESS negative feedback loop that enables plants to flexibly control nutrient exchange and ensure a mutually beneficial symbiosis.

Plant lysin motif extracellular proteins are required for arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal fungi (AMF) can form a mutually beneficial symbiotic relationship with most land plants. They are known to secrete lysin motif (LysM) effectors into host root cells for successful colonization. Intriguingly, plants secrete similar types of LysM proteins; however, their role in plant-microbe interactions is unknown. Here, we show that Medicago truncatula deploys LysM extracellular (LysMe) proteins to facilitate symbiosis with AMF. Promoter analyses demonstrated that three M. truncatula LysMe genes MtLysMe1/2/3, are expressed in arbuscule-containing cells and those adjacent to intercellular hyphae. Localization studies showed that these proteins are targeted to the periarbuscular space between the periarbuscular membrane and the fungal cell wall of the branched arbuscule. M. truncatula mutants in which MtLysMe2 was knocked out via CRISPR/Cas9-targeted mutagenesis exhibited a significant reduction in AMF colonization and arbuscule formation, whereas genetically complemented transgenic plants restored wild-type level AMF colonization. In addition, knocking out the ortholog of MtLysMe2 in tomato resulted in a similar defect in AMF colonization. In vitro binding affinity precipitation assays suggested binding of MtLysMe1/2/3 with chitin and chitosan, while microscale thermophoresis (MST) assays revealed weak binding of these proteins with chitooligosaccharides. Moreover, application of purified MtLysMe proteins to root segments could suppress chitooctaose (CO8)-induced reactive oxygen species production and expression of reporter genes of the immune response without impairing chitotetraose (CO4)-triggered symbiotic responses. Taken together, our results reveal that plants, like their fungal partners, also secrete LysM proteins to facilitate symbiosis establishment.

The good, the bad, and the phosphate: regulation of beneficial and detrimental plant-microbe interactions by the plant phosphate status

Phosphate (P_i ) is indispensable for life on this planet. However, for sessile land plants it is poorly accessible. Therefore, plants have developed a variety of strategies for enhanced acquisition and recycling of P_i . The mechanisms to cope with P_i limitation as well as direct uptake of P_i from the substrate via the root epidermis are regulated by a conserved P_i starvation response (PSR) system based on a family of key transcription factors (TFs) and their inhibitors. Furthermore, plants obtain P_i indirectly through symbiosis with mycorrhiza fungi, which employ their extensive hyphal network to drastically increase the soil volume that can be explored by plants for P_i . Besides mycorrhizal symbiosis, there is also a variety of other interactions with epiphytic, endophytic, and rhizospheric microbes that can indirectly or directly influence plant P_i uptake. It was recently discovered that the PSR pathway is involved in the regulation of genes that promote formation and maintenance of AM symbiosis. Furthermore, the PSR system influences plant immunity and can also be a target of microbial manipulation. It is known for decades that the nutritional status of plants influences the outcome of plant-microbe interactions. The first molecular explanations for these observations are now emerging.

Silencing of SlSPX1 and SlSPX2 promote growth and root mycorrhization in tomato (Solanum lycopersicum L.) seedlings

Owing to the essential requirement of phosphorus (P) for growth and development, plants tightly control inorganic phosphate (Pi) homeostasis. SPX-PHR regulatory circuit not only control phosphate homeostasis responses but also root mycorrhization by arbuscular mycorrhiza (AM) fungi. Besides sensing Pi deficiency, SPX (SYG1/Pho81/XPR1) proteins also control the transcription of P starvation inducible (PSI) genes by blocking the activity of PHR1 (PHOSPHATE STARVATION RESPONSE1) homologs in plants under Pi-sufficient conditions. However, the roles of SPX members in Pi homeostasis and AM fungi colonization remain to be fully recognized in tomato. In this study, we identified 17 SPX-domain containing members in the tomato genome. Transcript profiling revealed the high Pi-specific nature of their activation. Four SlSPX members have also induced in AM colonized roots. Interestingly, we found that SlSPX1 and SlSPX2 are induced by P starvation and AM colonization. Further, SlSPX1 and SlSPX2 exhibited varying degrees of interaction with the PHR homologs in this study. Virus-induced gene silencing-based (VIGS) transcript inhibition of these genes alone or together promoted the accumulation of higher total soluble Pi in tomato seedlings and improved their growth. It also enhanced AM fungi colonization in the roots of SlSPX1 and SlSPX2 silenced seedlings. Overall, the present study provides evidence in support of SlSPX members being good candidates for improving AM fungi colonization potential in tomato.

Improving phosphorus acquisition efficiency through modification of root growth responses to phosphate starvation in legumes

Phosphorus (P) is one of the essential macronutrients for plant growth and development, and it is an integral part of the major organic components, including nucleic acids, proteins and phospholipids. Although total P is abundant in most soils, a large amount of P is not easily absorbed by plants. Inorganic phosphate (Pi) is the plant-available P, which is generally immobile and of low availability in soils. Hence, Pi starvation is a major constraint limiting plant growth and productivity. Enhancing plant P efficiency can be achieved by improving P acquisition efficiency (PAE) through modification of morpho-physiological and biochemical alteration in root traits that enable greater acquisition of external Pi from soils. Major advances have been made to dissect the mechanisms underlying plant adaptation to P deficiency, especially for legumes, which are considered important dietary sources for humans and livestock. This review aims to describe how legume root growth responds to Pi starvation, such as changes in the growth of primary root, lateral roots, root hairs and cluster roots. In particular, it summarizes the various strategies of legumes to confront P deficiency by regulating root traits that contribute towards improving PAE. Within these complex responses, a large number of Pi starvation-induced (PSI) genes and regulators involved in the developmental and biochemical alteration of root traits are highlighted. The involvement of key functional genes and regulators in remodeling root traits provides new opportunities for developing legume varieties with maximum PAE needed for regenerative agriculture.

The RNAome landscape of tomato during arbuscular mycorrhizal symbiosis reveals an evolving RNA layer symbiotic regulatory network

Arbuscular mycorrhizal symbiosis (AMS) is an ancient plant-fungus relationship that is widely distributed in terrestrial plants. The formation of symbiotic structures and bidirectional nutrient exchange requires the regulation of numerous genes. However, the landscape of RNAome during plant AMS involving different types of regulatory RNA is poorly understood. In this study, a combinatorial strategy utilizing multiple sequencing approaches was used to decipher the landscape of RNAome in tomato, an emerging AMS model. The annotation of the tomato genome was improved by a multiple-platform sequencing strategy. A total of 3,174 protein-coding genes were upregulated during AMS, 42% of which were alternatively spliced. Comparative-transcriptome analysis revealed that genes from 24 orthogroups were consistently induced by AMS in eight phylogenetically distant angiosperms. Seven additional orthogroups were specifically induced by AMS in all surveyed dicot AMS host plants. However, these orthogroups were absent or not induced in monocots and/or non-AMS hosts, suggesting a continuously evolving AMS-responsive network in addition to a conserved core regulatory module. Additionally, we detected 587 lncRNAs, ten miRNAs, and 146 circRNAs that responded to AMS, which were incorporated to establish a tomato AMS-responsive, competing RNA-responsive endogenous RNA (ceRNA) network. Finally, a tomato symbiotic transcriptome database (TSTD, https://efg.nju.edu.cn/TSTD) was constructed to serve as a resource for deep deciphering of the AMS regulatory network. These results help elucidate the reconfiguration of the tomato RNAome during AMS and suggest a sophisticated and evolving RNA layer responsive network during AMS processes.

Dissection of the response mechanism of alfalfa under phosphite stress based on metabolomic and transcriptomic data

Phosphite, a reduced form of phosphate, inhibits the growth and even has toxic effect on plants. To learn more about the mechanism of alfalfa responses to phosphite, the morphological and physiological characteristics, and the metabolites and transcript levels were comprehensively analyzed following the exposure of alfalfa seedlings to phosphite and phosphate under greenhouse conditions. The results showed that phosphite inhibited seedling growth and photosynthesis. However, the absorption efficiency of phosphite was higher than that of phosphate in roots, which was supported by increased total phosphorus concentration of 16.29% and 52.30% on days 8 and 12. Moreover, phosphite stress affected the synthesis of lipids and carbohydrates, which were reflected in enhanced glycolipid and sulfolipid in roots and amylose in shoots. Phosphite stress resulted in a decrease in indole acetic acid (IAA) in the whole plant and zeatin in the shoots, which could enable alfalfa to adapt to the phosphite environment. Some genes involved in phosphate starvation response included SPX, phosphate response regulator2, and inorganic phosphate transporter 1-4 (PHT1;4) in roots were affected by phosphite stress. In addition, some genes that are involved in stress responses and DNA repair were induced by phosphite stress. These observations together suggest that alfalfa responds to phosphite stress by inhibiting growth, regulating the genes induced by phosphate starvation, improving oxidative protection, promoting DNA repair, and adjusting the IAA and zeatin signaling transductions. Our findings provide novel insights into the molecular response to phosphite stress in alfalfa.

The Perspective of Arbuscular Mycorrhizal Symbiosis in Rice Domestication and Breeding

In nature, symbiosis with arbuscular mycorrhizal (AM) fungi contributes to sustainable acquisition of phosphorus and other elements in over 80% of plant species; improving interactions with AM symbionts may mitigate some of the environmental problems associated with fertilizer application in grain crops such as rice. Recent developments of high-throughput genome sequencing projects of thousands of rice cultivars and the discovery of the molecular mechanisms underlying AM symbiosis suggest that interactions with AM fungi might have been an overlooked critical trait in rice domestication and breeding. In this review, we discuss genetic variation in the ability of rice to form AM symbioses and how this might have affected rice domestication. Finally, we discuss potential applications of AM symbiosis in rice breeding for more sustainable agriculture.

SlSPX1-SlPHR complexes mediate the suppression of arbuscular mycorrhizal symbiosis by phosphate repletion in tomato

Forming mutualistic symbioses with arbuscular mycorrhizae (AMs) improves the acquisition of mineral nutrients for most terrestrial plants. However, the formation of AM symbiosis usually occurs under phosphate (Pi)-deficient conditions. Here, we identify SlSPX1 (SYG1 (suppressor of yeast GPA1)/Pho81(phosphate 81)/XPR1 (xenotropic and polytropic retrovirus receptor 1) as the major repressor of the AM symbiosis in tomato (Solanum lycopersicum) under phosphate-replete conditions. Loss of SlSPX1 function promotes direct Pi uptake and enhances AM colonization under phosphate-replete conditions. We determine that SlSPX1 integrates Pi signaling and AM symbiosis by directly interacting with a set of arbuscule-induced SlPHR proteins (SlPHR1, SlPHR4, SlPHR10, SlPHR11, and SlPHR12). The association with SlSPX1 represses the ability of SlPHR proteins to activate AM marker genes required for the arbuscular mycorrhizal symbiosis. SlPHR proteins exhibit functional redundancy, and no defective AM symbiosis was detected in the single mutant of SlPHR proteins. However, silencing SlPHR4 in the Slphr1 mutant background led to reduced AM colonization. Therefore, our results support the conclusion that SlSPX1-SlPHRs form a Pi-sensing module to coordinate the AM symbiosis under different Pi-availability conditions.

The Roles of Phosphorus and Nitrogen Nutrient Transporters in the Arbuscular Mycorrhizal Symbiosis

More than 80% of land plant species can form symbioses with arbuscular mycorrhizal (AM) fungi, and nutrient transfer to plants is largely mediated through this partnership. Over the last few years, great progress has been made in deciphering the molecular mechanisms underlying the AM-mediated modulation of nutrient uptake progress, and a growing number of fungal and plant genes responsible for the uptake of nutrients from soil or transfer across the fungal–root interface have been identified. In this review, we outline the current concepts of nutrient exchanges within this symbiosis (mechanisms and regulation) and focus on P and N transfer from the fungal partner to the host plant, with a highlight on a possible interplay between P and N nutrient exchanges. Transporters belonging to the plant or AM fungi can synergistically process the transmembrane transport of soil nutrients to the symbiotic interface for further plant acquisition. Although much progress has been made to elucidate the complex mechanism for the integrated roles of nutrient transfers in AM symbiosis, questions still remain to be answered; for example, P and N transporters are less studied in different species of AM fungi; the involvement of AM fungi in plant N uptake is not as clearly defined as that of P; coordinated utilization of N and P is unknown; transporters of cultivated plants inoculated with AM fungi and transcriptomic and metabolomic networks at both the soil–fungi interface and fungi–plant interface have been insufficiently studied. These findings open new perspectives for fundamental research and application of AM fungi in agriculture.

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.

Host SPX-PHR regulatory circuit: the molecular dynamo steering mycorrhization in plants

We highlight the newly emerged roles of plant SPX-PHR proteins beyond phosphate starvation responses in controlling arbuscular mycorrhizal colonization success in roots.

Orchestrating plant direct and indirect phosphate uptake pathways

A recent groundbreaking study by Shi et al. reveals an extensive transcriptional regulatory network for arbuscular mycorrhizal (AM) symbiosis in rice. The finding that phosphate starvation response (PHR) transcription factors centrally orchestrate the direct and indirect AM pathways for inorganic phosphate (Pi) uptake in rice opens a wealth of opportunities for plant breeding to enhance nutrient acquisition.

Primary nutrient sensors in plants

Nutrients are scarce and valuable resources, so plants developed sophisticated mechanisms to optimize nutrient use efficiency. A crucial part of this is monitoring external and internal nutrient levels to adjust processes such as uptake, redistribution, and cellular compartmentation. Measurement of nutrient levels is carried out by primary sensors that typically involve either transceptors or transcription factors. Primary sensors are only now starting to be identified in plants for some nutrients. In particular, for nitrate, there is detailed insight concerning how the external nitrate status is sensed by members of the nitrate transporter 1 (NRT1) family. Potential sensors for other macronutrients such as potassium and sodium have also been identified recently, whereas for micronutrients such as zinc and iron, transcription factor type sensors have been reported. This review provides an overview that interprets and evaluates our current understanding of how plants sense macro and micronutrients in the rhizosphere and root symplast.

A SPX domain-containing phosphate transporter from Rhizophagus irregularis handles phosphate homeostasis at symbiotic interface of arbuscular mycorrhizas

Reciprocal symbiosis of > 70% of terrestrial vascular plants with arbuscular mycorrhizal (AM) fungi provides the fungi with fatty acids and sugars. In return, AM fungi facilitate plant phosphate (Pi) uptake from soil. However, how AM fungi handle Pi transport and homeostasis at the symbiotic interface of AM symbiosis is poorly understood. Here, we identify an SPX (SYG1/Pho81/XPR1) domain-containing phosphate transporter, RiPT7 from Rhizophagus irregularis. To characterize the RiPT7 transporter, we combined subcellular localization and heterologous expression studies in yeasts with reverse genetics approaches during the in planta phase. The results show that RiPT7 is conserved across fungal species and expressed in the intraradical mycelia. It is expressed in the arbuscules, intraradical hyphae and vesicles, independently of Pi availability. The plasma membrane-localized RiPT7 facilitates bidirectional Pi transport, depending on Pi gradient across the plasma membrane, whereas the SPX domain of RiPT7 inhibits Pi transport activity and mediates the vacuolar targeting of RiPT7 in yeast in response to Pi starvation. Importantly, RiPT7 silencing hampers arbuscule development of R. irregularis and symbiotic Pi delivery under medium- to low-Pi conditions. Collectively, our findings reveal a role for RiPT7 in fine-tuning of Pi homeostasis across the fungal membrane to maintain the AM development.

Sculpting the soil microbiota

Soil is a living ecosystem, the health of which depends on fine interactions among its abiotic and biotic components. These form a delicate equilibrium maintained through a multilayer network that absorbs certain perturbations and guarantees soil functioning. Deciphering the principles governing the interactions within soils is of critical importance for their management and conservation. Here, we focus on soil microbiota and discuss the complexity of interactions that impact the composition and function of soil microbiota and their interaction with plants. We discuss how physical aspects of soils influence microbiota composition and how microbiota-plant interactions support plant growth and responses to nutrient deficiencies. We predict that understanding the principles determining the configuration and functioning of soil microbiota will contribute to the design of microbiota-based strategies to preserve natural resources and develop more environmentally friendly agricultural practices.

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.

Symbiotic regulation: How plants seek salvation in starvation

When plants are starved of phosphate, many rely on an ancient symbiosis with arbuscular mycorrhizal fungi to secure a critical supply. A new study demonstrates a molecular basis for the regulation of symbiosis by phosphate starvation.

Phosphate starvation responsive GmSPX5 mediates nodule growth through interaction with GmNF-YC4 in soybean (Glycine max)

Phosphorus (P) deficiency adversely affects nodule development as reflected by reduced nodule fresh weight in legume plants. Though mechanisms underlying nodule adaptation to P deficiency have been studied extensively, it remains largely unknown which regulator mediates nodule adaptation to P deficiency. In this study, GUS staining and quantitative reverse transcription-PCR analysis reveal that the SPX member GmSPX5 is preferentially expressed in soybean (Glycine max) nodules. Overexpression of GmSPX5 enhanced soybean nodule development particularly under phosphate (Pi) sufficient conditions. However, the Pi concentration was not affected in soybean tissues (i.e., leaves, roots, and nodules) of GmSPX5 overexpression or suppression lines, which distinguished it from other well-known SPX members functioning in control of Pi homeostasis in plants. Furthermore, GmSPX5 was observed to interact with the transcription factor GmNF-YC4 in vivo and in vitro. Overexpression of either GmSPX5 or GmNF-YC4 significantly upregulated the expression levels of five asparagine synthetase-related genes (i.e., GmASL2-6) in soybean nodules. Meanwhile, yeast one-hybrid and luciferase activity assays strongly suggested that interactions of GmSPX5 and GmNF-YC4 activate GmASL6 expression through enhancing GmNF-YC4 binding of the GmASL6 promoter. These results not only demonstrate the GmSPX5-GmNF-YC4-GmASL6 regulatory pathway mediating soybean nodule development, but also considerably improve our understanding of SPX functions in legume crops.