[Relationship between chloride tolerance and polyamine accumulation in Glycine max, Glycine soja, and their hybrid seedlings]

The seedlings of the F4 hybrid strain 'JB185' selected for salt tolerance generation by generation, their parents Glycine max cv. Jackson and Glycine soja population 'BB52' were treated with different NaCl concentrations and iso-osmotic (-0.53 MPa) PEG-6000, NaCl, Na+ (without Cl-) and Cl- (without Na+) solutions for 6 d. The results showed that: (1) The relative electrolyte leakage and malondialdehyde (MDA) content in leaves of the above three soybean seedlings showed an increase trend when the NaCl concentration was elevated, but chlorophyll contents decreased except the significant increase in 'BB52' and 'JB185' under NaCl 50 mmol/L stress. The change in 'JB185' was between its parents. (2) Under different iso-osmotic stresses, the relative electrolyte leakage and MDA contents in leaves of three soybean seedlings also increased mostly, the changes in 'BB52' and 'JB185' under Na+ (without Cl-) stress were more than those under Cl- (without Na+) stress. The free and bound Put, Spd and Spm contents in leaves all increased when compared with the control, the ratios of free (Spd+Spm)/Put and total bound polyamines in 'BB52' and 'JB185' seedlings under Na+ (without Cl-) treatment were the lowest one among three iso-osmotic salt stresses. The results indicate that the F4 hybrid strain 'JB185' is more sensitive to Na+ than Cl- as its wild parent 'BB52' population.

Fractionations of rare earth elements in plants and their conceptive model

Fractionations of rare earth elements (REEs) and their mechanisms in soybean were studied through application of exogenous mixed REEs under hydroponic conditions. Significant enrichment of middle REEs (MREEs) and heavy REEs (HREEs) was observed in plant roots and leaves respectively, with slight fractionation between light REEs (LREEs) and HREEs in stems. Moreover, the tetrad effect was observed in these organs. Investigations into REE speciation in roots and in the xylem sap using X-ray absorption spectroscopy (XAS) and nanometer-sized TiO2 adsorption techniques, associated with other controlled experiments, demonstrated that REE fractionations should be dominated by fixation mechanism in roots caused by cell wall absorption and phosphate precipitation, and by the combined effects of fixation mechanism and transport mechanism in aboveground parts caused by solution complexation by intrinsic organic ligands. A conceptive model was established for REE fractionations in plants based on the above studies.

Protein storage vacuole acidification as a control of storage protein mobilization in soybeans

Soybean protease C1 (EC 3.4.21.25), the subtilisin-like serine protease that initiates the proteolysis of seed storage proteins in germinating soybean [Glycine max (L.) Merrill], was localized to the protein storage vacuoles of parenchyma cells in the cotyledons by immunoelectron microscopy. This was demonstrated not only in germination and early seedling growth as expected, but also in two stages of protein storage vacuole development during seed maturation. Thus, the plant places the proteolytic enzyme in the same compartment as the storage proteins, but is still able to accumulate those protein reserves. Since soybean protease C1 activity requires acidic conditions for activity, the hypothesis that the pH condition in the protein storage vacuole would support protease C1 activity in germination, but not in seed maturation, was tested. As hypothesized, acridine orange accumulation in the protein storage vacuole of storage parenchyma cells was detected by fluorescence confocal microscopy in seedlings before the onset of mobilization of reserve proteins as noted by SDS-PAGE. Accumulation of the dye was reversed by inclusion of the weak base methylamine to dissipate the pH gradient across the vacuolar membrane. Also as hypothesized, acridine orange did not accumulate in the protein storage vacuole of those parenchyma cells during seed maturation. These results were obtained using cells separated by pectolyase treatment and also using cotyledon slices.

Acclimation of photosystem II to high temperature in a suspension culture of soybean (Glycine max) cells requires proteins that are associated with the thylakoid membrane

In a study of the responses of photosystem II (PSII) to high temperature in suspension-cultured cells of soybean (Glycine max L. Merr.), we found that high temperatures inactivated PSII via two distinct pathways. Inactivation of PSII by moderately high temperatures, such as 41 degrees C, was reversed upon transfer of cells to 25 degrees C. The recovery of PSII required light, but not the synthesis of proteins de novo. By contrast, temperatures higher than 45 degrees C inactivated PSII irreversibly. An increase in the growth temperature from 25 to 35 degrees C resulted in an upward shift of 3 degrees C in the profile of the heat-induced inactivation of PSII, which indicated that the thermal stability of PSII had been enhanced. This acclimative response was reflected by the properties of isolated thylakoid membranes: PSII in thylakoid membranes from cells that had been grown at 35 degrees C exhibited greater thermal stability than that from cells grown at 25 degrees C. Disruption of the vesicular structure of thylakoid membranes with 0.05% Triton X-100 decreased the thermal stability of PSII to a similar level in both types of thylakoid membrane. Proteins released by Triton X-100 from thylakoid membranes from cells grown at 35 degrees C were able to increase the thermal stability of Triton-treated thylakoid membranes. These observations suggest that proteins that are associated with thylakoid membranes might be involved in the enhancement of the thermal stability of PSII.

Chloroplasts as a nitric oxide cellular source. Effect of reactive nitrogen species on chloroplastic lipids and proteins

Nitric oxide (NO) generation by soybean (Glycine max var. ADM 4800) chloroplasts was studied as an endogenous product assessed by the electron paramagnetic resonance spin-trapping technique. Nitrite and l-arginine (Arg) are substrates for enzymatic activities considered to be the possible sources of NO in plants. Soybean chloroplasts showed a NO production of 3.2 +/- 0.2 nmol min(-1) mg(-1) protein in the presence of 1 mm NaNO(2). Inhibition of photosynthetic electron flow by 3-(3,4-dichlorophenyl)-1,1-dimethyl urea resulted in a lower rate (1.21 +/- 0.04 nmol min(-1) mg(-1) protein) of NO generation. Chloroplasts incubated with 1 mm Arg showed NO production of 0.76 +/- 0.04 nmol min(-1) mg(-1) protein that was not affected either by omission of Ca(2+) or by supplementation with Ca(2+) and calmodulin to the incubation medium. This production was inhibited when chloroplasts were incubated in the presence of NO synthase inhibitors N(omega)-nitro-l-Arg methyl ester hydrochloride and N(omega)-nitro-l-Arg. In vitro exposure of chloroplasts to an NO donor (250 mum S-nitrosoglutathione) decreased lipid radical content in membranes by 29%; however, incubation in the presence of 25 mum peroxynitrite (ONOO(-)) led to an increase in lipid-derived radicals (34%). The effect of ONOO(-) on protein oxidation was determined by western blotting, showing an increase in carbonyl content either in stroma or thylakoid proteins as compared to controls. Moreover, ONOO(-) treatment significantly affected both O(2) evolution and chlorophyll fluorescence in thylakoids. Data reported here suggest that NO is an endogenous metabolite in soybean chloroplasts and that reactive nitrogen species could exert either antioxidant or prooxidant effects on chloroplast macromolecules.

[Cloning of GmHSFA1 gene and its overexpression leading to enhancement of heat tolerance in transgenic soybean]

Heat shock transcription factors (HSFs) are important in regulating heat stress response by mediating expression of heat shock protein (HSP) genes in various plant species. In the present study, a novel GmHSFA1 with an ORF of 1,533 bp (full-length cDNA sequence of 1,781 bp) was cloned from soybean genome via comparative genomic approach and RACE (rapid amplification of cDNA ends). This gene encodes 510 amino acids consisting of a protein of 56.2 kDa (GenBank accession number: AY458843). Similar to other HSFs, GmHSFA1 has the basic modular structure including DBD, OD, NLS, and CTAD. BLAST analysis revealed the identity of 52.46% between amino acid sequences between GmHSFA1 and LpHSFA1 that has the highest similarity to GmHSFA1 in all HSFA1s in various plant species. The results from RT-PCR, Northern blotting, and transformation showed: 1) GmHsfA1 exhibited the constitutive expression patterns in different tissues of soybean; 2) The expression level of GmHsfA1 in transgenic plants was notably higher than that in non-transgenic plants; 3) Overexpression of GmHsfA1 activated transcription of GmHSP22 in transgenic plants under normal conditions and enhanced obviously expressions of GmHSP23 and GmHSP70 in transgenic plants under heat stress conditions; 4) Heat tolerant temperature (as high as 52 degrees C) of transgenic plants was remarkably higher than that of non-transgenic plants. These results preliminarily proved that the overexpression of GmHsfA1 possibly led to the notable enhancement of heat-tolerant level of transgenic plants by mediating the activation of transcription or improvement of expression of some GmHSPs in the GmHsfA1's downstream in transgenic plants, suggesting GmHSFA1 is a novel and functional heat shock transcription factor of soybean.

Proteins induced by cadmium in soybean cells

The cadmium (Cd)-induced changes in protein pattern and identification of metal-stimulated polypeptides were analyzed in soybean cell suspension culture. The cell cultures were treated with various concentrations of Cd(2+) (3-10microM) for 24, 48 and 72h. The synthesis of [(35)S]-labeled proteins and their accumulation were analyzed by SDS-PAGE, whereas the identification of selected protein bands was performed by mass spectrometry. It is shown that Cd induced the appearance of the following proteins in soybean cells: superoxide dismutase, histone H2B, chalcone synthase and glutathione transferase.

An isoflavone conjugate-hydrolyzing beta-glucosidase from the roots of soybean (Glycine max) seedlings: purification, gene cloning, phylogenetics, and cellular localization

Soybeans (Glycine max (L.) Merr.) and certain other legumes excrete isoflavones from their roots, which participate in plantmicrobe interactions such as symbiosis and as a defense against infections by pathogens. In G. max, the release of free isoflavones from their conjugates, the latent forms, is mediated by an isoflavone conjugate-hydrolyzing beta-glucosidase. Here we report on the purification and cDNA cloning of this important beta-glucosidase from the roots of G. max seedlings as well as related phylogenetic and cellular localization studies. The purified enzyme, isoflavone conjugate-hydrolyzing beta-glucosidase from roots of G. max seedling (GmICHG), is a homodimeric glycoprotein with a subunit molecular mass of 58 kDa and is capable of directly hydrolyzing genistein 7-O-(6 ''-O-malonyl-beta-d-glucoside) to produce free genistein (k(cat), 98 s(-1); K(m), 25 microM at 30 degrees C, pH 7.0). GmICHG cDNA was isolated based on the amino acid sequence of the purified enzyme. GmICHG cDNA was abundantly expressed in the roots of G. max seedlings but only negligibly in the hypocotyl and cotyledon. An immunocytochemical analysis using anti-GmICHG antibodies, along with green fluorescent protein imaging analyses of Arabidopsis cultured cells transformed by the GmICHG:GFP fusion gene, revealed that the enzyme is exclusively localized in the cell wall and intercellular space of seedling roots, particularly in the cell wall of root hairs. A phylogenetic analysis revealed that GmICHG is a member of glycoside hydrolase family 1 and can be co-clustered with many other leguminous beta-glucosidases, the majority of which may also be involved in flavonoid-mediated interactions of legumes with microbes.

Effects of the overexpression of a soybean cytosolic glutamine synthetase gene (GS15) linked to organ-specific promoters on growth and nitrogen accumulation of pea plants supplied with ammonium

A soybean cytosolic glutamine synthetase gene (GS15) fused to a constitutive promoter (CaMV 35S), a putative nodule-specific promoter (LBC(3)), or a putative root-specific promoter (rolD) was transformed into Pisum sativum L. cv. Greenfeast. Four lines with single copies (Lines 1, 7, 8 and 9) and four lines with two copies each of GS15 (Lines 2, 4, 6 and 11) were compared to the wild-type (WT) parental line for levels of cytosolic glutamine synthetase (GS1), glutamine synthetase (GS) activity, N accumulation, N derived form the atmosphere (NDFA), and biomass of plants grown on 0.0, 0.1, 1.0 or 10.0 mM NH(4)(+). Enhanced levels of GS1 were detected in leaves of one of the two lines transformed with the 35S-GS15 construct, and all three lines containing the rolD-GS15 construct. All three lines containing the LBC(3)-GS15 construct had increased levels of GS1 in nodules. Despite the increased levels of GS1 in many transformants, only the roots of lines containing the rolD-GS15 construct consistently demonstrated enhanced levels of GS activity (up to 12-fold). Positive responses in plant N content, NDFA, and biomass were rare, but increases in plant biomass and N content of up to 17% and 54%, respectively, occurred in some of the rolD-GS15 lines at certain levels of ammonium. In general, GS15 copy number did not seem to differentially affect phenotype of the transformants, and transformants respond to ammonium concentrations in similar patterns to that previously observed with nitrate. Despite the fact that the rolD-GS15 transformants consistently resulted in increased GS activity in roots and resulted in some occurrences of increases in biomass and plant N content, the lack of consistent positive growth effect across all transformants indicates that the generalized overexpression of GS1 in tissues holds little potential for positive growth responses in pea.

A light-sensitive mechanism differently regulates transcription and transcript stability of omega3 fatty-acid desaturases (FAD3, FAD7 and FAD8) in soybean photosynthetic cell suspensions

The omega3 fatty-acid desaturases: FAD7 and FAD8 (plastid) and FAD3 (reticular) are responsible for trienoic fatty-acid (TA) production in plants. The expression of these enzymes seemed to be regulated differently in response to light. Darkness leads to a decrease in total TA level. Under such conditions, FAD3 and FAD8 transcript levels were undetectable but increased after re-illumination concomitant with TA levels, indicating a transcriptional control. On the contrary, FAD7 transcript levels were similar to illuminated control cells, suggesting the presence of a post-transcriptional control mechanism. Furthermore, FAD7 mRNA stability increased dramatically in darkness. Analysis of FAD7 protein accumulation using specific antibodies revealed that FAD7 was very stable whatever the light or darkness conditions. These results indicate that FAD7 enzyme availability is not limiting for 18:3 production in darkness. Our data point to an additional post-translational regulatory mechanism that controls the activity of FAD7 in response to light.

Isolation of a novel nodulin: a molecular marker of osmotic stress in Glycine max/Bradyrhizobium japonicum nodule

Symbiotic N(2) fixation of legume crops is highly sensitive to drought, which results in a dramatic drop of N accumulation and yield. The symbiosis between soybean (Glycine max) and Bradyrhizobium japonicum, because of its extreme sensitivity to drought, was chosen as a model to analyse the response to drought stress at a molecular level. The mRNA differential display technique was performed to isolate cDNA markers differentially expressed in well-watered [100% of N(2) fixation capacity (NFC)] and drought-stressed nodules (40% NFC). One gene noted, G93, appeared strongly down-regulated by drought and fully recovered after rehydration. In situ hybridization showed that G93 transcripts were localized in N(2)-fixing cells of mature nodules, indicating that G93 could be considered as a late nodulin. However, G93 expression was not directly correlated to N(2) fixation but mainly responded to osmotic stress. Other stresses that lead to decrease of N(2) fixation did not affect G93 expression. Sequence analyses showed that G93 presented a strong homology with two soybean expressed sequence tags (ESTs) and with the ZR1 protein of Medicago sativa. Putative roles of this nodulin in adaptation of soybean nodule to osmotic stress are proposed.

Paleopolyploidy and gene duplication in soybean and other legumes

Two of the most important observations from whole-genome sequences have been the high rate of gene birth and death and the prevalence of large-scale duplication events, including polyploidy. There is also a growing appreciation that polyploidy is more than the sum of the gene duplications it creates, in part because polyploidy duplicates the members of entire regulatory networks. Thus, it may be important to distinguish paralogs that are produced by individual gene duplications from the homoeologous sequences produced by (allo)polyploidy. This is not a simple task, for several reasons, including the chromosomally cryptic nature of many duplications and the variable rates of gene evolution. Recent progress has been made in understanding patterns of gene and genome duplication in the legume family, specifically in soybean.

Effects of feeding Spodoptera littoralis on lima bean leaves. III. Membrane depolarization and involvement of hydrogen peroxide

In response to herbivore (Spodoptera littoralis) attack, lima bean (Phaseolus lunatus) leaves produced hydrogen peroxide (H(2)O(2)) in concentrations that were higher when compared to mechanically damaged (MD) leaves. Cellular and subcellular localization analyses revealed that H(2)O(2) was mainly localized in MD and herbivore-wounded (HW) zones and spread throughout the veins and tissues. Preferentially, H(2)O(2) was found in cell walls of spongy and mesophyll cells facing intercellular spaces, even though confocal laser scanning microscopy analyses also revealed the presence of H(2)O(2) in mitochondria/peroxisomes. Increased gene and enzyme activations of superoxide dismutase after HW were in agreement with confocal laser scanning microscopy data. After MD, additional application of H(2)O(2) prompted a transient transmembrane potential (V(m)) depolarization, with a V(m) depolarization rate that was higher when compared to HW leaves. In transgenic soybean (Glycine max) suspension cells expressing the Ca(2+)-sensing aequorin system, increasing amounts of added H(2)O(2) correlated with a higher cytosolic calcium ([Ca(2+)](cyt)) concentration. In MD and HW leaves, H(2)O(2) also triggered the increase of [Ca(2+)](cyt), but MD-elicited [Ca(2+)](cyt) increase was more pronounced when compared to HW leaves after addition of exogenous H(2)O(2). The results clearly indicate that V(m) depolarization caused by HW makes the membrane potential more positive and reduces the ability of lima bean leaves to react to signaling molecules.

DNA recombination activity in soybean mitochondria

Mitochondrial genomes in higher plants are much larger and more complex as compared to animal mitochondrial genomes. There is growing evidence that plant mitochondrial genomes exist predominantly as a collection of linear and highly branched DNA molecules and replicate by a recombination-dependent mechanism. However, biochemical evidence of mitochondrial DNA (mtDNA) recombination activity in plants has previously been lacking. We provide the first report of strand-invasion activity in plant mitochondria. Similar to bacterial RecA, this activity from soybean is dependent on the presence of ATP and Mg(2+). Western blot analysis using an antibody against the Arabidopsis mitochondrial RecA protein shows cross-reaction with a soybean protein of about 44 kDa, indicating conservation of this protein in at least these two plant species. mtDNA structure was analyzed by electron microscopy of total soybean mtDNA and molecules recovered after field-inversion gel electrophoresis (FIGE). While most molecules were found to be linear, some molecules contained highly branched DNA structures and a small but reproducible proportion consisted of circular molecules (many with tails) similar to recombination intermediates. The presence of recombination intermediates in plant mitochondria preparations is further supported by analysis of mtDNA molecules by 2-D agarose gel electrophoresis, which indicated the presence of complex recombination structures along with a considerable amount of single-stranded DNA. These data collectively provide convincing evidence for the occurrence of homologous DNA recombination in plant mitochondria.

Soybean (Glycine max) transformation using immature cotyledon explants

Agrobacterium tumefaciens-mediated transformation of soybeans can be accomplished using immature zygotic cotyledons as target tissues providing an alternate explant to embryogenic tissue cultures, proliferating meristems, and cotyledonary nodes. The immature cotyledon method includes direct induction of transgenic somatic embryos from the explant plated on selective media after cocultivation, followed by maturation and regeneration of individual somatic embryos into whole plants. Although this method has been improved to be simple, rapid, reproducible, and applicable to a range of cultivars in different maturity groups, the transformation efficiency (Southern-positive, independent plants produced per 100 immature cotyledon explants) is 1.7% and needs to be further increased to make this a robust soybean transformation system. Further refinements of cocultivation conditions, tissue culture, and selection of regenerated transgenic plants will probably result in increases in transformation efficiency.

Sinorhizobium fredii HH103 mutants affected in capsular polysaccharide (KPS) are impaired for nodulation with soybean and Cajanus cajan

The Sinorhizobium fredii HH103 rkp-1 region, which is involved in capsular polysaccharides (KPS) production, was isolated and sequenced. The organization of the S. fredii genes identified, rkpUAGHIJ and kpsF3, was identical to that described for S. meliloti 1021 but different from that of S. meliloti AK631. The long rkpA gene (7.5 kb) of S. fredii HH103 and S. meliloti 1021 appears as a fusion of six clustered AK631 genes, rkpABCDEF. S. fredii HH103-Rif(r) mutants affected in rkpH or rkpG were constructed. An exoA mutant unable to produce exopolysaccharide (EPS) and a double mutant exoA rkpH also were obtained. Glycine max (soybean) and Cajanus cajan (pigeon pea) plants inoculated with the rkpH, rkpG, and rkpH exoA derivatives of S. fredii HH103 showed reduced nodulation and severe symptoms of nitrogen starvation. The symbiotic capacity of the exoA mutant was not significantly altered. All these results indicate that KPS, but not EPS, is of crucial importance for the symbiotic capacity of S. fredii HH103-Rif(r). S. meliloti strains that produce only EPS or KPS are still effective with alfalfa. In S. fredii HH103, however, EPS and KPS are not equivalent, because mutants in rkp genes are symbiotically impaired regardless of whether or not EPS is produced.

Inside-out but not right side-out plasma membrane vesicles from soybean enlarge when treated with ATP + 2,4-D as determined by electron microscopy and light scattering: evidence for involvement of a plasma membrane AAA-ATPase

The concept that the location of an AAA-ATPase associated with the plant plasma membrane may be indicative of a functional relationship to growth or cell enlargement by analogy with roles in physical membrane displacements as proposed for AAA-ATPases associated with internal membranes was tested. A plant growth hormone-responsive and nucleoside triphosphate-dependent enlargement of inside-out vesicles of plasma membranes from soybeans was utilized in a completely cell-free system. The rate of enlargement was accelerated by the synthetic plant growth factor 2,4-dichlorophenoxyacetic acid (2,4-D) in a log dose-dependent manner and was increased approximately 2-fold with the addition of 1 microM 2,4-D plus 100 microM ATP compared to 100 microM ATP alone, 1 microM 2,4-D alone or no additions. The cell-free enlargement was inhibited by AAA-ATPase-specific antisera and by CoCl2, an inhibitor specific for AAA-ATPases. The responsible ATP site appears to be on the inside of the cell, since right side-out vesicles did not enlarge in response to either ATP, 2,4-D or the two in combination.

The diphenylether herbicide lactofen induces cell death and expression of defense-related genes in soybean

Lactofen belongs to the diphenylether class of herbicides, which targets protoporphyrinogen oxidase, which in turn causes singlet oxygen generation. In tolerant plants like soybean (Glycine max), the chemical nonetheless causes necrotic patches called "bronzing" in contact areas. Here it is shown that such bronzing is accompanied by cell death, which was quantified from digital microscopic images using Assess Software. Cellular autofluorescence accompanied cell death, and a homolog of the cell death marker gene, Hsr203j, was induced by lactofen in treated soybean tissues. Thus, this form of chemically induced cell death shares some hallmarks of certain types of programmed cell death. In addition to the cell death phenotype, lactofen caused enhanced expressions of chalcone synthase and chalcone reductase genes, mainly in the exposed and immediately adjacent (proximal) cells. Furthermore, isoflavone synthase genes, which are wound inducible in soybean, were up-regulated by lactofen in both proximal and distal cell zones in minimally wounded cotyledons and further enhanced in wounded tissues. Moreover, if the wall glucan elicitor from Phytophthora sojae was present during lactofen treatment, the induction of isoflavone synthase was even more rapid. These results are consistent with the fact that lactofen triggers massive isoflavone accumulations and activates the capacity for glyceollin elicitation competency. In addition, lactofen induces late expression of a selective set of pathogenesis-related (PR) protein genes, including PR-1a, PR-5, and PR-10, mainly in treated proximal tissues. These various results are discussed in the context of singlet oxygen-induced responses and lactofen's potential as a disease resistance-inducing agent.

The role of wall Ca2+ in the regulation of wall extensibility during the acid-induced extension of soybean hypocotyl cell walls

We examined the acid-facilitated yielding properties of cell walls of soybean hypocotyls and the effects of Ca(2+) upon the properties by stress-strain analyses using glycerinated hollow cylinders (GHCs) from the elongating regions of the hypocotyls. Stress-extension rate curves of native GHCs showed characteristic changes with pH, all indicating the existence of yield threshold tension (y) as well as wall extensibility (phi), i.e. a downward shift of y and an increase in phi with wall acidification. The acid-induced downward shift of y was inhibited by boiling of GHCs. In contrast, a considerable increase in phi with acidification remained even after boiling. This indicates that phi consists of two components, i.e. heat-sensitive and heat-resistant, both being pH sensitive. A Ca(2+) chelator (Quin 2) dramatically increased phi at a neutral pH. Subsequent addition of Ca(2+) or ruthenium red suppressed the chelator-induced increase in phi. These findings suggest that wall Ca(2+) plays an important role in the regulation of wall extensibility during the acid-induced wall extension by reacting with carboxyl groups of wall pectin.

An endopolygalacturonase from Sclerotinia sclerotiorum induces calcium-mediated signaling and programmed cell death in soybean cells

A basic endopolygalacturonase (PG) isoform, produced early by Sclerotinia sclerotiorum when infecting soybean seedlings, was used to examine the signaling role of the enzyme in aequorin-expressing soybean cells. A cytosolic Ca2+ elevation was induced, with a rapid increase (phase 1) and a very slow decrease (phase 2) of Ca2+ concentration, indicating the involvement of Ca2+ ions in PG signaling. Within 1 h of PG-cell contact a remarkable level of cell death was recorded, significantly higher than the control cell culture turnover. The observed morphological and biochemical changes were indicative of the activation of programmed cell death; in particular, cytochrome c release in the cytoplasm and activation of both caspase 9-like and caspase 3-like proteases were found. When a polygalacturonase-inhibiting protein (PGIP) and the PG were simultaneously applied to cells, both the Ca2+ increase and cell death were annulled. The possible roles of prolonged sustained cytosolic Ca2+ concentrations in inducing cell death and of the PG-PGIP interaction in preventing PG signaling are discussed.

Involvement of the mitochondrial K(+)ATP channel in H2O2- or NO-induced programmed death of soybean suspension cell cultures

Soybean suspension cell cultures were treated by H2O2 or nitric oxide (NO), to assess the mechanism leading to programmed cell death (PCD). Hydrogen peroxide (5 mM) induced PCD. Cells become necrotic at 20 mM H2O2, with cells exhibiting intermediate hallmarks before that (necrapoptotic cells). The level of ATP and of glucose-6-phosphate remained constant in cells undergoing PCD, while it decreased significantly in the necrotic ones. Mitochondria, isolated from 5 mM H2O2-treated (apoptotic) cells, showed that succinate-dependent oxygen consumption was slightly uncoupled, and the electrical potential difference (delta psi) weakly decreased. The addition of KCl to the delta psi formed determined a partial dissipation, which was higher than the dissipation observed in mitochondria from control cells. The addition of cyclosporin A (CsA) to de-energized mitochondria also induced delta psi formation, due to a K+ efflux from the matrix, which was decreased in mitochondria from treated cells. The same pattern of response was also observed in mitochondria isolated from 1 mM sodium nitroprusside (NO)-treated cells, exhibiting apoptotic symptoms. In mitochondria isolated from 20 mM H2O2-treated (necrotic) cells, succinate-dependent oxygen consumption was completely uncoupled, delta psi generation significantly inhibited, and CsA-dependent delta psi formation prevented. In addition, mitochondria isolated from control cells still underwent swelling, which was partially or completely prevented in mitochondria isolated from apoptotic or necrotic cells, respectively. The moderate swelling was accompanied by a slight rupture of the outer membrane and by a release of cytochrome c. These results point to the involvement of a K(+)ATP channel during the manifestation of PCD induced by H2O2 or NO in plants.

The composition of newly synthesized proteins in the endoplasmic reticulum determines the transport pathways of soybean seed storage proteins

Glycinin (11S) and beta-conglycinin (7S) are major storage proteins in soybean (Glycine max L.) seeds and accumulate in the protein storage vacuole (PSV). These proteins are synthesized in the endoplasmic reticulum (ER) and transported to the PSV by vesicles. Electron microscopic analysis of developing soybean cotyledons of the wild type and mutants with storage protein composition different from that of the wild type showed that there are two transport pathways: one is via the Golgi and the other bypasses it. Golgi-derived vesicles were observed in all lines used in this study and formed smooth dense bodies with a diameter of 0.5 to several micrometers. ER-derived protein bodies (PBs) with a diameter of 0.3-0.5 microm were observed at high frequency in the mutants containing higher amount of 11S group I subunit than the wild type, whereas they were hardly observed in the mutants lacking 11S group I subunit. These indicate that pro11S group I may affect the formation of PBs. Thus, the composition of newly synthesized proteins in the ER is important in the selection of the transport pathways.

Use of Tn KPK2 for sequencing a 10.6-kb PstI DNA fragment of Bradyrhizobium japonicum and for the construction of aspA and ndvA mutants

Transposon Tn KPK2 was used to saturate a randomly cloned Bradyrhizobium japonicum PstI fragment and the insertions were used as starting points for the sequence determination. The first gene of the 10.6-kb DNA insert encodes a homologue to ndvA, the product of which is known to be involved in the formation of periplasmic cyclic glucans. Selected Tn KPK2 insertions were introduced into the B. japonicum wild-type strain. The resulting mutants were subsequently tested for their symbiotic interactions with soybeans. As in Sinorhizobium meliloti, a B. japonicum ndvA mutant was affected in salt-stress tolerance and exhibited symbiotic defects in that it induced the formation of ineffective soybean nodules. The central nodule tissue was infected by bacteroids, but within the infected cells the mutant was not properly maintained. Another gene was found to be highly similar to bacterial aspartases and thus was named aspA. The putative function of the product of this gene was confirmed by genetic complementation of aspartase-less Escherichia coli strain TK237. The symbiotic phenotype of a B. japonicum aspA:Tn KPK2 mutant consisted of enlarged symbiosomes that made the system ineffective. In general, Tn KPK2 is a suitable means for fast sequencing. In combination with pJQ200SK, the resulting recombinant plasmids can be directly used to create genetically defined mutants.

Endoplasmic reticulum stress-induced programmed cell death in soybean cells

In animal cells, the endoplasmic reticulum may participate in programmed cell death by sensing and transducing apoptotic signals. In an attempt to analyze the role of the endoplasmic reticulum in plant programmed cell death we investigated the effect of cyclopiazonic acid, a specific blocker of plant endoplasmic reticulum-type IIA Ca2+-pumps, in soybean cells. Cyclopiazonic acid treatment elicited endoplasmic reticulum stress and a biphasic increase in cytosolic Ca2+ concentration, followed by the induction of a cell death program. Cyclopiazonic acid-induced programmed cell death occurred with accumulation of H2O2, cytochrome c release from mitochondria, caspase 9- and caspase 3-like protease activation, cytoplasmic shrinkage and chromatin condensation. Chelation of cytosolic Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethil ester) failed to inhibit cyclopiazonic acid-induced cell death. Taken together, our results provide evidence for a role of the endoplasmic reticulum and mitochondria in regulating cyclopiazonic acid-induced programmed cell death in soybean cells, probably via a cross-talk between the two organelles.

Cloning of soybean genes induced during hypersensitive cell death caused by syringolide elicitor

Syringolide elicitors produced by bacteria expressing Pseudomonas syringae pv. glycinea avirulence gene D (avrD) induce hypersensitive cell death (HCD) only in soybean (Glycine max [L.] Merr.) plants carrying the Rpg4 disease resistance gene. Employing a differential display method, we isolated 13 gene fragments induced in cultured cells of a soybean cultivar Harosoy (Rpg4) treated with syringolides. Several genes for isolated fragments were induced by syringolides in an rpg4 cultivar Acme as well as in Harosoy; however, the genes for seven fragments designated as SIH (for syringolide-induced/ HCD associated) were induced exclusively or strongly in Harosoy. cDNA clones for SIH genes were obtained from a cDNA library of Harosoy treated with syringolide. Several sequences are homologous to proteins associated with plant defense responses. The SIH genes did not respond to a non-specific beta-glucan elicitor, which induces phytoalexin accumulation but not HCD, suggesting that the induction of the SIH genes is specific for the syringolide-Harosoy interaction. HCD and the induction of SIH genes by syringolides were independent of H(2)O(2). On the other hand, Ca(2+) was required for HCD and the induction of some SIH genes. These results suggest that the induction of SIH genes by syringolides could be activated through the syringolide-specific signaling pathway and the SIH gene products may play an important role(s) in the processes of HCD induced by syringolides.

[Light and nitrate utilization by soybean callus cells]

We studied the role of light during exogenous assimilation of nitrate (as the only source of nitrogen) by the callus cells of soybean (Glycine max). The nitrate absorbed and assimilated by the photosynthetic (mixotrophic) and non-photosynthetic cells (heterotrophic and chlorophyll-containing cells cultivated in the light in the same medium with diuron) was quantified. The assimilated nitrate was quantified at the final stage of the growth cycle as the difference between the amount of nitrogen absorbed from the medium and the amount of endogenous nitrate in the cells. Comparison of the assimilated nitrate quantities per accumulated dry biomass between the photosynthetic and non-photosynthetic cells demonstrated that nearly 30% of nitrate is assimilated with the help of photosynthesis in a mixotrophic culture when nitrate is the only source of nitrogen.

Analysis of the alternative oxidase promoters from soybean

Alternative oxidase (Aox) is a nuclear-encoded mitochondrial protein. In soybean (Glycine max), the three members of the gene family have been shown to be differentially expressed during normal plant development and in response to stresses. To examine the function of the Aox promoters, genomic fragments were obtained for all three soybean genes: Aox1, Aox2a, and Aox2b. The regions of these fragments immediately upstream of the coding regions were used to drive beta-glucuronidase (GUS) expression during transient transformation of soybean suspension culture cells and stable transformation of Arabidopsis. The expression patterns of the GUS reporter genes in soybean cells were in agreement with the presence or absence of the various endogenous Aox proteins, determined by immunoblotting. Deletion of different portions of the upstream regions identified sequences responsible for both positive and negative regulation of Aox gene expression in soybean cells. Reporter gene analysis in Arabidopsis plants showed differential tissue expression patterns driven by the three upstream regions, similar to those reported for the endogenous proteins in soybean. The expression profiles of all five members of the Arabidopsis Aox gene family were examined also, to compare with GUS expression driven by the soybean upstream fragments. Even though the promoter activity of the upstream fragments from soybean Aox2a and Aox2b displayed the same tissue specificity in Arabidopsis as they do in soybean, the most prominently expressed endogenous genes in all tissues of Arabidopsis were of the Aox1 type. Thus although regulation of Aox expression generally appears to involve the same signals in different species, different orthologs of Aox may respond variously to these signals. A comparison of upstream sequences between soybean Aox genes and similarly expressed Arabidopsis Aox genes identified common motifs.

Extraction of soybean oil from single cells

Single cells prepared from autoclaved soybeans and cellulase treatment of the cells were effective in digesting the cell walls of and extracting the oil from soybeans. The first cell wall of the soybean single cell was completely removed using cellulases; the thin and transparent second cell wall of the cell was swollen. Oil in the cell formed spherical or hemispherical oil drops, and oil leaking from the oil bodies was observed. The oil was almost retained within the second cell wall. Water-extractable substances were obtained at approximately >60% of the weight. Flotation of oil drops by centrifugation was easily done. Ambient n-hexane extraction was also possible; however, residual oil remained in the oil bodies. Protease or peptidase digested the structure of the oil bodies; however, separation of the oil and the hydrolysates was impossible. The oil from the oil bodies was obtained effectively (>85%) by pressing the single cells and/or cellulase-treated single cells.

Characterization of the targeting signal of dual-targeted pea glutathione reductase

We investigated the dual targeting signal of pea glutathione reductase (GR) that had been previously shown to be capable of targeting the passenger protein phosphinothricin acetyl transferase to mitochondria and chloroplasts in vivo. We confirmed that GR was imported into mitochondria and chloroplasts in vitro. Rupture of the outer mitochondrial membrane after the import assay indicated that GR was imported into both the intermembrane space and the matrix. Changing positive and hydrophobic residues in the targeting signal we investigated if dual targeting of GR was due to an overlapping or separate signal. Overall single mutations had a greater effect on mitochondrial import compared to chloroplasts, especially those on positive residues. Precursors containing both positive and hydrophobic residue mutations (double mutants) indicated that there might be some redundancy in targeting information for chloroplastic import as double mutants had a greater effect than predicted from the single mutants. Fusion of the targeting signal to the green fluorescent protein (GFP) followed by transient transformation indicated that this signal was only capable of targeting this passenger protein to plastids. Additionally, fusion of the complete coding sequence of GR to GFP also resulted in an exclusive chloroplastic localization. Mutations in the targeting signal that reduced import into plastids in vitro also displayed altered patterns of GFP localizations in vivo. These results indicate that some residues in the signal for dual localisation of GR play a role in both mitochondrial and chloroplastic import, and thus the signal is overlapping.

Structural characterization of plant-derived hepatitis B surface antigen employed in oral immunization studies

Several subunit vaccine antigens have been successfully expressed in plants and recently the hepatitis B surface antigen (HBsAg), expressed in potatoes, was shown to be orally immunogenic in animal studies. However, to date, a detailed analysis of the plant-derived antigen is lacking. Herein, we comprehensively characterize the structure and post-translational processing of HBsAg from potato tuber and two plant cell suspension cultures. The HBsAg was found to accumulate intracellularly as tubular structures, with a complex size distribution, differing substantially from the virus-like particle (VLP) preparations of the current commercial vaccines. Extensive disulfide-bond cross-linking, which is important for immunogenicity, was evident and 21-37% of total HBsAg protein displayed epitopes which correlate with vaccine potency. The significance of these results with regard to the production of cost-effective orally delivered vaccines is discussed.

Meloidogyne javanica chorismate mutase 1 alters plant cell development

Root-knot nematodes are obligate plant parasites that alter plant cell growth and development by inducing the formation of giant cells for feeding. Nematodes inject secretions from their esophageal glands through their stylet and into plant cells to induce giant cell formation. Meloidogyne javanica chorismate mutase 1 (MjCM-1) is one such esophageal gland protein likely to be secreted from the nematode as giant cells form. MjCM-1 has two domains, an N-terminal chorismate mutase (CM) domain and a C-terminal region of unknown function. It is the N-terminal CM domain of the protein that is the predominant form produced in root-knot nematodes. Transgenic expression of MjCM-1 in soybean hairy roots results in a phenotype of reduced and aborted lateral roots. Histological studies demonstrate the absence of vascular tissue in hairy roots expressing MjCM-1. The phenotype of MjCM-1 expressed at low levels can be rescued by the addition of indole-3-acetic acid (IAA), indicating MjCM-1 overexpression reduces IAA biosynthesis. We propose MjCM-1 lowers IAA by causing a competition for chorismate, resulting in an alteration of chorismate-derived metabolites and, ultimately, in plant cell development. Therefore, we hypothesize that MjCM-1 is involved in allowing nematodes to establish a parasitic relationship with the host plant.

N-terminal domain of the dual-targeted pea glutathione reductase signal peptide controls organellar targeting efficiency

Import of nuclear-encoded proteins into mitochondria and chloroplasts is generally organelle specific and its specificity depends on the N-terminal signal peptide. Yet, a group of proteins known as dual-targeted proteins have a targeting peptide capable of leading the mature protein to both organelles. We have investigated the domain structure of the dual-targeted pea glutathione reductase (GR) signal peptide by using N-terminal truncations. A mutant of the GR precursor (pGR) starting with the second methionine residue of the targeting peptide, pGRdelta2-4, directed import into both organelles, negating the possibility that dual import was controlled by the nature of the N terminus. The deletion of the 30 N-terminal residues (pGRdelta2-30) inhibited import efficiency into chloroplasts substantially and almost completely into mitochondria, whereas the removal of only 16 N-terminal amino acid residues (pGRdelta2-16) resulted in the strongly stimulated mitochondrial import without significantly affecting chloroplast import. Furthermore, N-terminal truncations of the signal peptide (pGRdelta2-16 and pGRdelta2-30) greatly stimulated the mitochondrial processing activity measured with the isolated processing peptidase. These results suggest a domain structure for the dual-targeting peptide of pGR and the existence of domains controlling organellar import efficiency therein.

An original mutation in soybean (Glycine max (L.) Merrill) involving degeneration of the generative cell and causing male sterility

A spontaneous mutation causing male sterility has been detected in line BR97-17739 from the soybean breeding program conducted by Embrapa-National Soybean Research Center. Meiotic division and male gametophyte development were analyzed in 10 male-sterile, female-fertile plants. Meiotic process had few irregularities related to chromosome segregation and affected about 2% of tetrads. Despite the high frequency of normal microspores, pollen sterility was total. After callose dissolution, microspores were released into the anther loculle and interphase nucleus was displaced from the center to one side of the cell. Displacement continued throughout normal microspore mitosis (PMI). After telophase, the hemispherical phragmoplast marked the place of cytokinesis. A typical generative cell, adjacent to the plasma membrane, and the vegetative one, containing most of the cytoplasm, were formed. In spite of the well-formed generative cell, pollen mitosis (PMII) failed to occur. The generative cell degenerated and was completely destroyed. The 3:1 segregation for male sterility in this line and its progenies indicate that a single recessive gene controls mutation.

Regulation of alternative oxidase gene expression in soybean

Soybean (Glycine max cv. Stevens) suspension cells were used to investigate the expression of the alternative oxidase (Aox) multigene family. Suspension cells displayed very high rates of cyanide-insensitive respiration, but Aox3 was the only isoform detected in untreated cells. Incubation with antimycin A, citrate, salicylic acid or at low temperature (10 degrees C) specifically induced the accumulation of the Aox1 isoform. Aox2 was not observed under any conditions in the cells. Increases in Aox1 protein correlated with increases in Aox1 mRNA. Treatment of soybean cotyledons with norflurazon also induced expression of Aox1. Reactive oxygen species (ROS) were detected upon incubation of cells with antimycin, salicylic acid or at low temperature, but not during incubation with citrate. Aox1 induction by citrate, but not by antimycin, was prevented by including the protein kinase inhibitor staurosporine in the medium. The results suggest that multiple pathways exist in soybean to regulate expression of Aox genes and that Aox1 specifically is induced by a variety of stress and metabolic conditions via at least two independent signal transduction pathways.

Transcriptional response of soybean suspension-cultured cells induced by Nod factors obtained from Bradyrhizobium japonicum USDA110

Genes responding to Nod factors were picked up by the application of a differential display method for soybean suspension-cultured cells. Forty-five cDNA fragments derived from such genes were detected. Seven fragments (ssc1-ssc7) were successfully cloned. The putative product of genes corresponding to ssc1 was estimated to be a disease-resistance protein relating to the induction of the plant defense response against pathogens, and that corresponding to ssc7 was a sucrose transporter. Amino acid sequences deduced from full-length cDNA corresponding to ssc2 and ssc4 were investigated, and it was shown that these polypeptides were equipped with a leucine zipper motif and with phosphorylation sites that were targeted by tyrosin kinase and cAMP-dependent protein kinase, respectively. In a differential display experiment, the transcriptional levels of three genes corresponding to ssc2, ssc3 and ssc5 were estimated to be up-regulated at 6 h after initiation of the treatment and the remaining four were estimated to be down-regulated. However, transcription of the genes corresponding to all ssc was clearly repressed within 2 h after initiation of the treatment. Five of them were restored to their transcriptional level 6 h after initiation of the treatment, although the others were repressed throughout the experimental period.

The role of calcium in oligogalacturonide-activated signalling in soybean cells

Alpha-1,4-Linked oligogalacturonides (OGs) are pectic fragments of the plant cell wall that are perceived by the plant cell as signalling molecules. Using cytosolic aequorin-expressing soybean (Glycine max L.) cells, we have analysed cytosolic Ca(2+) changes and the oxidative burst induced by OGs with different degrees of polymerization. Our results provide evidence that different OGs are sensed through transient elevations of cytosolic Ca(2+) that show different kinetics. Specificity of the Ca(2+) signature relies also on the precise structural characteristics of the OG molecules, such as the methylesterification of galacturonic acid residues and the steric conformation. Inhibition of the OG-induced Ca(2+) transient also blocks the oxidative burst, indicating that the cytosolic Ca(2+) increase is one of the earliest steps in OG-activated signalling. However, a phosphorylation event seems to precede the Ca(2+) rise, because the Ca(2+) transient could be abolished by the protein kinase inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB). A pharmacological approach with different antagonists that interfere with the induction of the cytosolic Ca(2+) rise indicates that both extracellular Ca(2+) influx and intracellular Ca(2+) release participate in transducing the OG signal. Treatment of cells with OGs establishes a refractory state, which impairs the ability of the cell to respond to a second stimulus with the same elicitor for up to 16 h. This desensitization period could be prolonged with the phosphatase inhibitor okadaic acid, and eliminated with the protein kinase inhibitor Ro 31-8220, suggesting that phosphorylation events may be involved in the establishment of the cell refractory state.

SCB1, a BURP-domain protein gene, from developing soybean seed coats

We describe a gene, SCB1 (Seed Coat BURP-domain protein 1), that is expressed specifically within the soybean (Glycine max [L.] Merrill) seed coat early in its development. Northern blot analysis and mRNA in situ hybridization revealed novel patterns of gene expression during seed development. SCB1 mRNA accumulated first within the developing thick-walled parenchyma cells of the inner integument and later in the thick- and thin-walled parenchyma cells of the outer integument. This occurred prior to the period of seed coat maturation and seed filling and before either of the layers started to degrade. SCB1 may therefore play a role in the differentiation of the seed coat parenchyma cells. In addition, the protein product appears to be located within cell walls. The SCB1 gene codes for a new member of a class of modular proteins that possess a carboxy-terminal BURP domain and a variety of different repeated sequences. The sequence of the genomic clone revealed the insertion of a Tgm transposable element in the upstream promoter region but it is not certain whether it contributes to the tissue-specific pattern of SCB1 expression.

A novel in vitro system for simultaneous import of precursor proteins into mitochondria and chloroplasts

Most chloroplast and mitochondrial precursor proteins are targeted specifically to either chloroplasts or mitochondria. However, there is a group of proteins that are dual targeted to both organelles. We have developed a novel in vitro system for simultaneous import of precursor proteins into mitochondria and chloroplasts (dual import system). The mitochondrial precursor of alternative oxidase, AOX was specifically targeted only to mitochondria. The chloroplastic precursor of small subunit of pea ribulose bisphosphate carboxylase/oxygenase, Rubisco, was mistargeted to pea mitochondria in a single import system, but was imported only into chloroplasts in the dual import system. The dual targeted glutathione reductase GR precursor was targeted to both mitochondria and chloroplasts in both systems. The GR pre-sequence could support import of the mature Rubisco protein into mitochondria and chloroplasts in the single import system but only into chloroplasts in the dual import system. Although the GR pre-sequence could support import of the mature portion of the mitochondrial FAd subunit of the ATP synthase into mitochondria and chloroplasts, mature AOX protein was only imported into mitochondria under the control of the GR pre-sequence in both systems. These results show that the novel dual import system is superior to the single import system as it abolishes mistargeting of chloroplast precursors into pea mitochondria observed in a single organelle import system. The results clearly show that although the GR pre-sequence has dual targeting ability, this ability is dependent on the nature of the mature protein.

Ribozyme termination of RNA transcripts down-regulate seed fatty acid genes in transgenic soybean

We investigated whether termination of transcripts with a self-cleaving ribozyme can enhance nuclear retention and serve as a tool to decrease specific plant gene expression. Nuclear retention was first monitored in tobacco using the beta-glucuronidase gene terminated with either the 35S CaMV 3' untranslated sequence (UTR) or a cis-acting ribozyme. Northern blot analysis of nuclear RNA and total RNA, and in situ hybridizations showed that the ribozyme-terminated transcripts were preferentially retained in the nucleus of transgenic tobacco. Ribozyme-terminated transcripts were subsequently tested as a gene down-regulation strategy in soybean. The embryo-specific Delta-12 fatty acid desaturase FAD2-1 gene was targeted because its down-regulation elevates oleic acid content of seed storage lipids. Both ribozyme-terminated antisense and standard antisense constructs were capable of gene down-regulation, producing over 57% oleic acid compared with less than 18% in wild-type seed. Ribozyme termination cassettes were also constructed to evaluate sense transcripts for single gene down-regulation and the simultaneous down-regulation of two embryo-specific genes in soybean using a single promoter. Eight independent soybean transformants were screened that harboured standard plus sense or ribozyme terminated FAD2-1 cassette. Two of the eight ribozyme terminated transformants displayed oleic acids levels in the seed storage lipids of over 75%, while none of the standard plus sense FAD2-1 lines showed elevated oleic acid phenotypes. The dual constructs targeted FAD2-1 and the FatB gene encoding a palmitoyl-thioesterase. Five transgenic soybean lines harbouring the dual constructs had oleic acid levels, greater than 85%, and saturated fatty acids levels, less than 6%. Thus, ribozyme termination of transcripts can be utilized to specifically down-regulate endogenous gene expression in soybean.

Cell enlargement of plant tissue explants oscillates with a temperature-compensated period of ca. 24 min

Rate of plant cell enlargement, measured at intervals of 3 min using a sensitive linear transducer, oscillates with a minimum period of about 24 min that parallels the 24-min periodicity observed with the oxidation of NADH by the external plasma membrane NADH oxidase and of single cells measured previously by video-enhanced light microscopy. Also exhibiting 24-min oscillations is the steady-state rate of cell enlargement induced by the addition of the auxin herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) or the natural auxin indole-3-acetic acid (IAA). Immediately following 2,4-D addition, a very complex pattern of oscillations is frequently observed. However, after several hours a dominant 24-min period emerges. The length of the 24-min period is temperature compensated and remains constant at 24 min when measured at 15, 25 or 35 degrees C, despite the fact that the rate of cell enlargement approximately doubles for each 10 degree C rise over this same range of temperatures.

Induction of H(2)O(2) synthesis by beta-glucan elicitors in soybean is independent of cytosolic calcium transients

Soybean cell suspension cultures have been used to investigate the role of the elevation of the cytosolic Ca(2+) concentration in beta-glucan elicitors-induced defence responses, such as H(2)O(2) and phytoalexin production. The intracellular Ca(2+) concentration was monitored in transgenic cells expressing the Ca(2+)-sensing aequorin. Two lines of evidence showed that a transient increase of the cytosolic Ca(2+) concentration is not necessarily involved in the induction of H(2)O(2) generation: (i) a Bradyrhizobium japonicum cyclic beta-glucan induced the H(2)O(2) burst without increasing the cytosolic Ca(2+) concentration; (ii) two ion channel blockers (anthracene-9-carboxylate, A9C; 5-nitro-2-(3-phenylpropylamino)-benzoate, NPPB) could not prevent a Phytophthora soja beta-glucan elicitor-induced H(2)O(2) synthesis but did prevent a cytosolic Ca(2+) concentration increase. Moreover, A9C and NPPB inhibited P. sojae beta-glucan-elicited defence-related gene inductions as well as the inducible accumulation of phytoalexins, suggesting that the P. sojae beta-glucan-induced transient cytosolic Ca(2+) increase is not necessary for the elicitation of H(2)O(2) production but is very likely required for phytoalexin synthesis.

Signal interactions between nitric oxide and reactive oxygen intermediates in the plant hypersensitive disease resistance response

Nitric oxide (NO) and reactive oxygen intermediates (ROIs) play key roles in the activation of disease resistance mechanisms both in animals and plants. In animals NO cooperates with ROIs to kill tumor cells and for macrophage killing of bacteria. Such cytotoxic events occur because unregulated NO levels drive a diffusion-limited reaction with O(2)(-) to generate peroxynitrite (ONOO(-)), a mediator of cellular injury in many biological systems. Here we show that in soybean cells unregulated NO production at the onset of a pathogen-induced hypersensitive response (HR) is not sufficient to activate hypersensitive cell death. The HR is triggered only by balanced production of NO and ROIs. Moreover, hypersensitive cell death is activated after interaction of NO not with O(2)- but with H(2)O(2) generated from O(2)(-) by superoxide dismutase. Increasing the level of O(2)(-) reduces NO-mediated toxicity, and ONOO(-) is not a mediator of hypersensitive cell death. During the HR, superoxide dismutase accelerates O(2)(-) dismutation to H(2)O(2) to minimize the loss of NO by reaction with O(2)(-) and to trigger hypersensitive cell death through NO/H(2)O(2) cooperation. However, O(2)(-) rather than H(2)O(2) is the primary ROI signal for pathogen induction of glutathione S-transferase, and the rates of production and dismutation of O(2)(-) generated during the oxidative burst play a crucial role in the modulation and integration of NO/H(2)O(2) signaling in the HR. Thus although plants and animals use a similar repertoire of signals in disease resistance, ROIs and NO are deployed in strikingly different ways to trigger host cell death.

Resistance of a soybean cell line to oxyfluorfen by overproduction of mitochondrial protoporphyrinogen oxidase

The diphenyl ether herbicide oxyfluorfen (2-chloro-4-trifluoromethylphenyl 3-ethoxy-4-nitrophenyl ether) inhibits protoporphyrinogen oxidase (Protox) which catalyzes the oxidation of protoporphyrinogen IX (Protogen) to protoporphyrin IX (Proto IX), the last step of the common pathway to chlorophyll and haeme biosynthesis. We have selected an oxyfluorfen-resistant soybean cell line by stepwise selection methods, and the resistance mechanism has been investigated. No growth inhibition was observed in resistant cells at a concentration of 10(-7) M oxyfluorfen, a concentration at which normal cells did not survive. While the degree of inhibition of total extractable Protox by oxyfluorfen was the same in both cell types, the enzyme activity in the mitochondrial fraction from non-treated resistant cells was about nine-fold higher than that from normal cells. Northern analysis of mitochondrial Protox revealed that the concentration of mitochondrial Protox mRNA was much higher in resistant cells than that in normal cells. There were no differences in the absorption and metabolic breakdown of oxyfluorfen. The growth of resistant cells was also insensitive to oxadiazon [5-tert-butyl-3-(2,4-dichloro-5-isopropoxyphenyl)-1,3,4-oxadiazol-2-(3H)- one], the other chemical class of Protox inhibitor. Therefore, the resistance of the selected soybean cell line to oxyfluorfen is probably mainly due to the overproduction of mitochondrial Protox.

Differential mRNA degradation of two beta-tubulin isoforms correlates with cytosolic Ca2+ changes in glucan-elicited soybean cells

Transgenic soybean (Glycine max) culture cells expressing apoaequorin, a Ca2+ indicator, were exposed to glucan fragments derived from Phytophthora sojae or to chitin oligomers. The effects of these elicitors on cytosolic Ca2+ concentrations and on mRNA levels of two beta-tubulin isoforms, tubB1 and tubB2, were investigated. The glucan elicitors, to which the cells are known to react with a biphasic cytosolic Ca2+ increase, induced a down-regulation of the tubB1 mRNA levels while the tubB2 mRNA level remained constant. The decrease of tubB1 mRNA level was observed after 1 hour of glucan treatment. In contrast, chitin oligomers, known to provoke a monophasic Ca2+ increase of short duration, did not affect the tubB1 mRNA level. Pre-incubation with 10 mM 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, an extracellular Ca2+ chelator, blocked the cytosolic Ca2+ increase as well as the decrease of tubB1 mRNA levels induced by glucan elicitors. Likewise, pre-incubation with 1 mM neomycin, which reduced only the second glucan-induced Ca2+ peak, blocked the decrease of tubB1 mRNA level. Experiments with cordycepin, a transcription inhibitor, indicated that glucan fragments induced the degradation of tubB1 mRNA. In conclusion, the glucan-induced cytosolic Ca2+ changes are correlated with a strong increase in tubB1 mRNA degradation.

Modulations in gene expression and mapping of genes associated with cyst nematode infection of soybean

Infection of the soybean root by the soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) induces a well-documented, yet poorly understood, response by the host plant. The plant response, involving the differentiation of a feeding structure, or "syncytium," facilitates the feeding and reproduction of the nematode to the detriment of the host. We used a genetic system involving a single dominant soybean gene conferring susceptibility to an inbred nematode strain, VL1, to characterize the nematode-host interaction in susceptible line PI 89008. The restriction fragment length polymorphism marker pB053, shown to map to a major SCN resistance locus, cosegregates with resistance among F2 progeny from the PI 89008 x PI 88287 cross. Cytological examination of the infection process confirmed that syncytium development in this genetic system is similar to that reported by others who used noninbred nematode lines. Our study of infected root tissue in the susceptible line PI 89008 revealed a number of genes enhanced in expression. Among these are catalase, cyclin, elongation factor 1alpha, beta-1,3-endoglucanase, hydroxy-methylglutaryl coenzyme A reductase, heat shock protein 70, late embryonic abundant protein 14, and formylglycinamidine ribonucleotide synthase, all of which we have genetically positioned on the public linkage map of soybean. Formylglycinamidine ribonucleotide synthase was found to be tightly linked with a major quantitative trait locus for SCN resistance. Our observations are consistent with the hypothesis proposed by others that feeding site development involves the dramatic modulation of gene expression relative to surrounding root cells.

Soybean cell enlargement oscillates with a temperature-compensated period length of ca. 24 min

Rate of enlargement of epidermal cells from soybean, when measured at intervals of 1 min using a light microscope equipped with a video measurement system, oscillated with a period length of about 24 min. This oscillation parallels the 24-min periodicity observed for the oxidation of NADH by the external plasma membrane NADH oxidase. The increase in length was not only non-linear, but intervals of rapid increase in area alternated with intervals of rapid decrease in area. The length of the period was temperature compensated, and was approximately the same when measured at 14, 24 and 34 degrees C even though the rate of cell enlargement varied over this same range of temperatures. These observations represent the first demonstration of an oscillatory growth behavior correlated with a biochemical activity where the period length of both is independent of temperature (temperature compensated) as is the hallmark of clock-related biological phenomena.

The role of lipid peroxidation in aluminium toxicity in soybean cell suspension cultures

The primary reactions leading to Al toxicity in plant cells have not yet been elucidated. We used soybean (Glycine max [L.] Merr.) cell suspension cultures to address the question whether lipid peroxidation plays an important role in Al toxicity. Upon transfer to an Al-containing culture medium with a calculated Al3+ activity of 15 microM soybean cells showed a distinct and longtime increase in lipid peroxidation within 4 h. At the same time a drastic loss of cell viability was observed. Butylated hydroxyanisole (BHA) and N,N'-diphenyl-p-phenylenediamine (DPPD), two lipophilic antioxidants, were able to almost completely suppress lipid peroxidation in Al-treated cells at a concentration of 20 microM. This effect was dose-dependent for DPPD and was observed at minimum concentrations of 1-2 microM. When lipid peroxidation was suppressed by DPPD or BHA cell viability remained high even in the presence of toxic Al concentrations. These results suggest that Al-induced enhancement of lipid peroxidation is a decisive factor for Al toxicity in suspension cultured soybean cells.

Pattern recognition analysis of endogenous cell metabolites for high throughput mode of action identification: removing the postscreening dilemma associated with whole-organism high throughput screening

Although whole-organism HTS can give clear indications of in vivo activity, typically few clues are given as to the mechanism of action (MOA), and determining the MOA for large numbers of active compounds can be costly and complex-an alternative approach is required. This report demonstrates that it is possible to conduct relatively high throughput MOA characterization of HTS hits utilizing a single sample preparation and analytical method. By monitoring a wide range of endogenous cellular metabolites via (1)H nuclear magnetic resonance spectroscopy, the MOA of herbicides can be predicted using computational methods to compare the metabolite perturbation patterns. Herbicides that induce a characteristic pattern of metabolic perturbation in maize include inhibitors of acetolactate synthase, acetyl co-enzyme A carboxylase, protoporphyrinogen oxidase, 5-enolpyruvylshikimate-3-phosphate synthase, and phytoene desaturase. In soya, photosystem II inhibitors can also be detected, further demonstrating that this method is not limited to inhibitors of enzymes that directly act upon endogenous metabolites, or a single species. The methods, including data analysis, can be readily automated, enabling relatively high throughput MOA elucidation of whole-organism screen hits. Additionally, for compounds with a novel MOA, this approach may lead to MOA identification faster than traditional methods. It is envisaged that application of these data analysis methods to other data types-for example, transcription (mRNA) or translation (protein) profiles-is likely to permit higher throughput with smaller sample requirements, along with ability to discriminate MOAs that are not adequately discriminated based upon endogenous metabolite profiles.

Rac-related GTP-binding protein in elicitor-induced reactive oxygen generation by suspension-cultured soybean cells

Plant cells produce reactive oxygen species (ROS) in response to many stimuli. However, the mechanism of ROS biosynthesis remains unclear. We have explored the hypothesis that the superoxide burst in plants mechanistically resembles the oxidative burst in neutrophils. First we have confirmed that ROS production, which occurs in suspension-cultured soybean (Glycine max) cells in response to hypo-osmotic shock, is inhibited by diphenylene iodonium, an inhibitor of the flavin-dependent oxidase of neutrophils. Because a Rac family G protein is an essential regulator of this NADPH oxidase, and because many plant homologs of Rac have been cloned, we next examined whether Rac-like proteins might be involved in the oxidative burst in the soybean cells. We identified a Rac-like 21-kD soybean protein that cross-reacts with antibodies to human Rac and garden pea Rop and also binds [gamma-(35)S] GTP, a diagnostic trait of small G proteins. This Rac-related protein translocated from the cytosol to microsomes during the oxidative burst. Moreover, soybean cells transiently transformed with either a dominant negative (RacN17) or a dominant positive (RacV12) form of Rac1 showed the anticipated altered responses to three different stimuli: hypo-osmotic shock, oligo-GalUA, and harpin. In response to these stimuli, cells transformed with RacN17 produced less ROS and cells transformed with RacV12 generated more ROS than control cells. These results strongly suggest that a Rac-related protein participates in the regulation of ROS production in soybean cells, possibly via activation of an enzyme complex similar to the NADPH oxidase of phagocytes in animal systems.

Homologous and heterologous desensitization and synergy in pathways leading to the soybean oxidative burst

Because the H2O2 and O2- generated during a pathogen-triggered oxidative burst could either protect or destroy a besieged plant cell, their synthesis might be expected to be tightly regulated. We have examined the nature of this regulation as it is communicated between homologous and heterologous oxidative-burst pathways, using both chemical (oligogalacturonic acid, harpin, fensulfothion) and mechanical (osmotic stress) stimuli to induce the burst. We report here that the above three chemical elicitors attenuate a subsequent oxidative burst induced in cultured soybean (Glycine max L.) cells by either the same (homologous desensitization) or a different chemical elicitor (heterologous desensitization). Further, when the magnitude of the initial oxidative burst is maximal, the cells remain refractory to subsequent elicitation for at least 10 min and then revive their sensitivities to re-stimulation with a half-time of >20 min. Mechanical stimulation of the oxidative burst appears to be regulated by a different set of constraints. Although initiation of a mechanically induced burst leads to attenuation of a subsequent mechanically induced burst, the same mechanical stimulus is peculiarly unable to reduce a subsequent chemically induced burst. The converse is also true, suggesting that heterologous desensitization of the oxidative burst does not extend to mixed chemical and mechanical/osmotic stimuli. However, communication between these disparate forms of elicitation is still demonstrated to occur, since low-level chemical stimuli strongly synergize concurrent low-level osmotic stimuli and vice versa. Furthermore, the pattern of synergy changes dramatically if one stimulus is administered immediately prior to the other. Taken together, these data demonstrate that significant cross-talk occurs among the different signaling pathways of the oxidative burst and that the overall process is tightly regulated.