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1. VPT ‐like genes modulate Rhizobium –legume symbiosis and phosphorus adaptation

   https://doi.org/10.1111/tpj.16363  

   Liu, Jinlong; Yang, Rongchen; Yan, Jun; Li, Chun; Lin, Xizhen; Lin, Lin; Cao, Yanyan; Xu, Tiandong; Li, Jianxuan; Yuan, Yangyang; et al (October 2023, The Plant Journal)

   SUMMARY Although vacuolar phosphate transporters (VPTs) are essential for plant phosphorus adaptation, their role inRhizobium–legume symbiosis is unclear. In this study, homologous genes ofVPT1(MtVPTs)were identified inMedicago truncatulato assess their roles inRhizobium–legume symbiosis and phosphorus adaptation.MtVPT2andMtVPT3mainly positively responded to low and high phosphate, respectively. However, bothmtvpt2andmtvpt3mutants displayed shoot phenotypes with high phosphate sensitivity and low phosphate tolerance. The root‐to‐shoot phosphate transfer efficiency was significantly enhanced inmtvpt3but weakened inmtvpt2, accompanied by lower and higher root cytosolic inorganic phosphate (Pi) concentration, respectively. Low phosphate inducedMtVPT2andMtVPT3expressions in nodules.MtVPT2andMtVPT3mutations markedly reduced the nodule number and nitrogenase activity under different phosphate conditions. Cytosolic Pi concentration in nodules was significantly lower inmtvpt2andmtvpt3than in the wildtype, especially in tissues near the base of nodules, probably due to inhibition of long‐distance Pi transport and cytosolic Pi supply. Also,mtvpt2andmtvpt3could not maintain a stable cytosolic Pi level in the nodule fixation zone as the wildtype under low phosphate stress. These findings show thatMtVPT2and MtVPT3modulate phosphorus adaptation and rhizobia–legume symbiosis, possibly by regulating long‐distance Pi transport. 

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2. Characterization of the Sinorhizobium meliloti HslUV and ClpXP Protease Systems in Free-Living and Symbiotic States

   https://doi.org/10.1128/JB.00498-18  

   Ogden, Aaron J.; McAleer, Jacqueline M.; Kahn, Michael L.; Becker, Anke (April 2019, Journal of Bacteriology)

   ABSTRACT Symbiotic nitrogen fixation (SNF) in the interaction between the soil bacteria Sinorhizobium meliloti and legume plant Medicago sativa is carried out in specialized root organs called nodules. During nodule development, each symbiont must drastically alter their proteins, transcripts, and metabolites in order to support nitrogen fixation. Moreover, bacteria within the nodules are under stress, including challenges by plant antimicrobial peptides, low pH, limited oxygen availability, and strongly reducing conditions, all of which challenge proteome integrity. S. meliloti stress adaptation, proteome remodeling, and quality control are controlled in part by the large oligomeric protease complexes HslUV and ClpXP1. To improve understanding of the roles of S. meliloti HslUV and ClpXP1 under free-living conditions and in symbiosis with M. sativa , we generated Δ hslU , Δ hslV , Δ hslUV , and Δ clpP1 knockout mutants. The shoot dry weight of M. sativa plants inoculated with each deletion mutant was significantly reduced, suggesting a role in symbiosis. Further, slower free-living growth of the Δ hslUV and Δ clpP1 mutants suggests that HslUV and ClpP1 were involved in adapting to heat stress, the while Δ hslU and Δ clpP1 mutants were sensitive to kanamycin. All deletion mutants produced less exopolysaccharide and succinoglycan, as shown by replicate spot plating and calcofluor binding. We also generated endogenous C-terminal enhanced green fluorescent protein (eGFP) fusions to HslU, HslV, ClpX, and ClpP1 in S. meliloti . Using anti-eGFP antibodies, native coimmunoprecipitation experiments with proteins from free-living and nodule tissues were performed and analyzed by mass spectrometry. The results suggest that HslUV and ClpXP were closely associated with ribosomal and proteome quality control proteins, and they identified several novel putative protein-protein interactions. IMPORTANCE Symbiotic nitrogen fixation (SNF) is the primary means by which biologically available nitrogen enters the biosphere, and it is therefore a critical component of the global nitrogen cycle and modern agriculture. SNF is the result of highly coordinated interactions between legume plants and soil bacteria collectively referred to as rhizobia, e.g., Medicago sativa and S. meliloti , respectively. Accomplishing SNF requires significant proteome changes in both organisms to create a microaerobic environment suitable for high-level bacterial nitrogenase activity. The bacterial protease systems HslUV and ClpXP are important in proteome quality control, in metabolic remodeling, and in adapting to stress. This work shows that S. meliloti HslUV and ClpXP are involved in SNF, in exopolysaccharide production, and in free-living stress adaptation. 

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

   https://doi.org/doi.org/10.1093/plphys/kiaa115  

   Dokwal, D; Romsdahl, TB; Kunz, DA; Alonso, AP; Dickstein, R (January 2021, Plant physiology)

   null (Ed.)

   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. 

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4. Comparison of nodule endophyte composition, diversity, and gene content between Medicago truncatula genotypes

   https://doi.org/10.1094/PBIOMES-10-20-0077-R  

   Burns, Mannix; Epstein, Brendan; Burghardt, Liana (May 2021, Phytobiomes Journal)

   null (Ed.)

   Leguminous plants form symbiotic relationships with rhizobia. These nitrogen-fixing bacteria live in specialized root organs called nodules. While rhizobia form the most notable host relationship within root nodules, other bacterial endophytes also inhabit these root nodules and can influence host-rhizobia interactions as well as exert effects of their own, whether beneficial or detrimental. In this study, we investigate differences in nodule communities between genotypes (A17 and R108) of a single plant species, the model legume Medicago truncatula. While diversity of endophytes in nodules was similar across hosts, both nodule endophyte composition and gene functional groups differed. In contrast to the significant direct effect of host genotype, neither the presence nor identity of a host in the previous generation (either A17 or R108) had a significant effect on the nodule endophyte diversity or composition. However, whether or not a host was present altered gene functional groups. We conclude that genetic variation within a legume host species can play an important role in the establishment of nodule microbiomes. Further studies, including GWAS and functional assays, can open the door for engineering and optimizing nodule endophyte communities that promote growth or have other beneficial qualities. 

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5. UMAMIT44 is a key player in glutamate export from Arabidopsis chloroplasts

   https://doi.org/10.1093/plcell/koad310  

   The, Samantha_Vivia; Santiago, James_P; Pappenberger, Clara; Hammes, Ulrich_Z; Tegeder, Mechthild (December 2023, The Plant Cell)

   Abstract Selective partitioning of amino acids among organelles, cells, tissues, and organs is essential for cellular metabolism and plant growth. Nitrogen assimilation into glutamine and glutamate and de novo biosynthesis of most protein amino acids occur in chloroplasts; therefore, various transport mechanisms must exist to accommodate their directional efflux from the stroma to the cytosol and feed the amino acids into the extraplastidial metabolic and long-distance transport pathways. Yet, Arabidopsis (Arabidopsis thaliana) transporters functioning in plastidial export of amino acids remained undiscovered. Here, USUALLY MULTIPLE ACIDS MOVE IN AND OUT TRANSPORTER 44 (UMAMIT44) was identified and shown to function in glutamate export from Arabidopsis chloroplasts. UMAMIT44 controls glutamate homeostasis within and outside of chloroplasts and influences nitrogen partitioning from leaves to sinks. Glutamate imbalances in chloroplasts and leaves of umamit44 mutants impact cellular redox state, nitrogen and carbon metabolism, and amino acid (AA) and sucrose supply of growing sinks, leading to negative effects on plant growth. Nonetheless, the mutant lines adjust to some extent by upregulating alternative pathways for glutamate synthesis outside the plastids and by mitigating oxidative stress through the production of other amino acids and antioxidants. Overall, this study establishes that the role of UMAMIT44 in glutamate export from chloroplasts is vital for controlling nitrogen availability within source leaf cells and for sink nutrition, with an impact on growth and seed yield. 

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