Identification and characterization of stress responsive homeodomain leucine zipper transcription factors in Medicago truncatula

BACKGROUND

Homeodomain leucine zipper (HD-ZIP) transcription factors play roles in regulating plant development and responses to abiotic stresses; however, how HD-ZIP genes in Medicago truncatula are involved in abiotic stress response remains elusive.

METHODS AND RESULTS

The HD-ZIP I genes in Medicago truncatula were identified and characterized, and their expression patterns in different tissues and under different abiotic stresses were analyzed. A total of 15 Medicago truncatula HD-ZIP I genes were identified and a phylogenetic analysis of HD-ZIP I proteins in Arabidopsis thaliana and Medicago truncatula was conducted. Fifteen HD-ZIP I genes showed diverse tissue preferences. Among them, expressions of MtHB22 and MtHB51 were specially detected in vegetative buds. In addition, they responded to various abiotic stresses, including salinity and osmotic stress and abscisic acid (ABA). For instance, MtHB7 and MtHB12 expression levels were found to be positively associated with salt, osmotic stress and ABA in both shoots and roots, while MtHB13 and MtHB23 were negatively associated with these stresses in Medicago truncatula.

CONCLUSION

The HD-ZIP I genes in Medicago truncatula are evolutionarily conserved, but also exhibit gene duplication and gene loss events. Differential expression analysis of Medicago truncatula HD-ZIP I genes indicated their crucial roles in abiotic stress responses. Our genome-wide analysis of the HD-ZIP I transcription factor family in Medicago truncatula provided a valuable reference for further research.

Root cortex development is fine-tuned by the interplay of MIGs, SCL3 and DELLAs during arbuscular mycorrhizal symbiosis

Root development is a crucial process that determines the ability of plants to acquire nutrients, adapt to the substrate and withstand changing environmental conditions. Root plasticity is controlled by a plethora of transcriptional regulators that allow, in contrast to tissue development in animals, post-embryonic changes that give rise to new tissue and specialized cells. One of these changes is the accommodation in the cortex of hyperbranched hyphae of symbiotic arbuscular mycorrhizal (AM) fungi, called arbuscules. Arbuscule-containing cells undergo massive reprogramming to coordinate developmental changes with transport processes. Here we describe a novel negative regulator of arbuscule development, MIG3. MIG3 induces and interacts with SCL3, both of which modulate the activity of the central regulator DELLA, restraining cortical cell growth. As in a tug-of-war, MIG3-SCL3 antagonizes the function of the complex MIG1-DELLA, which promotes the cell expansion required for arbuscule development, adjusting cell size during the dynamic processes of the arbuscule life cycle. Our results in the legume plant Medicago truncatula advance the knowledge of root development in dicot plants, showing the existence of additional regulatory elements not present in Arabidopsis that fine-tune the activity of conserved central modules.

Systemic control of nodule formation by plant nitrogen demand requires autoregulation-dependent and independent mechanisms

In legumes interacting with rhizobia, the formation of symbiotic organs involved in the acquisition of atmospheric nitrogen gas (N2) is dependent on the plant nitrogen (N) demand. We used Medicago truncatula plants cultivated in split-root systems to discriminate between responses to local and systemic N signaling. We evidenced a strong control of nodule formation by systemic N signaling but obtained no clear evidence of a local control by mineral nitrogen. Systemic signaling of the plant N demand controls numerous transcripts involved in root transcriptome reprogramming associated with early rhizobia interaction and nodule formation. SUPER NUMERIC NODULES (SUNN) has an important role in this control, but we found that major systemic N signaling responses remained active in the sunn mutant. Genes involved in the activation of nitrogen fixation are regulated by systemic N signaling in the mutant, explaining why its hypernodulation phenotype is not associated with higher nitrogen fixation of the whole plant. We show that the control of transcriptome reprogramming of nodule formation by systemic N signaling requires other pathway(s) that parallel the SUNN/CLE (CLAVATA3/EMBRYO SURROUNDING REGION-LIKE PEPTIDES) pathway.

A novel LRR-RLK (CTLK) confers cold tolerance through regulation on the C-repeat-binding factor pathway, antioxidants, and proline accumulation

Leucine-rich repeat-receptor-like kinase (LRR-RLK) is a large subfamily of plant RLKs; however, its role in cold tolerance is still unknown. A novel cold tolerance LRR-RLK gene (MtCTLK1) in Medicago truncatula was identified using the transgenic lines overexpressing MtCTLK1 (MtCTLK1-OE) and mtctlk1 lines with Tnt1 retrotransposon insertion. Compared with the wild-type, MtCTLK1-OE lines had increased cold tolerance and mtctlk1 showed decreased cold tolerance. The impaired cold tolerance in mtctlk1 could be complemented by the transgenic expression of MtCTLK1 or its homolog MfCTLK1 from Medicago falcata. Antioxidant enzyme activities and proline accumulation as well as transcript levels of the associated genes were increased in response to cold, with higher levels in MtCTLK1-OE or lower levels in mtctlk1 lines as compared with wild type. C-Repeat-Binding Factors (CBFs) and CBF-dependent cold-responsive genes were also induced in response to cold, and higher transcript levels of CBFs and CBF-dependent cold-responsive genes were observed in MtCTLK1-OE lines whereas lower levels in mtctlk1 mutants. The results validate the role of MtCTLK1 or MfCTLK1 in the regulation of cold tolerance through the CBF pathway, antioxidant defense system and proline accumulation. It also provides a valuable gene for the molecular breeding program to improve cold tolerance in crops.

MtNPF6.5 mediates chloride uptake and nitrate preference in Medicago roots

The preference for nitrate over chloride through regulation of transporters is a fundamental feature of plant ion homeostasis. We show that Medicago truncatula MtNPF6.5, an ortholog of Arabidopsis thaliana AtNPF6.3/NRT1.1, can mediate nitrate and chloride uptake in Xenopus oocytes but is chloride selective and that its close homologue, MtNPF6.7, can transport nitrate and chloride but is nitrate selective. The MtNPF6.5 mutant showed greatly reduced chloride content relative to wild type, and MtNPF6.5 expression was repressed by high chloride, indicating a primary role for MtNPF6.5 in root chloride uptake. MtNPF6.5 and MtNPF6.7 were repressed and induced by nitrate, respectively, and these responses required the transcription factor MtNLP1. Moreover, loss of MtNLP1 prevented the rapid switch from chloride to nitrate as the main anion in nitrate-starved plants after nitrate provision, providing insight into the underlying mechanism for nitrate preference. Sequence analysis revealed three sub-types of AtNPF6.3 orthologs based on their predicted substrate-binding residues: A (chloride selective), B (nitrate selective), and C (legume specific). The absence of B-type AtNPF6.3 homologues in early diverged plant lineages suggests that they evolved from a chloride-selective MtNPF6.5-like protein.

LATE MERISTEM IDENTITY1 regulates leaf margin development via the auxin transporter gene SMOOTH LEAF MARGIN1

Plant leaves have evolved into diverse shapes and LATE MERISTEM IDENTITY1 (LMI1) and its putative paralogous genes encode homeodomain leucine zipper transcription factors that are proposed evolutionary hotspots for the regulation of leaf development in plants. However, the LMI1-mediated regulatory mechanism underlying leaf shape formation is largely unknown. MtLMI1a and MtLMI1b are putative orthologs of LMI1 in the model legume barrelclover (Medicago truncatula). Here, we investigated the role of MtLMI1a and MtLMI1b in leaf margin morphogenesis by characterizing loss-of-function mutants. MtLMI1a and MtLMI1b are expressed along leaf margin in a near-complementary pattern, and they redundantly promote development of leaf margin serrations, as revealed by the relatively smooth leaf margin in their double mutants. Moreover, MtLMI1s directly activate expression of SMOOTH LEAF MARGIN1 (SLM1), which encodes an auxin efflux carrier, thereby regulating auxin distribution along the leaf margin. Further analysis indicates that MtLMI1s genetically interact with NO APICAL MERISTEM (MtNAM) and the ARGONAUTE7 (MtAGO7)-mediated trans-acting short interfering RNA3 (TAS3 ta-siRNA) pathway to develop the final leaf margin shape. The participation of MtLMI1s in auxin-dependent leaf margin formation is interesting in the context of functional conservation. Furthermore, the diverse expression patterns of LMI1s and their putative paralogs within key domains are important drivers for functional specialization, despite their functional equivalency among species.

Rhizobium symbiosis modulates the accumulation of arsenic in Medicago truncatula via nitrogen and NRT3.1-like genes regulated by ABA and linalool

Arsenic (As) contamination is a worldwide problem and threatens human health. Here, we found that Rhizobium symbiosis can improve the tolerance to arsenate [As(V)], and a wild type R. meliloti Rm5038 symbiosis can significantly decrease the accumulation of As in Medicago truncatula shoots. The As content in plants could be decreased by nitrogen and the mutation of nitrate transporter NRT3.1. The expression of M. truncatula NRT3.1-like gene NRT3.1L1 could reverse the As(V)-tolerance phenotype of the Arabidopsis nrt3.1 mutant. Rm5038 symbiosis significantly increased the level of nitrogen in the shoot and reduced the expression of NRT3.1Ls in plants afflicted by As(V). The genetic analyses of aba2-1, pyr1/pyl1/2/4/5/8, and abi1-2/abi2-2/hab1-1/pp2ca-1 mutants revealed that abscisic acid (ABA) signaling regulates the tolerance of plants to As(V). ABA and linalool could promote the expression of NRT3.1Ls, however, their root biosynthesis was inhibited by ammonium, the first form of nitrogen fixed by Rhizobium symbiosis. Moreover, ABA and linalool may also control As and nitrate accumulation in Rhizobium symbionts via signaling pathways other than ammonia and NRT3.1Ls. Thus, Rhizobium symbiosis modulates the accumulation of As in plants via nitrogen and NRT3.1Ls regulated by ABA and linalool, which provides novel approaches to reduce As accumulation in legume crops.

Three Common Symbiotic ABC Subfamily B Transporters in Medicago truncatula Are Regulated by a NIN-Independent Branch of the Symbiosis Signaling Pathway

Several ATP-binding cassette (ABC) transporters involved in the arbuscular mycorrhizal symbiosis and nodulation have been identified. We describe three previously unreported ABC subfamily B transporters, named AMN1, AMN2, and AMN3 (ABCB for mycorrhization and nodulation), that are expressed early during infection by rhizobia and arbuscular mycorrhizal fungi. These ABCB transporters are strongly expressed in symbiotically infected tissues, including in root-hair cells with rhizobial infection threads and arbusculated cells. During nodulation, the expression of these genes is highly induced by rhizobia and purified Nod factors and is dependent on DMI3 but is not dependent on other known major regulators of infection, such as NIN, NSP1, or NSP2. During mycorrhization their expression is dependent on DMI3 and RAM1 but not on NSP1 and NSP2. Therefore, they may be commonly regulated through a distinct branch of the common symbiotic pathway. Mutants with exonic Tnt1-transposon insertions were isolated for all three genes. None of the single or double mutants showed any differences in colonization by either rhizobia or mycorrhizal fungi, but the triple amn1 amn2 amn3 mutant showed an increase in nodule number. Further studies are needed to identify potential substrates of these transporters and understand their roles in these beneficial symbioses.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Structure of the unique tetrameric STENOFOLIA homeodomain bound with target promoter DNA

Homeobox transcription factors are key regulators of morphogenesis and development in both animals and plants. In plants, the WUSCHEL-related homeobox (WOX) family of transcription factors function as central organizers of several developmental programs ranging from embryo patterning to meristematic stem-cell maintenance through transcriptional activation and repression mechanisms. The Medicago truncatula STENOFOLIA (STF) gene is a master regulator of leaf-blade lateral development. Here, the crystal structure of the homeodomain (HD) of STF (STF-HD) in complex with its promoter DNA is reported at 2.1 Å resolution. STF-HD binds DNA as a tetramer, enclosing nearly the entire bound DNA surface. The STF-HD tetramer is partially stabilized by docking of the C-terminal tail of one protomer onto a conserved hydrophobic surface on the head of another protomer in a head-to-tail manner. STF-HD specifically binds TGA motifs, although the promoter sequence also contains TAAT motifs. Helix α3 not only serves a canonical role as a base reader in the major groove, but also provides DNA binding in the minor groove through basic residues located at its C-terminus. The structural and functional data in planta reported here provide new insights into the DNA-binding mechanisms of plant-specific HDs from the WOX family of transcription factors.

Plant-specific histone deacetylases are essential for early and late stages of Medicago nodule development

Legume and rhizobium species can establish a nitrogen-fixing nodule symbiosis. Previous studies have shown that several transcription factors that play a role in (lateral) root development are also involved in nodule development. Chromatin remodeling factors, like transcription factors, are key players in regulating gene expression. However, studies have not investigated whether chromatin remodeling genes that are essential for root development are also involved in nodule development. Here, we studied the role of Medicago (Medicago truncatula) histone deacetylases (MtHDTs) in nodule development. Arabidopsis (Arabidopsis thaliana) orthologs of HDTs have been shown to play a role in root development. MtHDT expression is induced in nodule primordia and is maintained in the nodule meristem and infection zone. Conditional, nodule-specific knockdown of MtHDT expression by RNAi blocks nodule primordium development. A few nodules may still form, but their nodule meristems are smaller, and rhizobial colonization of the cells derived from the meristem is markedly reduced. Although the HDTs are expressed during nodule and root development, transcriptome analyses indicate that HDTs control the development of each organ in a different manner. During nodule development, the MtHDTs positively regulate 3-hydroxy-3-methylglutaryl coenzyme a reductase 1 (MtHMGR1). Decreased expression of MtHMGR1 is sufficient to explain the inhibition of primordium formation.

A seed coat-specific β-ketoacyl-CoA synthase, KCS12, is critical for preserving seed physical dormancy

Physical dormancy in seeds exists widely in seed plants and plays a vital role in maintaining natural seed banks. The outermost cuticle of the seed coat forms a water-impermeable layer, which is critical for establishing seed physical dormancy. We previously set up the legume plant Medicago truncatula as an excellent model for studying seed physical dormancy, and our studies revealed that a class II KNOTTED-like homeobox, KNOX4, is a transcription factor critical for controlling hardseededness. Here we report the function of a seed coat β-ketoacyl-CoA synthase, KCS12. The expression level of KCS12 is significantly downregulated in the knox4 mutant. The KCS12 gene is predominantly expressed in the seed coat, and seed development in the M. truncatula kcs12 mutant is altered. Further investigation demonstrated that kcs12 mutant seeds lost physical dormancy and were able to absorb water without scarification treatment. Chemical analysis revealed that concentrations of C24:0 lipid polyester monomers are significantly decreased in mutant seeds, indicating that KCS12 is an enzyme that controls the production of very long chain lipid species in the seed coat. A chromatin immunoprecipitation assay demonstrated that the expression of KCS12 in the seed coat is directly regulated by the KNOX4 transcription factor. These findings define a molecular mechanism by which KNOX4 and KCS12 control formation of the seed coat and seed physical dormancy.

Overexpression of MtRAV3 enhances osmotic and salt tolerance and inhibits growth of Medicago truncatula

Related to ABI3/VP1 (RAV) transcription factors play important roles in regulating plant growth and stress tolerance, which have been studied in many plant species, but have remained largely unidentified in legumes. To functionally characterize RAV in legumes, MtRAV3 from legume model plant Medicago truncatula was isolated and its function was investigated by overexpressing MtRAV3 in M. truncatula. Expression analysis demonstrated that MtRAV3 was markedly induced by NaCl and polyethylene glycol (PEG). MtRAV3 overexpression enhanced tolerance of transgenic M. truncatula to mannitol, drought and salt stresses, and induced the expression of adversity-related genes, including MtWRKY76, MtMYB61, cold-acclimation specific protein 31 (MtCAS31), alternative oxidase 1 (MtAOX1) and ethylene response factor 1 (MtERF1). There were lower relative electrolyte leakage and higher chlorophyll content of leaves in the MtRAV3 overexpression plants than in wild type plants under both salt and drought stress. MtRAV3 overexpression M. truncatula were featured by some phenotypes of dwarfing, late flowering, more branches, smaller flower and leaf organs. Further investigations showed that the expression levels of DWARF14 (MtD14), CAROTENOID CLEAVAGE DIOXYGENASES 7 (MtCCD7) and GA3-oxidase1 (MtGA3ox1), which related to dwarf and branch phenotype, were obviously reduced, as well as MtGA3ox1' (MTR_1g011580), GA20-oxidase1 (MtGA20ox1), MtGA20ox1' (MTR_1g102070) and GA20-oxidase2 (MtGA20ox2) involved in gibberellins (GAs) pathway. Overall, our results revealed that MtRAV3 exerted an important role in adversity response and plant growth, was a multifunctional gene in M. truncatula, which provided reference for genetic improvement of alfalfa (Medicago sativa).

Interaction between the MtDELLA-MtGAF1 Complex and MtARF3 Mediates Transcriptional Control of MtGA3ox1 to Elaborate Leaf Margin Formation in Medicago truncatula

The molecular mechanisms underlying the diversity of leaf shapes have been of great interest to researchers. Leaf shape depends on the pattern of serrations and the degree of indentation of leaf margins. Multiple transcription factors and hormone signaling pathways are involved in this process. In this study, we characterized the developmental roles of SMALL AND SERRATED LEAF (SSL) by analyzing a recessive mutant in the model legume Medicago truncatula. An ortholog of Arabidopsis thaliana GA3-oxidase 1 (GA3ox1), MtGA3ox1/SSL, is required for GA biosynthesis. Loss of function in MtGA3ox1 results in the small plant and lateral organs. The prominent phenotype of the mtga3ox1 mutant is a more pronounced leaf margin, indicating the critical role of GA level in leaf margin formation. Moreover, 35S:MtDELLA2ΔDELLA and 35S:MtARF3 transgenic plants display leaves with a deeply wavy margin, which resembles those of mtga3ox1. Further investigations show that MtGA3ox1 is under the control of MtDELLA1/2/3-MtGAF1 complex-dependent feedback regulation. Further, MtARF3 behaves as a competitive inhibitor of MtDELLA2/3-MtGAF1 complexes to repress the expression of MtGA3ox1 indirectly. These findings suggest that GA feedback regulatory circuits play a fundamental role in leaf margin formation, in which the posttranslational interaction between transcription factors functions as an additional feature.

The R2R3-MYB transcription factor MtMYB134 orchestrates flavonol biosynthesis in Medicago truncatula

Our results provide insights into the flavonol biosynthesis regulation of M. truncatula. The R2R3-MYB transcription factor MtMYB134 emerged as tool to improve the flavonol biosynthesis. Flavonols are plant specialized metabolites with vital roles in plant development and defense and are known as diet compound beneficial to human health. In leguminous plants, the regulatory proteins involved in flavonol biosynthesis are not well characterized. Using a homology-based approach, three R2R3-MYB transcription factor encoding genes have been identified in the Medicago truncatula reference genome sequence. The gene encoding a protein with highest similarity to known flavonol regulators, MtMYB134, was chosen for further experiments and was characterized as a functional flavonol regulator from M. truncatula. MtMYB134 expression levels are correlated with the expression of MtFLS2, encoding a key enzyme of flavonol biosynthesis, and with flavonol metabolite content. MtMYB134 was shown to activate the promoters of the A. thaliana flavonol biosynthesis genes AtCHS and AtFLS1 in Arabidopsis protoplasts in a transactivation assay and to interact with the Medicago promoters of MtCHS2 and MtFLS2 in yeast 1-hybrid assays. To ascertain the functional aspect of the identified transcription factor, we developed a sextuple mutant, which is defective in anthocyanin and flavonol biosynthesis. Ectopic expression of MtMYB134 in a multiple myb A. thaliana mutant restored flavonol biosynthesis. Furthermore, overexpression of MtMYB134 in hairy roots of M. truncatula enhanced the biosynthesis of various flavonol derivatives. Taken together, our results provide insight into the understanding of flavonol biosynthesis regulation in M. truncatula and provides MtMYB134 as tool for genetic manipulation to improve flavonol synthesis.

UGT84F9 is the major flavonoid UDP-glucuronosyltransferase in Medicago truncatula

Mammalian phase II metabolism of dietary plant flavonoid compounds generally involves substitution with glucuronic acid. In contrast, flavonoids mainly exist as glucose conjugates in plants, and few plant UDP-glucuronosyltransferase enzymes have been identified to date. In the model legume Medicago truncatula, the major flavonoid compounds in the aerial parts of the plant are glucuronides of the flavones apigenin and luteolin. Here we show that the M. truncatula glycosyltransferase UGT84F9 is a bi-functional glucosyl/glucuronosyl transferase in vitro, with activity against a wide range of flavonoid acceptor molecules including flavones. However, analysis of metabolite profiles in leaves and roots of M. truncatula ugt84f9 loss of function mutants revealed that the enzyme is essential for formation of flavonoid glucuronides, but not most flavonoid glucosides, in planta. We discuss the use of plant UGATs for the semi-synthesis of flavonoid phase II metabolites for clinical studies.

Brassinosteroid homeostasis is critical for the functionality of the Medicago truncatula pulvinus

Many plant species open their leaves during the daytime and close them at night as if sleeping. This leaf movement is known as nyctinasty, a unique and intriguing phenomenon that been of great interest to scientists for centuries. Nyctinastic leaf movement occurs widely in leguminous plants, and is generated by a specialized motor organ, the pulvinus. Although a key determinant of pulvinus development, PETIOLULE-LIKE PULVINUS (PLP), has been identified, the molecular genetic basis for pulvinus function is largely unknown. Here, through an analysis of knockout mutants in barrelclover (Medicago truncatula), we showed that neither altering brassinosteroid (BR) content nor blocking BR signal perception affected pulvinus determination. However, BR homeostasis did influence nyctinastic leaf movement. BR activity in the pulvinus is regulated by a BR-inactivating gene PHYB ACTIVATION TAGGED SUPPRESSOR1 (BAS1), which is directly activated by PLP. A comparative analysis between M. truncatula and the non-pulvinus forming species Arabidopsis and tomato (Solanum lycopersicum) revealed that PLP may act as a factor that associates with unknown regulators in pulvinus determination in M. truncatula. Apart from exposing the involvement of BR in the functionality of the pulvinus, these results have provided insights into whether gene functions among species are general or specialized.

Phosphorus deprivation affects composition and spatial distribution of membrane lipids in legume nodules

In legumes, symbiotic nitrogen (N) fixation (SNF) occurs in specialized organs called nodules after successful interactions between legume hosts and rhizobia. In a nodule, N-fixing rhizobia are surrounded by symbiosome membranes, through which the exchange of nutrients and ammonium occurs between bacteria and the host legume. Phosphorus (P) is an essential macronutrient, and N2-fixing legumes have a higher requirement for P than legumes grown on mineral N. As in the previous studies, in P deficiency, barrel medic (Medicago truncatula) plants had impaired SNF activity, reduced growth, and accumulated less phosphate in leaves, roots, and nodules compared with the plants grown in P sufficient conditions. Membrane lipids in M. truncatula tissues were assessed using electrospray ionization-mass spectrometry. Galactolipids were found to increase in P deficiency, with declines in phospholipids (PL), especially in leaves. Lower PL losses were found in roots and nodules. Subsequently, matrix-assisted laser desorption/ionization-mass spectrometry imaging was used to spatially map the distribution of the positively charged phosphatidylcholine (PC) species in nodules in both P-replete and P-deficient conditions. Our results reveal heterogeneous distribution of several PC species in nodules, with homogeneous distribution of other PC classes. In P poor conditions, some PC species distributions were observed to change. The results suggest that specific PC species may be differentially important in diverse nodule zones and cell types, and that membrane lipid remodeling during P stress is not uniform across the nodule.

The F-box protein MIO1/SLB1 regulates organ size and leaf movement in Medicago truncatula

The size of leaf and seed organs, determined by the interplay of cell proliferation and expansion, is closely related to the final yield and quality of forage and crops. Yet the cellular and molecular mechanisms underlying organ size modulation remain poorly understood, especially in legumes. Here, MINI ORGAN1 (MIO1), which encodes an F-box protein SMALL LEAF AND BUSHY1 (SLB1) recently reported to control lateral branching in Medicago truncatula, was identified as a key regulator of organ size. We show that loss-of-function of MIO1/SLB1 severely reduced organ size. Conversely, plants overexpressing MIO1/SLB1 had enlarged organs. Cellular analysis revealed that MIO1/SLB1 controlled organ size mainly by modulating primary cell proliferation during the early stages of leaf development. Biochemical analysis revealed that MIO1/SLB1 could form part of SKP1/Cullin/F-box (SCF) E3 ubiquitin ligase complex, to target BIG SEEDS1 (BS1), a repressor of primary cell division, for degradation. Interestingly, we found that MIO1/SLB1 also played a key role in pulvinus development and leaf movement by modulating cell proliferation of the pulvinus as leaves developed. Our study not only demonstrates a conserved role of MIO1/SLB1 in the control of organ size in legumes, but also sheds light on the novel function of MIO1/SLB1 in leaf movement.

NIN is essential for development of symbiosomes, suppression of defence and premature senescence in Medicago truncatula nodules

NIN (NODULE INCEPTION) is a transcription factor that plays a key role during root nodule initiation. However, its role in later nodule developmental stages is unclear. Both NIN mRNA and protein accumulated at the highest level in the proximal part of the infection zone in Medicago truncatula nodules. Two nin weak allele mutants, nin-13/16, form a rather normal nodule infection zone, whereas a fixation zone is not formed. Instead, a zone with defence responses and premature senescence occurred and symbiosome development gets arrested. Mutations in nin-13/16 resulted in a truncated NIN lacking the conserved PB1 domain. However, this did not cause the nodule phenotype as nin mutants expressing NIN_ΔPB1 formed wild-type-like nodule. The phenotype is likely to be caused by reduced NIN mRNA levels in the cytoplasm. Transcriptome analyses of nin-16 nodules showed that expression levels of defence/senescence-related genes are markedly increased, whereas the levels of defence suppressing genes are reduced. Although defence/senescence seems well suppressed in the infection zone, the transcriptome is already markedly changed in the proximal part of infection zone. In addition to its function in infection and nodule organogenesis, NIN also plays a major role at the transition from infection to fixation zone in establishing a functional symbiosis.

The geometry of the compound leaf plays a significant role in the leaf movement of Medicago truncatula modulated by mtdwarf4a

In most legumes, two typical features found in leaves are diverse compound forms and the pulvinus-driven nyctinastic movement. Many genes have been identified for leaf-shape determination, but the underlying nature of leaf movement as well as its association with the compound form remains largely unknown. Using forward-genetic screening and whole-genome resequencing, we found that two allelic mutants of Medicago truncatula with unclosed leaflets at night were impaired in MtDWARF4A (MtDWF4A), a gene encoding a cytochrome P450 protein orthologous to Arabidopsis DWARF4. The mtdwf4a mutant also had a mild brassinosteroid (BR)-deficient phenotype bearing pulvini without significant deficiency in organ identity. Both mtdwf4a and dwf4 could be fully rescued by MtDWF4A, and mtdwf4a could close their leaflets at night after the application of exogenous 24-epi-BL. Surgical experiments and genetic analysis of double mutants revealed that the failure to exhibit leaf movement in mtdwf4a is a consequence of the physical obstruction of the overlapping leaflet laminae, suggesting a proper geometry of leaflets is important for their movement in M. truncatula. These observations provide a novel insight into the nyctinastic movement of compound leaves, shedding light on the importance of open space for organ movements in plants.

Strategies to Apply Water-Deficit Stress: Similarities and Disparities at the Whole Plant Metabolism Level in Medicago truncatula

Water-deficit stresses such as drought and salinity are the most important factors limiting crop productivity. Hence, understanding the plant responses to these stresses is key for the improvement of their tolerance and yield. In this study M. truncatula plants were subjected to 250 mM NaCl as well as reduced irrigation (No-W) and 250 g/L polyethylene glycol (PEG)-6000 to induce salinity and drought stress, respectively, provoking a drop to −1.7 MPa in leaf water potential. The whole plant physiology and metabolism was explored by characterizing the stress responses at root, phloem sap and leaf organ level. PEG treatment led to some typical responses of plants to drought stress, but in addition to PEG uptake, an important impairment of nutrient uptake and a different regulation of carbon metabolism could be observed compared to No-W plants. No-W plants showed an important redistribution of antioxidants and assimilates to the root tissue, with a distinctive increase in root proline degradation and alkaline invertase activity. On the contrary, salinity provoked an increase in leaf starch and isocitrate dehydrogenase activity, suggesting key roles in the plant response to this stress. Overall, results suggest higher protection of salt-stressed shoots and non-irrigated roots through different mechanisms, including the regulation of proline and carbon metabolism, while discarding PEG as safe mimicker of drought. This raises the need to understand the effect at the whole plant level of the different strategies employed to apply water-deficit stress.

The antagonistic MYB paralogs RH1 and RH2 govern anthocyanin leaf markings in Medicago truncatula

Patterned leaf coloration in plants generates remarkable diversity in nature, but the underlying mechanisms remain largely unclear. Here, using Medicago truncatula leaf marking as a model, we show that the classic M. truncatula leaf anthocyanin spot trait depends on two R2R3 MYB paralogous regulators, RED HEART1 (RH1) and RH2. RH1 mainly functions as an anthocyanin biosynthesis activator that specifically determines leaf marking formation depending on its C-terminal activation motif. RH1 physically interacts with the M. truncatula bHLH protein MtTT8 and the WDR family member MtWD40-1, and this interaction facilitates RH1 function in leaf anthocyanin marking formation. RH2 has lost transcriptional activation activity, due to a divergent C-terminal domain, but retains the ability to interact with the same partners, MtTT8 and MtWD40-1, as RH1, thereby acting as a competitor in the regulatory complex and exerting opposite effects. Moreover, our results demonstrate that RH1 can activate its own expression and that RH2-mediated competition can repress RH1 expression. Our findings reveal the molecular mechanism of the antagonistic gene paralogs RH1 and RH2 in determining anthocyanin leaf markings in M. truncatula, providing a multidimensional paralogous-antagonistic regulatory paradigm for fine-tuning patterned pigmentation.

Characterisation of Medicago truncatula CLE34 and CLE35 in nitrate and rhizobia regulation of nodulation

Legumes form a symbiosis with atmospheric nitrogen (N₂ )-fixing soil rhizobia, resulting in new root organs called nodules that enable N₂ -fixation. Nodulation is a costly process that is tightly regulated by the host through autoregulation of nodulation (AON) and nitrate-dependent regulation of nodulation. Both pathways require legume-specific CLAVATA/ESR-related (CLE) peptides. Nitrogen-induced nodulation-suppressing CLE peptides have not previously been investigated in Medicago truncatula, for which only rhizobia-induced MtCLE12 and MtCLE13 have been characterised. Here, we report on novel peptides MtCLE34 and MtCLE35 in nodulation control. The nodulation-suppressing CLE peptides of five legume species were classified into three clades based on sequence homology and phylogeny. This approached identified MtCLE34 and MtCLE35 and four new CLE peptide orthologues of Pisum sativum. Whereas MtCLE12 and MtCLE13 are induced by rhizobia, MtCLE34 and MtCLE35 respond to both rhizobia and nitrate. MtCLE34 was identified as a pseudogene lacking a functional CLE-domain. MtCLE35 was found to inhibit nodulation in a SUNN- and RDN1-dependent manner via overexpression analysis. Together, our findings indicate that MtCLE12 and MtCLE13 have a specific role in AON, while MtCLE35 regulates nodule numbers in response to both rhizobia and nitrate. MtCLE34 likely had a similar role to MtCLE35, but its function was lost due to a premature nonsense mutation.

Potassium content diminishes in infected cells of Medicago truncatula nodules due to the mislocation of channels MtAKT1 and MtSKOR/GORK

Rhizobia establish a symbiotic relationship with legumes that results in the formation of root nodules, where bacteria encapsulated by a membrane of plant origin (symbiosomes), convert atmospheric nitrogen into ammonia. Nodules are more sensitive to ionic stresses than the host plant itself. We hypothesize that such a high vulnerability might be due to defects in ion balance in the infected tissue. Low temperature SEM (LTSEM) and X-ray microanalysis of Medicago truncatula nodules revealed a potassium (K⁺) decrease in symbiosomes and vacuoles during the life span of infected cells. To clarify K⁺ homeostasis in the nodule, we performed phylogenetic and gene expression analyses, and confocal and electron microscopy localization of two key plant Shaker K⁺ channels, AKT1 and SKOR/GORK. Phylogenetic analyses showed that the genome of some legume species, including the Medicago genus, contained one SKOR/GORK and one AKT1 gene copy, while other species contained more than one copy of each gene. Localization studies revealed mistargeting and partial depletion of both channels from the plasma membrane of M. truncatula mature nodule-infected cells that might compromise ion transport. We propose that root nodule-infected cells have defects in K⁺ balance due to mislocation of some plant ion channels, as compared with non-infected cells. The putative consequences are discussed.

WOX9 functions antagonistic to STF and LAM1 to regulate leaf blade expansion in Medicago truncatula and Nicotiana sylvestris

WOX family transcription factors regulate multiple developmental programs. The intermediate clade transcriptional activator WOX9 functions together with the modern clade transcriptional repressor WOX genes in embryogenesis and meristems maintenance, but the mechanism of this interaction is unclear. STF and LAM1 are WOX1 orthologs required for leaf blade outgrowth in Medicago truncatula and Nicotiana sylvestris, respectively. Using biochemical methods and genome editing technology, here we show that WOX9 is an abaxial factor and functions antagonistically to STF and LAM1 to regulate leaf blade development. While NsWOX9 ectopic expression enhances the lam1 mutant phenotype, and antisense expression partially rescues the lam1 mutant, both overexpression and knockout of NsWOX9 in N. sylvestris resulted in a range of severe leaf blade distortions, indicating important role in blade development. Our results indicate that direct repression of WOX9 by WUS clade repressor STF/LAM1 is required for correct blade architecture and patterning in M. truncatula and N. sylvestris. These findings suggest that controlling transcriptional activation and repression mechanisms by direct interaction of activator and repressor WOX genes may be required for cell proliferation and differentiation homeostasis, and could be an evolutionarily conserved mechanism for the development of complex and diverse morphology in flowering plants.