Crosstalk between the nodulation signaling pathway and the autoregulation of nodulation in Medicago truncatula

A subset of CLAVATA3/endosperm-surrounding region-related (CLE) peptides are involved in autoregulation of nodulation (AON) in Medicago truncatula (e.g. MtCLE12 and MtCLE13). However, their linkage to other components of the AON pathways downstream of the shoot-derived inhibitor (SDI) is not understood. We have ectopically expressed the putative peptide ligand encoding genes MtCLE12 and MtCLE13 in M. truncatula which abolished nodulation completely in wild-type roots but not in the supernodulating null mutant sunn-4. Further, root growth inhibition was detected when MtCLE12 was ectopically expressed in wild-type roots or synthetic CLE12 peptide was applied exogenously. To identify downstream genes, roots of wild-type and sunn-4 mutant overexpressing MtCLE12 were used for quantitative gene expression analysis. We found that, in 35S:MtCLE12 roots, NODULE INCEPTION (NIN, a central regulator of nodulation) was down-regulated, whereas MtEFD (ethylene response factor required for nodule differentiation) and MtRR8 (a type-A response regulator thought to be involved in the negative regulation of cytokinin signaling), were up-regulated. Moreover, we found that the up-regulation of MtEFD and MtRR8 caused by overexpressing MtCLE12 is SUNN-dependent. Hence, our data link for the first time the pathways for Nod factor signaling, cytokinin perception and AON.     

Functional genomics dissection of the nodulation autoregulation pathway(AON) in soybean(Glycine max)

The combination of mutation-based genetics and functional genomics has allowed a detailed dissection of the nodulation-induction and autoregulation of nodulation(AON) pathways of soybean. Applicable to all legumes, nodulation is induced by Rhizobium/Bradyrhizobium-produced lipopolysaccharides(Nod factors), perceived by Nod factor receptors(NFR1/NFR5 dimers), leading to cortical and pericycle cell divisions. These induce the production of CLAVATA3-like(CLE) peptides, which travel in the xylem to the shoot, where they are perceived by a receptor complex including a leucine-rich repeat(LRR) receptor kinase, encoded by Gm NARK, Lj HAR1, Mt SUNN and closely related receptors in other legumes like Phaseolus vulgaris(common bean), Pisum sativum(pea), and Glycine soja. The activated receptor complex negatively regulates by phosphorylation of the constitutive synthesis of mi R2111 in the shoot. This is normally is translocated via the phloem to the entire plant body, initiating suppression of a rootexpressed Kelch repeat-containing F-box protein“Too Much Love(TML),” which in turn suppresses the nodule initiation cascade. Nodulation is therefore permitted during a developmental window between the induction and progress of the nodulation/cell division/infection cascade during the first few days after inoculation and the functional“readiness” of the AON cascade, delayed by the root–shoot–root loop. Loss-of-function mutations in Gm NARK and Lj TML result in excessive nodulation(supernodulation/hypernodulation/supernummary nodulation) as well as localized tolerance to externally applied nitrate. Recent analyses have indicated an interaction of the AON with lateral root formation as well as with the autoregulation of mycorrhization(AOM). Further details of the parallel functions of key points in this regulatory loop remain to be elucidated.     

Changes in the mineral composition of soybean xylem sap during monocarpic senescence and alterations by depodding

Evidence from earlier studies with explants (stem cutting with a leaf and a pod) indicates that a decline in the supply of mineral nutrients from the roots may prepare the leaves for induction of monocarpic senescence in soybean [ Glycine max (L.) Merrill cv. Anoka). In order to assess the changes in mineral flux from the root system, xylem sap was collected from a decapitated plant under 100 kPa pressure over 50 min. The sap volume yield declines after flowering starts, but increases during pod extension and then decreases again during podfill. The concentrations of K, Ca, Mg, P, S, Zn, Fe, Mn, Cu, Mo and Si rise and then fall during reproductive development, but the exact timing differs among the elements. In contrast, B, Al and Na concentrations show a slow rise initially with a large increase in late pod development. Depodding, which prevents the early death of the plant, inhibits the changes of some elements (K, Mg) but not others (Ca. Mg, P, S, Zn. Fe. Mn, B, Cu, Al), and it does not prevent the decrease in sap volume delivered. Inasmuch as the mineral concentration of xylem sap quantitatively reflects upward mineral flux, the supply of most minerals to the shoot declines, and this decrease seems to be an important factor in the preparatory phase of monocarpic senescence. The different minerals show different patterns of change, which indicate differences in the transport mechanisms and their regulation.     

The potential roles of strigolactones and brassinosteroids in the autoregulation of nodulation pathway

Background and Aims

The number of nodules formed on a legume root system is under the strict genetic control of the autoregulation of nodulation (AON) pathway. Plant hormones are thought to play a role in AON; however, the involvement of two hormones recently described as having a largely positive role in nodulation, strigolactones and brassinosteroids, has not been examined in the AON process.

Methods

A genetic approach was used to examine if strigolactones or brassinosteroids interact with the AON system in pea (Pisum sativum). Double mutants between shoot-acting (Psclv2, Psnark) and root-acting (Psrdn1) mutants of the AON pathway and strigolactone-deficient (Psccd8) or brassinosteroid-deficient (lk) mutants were generated and assessed for various aspects of nodulation. Strigolactone production by AON mutant roots was also investigated.

Key Results

Supernodulation of the roots was observed in both brassinosteroid- and strigolactone-deficient AON double-mutant plants. This is despite the fact that the shoots of these plants displayed classic strigolactone-deficient (increased shoot branching) or brassinosteroid-deficient (extreme dwarf) phenotypes. No consistent effect of disruption of the AON pathway on strigolactone production was found, but root-acting Psrdn1 mutants did produce significantly more strigolactones.

Conclusions

No evidence was found that strigolactones or brassinosteroids act downstream of the AON genes examined. While in pea the AON mutants are epistatic to brassinosteroid and strigolactone synthesis genes, we argue that these hormones are likely to act independently of the AON system, having a role in the promotion of nodule formation.     

Bicarbonate-induced alkalinization of the xylem sap in intact maize seedlings as measured in situ with a novel xylem pH probe

In higher plants the pH of the xylem sap plays an important role in drought signaling, growth regulation, and plant nutrition. However, the interpretation of the data is very controversial. The main reason for this is that the xylem pH in intact plants was not directly accessible hitherto. We present here a novel, minimally-invasive probe based on the xylem pressure-potential probe (used for measuring directly xylem pressure and the electrical potential between root xylem sap and medium). Single-tipped, double-barreled capillaries were used, one barrel served as H(+)-selective electrode, whereas pressure and electrical potential were recorded by the other one. Upon insertion of the probe into the root xylem of maize (Zea mays) seedlings, pH values ranging between about 4.2 and 4.9 were monitored when the roots were immersed in standard nutrient solution. The pH did not respond to changes in light irradiation (up to 300 micromol m(-2) s(-1)), but increased upon exposure of the root to 5 or 20 mm bicarbonate in the bath solution. Changes in pH could also be recorded in transpiring plants when the root was cut below the insertion point of the probe and placed in media with different pH. The data support the hypothesis of Mengel ([1994] Plant Soil 165: 275-283) that upon external supply with bicarbonate Fe is immobilized in the leaf apoplast via changes in xylem pH.     

cDNA cloning of a novel lectin-like xylem sap protein and its root-specific expression in cucumber

A novel cDNA encoding a major 30-kDa protein in xylem sap of cucumber Cucumis sativus (XSP30) was homologous to the B chains of galactose-binding lectins such as ricin and abrin. XSP30 gene was specifically expressed in roots, and XSP30 was immunologically detected only in the xylem sap, but not in any organs.     

Identification of soybean microRNAs involved in soybean cyst nematode infection by deep sequencing

Soybean cyst nematode (SCN), Heterodera glycines, is the most devastating pathogen of soybean worldwide. MicroRNAs (miRNAs) are a class of small, non-coding RNAs that are known to play important role in plant stress response. However, there are few reports profiling the miRNA expression patterns during pathogen stress. We sequenced four small RNA libraries from two soybean cultivar (Hairbin xiaoheidou, SCN race 3 resistant, Liaodou 10, SCN race 3 susceptible) that grown under un-inoculated and SCN-inoculated soil. Small RNAs were mapped to soybean genome sequence, 364 known soybean miRNA genes were identified in total. In addition, 21 potential miRNA candidates were identified. Comparative analysis of miRNA profiling indicated 101 miRNAs belong to 40 families were SCN-responsive. We also found 20 miRNAs with different express pattern even between two cultivars of the same species. These findings suggest that miRNA paly important role in soybean response to SCN and have important implications for further identification of miRNAs under pathogen stress.     

Relationships between feeding of Philaenus spumarius (L.) and the amino acid concentration in the xylem sap

Summary. 1. P.spwnarius nymphs selected young, upper leaves of Xanthium strumarium plants, which have a relatively high amino acid concentration in the xylem sap.
2. Nymphs selected or rejected a host leaf as a feeding site after a test ingestion of plant sap.
3. Nymphs fed on detached leaves kept with their petioles in a solution of amino acids, in preference to leaves with petioles in a solution containing no amino acids.
4. Nymphs caged on leaves with relatively high amino acid concentrations in the xylem sap suffered a lower mortality after 7 days than nymphs caged on leaves with low amino acid concentrations in the xylem sap.
5. Mean excretory rate increased with the increase in the amino acid concentration in the xylem sap for third and fourth instar nymphs, and adults, but not in fifth instars.     

Identification of the silicon form in xylem sap of rice (Oryza sativa L.)

Rice (Oryza sativa L.) is a typical silicon (Si)-accumulating plant, but the mechanism responsible for the translocation from the root to the shoot is poorly understood. In this study, the form of Si in xylem sap was identified by (29)Si-nuclear magnetic resonance (NMR) spectroscopy. In rice (cv. Oochikara) cultured in a monosilicic acid solution containing 0.5 mM Si, the Si concentration in the xylem reached 6 mM within 30 min. In the (29)Si-NMR spectra of the xylem sap, only one signal was observed at a chemical shift of -72.6 ppm, which is consistent with that of monosilicic acid. A (1)H-NMR study of xylem sap did not show any significant difference between the wild-type rice and mutant rice defective in Si uptake, and the components of the xylem sap were not affected by the Si supply. The Si concentration in the xylem sap in vitro decreased from an initial 18 mM to 2.6 mM with time. Addition of xylem sap to a solution containing 8 mM Si did not prevent the polymerization of silicic acid. All these results indicate that Si is translocated in the form of monosilicic acid through the xylem and that the concentration of monosilicic acid is high in the xylem only transiently.     

Identification and distribution of microsymbionts associated with soybean nodulation in Mozambican soils

Indigenous soybean rhizobial strains were isolated from root nodules sampled from farmers’ fields in Mozambique to determine their identity, distribution and symbiotic relationships. Plant infection assays revealed variable nodulation and symbiotic effectiveness among the 43 bacterial isolates tested. Strains from Ruace generally promoted greater whole-plant growth than the others. 16S rRNA-RFLP analysis of genomic DNA extracted from the rhizobial isolates produced different banding patterns, a clear indication of high bacterial diversity. However, the multilocus sequence analysis (MLSA) data showed alignment of the isolates with B. elkanii species. The 16S rRNA sequences of representative soybean isolates selected from each 16S rRNA-RFLP cluster showed their relatedness to B. elkanii, as well as to other Bradyrhizobium species. But a concatenated phylogeny of two housekeeping genes (glnII and gyrB) identified the soybean nodulating isolates as Bradyrhizobium, with very close relatedness to B. elkanii. The nifH and nodC sequences also showed that the majority of the test soybean isolates were closely related to B. elkanii, albeit the inconsistency with some isolates. Taken together, these findings suggest that the B. elkanii group are the preferred dominant microsymbiont of soybean grown in Mozambican soils. Furthermore, the distribution of soybean rhizobia in the agricultural soils of Mozambique was found to be markedly influenced by soil pH, followed by the concentrations of plant-available P and Mn. This study suggested that the identified isolates TUTMJM5, TUTMIITA5A and TUTLBC2B can be used as inoculants for increased soybean production in Mozambique.     

The xylem sap of maple (Acer platanoides) trees--sap obtained by a novel method shows changes with season and height

Xylem sap of log pieces of maple trees was collected by a novelpressure/decompression method developed recently for the mechanicaldrying of timber. Seasonal changes and spatial distributionsof the osmotic potential, the pH and the concentrations of majorsolutes and of the plant stress-hormone abscisic acid (ABA)were investigated. Sucrose and quebrachitol were the main components of the xylemsap. The sucrose concentration varied between 10 mM and 25 mMduring the winter months and declined to a minimum during theperiod of bud burst. Quebrachitol was found in concentrationsof up to 7 mM with a high variability throughout the year. Highconcentrations of ABA were measured during the summer seasonand in mid-winter. Rainfall caused an increase of ABA in somesamples. The osmotic potential of the xylem sap increased with the heightof the collection site. The pH of the sap decreased by approximatelyone unit between the base of the trunk and the crown. The increaseof the osmotic potential was mainly due to sucrose, quebrachitoland potassium. Malate contributed to the decrease of the pH.ABA of the xylem sap increased with decreasing moisture contentof the wood. Key words: Pressure/decompression method, xylem sap, abscisic acid, sucrose, quebrachitol, Acer platanoides     

Enhanced nodulation of soybean by Bradyrhizobium in the presence ofPseudomonas fluorescens

Experiments were undertaken to determine the effect ofPseudomonas fluorescens on nodulation of soybean by two strains ofBradyrhizobium japonicum, USDA I-110 and 61A76.Pseudomonas fluorescens can enhance the nodulation ability ofB. japonicum. Preincubation ofB. japonicum withP. fluorescens before inoculation further increased the level of nodulation.