RPG interacts with E3-ligase CERBERUS to mediate rhizobial infection in Lotus japonicus

Symbiotic interactions between rhizobia and legumes result in the formation of root nodules, which fix nitrogen that can be used for plant growth. Rhizobia usually invade legume roots through a plant-made tunnel-like structure called an infection thread (IT). RPG (Rhizobium-directed polar growth) encodes a coiled-coil protein that has been identified in Medicago truncatula as required for root nodule infection, but the function of RPG remains poorly understood. In this study, we identified and characterized RPG in Lotus japonicus and determined that it is required for IT formation. RPG was induced by Mesorhizobium loti or purified Nodulation factor and displayed an infection-specific expression pattern. Nodule inception (NIN) bound to the RPG promoter and induced its expression. We showed that RPG displayed punctate subcellular localization in L. japonicus root protoplasts and in root hairs infected by M. loti. The N-terminal predicted C2 lipid-binding domain of RPG was not required for this subcellular localization or for function. CERBERUS, a U-box E3 ligase which is also required for rhizobial infection, was found to be localized similarly in puncta. RPG co-localized and directly interacted with CERBERUS in the early endosome (TGN/EE) compartment and near the nuclei in root hairs after rhizobial inoculation. Our study sheds light on an RPG-CERBERUS protein complex that is involved in an exocytotic pathway mediating IT elongation.

Serum cytokine patterns are modulated in infants fed formula with probiotics or milk fat globule membranes: A randomized controlled trial

Background

Proteins and lipids of milk fat globule membrane (MFGM) and probiotics are immunomodulatory. We hypothesized that Lactobacillus paracasei ssp. paracasei strain F19 (F19) would augment vaccine antibody and T helper 1 type immune responses whereas MFGM would produce an immune response closer to that of breastfed (BF) infants.

Objective

To compare the effects of supplementing formula with F19 or bovine MFGM on serum cytokine and vaccine responses of formula-fed (FF) and BF infants.

Design

FF infants were randomized to formula with F19 (n = 195) or MFGM (n = 192), or standard formula (SF) (n = 194) from age 21±7 days until 4 months. A BF group served as reference (n = 208). We analyzed seven cytokines (n = 398) in serum at age 4 months using magnetic bead-based multiplex technology. Using ELISA, we analyzed anti-diphtheria IgG (n = 258) and anti-poliovirus IgG (n = 309) concentrations in serum before and after the second and third immunization, respectively.

Results

Compared with SF, the F19 group had greater IL-2 and lower IFN-γ concentrations (p<0.05, average effect size 0.14 and 0.39). Compared with BF, the F19 group had greater IL-2, IL-4 and IL-17A concentrations (p<0.05, average effect size 0.42, 0.34 and 0.26, respectively). The MFGM group had lower IL-2 and IL-17A concentrations compared with SF (p<0.05, average effect size 0.34 and 0.31). Cytokine concentrations were comparable among the MFGM and BF groups. Vaccine responses were comparable among the formula groups.

Conclusions

Contrary to previous studies F19 increased IL-2 and lowered IFN-γ production, suggesting that the response to probiotics differs across populations. The cytokine profile of the MFGM group approached that of BF infants, and may be associated with the previous finding that infectious outcomes for the MFGM group in this cohort were closer to those of BF infants, as opposed to the SF group. These immunomodulatory effects support future clinical evaluation of infant formula with F19 or MFGM.

Dairy-Derived Emulsifiers in Infant Formula Show Marginal Effects on the Plasma Lipid Profile and Brain Structure in Preterm Piglets Relative to Soy Lecithin

Breastfed infants have higher intestinal lipid absorption and neurodevelopmental outcomes compared to formula-fed infants, which may relate to a different surface layer structure of fat globules in infant formula. This study investigated if dairy-derived emulsifiers increased lipid absorption and neurodevelopment relative to soy lecithin in newborn preterm piglets. Piglets received a formula diet containing soy lecithin (SL) or whey protein concentrate enriched in extracellular vesicles (WPC-A-EV) or phospholipids (WPC-PL) for 19 days. Both WPC-A-EV and WPC-PL emulsions, but not the intact diets, increased in vitro lipolysis compared to SL. The main differences of plasma lipidomics analysis were increased levels of some sphingolipids, and lipid molecules with odd-chain (17:1, 19:1, 19:3) as well as mono- and polyunsaturated fatty acyl chains (16:1, 20:1, 20:3) in the WPC-A-EV and WPC-PL groups and increased 18:2 fatty acyls in the SL group. Indirect monitoring of intestinal triacylglycerol absorption showed no differences between groups. Diffusor tensor imaging measurements of mean diffusivity in the hippocampus were lower for WPC-A-EV and WPC-PL groups compared to SL indicating improved hippocampal maturation. No differences in hippocampal lipid composition or short-term memory were observed between groups. In conclusion, emulsification of fat globules in infant formula with dairy-derived emulsifiers altered the plasma lipid profile and hippocampal tissue diffusivity but had limited effects on other absorptive and learning abilities relative to SL in preterm piglets.

Milk Fat Globule Membrane as a Modulator of Infant Metabolism and Gut Microbiota: A Formula Supplement Narrowing the Metabolic Differences between Breastfed and Formula-Fed Infants

SCOPE

Milk fat globule membrane (MFGM) is an important component of milk that has previously been removed in the manufacture of infant formulas, but has recently gained attention owing to its potential to improve immunological, cognitive, and metabolic health. The goal of this study is to determine whether supplementing MFGM in infant formula would drive desirable changes in metabolism and gut microbiota to elicit benefits observed in prior studies.

METHODS AND RESULTS

The serum metabolome and fecal microbiota are analyzed using ¹ H NMR spectroscopy and 16S rRNA gene sequencing respectively in a cohort of Chinese infants given a standard formula or a formula supplemented with an MFGM-enriched whey protein fraction. Supplementing MFGM suppressed protein degradation pathways and the levels of insulinogenic amino acids that are typically enhanced in formula-fed infants while facilitating fatty acid oxidation and ketogenesis, a feature that may favor brain development. MFGM supplementation did not induce significant compositional changes in the fecal microbiota but suppressed microbial diversity and altered microbiota-associated metabolites.

CONCLUSION

Supplementing MFGM in a formula reduced some metabolic gaps between formula-fed and breastfed infants.

Early-Life Supplementation of Bovine Milk Osteopontin Supports Neurodevelopment and Influences Exploratory Behavior

Introduction: Osteopontin (OPN) is a whey protein found at high concentration in human milk and is involved in processes such as bone cell proliferation and differentiation. Milk OPN has shown to be involved in various aspects of development, including the immune system and gut health. However, the influence of dietary bovine milk OPN inclusion on brain and cognitive development has not been studied extensively until recently. This research examines whether dietary supplementation of bovine milk OPN supports brain and cognitive development in the translational pig model. Methods: From postnatal day (PND) 2 to 34, twenty-one intact male pigs were provided ad libitum access to one of two dietary treatments, a standard soy protein isolate-based milk replacer to serve as a control diet (n = 11) and the same base diet supplemented with bovine milk OPN to serve as a test diet (n = 10). In addition to growth and health outcomes, recognition memory was tested using the novel object recognition (NOR) task from PND 28 to 32, and magnetic resonance imaging was conducted at PND 34 to evaluate brain development. Results: No dietary effects were observed for growth performance or health indices. For the behavioral analysis, pigs that received the test diet exhibited shorter (p < 0.05) latency to the first object visited compared with pigs fed the control diet. Although the control group exhibited novelty preference, there was no difference in recognition index between dietary groups. Neuroimaging outcomes revealed increased (p < 0.05) relative brain volumes of the corpus callosum, lateral ventricle, left and right internal capsule, left and right putamen-globus pallidus, and right hippocampus, and right cortex in the test group. Diffusion tensor imaging revealed higher (p < 0.05) radial diffusivity in the corpus callosum and lower (p < 0.05) fractional anisotropy in pigs provided the test diet. Conclusion: Dietary supplementation of bovine milk OPN increased the relative volume of several brain regions and altered behaviors in the NOR task. Underlying mechanisms of bovine milk OPN influencing the development of brain structures and additional behaviors warrant further investigation.

Feeding Infants Formula With Probiotics or Milk Fat Globule Membrane: A Double-Blind, Randomized Controlled Trial

Purpose: To evaluate effects on growth and infection rates of supplementing infant formula with the probiotic Lactobacillus paracasei ssp. paracasei strain F19 (F19) or bovine milk fat globule membrane (MFGM). Methods: In a double-blind, randomized controlled trial, 600 infants were randomized to a formula supplemented with F19 or MFGM, or to standard formula (SF). A breastfed group was recruited as reference (n = 200).The intervention lasted from age 21 ± 7 days until 4 months, and infants were followed until age one year. Results: Both experimental formulas were well tolerated and resulted in high compliance. The few reported adverse events were not likely related to formula, with the highest rates in the SF group, significantly higher than for the F19-supplemented infants (p = 0.046). Weight or length gain did not differ during or after the intervention among the formula-fed groups, with satisfactory growth. During the intervention, overall, the experimental formula groups did not have more episodes of diarrhea, fever, or days with fever than the breastfed infants. However, compared to the breastfed infants, the SF group had more fever episodes (p = 0.021) and days with fever (p = 0.036), but not diarrhea. Compared with the breastfed group, the F19-supplemented infants but not the other two formula groups had more visits/unscheduled hospitalizations (p = 0.015) and borderline more episodes of upper respiratory tract infections (p = 0.048). Conclusions: Both the MFGM- and F19-supplemented formulas were safe and well-tolerated, leading to few adverse effects, similar to the breastfed group and unlike the SF group. During the intervention, the MFGM-supplemented infants did not differ from the breastfed infants in any primary outcome.

Cytokinin Biosynthesis Promotes Cortical Cell Responses during Nodule Development

Legume mutants have shown the requirement for receptor-mediated cytokinin signaling in symbiotic nodule organogenesis. While the receptors are central regulators, cytokinin also is accumulated during early phases of symbiotic interaction, but the pathways involved have not yet been fully resolved. To identify the source, timing, and effect of this accumulation, we followed transcript levels of the cytokinin biosynthetic pathway genes in a sliding developmental zone of Lotus japonicus roots. LjIpt2 and LjLog4 were identified as the major contributors to the first cytokinin burst. The genetic dependence and Nod factor responsiveness of these genes confirm that cytokinin biosynthesis is a key target of the common symbiosis pathway. The accumulation of LjIpt2 and LjLog4 transcripts occurs independent of the LjLhk1 receptor during nodulation. Together with the rapid repression of both genes by cytokinin, this indicates that LjIpt2 and LjLog4 contribute to, rather than respond to, the initial cytokinin buildup. Analysis of the cytokinin response using the synthetic cytokinin sensor, TCSn, showed that this response occurs in cortical cells before spreading to the epidermis in L. japonicus While mutant analysis identified redundancy in several biosynthesis families, we found that mutation of LjIpt4 limits nodule numbers. Overexpression of LjIpt3 or LjLog4 alone was insufficient to produce the robust formation of spontaneous nodules. In contrast, overexpressing a complete cytokinin biosynthesis pathway leads to large, often fused spontaneous nodules. These results show the importance of cytokinin biosynthesis in initiating and balancing the requirement for cortical cell activation without uncontrolled cell proliferation.

The ERN1 transcription factor gene is a target of the CCaMK/CYCLOPS complex and controls rhizobial infection in Lotus japonicus

Bacterial accommodation inside living plant cells is restricted to the nitrogen-fixing root nodule symbiosis. In many legumes, bacterial uptake is mediated via tubular structures called infection threads (ITs). To identify plant genes required for successful symbiotic infection, we screened an ethyl methanesulfonate mutagenized population of Lotus japonicus for mutants defective in IT formation and cloned the responsible gene, ERN1, encoding an AP2/ERF transcription factor. We performed phenotypic analysis of two independent L. japonicus mutant alleles and investigated the regulation of ERN1 via transactivation and DNA-protein interaction assays. In ern1 mutant roots, nodule primordia formed, but most remained uninfected and bacterial entry via ITs into the root epidermis was abolished. Infected cortical nodule cells contained bacteroids, but transcellular ITs were rarely observed. A subset exhibited localized cell wall degradation and loss of cell integrity associated with bacteroid spread into neighbouring cells and the apoplast. Functional promoter studies revealed that CYCLOPS binds in a sequence-specific manner to a motif within the ERN1 promoter and in combination with CCaMK positively regulates ERN1 transcription. We conclude that the activation of ERN1 by CCaMK/CYCLOPS complex is an important step controlling IT-mediated bacterial progression into plant cells.

CYTOKININ OXIDASE/DEHYDROGENASE3 Maintains Cytokinin Homeostasis during Root and Nodule Development in Lotus japonicus

Cytokinins are required for symbiotic nodule development in legumes, and cytokinin signaling responses occur locally in nodule primordia and in developing nodules. Here, we show that the Lotus japonicus Ckx3 cytokinin oxidase/dehydrogenase gene is induced by Nod factor during the early phase of nodule initiation. At the cellular level, pCkx3::YFP reporter-gene studies revealed that the Ckx3 promoter is active during the first cortical cell divisions of the nodule primordium and in growing nodules. Cytokinin measurements in ckx3 mutants confirmed that CKX3 activity negatively regulates root cytokinin levels. Particularly, tZ and DHZ type cytokinins in both inoculated and uninoculated roots were elevated in ckx3 mutants, suggesting that these are targets for degradation by the CKX3 cytokinin oxidase/dehydrogenase. The effect of CKX3 on the positive and negative roles of cytokinin in nodule development, infection and regulation was further clarified using ckx3 insertion mutants. Phenotypic analysis indicated that ckx3 mutants have reduced nodulation, infection thread formation and root growth. We also identify a role for cytokinin in regulating nodulation and nitrogen fixation in response to nitrate as ckx3 phenotypes are exaggerated at increased nitrate levels. Together, these findings show that cytokinin accumulation is tightly regulated during nodulation in order to balance the requirement for cell divisions with negative regulatory effects of cytokinin on infection events and root development.

The molecular network governing nodule organogenesis and infection in the model legume Lotus japonicus

Bacterial infection of interior tissues of legume root nodules is controlled at the epidermal cell layer and is closely coordinated with progressing organ development. Using spontaneous nodulating Lotus japonicus plant mutants to uncouple nodule organogenesis from infection, we have determined the role of 16 genes in these two developmental processes. We show that host-encoded mechanisms control three alternative entry processes operating in the epidermis, the root cortex and at the single cell level. Single cell infection did not involve the formation of trans-cellular infection threads and was independent of host Nod-factor receptors and bacterial Nod-factor signals. In contrast, Nod-factor perception was required for epidermal root hair infection threads, whereas primary signal transduction genes preceding the secondary Ca2+ oscillations have an indirect role. We provide support for the origin of rhizobial infection through direct intercellular epidermal invasion and subsequent evolution of crack entry and root hair invasions observed in most extant legumes.

LIN, a novel type of U-box/WD40 protein, controls early infection by rhizobia in legumes

The formation of a nitrogen-fixing nodule requires the coordinated development of rhizobial colonization and nodule organogenesis. Based on its mutant phenotype, lumpy infections (lin), LIN functions at an early stage of the rhizobial symbiotic process, required for both infection thread growth in root hair cells and the further development of nodule primordia. We show that spontaneous nodulation activated by the calcium- and calmodulin-dependent protein kinase is independent of LIN; thus, LIN is not necessary for nodule organogenesis. From this, we infer that LIN predominantly functions during rhizobial colonization and that the abortion of this process in lin mutants leads to a suppression of nodule development. Here, we identify the LIN gene in Medicago truncatula and Lotus japonicus, showing that it codes for a predicted E3 ubiquitin ligase containing a highly conserved U-box and WD40 repeat domains. Ubiquitin-mediated protein degradation is a universal mechanism to regulate many biological processes by eliminating rate-limiting enzymes and key components such as transcription factors. We propose that LIN is a regulator of the component(s) of the nodulation factor signal transduction pathway and that its function is required for correct temporal and spatial activity of the target protein(s).

GRAS proteins form a DNA binding complex to induce gene expression during nodulation signaling in Medicago truncatula

The symbiotic association of legumes with rhizobia involves bacterially derived Nod factor, which is sufficient to activate the formation of nodules on the roots of the host plant. Perception of Nod factor by root hair cells induces calcium oscillations that are a component of the Nod factor signal transduction pathway. Perception of the calcium oscillations is a function of a calcium- and calmodulin-dependent protein kinase, and this activates nodulation gene expression via two GRAS domain transcriptional regulators, Nodulation Signaling Pathway1 (NSP1) and NSP2, and an ERF transcription factor required for nodulation. Here, we show that NSP1 and NSP2 form a complex that is associated with the promoters of early nodulin genes. We show that NSP1 binds directly to ENOD promoters through the novel cis-element AATTT. While NSP1 shows direct binding to the ENOD11 promoter in vitro, this association in vivo requires NSP2. The NSP1-NSP2 association with the ENOD11 promoter is enhanced following Nod factor elicitation. Mutations in the domain of NSP2 responsible for its interaction with NSP1 highlight the significance of the NSP1-NSP2 heteropolymer for nodulation signaling. Our work reveals direct binding of a GRAS protein complex to DNA and highlights the importance of the NSP1-NSP2 complex for efficient nodulation in the model legume Medicago truncatula.

Structural implications of mutations in the pea SYM8 symbiosis gene, the DMI1 ortholog, encoding a predicted ion channel

The Pisum sativum SYM8 gene plays an essential part in both rhizobial and mycorrhizal symbioses. Mutation of sym8 in the original type line R25 blocks nodulation, mycorrhization, and Nod-factor-induced calcium spiking, an early component of the nodulation signaling pathway. We describe four new sym8 alleles of pea, which fall into the same complementation group as R25. The sym8 mutants are phenotypically similar to Medicago truncatula dmi1 mutants and map to a syntenic location. We used sequence homology to isolate the pea ortholog of M. truncatula DMI1 and have shown that the cloned pea ortholog can complement a M. truncatula dmi1 mutant for nodulation. Each of the five pea sym8 mutants carries a mutation in the DMI1 ortholog, confirming that the pea SYM8 is the DMI1 ortholog. Based on predicted structural similarities with an archaebacterial ion channel, we propose that SYM8 forms a tetrameric calcium-gated channel of a predicted structure similar to the archaebacterial potassium channel but containing a filter region that is different. The predicted structure identifies four aspartate residues (one from each subunit) forming the channel opening. We made a mutation changing the aspartate to valine and identified a missense mutation (changing alanine to valine adjacent to the aspartate residues) in this predicted filter region; both mutations caused a loss of function. We also identified a loss-of-function missense mutation (changing arginine to isoleucine) in a domain proposed to link the predicted channel and the gating ring domains, indicating that this mutation may block function by preventing a protein conformational change being transmitted from the gating-ring domain to the pore domain.

The Medicago truncatula DMI1 protein modulates cytosolic calcium signaling

In addition to establishing symbiotic relationships with arbuscular mycorrhizal fungi, legumes also enter into a nitrogen-fixing symbiosis with rhizobial bacteria that results in the formation of root nodules. Several genes involved in the development of both arbuscular mycorrhiza and legume nodulation have been cloned in model legumes. Among them, Medicago truncatula DMI1 (DOESN'T MAKE INFECTIONS1) is required for the generation of nucleus-associated calcium spikes in response to the rhizobial signaling molecule Nod factor. DMI1 encodes a membrane protein with striking similarities to the Methanobacterium thermoautotrophicum potassium channel (MthK). The cytosolic C terminus of DMI1 contains a RCK (regulator of the conductance of K(+)) domain that in MthK acts as a calcium-regulated gating ring controlling the activity of the channel. Here we show that a dmi1 mutant lacking the entire C terminus acts as a dominant-negative allele interfering with the formation of nitrogen-fixing nodules and abolishing the induction of calcium spikes by the G-protein agonist Mastoparan. Using both the full-length DMI1 and this dominant-negative mutant protein we show that DMI1 increases the sensitivity of a sodium- and lithium-hypersensitive yeast (Saccharomyces cerevisiae) mutant toward those ions and that the C-terminal domain plays a central role in regulating this response. We also show that DMI1 greatly reduces the release of calcium from internal stores in yeast, while the dominant-negative allele appears to have the opposite effect. This work suggests that DMI1 is not directly responsible for Nod factor-induced calcium changes, but does have the capacity to regulate calcium channels in both yeast and plants.

Lotus japonicus nodulation requires two GRAS domain regulators, one of which is functionally conserved in a non-legume

A new nodulation-defective mutant of Lotus japonicus does not initiate nodule cortical cell division in response to Mesorhizobium loti, but induces root hair deformation, Nod factor-induced calcium spiking, and mycorrhization. This phenotype, together with mapping data, suggested that the mutation could be in the ortholog of the Medicago truncatula NSP1 gene (MtNSP1). The sequence of the orthologous gene (LjNSP1) in the L. japonicus mutant (Ljnsp1-1) revealed a mutation causing a premature stop resulting in loss of the C-terminal 23 amino acids. We also sequenced the NSP2 gene from L. japonicus (LjNSP2). A mutant (Ljnsp2-3) with a premature stop codon was identified by TILLING showing a similar phenotype to Ljnsp1-1. Both LjNSP1 and LjNSP2 are predicted GRAS (GAI, RGA, SCR) domain transcriptional regulators. Transcript steady-state levels of LjNSP1 and LjNSP2 initially decreased and then increased following infection by M. loti. In hairy root transformations, LjNSP1 and MtNSP1 complemented both Mtnsp1-1 and Ljnsp1-1 mutants, demonstrating that these orthologous proteins have a conserved biochemical function. A Nicotiana benthamiana NSP1-like gene (NbNSP1) was shown to restore nodule formation in both Ljnsp1-1 and Mtnsp1-1 mutants, indicating that NSP1 regulators from legumes and non-legumes can propagate the Nod factor-induced signal, activating appropriate downstream targets. The L. japonicus nodules complemented with NbNSP1 contained some cells with abnormal bacteroids and could fix nitrogen. However, the NbNSP1-complemented M. truncatula nodules did not fix nitrogen and contained very few bacteria released from infection threads. These observations suggest that NSP1 is also involved in infection, bacterial release, and normal bacteroid formation in nodule cells.

Identification of symbiotically defective mutants of Lotus japonicus affected in infection thread growth

During the symbiotic interaction between legumes and rhizobia, the host cell plasma membrane and associated plant cell wall invaginate to form a tunnel-like infection thread, a structure in which bacteria divide to reach the plant root cortex. We isolated four Lotus japonicus mutants that make infection pockets in root hairs but form very few infection threads after inoculation with Mesorhizobium loti. The few infection threads that did initiate in the mutants usually did not progress further than the root hair cell. These infection-thread deficient (itd) mutants were unaffected for early symbiotic responses such as calcium spiking, root hair deformation, and curling, as well as for the induction of cortical cell division and the arbuscular mycorrhizal symbiosis. Complementation tests and genetic mapping indicate that itd2 is allelic to Ljsym7, whereas the itdl, itd3, and itd4 mutations identified novel loci. Bacterial release into host cells did occur occasionally in the itdl, itd2, and itd3 mutants suggesting that some infections may succeed after a long period and that infection of nodule cells could occur normally if the few abnormal infection threads that were formed reached the appropriate nodule cells.

A single hemoglobin gene in Myrica gale retains both symbiotic and non-symbiotic specificity

Here, a hemoglobin gene from the nitrogen-fixing actinorhizal plant Myrica gale was isolated, cloned and sequenced. The gene (MgHb) was a class I hemoglobin with strong sequence homology to non-symbiotic hemoglobin genes. MgHb is highly expressed in symbiotic root nodules, but transcripts and protein were also detected in leaves of M. gale. In Arabidopsis thaliana the MgHb promoter, linked to a beta-glucuronidase coding region, directed expression in the vascular tissue, in shoot meristem and at root branch point--a pattern very similar to the combined expression pattern of the two non-symbiotic A. thaliana hemoglobin promoters AHb1 and AHb2. The results points to a symbiotic as well as a non-symbiotic specificity of MgHb similar to a hemoglobin gene identified in Parasponia andersonii, but different from the situation in Casuarina glauca--a close actinorhizal relative of M. gale.