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PubMed2004

Characterisation of new symbiotic Medicago truncatula (Gaertn.) mutants, and phenotypic or genotypic complementary information on previously described mutants.

From a pool of Medicago truncatula mutants--obtained by gamma-irradiation or ethyl methanesulfonate mutagenesis--impaired in symbiosis with the N-fixing bacterium Sinorhizobium meliloti, new mutants are described and genetically analysed, and for already reported mutants, complementary data are given on their phenotypic and genetic analysis. Phenotypic data relate to nodulation and mycorrhizal phenotypes. Among the five new mutants, three were classified as [Nod+ Fix- Myc+] and the mutations were ascribed to two loci, Mtsym20 (TRV43, TRV54) and Mtsym21 (TRV49). For the two other new mutants, one was classified as [Nod-/+ Myc+] with a mutation ascribed to gene Mtsym15 (TRV48), and the other as [Nod- Myc-/+] with a mutation ascribed to gene Mtsym16 (TRV58). Genetic analysis of three previously described mutants has shown that [Nod-/+ Myc+] TR74 mutant can be ascribed to gene Mtsym14, and that [Nod-/+ Myc-/+] TR89 and TRV9 mutants are ascribed to gene Mtsym2 (dmi2). Using a detailed analysis of mycorrhizal phenotype, we have observed a delayed typical arbuscular mycorrhizal formation on some mutants that present thick lens-shaped appressoria. This phenotype was called [Myc-/+] and mutants TR25, TR26, TR89, TRV9, P1 and Y6 were reclassified as [Myc-/+]. Mutant P1 was reclassified as [Nod-/+] because of a late nodulation observed on roots of this mutant.

Gamma Rays↗
PubMed2002

Isolation of photorespiratory mutants from Lotus japonicus deficient in glutamine synthetase.

A mutagenesis programme using ethyl methanesulphonate (EMS) was carried out on Lotus japonicus (Regel) Larsen cv. Gifu in order to isolate photorespiratory mutants in this model legume. These mutants were able to grow in a CO2-enriched atmosphere [0.7% (v/v) CO2] but showed stress symptoms when transferred to air. Among them, three mutants displayed low levels of glutamine synthetase (GS; EC 6.3.1.2) activity in leaves. The mutants accumulated ammonium in leaves upon transfer from 0.7% (v/v) CO2 to air. F1 plants of back crosses to wild type were viable in air and F2 populations segregated 3 : 1 (viable in air : air-sensitive) indicative of a single Mendelian recessive trait. Complementation tests showed that the three mutants obtained were allelic. Chromatography on DEAE-Sephacel used to separate the cytosolic and plastidic GS isoenzymes together with immunological data showed that: (1) mutants were specifically affected in the plastidic GS isoform, and (2) in L. japonicus the plastidic GS isoform eluted at lower ionic strength than the cytosolic isoform, contrary to what happens in most plants. The plastidic GS isoform present in roots of wild type L. japonicus was also absent in roots of the mutants, indicating that this plastidic isoform from roots was encoded by the same gene than the GS isoform expressed in leaf tissue. Viability of mutant plants in high-CO2 conditions indicates that plastidic GS is not essentially required for primary ammonium assimilation. Nevertheless, mutant plants did not grow as well as wild type plants in high-CO2 conditions.

Journal Article↗
PubMed2005

Morphometric analysis of root shape.

Alterations in the root shape in plant mutants indicate defects in hormonal signalling, transport and cytoskeleton function. To quantify the root shape, we introduced novel parameters designated vertical growth index (VGI) and horizontal growth index (HGI). VGI was defined as a ratio between the root tip ordinate and the root length. HGI was the ratio between the root tip abscissa and the root length. To assess the applicability of VGI and HGI for quantification of root shape, we analysed root development in agravitropic Arabidopsis mutants. Statistical analysis indicated that VGI is a sensitive morphometric parameter enabling detection of weak gravitropic defects. VGI dynamics were qualitatively similar in auxin-transport mutants aux1, pin2 and trh1, but different in the auxin-signalling mutant axr2. Analysis of VGI and HGI of roots grown on tilted plates showed that the trh1 mutation affected downstream cellular responses rather than perception of the gravitropic stimulus. All these tests indicate that the VGI and HGI analysis is a versatile and sensitive method for the study of root morphology.

Arabidopsis↗
PubMed1999

Agravitropic mutant for the study of hydrotropism in seedling roots.

Roots have been shown to respond to a moisture gradient by positive hydrotropism. Agravitropic mutant plants are useful for the study of the hydrotropism in roots because on Earth hydrotropism is obviously altered by the gravity response in the roots of normally gravitropic plants. The roots are able to sense water potential gradient as small as 0.5 MPa mm(-1). The root cap includes the sensing apparatus that causes a differential growth at the elongation region of roots. A gradient in apoplastic calcium and calcium influx through plasmamembrane in the root cap is somehow involved in the signal transduction mechanism in hydrotropism, which may cause a differential change in cell wall extensibility at the elongation region. We have isolated an endoxy loglucan transferase (EXGT) gene that is strongly expressed in pea roots and appears to be involved in the differential growth in hydrotropically responding roots. Thus, it is now possible to study hydrotropism in roots by comparing with or separate from gravitropism. These results also imply that microgravity conditions in space are useful for the study of hydrotropism and its interaction with gravitropism.

Calcium Channels↗
PubMed2005

Early responses to Nod factors and mycorrhizal colonization in a non-nodulating Phaseolus vulgaris mutant.

Legumes can acquire nitrogen through a symbiotic interaction with rhizobial bacteria. The initiation of this process is determined by a molecular dialogue between the two partners. Legume roots exude flavonoids that induce the expression of the bacterial nodulation genes, which encode proteins involved in the synthesis and secretion of signals called Nod factors (NFs). NFs signal back to the plant root and trigger several responses, leading to bacterial invasion and nodule formation. Here, we describe the molecular and cellular characterization of a Phaseolus vulgaris non-nodulating mutant (NN-mutant). Root hair cells of the NN-mutant plant respond with swelling and branching when inoculated with Rhizobium etli, albeit without curling induction. Furthermore, neither initiation of cell division in the outer cortex, nor entrapment of bacteria nor infection thread formation was observed. Both the bean wild-type and the NN-mutant responded with elevated intracellular calcium changes in the root hairs. Although the NN-mutant is deficient in early nodulin gene expression when inoculated with R. etli, it can be effectively colonized by arbuscular mycorrhizal fungi (Glomus intraradices). Our data indicate that the P. vulgaris NN-mutant is not blocked at the NFs early perception stage, but at later downstream stages between Ca(2+) signaling and early nodulin induction. This supports the idea that both microsymbionts are perceived and trigger different downstream pathways in the host plant.

Bacterial Proteins↗
PubMed1988

A gradient of endogenous calcium forms in mucilage of graviresponding roots of Zea mays.

Agar blocks that contacted the upper sides of tips of horizontally-oriented roots of Zea mays contain significantly less calcium (Ca) than blocks that contacted the lower sides of such roots. This gravity-induced gradient of Ca forms prior to the onset of gravicurvature, and does not form across tips of vertically-oriented roots or roots of agravitropic mutants. These results indicate that (1) Ca can be collected from mucilage of graviresponding roots, (2) gravity induces a downward movement of endogenous Ca in mucilage overlying the root tip, (3) this gravity-induced gradient of Ca does not form across tips of agravitropic roots, and (4) formation of a Ca gradient is not a consequence of gravicurvature. These results are consistent with gravity-induced movement of Ca being a trigger for subsequent redistribution of growth effectors (e.g. auxin) that induce differential growth and gravicurvature.

Biological Transport↗
PubMed2006

A novel polar surface polysaccharide from Rhizobium leguminosarum binds host plant lectin.

Rhizobium bacteria produce different surface polysaccharides which are either secreted in the growth medium or contribute to a capsule surrounding the cell. Here, we describe isolation and partial characterization of a novel high molecular weight surface polysaccharide from a strain of Rhizobium leguminosarum that nodulates Pisum sativum (pea) and Vicia sativa (vetch) roots. Carbohydrate analysis showed that the polysaccharide consists for 95% of mannose and glucose, with minor amounts of galactose and rhamnose. Lectin precipitation analysis revealed high binding affinity of pea and vetch lectin for this polysaccharide, in contrast to the other known capsular and extracellular polysaccharides of this strain. Expression of the polysaccharide was independent of the presence of a Sym plasmid or the nod gene inducer naringenin. Incubation of R. leguminosarum with labelled pea lectin showed that this polysaccharide is exclusively localized on one of the poles of the bacterial cell. Vetch roots incubated with rhizobia and labelled pea lectin revealed that this bacterial pole is involved in attachment to the root surface. A mutant strain deficient in the production of this polysaccharide was impaired in attachment and root hair infection under slightly acidic conditions, in contrast to the situation at slightly alkaline conditions. Our data are consistent with the hypothesis that rhizobia can use (at least) two mechanisms for docking at the root surface, with use of a lectin-glycan mechanism under slightly acidic conditions.

Carbohydrates↗
PubMed1999

Age-related atrophy of motor axons in mice deficient in the mid-sized neurofilament subunit.

Neurofilaments are central determinants of the diameter of myelinated axons. It is less clear whether neurofilaments serve other functional roles such as maintaining the structural integrity of axons over time. Here we show that an age-dependent axonal atrophy develops in the lumbar ventral roots of mice with a null mutation in the mid-sized neurofilament subunit (NF-M) but not in animals with a null mutation in the heavy neurofilament subunit (NF-H). Mice with null mutations in both genes develop atrophy in ventral and dorsal roots as well as a hind limb paralysis with aging. The atrophic process is not accompanied by significant axonal loss or anterior horn cell pathology. In the NF-M-null mutant atrophic ventral root, axons show an age-related depletion of neurofilaments and an increased ratio of microtubules/neurofilaments. By contrast, the preserved dorsal root axons of NF-M-null mutant animals do not show a similar depletion of neurofilaments. Thus, the lack of an NF-M subunit renders some axons selectively vulnerable to an age-dependent atrophic process. These studies argue that neurofilaments are necessary for the structural maintenance of some populations of axons during aging and that the NF-M subunit is especially critical.

Aging↗
PubMed1987

Tn5-induced cytochrome mutants of Bradyrhizobium japonicum: effects of the mutations on cells grown symbiotically and in culture.

Two Bradyrhizobium japonicum cytochrome mutants were obtained by Tn5 mutagenesis of strain LO and were characterized in free-living cultures and in symbiosis in soybean root nodules. One mutant strain, LO501, expressed no cytochrome aa3 in culture; it had wild-type levels of succinate oxidase activity but could not oxidize NADH or N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). The cytochrome content of LO501 root nodule bacteroids was nearly identical to that of the wild type, but the mutant expressed over fourfold more bacteroid cytochrome c oxidase activity than was found in strain LO. The Tn5 insertion of the second mutant, LO505, had a pleiotropic effect; this strain was missing cytochromes c and aa3 in culture and had a diminished amount of cytochrome b as well. The oxidations of TMPD, NADH, and succinate by cultured LO505 cells were very similar to those by the cytochrome aa3 mutant LO501, supporting the conclusion that cytochromes c and aa3 are part of the same branch of the electron transport system. Nodules formed from the symbiosis of strain LO505 with soybean contained no detectable amount of leghemoglobin and had no N2 fixation activity. LO505 bacteroids were cytochrome deficient but contained nearly wild-type levels of bacteroid cytochrome c oxidase activity. The absence of leghemoglobin and the diminished bacterial cytochrome content in nodules from strain LO505 suggest that this mutant may be deficient in some aspect of heme biosynthesis.

Cytochromes↗
PubMed2001

Arabidopsis thaliana Rop GTPases are localized to tips of root hairs and control polar growth.

Plants contain a novel unique subfamily of Rho GTPases, vital components of cellular signalling networks. Here we report a general role for some members of this family in polarized plant growth processes. We show that Arabidopsis AtRop4 and AtRop6 encode functional GTPases with similar intrinsic GTP hydrolysis rates. We localized AtRop proteins in root meristem cells to the cross-wall and cell plate membranes. Polar localization of AtRops in trichoblasts specifies the growth sites for emerging root hairs. These sites were visible before budding and elongation of the Arabidopsis root hair when AtRops accumulated at their tips. Expression of constitutively active AtRop4 and AtRop6 mutant proteins in root hairs of transgenic Arabidopsis plants abolished polarized growth and delocalized the tip-focused Ca2+ gradient. Polar localization of AtRops was inhibited by brefeldin A, but not by other drugs such as latrunculin B, cytochalasin D or caffeine. Our results demonstrate a general function of AtRop GTPases in tip growth and in polar diffuse growth.

Arabidopsis↗
PubMed2026

Functional divergence of two soybean cytosolic serine hydroxymethyltransferases in development and defense against soybean cyst nematode.

Serine hydroxymethyltransferase (SHMT) is an enzyme essential for one-carbon metabolism. In higher plants, multiple SHMT genes code for isoforms that function in the cytosol, nucleus, mitochondria, and chloroplasts. The soybean genome contains two cytosolic SHMTs, GmSHMT05 and GmSHMT08, sharing high sequence identity and similar expression throughout soybean development. In certain soybean genotypes, two amino acid substitutions negatively impact GmSHMT08's ability to bind to tetrahydrofolate (THF), leading to a gain-of-function in resistance to the soybean cyst nematode (SCN). Whether this perturbation to the enzyme has other functional consequences for soybean growth and development remains unknown. Here, we investigated the roles of cytosolic GmSHMTs in soybean growth and development. We determined that the 3D structure and folate-binding affinity of GmSHMT05 are highly similar to the version of GmSHMT08 found in susceptible soybeans. We further measured phenotypic traits of two ethyl methanesulfonate-derived Gmshmt08 mutant plants in an SCN-resistant soybean background. Aboveground soybean growth and development were similar, except the Gmshmt08 mutant plants showed a significant increase in pods/plant in field phenotyping trials. Belowground analyses revealed a significant increase in lateral root and total root length in mutant plants, and CRISPR-Cas9 editing demonstrated an essential role of cytosolic SHMTs in root growth. Taken together, our results indicate that GmSHMT05 sustains overall soybean growth and development in the absence of GmSHMT08; however, GmSHMT08's gain-of-function in SCN resistance negatively influences pod and root growth, highlighting a potential trade-off between soybean defense and development that may impact yield when breeding with GmSHMT08 to develop SCN-resistant varieties.

1-C folate metabolism↗
PubMed1986

Regulation of the soybean-Rhizobium nodule symbiosis by shoot and root factors.

The availability of soybean mutants with altered symbiotic properties allowed an investigation of the shoot or root control of the relevant phenotype. By means of grafts between these mutants and wild-type plants (cultivar Bragg and Williams), we demonstrated that supernodulation as well as hypernodulation (nitrate tolerance in nodulation and lack of autoregulation) is shoot controlled in two mutants (nts382 and nts1116) belonging most likely to two separate complementation groups. The supernodulation phenotype was expressed on roots of the parent cultivar Bragg as well as the roots of cultivar Williams. Likewise it was shown that non-nodulation (resistance to Bradyrhizobium) is root controlled in mutant nod49. The shoot control of nodule initiation is epistatically suppressed by the non-nodulation, root-expressed mutation. These findings suggest that different plant organs can influence the expression of the nodulation phenotype.

Journal Article↗
PubMed2005

Nodule development induced by Mesorhizobium loti mutant strains affected in polysaccharide synthesis.

The role of Mesorhizobium loti surface polysaccharides on the nodulation process is not yet fully understood. In this article, we describe the nodulation phenotype of mutants affected in the synthesis of lipopolysaccharide (LPS) and beta(1,2) cyclic glucan. M. loti lpsbeta2 mutant produces LPS with reduced amount of O-antigen, whereas M. loti lpsbeta1 mutant produces LPS totally devoid of O-antigen. Both genes are clustered in the chromosome. Based on amino acid sequence homology, LPS sugar composition, and enzymatic activity, we concluded that lpsbeta2 codes for an enzyme involved in the transformation of dTDP-glucose into dTDP-rhamnose, the sugar donor of rhamnose for the synthesis of O-antigen. On the other hand, lpsbeta1 codes for a glucosyltransferase involved in the biosynthesis of the O-antigen. Although LPS mutants elicited normal nodules, both show reduced competitiveness compared with the wild type. M. loti beta(1-2) cyclic glucan synthase (cgs) mutant induces white, empty, ineffective pseudonodules in Lotus tenuis. Cgs mutant induces normal root hair curling but is unable to induce the formation of infection threads. M. loti cgs mutant was more sensitive to deoxycholate and displayed motility impairment compared with the wild-type strain. This pleiotropic effect depends on calcium concentration and temperature.

Calcium↗
PubMed1993

Restoration of attachment, virulence and nodulation of Agrobacterium tumefaciens chvB mutants by rhicadhesin.

In contrast to wild-type Agrobacterium tumefaciens strains, beta-1,2-glucan-deficient chvB mutants were found to be unable to attach to pea root hair tips. The mutants appeared to produce rhicadhesin, the protein that mediates the first step in attachment of Rhizobiaceae cells to plant root hairs, but the protein was inactive. Both attachment to root hairs and virulence of the chvB mutants could be restored by treatment of the plants with active rhicadhesin, whereas treatment of plants with beta-1,2-glucan had no effect on attachment or virulence. Moreover, nodulation ability of a chvB mutant carrying a Sym plasmid could be restored by pretreatment of the host plant with rhicadhesin. Apparently the attachment-minus and avirulence phenotype of chvB mutants is caused by lack of active rhicadhesin, rather than directly being caused by a deficiency in beta-1,2-glucan synthesis. The results strongly suggest that rhicadhesin is essential for attachment and virulence of A. tumefaciens cells. They also indicate that the mechanisms of binding of Agrobacterium and Rhizobium bacteria to plant target cells are similar, despite differences between these target cells.

Adhesins, Bacterial↗
PubMed2005

The roothairless1 gene of maize encodes a homolog of sec3, which is involved in polar exocytosis.

The roothairless1 (rth1) mutant is impaired in root hair elongation and exhibits other growth abnormalities. Unicellular root hairs elongate via localized tip growth, a process mediated by polar exocytosis of secretory vesicles. We report here the cloning of the rth1 gene that encodes a sec3 homolog. In yeast (Saccharomyces cerevisiae) and mammals, sec3 is a subunit of the exocyst complex, which tethers exocytotic vesicles prior to their fusion. The cloning of the rth1 gene associates the homologs of exocyst subunits to an exocytotic process in plant development and supports the hypothesis that exocyst-like proteins are involved in plant exocytosis. Proteomic analyses identified four proteins that accumulate to different levels in wild-type and rth1 primary roots. The preferential accumulation in the rth1 mutant proteome of a negative regulator of the cell cycle (a prohibitin) may at least partially explain the delayed development and flowering of the rth1 mutant.

Chromosome Mapping↗
PubMed1989

Requirement for chemotaxis in pathogenicity of Agrobacterium tumefaciens on roots of soil-grown pea plants.

Agrobacterium tumefaciens Tn5 mutants deficient in chemotaxis to root exudates were used to study the significance of chemotaxis in crown gall pathogenesis. Mutants deficient in motility and in chemotaxis were fully virulent when inoculated by direct immersion in inoculum, followed by growth for 2 weeks in moist growth pouches. Ability of mutant bacteria to move through soil to infect roots was tested by planting wounded seedlings into air-dried soil or sand that had been infested with inoculum. Mutant bacteria were almost as virulent as the parent on plants grown in sand but were avirulent on soil-grown plants.

Chemotaxis↗
PubMed1998

The tropic response of plant roots to oxygen: oxytropism in Pisum sativum L.

Plant roots are known to orient growth through the soil by gravitropism, hydrotropism, and thigmotropism. Recent observations of plant roots that developed in a microgravity environment in space suggested that plant roots may also orient their growth toward oxygen (oxytropism). Using garden pea (Pisum sativum L. cv. Weibul's Apollo) and an agravitropic mutant (cv. Ageotropum), root oxytropism was studied in the controlled environment of a microrhizotron. A series of channels in the microrhizotron allowed establishment of an oxygen gradient of 0.8 mmol mol-1 mm-1. Curvature of seedling roots was determined prior to freezing the roots for subsequent spectrophotometric determinations of alcohol dehydrogenase activity. Oxytropic curvature was observed all along the gradient in both cultivars of pea. The normal gravitropic cultivar showed a maximal curvature of 45 degrees after 48 h, while the agravitropic mutant curved to 90 degrees. In each cultivar, the amount of curvature declined as the oxygen concentration decreased, and was linearly related to the root elongation rate. Since oxytropic curvature occurred in roots exposed to oxygen concentrations that were not low enough to induce the hypoxically responsive protein alcohol dehydrogenase, we suspect that the oxygen sensor associated with oxytropism does not control the induction of hypoxic metabolism. Our results indicate that oxygen can play a critical role in determining root orientation as well as impacting root metabolic status. Oxytropism allows roots to avoid oxygen-deprived soil strata and may also be the basis of an auto-avoidance mechanism, decreasing the competition between roots for water and nutrients as well as oxygen.

Alcohol Dehydrogenase↗
PubMed2006

Spontaneous root-nodule formation in the model legume Lotus japonicus: a novel class of mutants nodulates in the absence of rhizobia.

Root-nodule development in legumes is an inducible developmental process initially triggered by perception of lipochitin-oligosaccharide signals secreted by the bacterial microsymbiont. In nature, rhizobial colonization and invasion of the legume root is therefore a prerequisite for formation of nitrogen-fixing root nodules. Here, we report isolation and characterization of chemically induced spontaneously nodulating mutants in a model legume amenable to molecular genetics. Six mutant lines of Lotus japonicus were identified in a screen for spontaneous nodule development under axenic conditions, i.e., in the absence of rhizobia. Spontaneous nodules do not contain rhizobia, bacteroids, or infection threads. Phenotypically, they resemble ineffective white nodules formed by some bacterial mutants on wild-type plants or certain plant mutants inoculated with wild-type Mesorhizobium loti. Spontaneous nodules formed on mutant lines show the ontogeny and characteristic histological features described for rhizobia-induced nodules on wild-type plants. Physiological responses to nitrate and ethylene are also maintained, as elevated levels inhibit spontaneous nodulation. Activation of the nodule developmental program in spontaneous nodules was shown for the early nodulin genes Enod2 and Nin, which are both upregulated in spontaneous nodules as well as in rhizobial nodules. Both monogenic recessive and dominant spontaneous nodule formation (snf) mutations were isolated in this mutant screen, and map positions were determined for three loci. We suggest that future molecular characterization of these mutants will identify key plant determinants involved in regulating nodulation and provide new insight into plant organ development.

Gene Expression Regulation, Plant↗