Sodium stimulation of uptake hydrogenase activity in symbiotic Rhizobium

Initial observations showed a 100% increase in H₂-uptake (Hup) activity of Rhizobium leguminosarum strain 3855 in pea root nodules (Pisum sativum L. cv Alaska) on plants growing in a baked clay substrate relative to those growing in vermiculite, and an investigation of nutrient factors responsible for the phenomenon was initiated. Significantly greater Hup activity was first measured in the clay-grown plants 24 days after germination, and higher activity was maintained relative to the vermiculite treatment until experiments were terminated at day 32. The increase in Hup activity was associated with a decrease in H₂ evolution for plants with comparable rates of acetylene reduction. Analyses of the clay showed that it contained more Na⁺ (29 versus 9 milligrams per kilogram) and less K⁺ (6 versus 74 milligrams per kilogram) than the vermiculite. Analyses of plants, however, showed a large increase in Na⁺ concentration of clay-grown plants with a much smaller reduction in K⁺ concentration. In tests with the same organisms in a hydroponic system with controlled pH, 40 millimolar NaCl increased Hup activity more than 100% over plants grown in solutions lacking NaCl. Plants with increased Hup activity, however, did not have greater net carbon or total nitrogen assimilation. KCl treatments from 5 to 80 millimolar produced slight increased in Hup activity at 10 millimolar KCl, and tests with other salts in the hydroponic system indicated that only Na⁺ strongly promoted Hup activity. Treating vermiculite with 50 millimolar NaCl increased Na⁺ concentration in pea plant tissue and greatly promoted Hup activity of root nodules in a manner analogous to the original observation with the clay rooting medium. A wider generality of the phenomenon was suggested by demonstrating that exogenous Na⁺ increased Hup activity of other R. leguminosarum strains and promoted Hup activity of R. meliloti strain B300 in alfalfa (Medicago sativa L.).     

Salicylic Acid induces cyanide-resistant respiration in tobacco cell-suspension cultures

Cyanide-resistant, alternative respiration in Nicotiana tabacum L. cv Xanthi-nc was analyzed in liquid suspension cultures using O₂ uptake and calorimetric measurements. In young cultures (4-8 d after transfer), cyanide inhibited O₂ uptake by up to 40% as compared to controls. Application of 20 μm salicylic acid (SA) to young cells increased cyanide-resistant O₂ uptake within 2 h. Development of KCN resistance did not affect total O₂ uptake, but was accompanied by a 60% increase in the rate of heat evolution from cells as measured by calorimetry. This stimulation of heat evolution by SA was not significantly affected by 1 mm cyanide, but was reduced by 10 mm salicylhydroxamic acid (SHAM), an inhibitor of cyanide-resistant respiration. Treatment of SA-induced or uninduced cells with a combination of cyanide and SHAM blocked most of the O₂ consumption and heat evolution. Fifty percent of the applied SA was taken up within 10 min, with most of the intracellular SA metabolized in 2 h. 2,6-Dihydroxybenzoic and 4-hydroxybenzoic acids also induced cyanide-resistant respiration. These data indicate that in tobacco cell-suspension culture, SA induces the activity and the capacity of cyanide-resistant respiration without affecting the capacity of the cytochrome c respiration pathway.     

Development and Partial Characterization of Nearly Isogenic Pea Lines (Pisum sativum L.) that Alter Uptake Hydrogenase Activity in Symbiotic Rhizobium

Some Rhizobium bacteria have H₂-uptake (Hup) systems that oxidize H₂ evolved from nitrogenase in leguminous root nodules. Pea (Pisum sativum L.) cultivars `JI1205' and `Alaska' produce high Hup (Hup⁺⁺) and moderate Hup (Hup⁺) phenotypes, respectively, in Rhizobium leguminosarum 128C53. The physiological significance and biochemical basis of this host plant genetic effect are unknown. The purpose of this investigation was to advance basic Hup studies by developing nearly isogenic lines of peas that alter Hup phenotypes in R. leguminosarum strains containing hup genes. Eight pairs of nearly isogenic pea lines that produce Hup⁺⁺ and Hup⁺ phenotypes in R. leguminosarum 128C53 were identified in 173 F₂-derived F₆ families produced from crosses between JI1205 and Alaska. Tests with the pea isolines and three strains of hup-containing R. leguminosarum showed that the isolines altered Hup activity significantly (P ≤ 0.05) in 19 of 24 symbiotic combinations. Analyses of Hup phenotypes in F₆ families, the F₁ population, and two backcrosses suggested involvement of a single genetic locus. Three of the eight pairs of isolines were identified as being suitable for physiological studies, because the two lines in each pair showed similar growth, N assimilation, and flowering traits under nonsymbiotic conditions. Tests of those lines under N₂-dependent conditions with isogenic Hup⁺ and negligible Hup (Hup⁻) mutants of R. leguminosarum 128C53 showed that, in symbioses with Hup⁺ rhizobia, two out of three Hup⁺⁺ pea lines decreased N₂ fixation relative to Hup⁺ peas. In one of those cases, however, the Hup⁺⁺ plant line also decreased fixation by Hup⁻ rhizobia. When results were averaged across all rhizobia tested, Hup⁺ pea isolines had 8.2% higher dry weight (P ≤ 0.05) and fixed 12.6% more N₂ (P ≤ 0.05) than Hup⁺⁺ isolines. Pea lines described here may help identify host plant factors that influence rhizobial Hup activity and should assist in clarifying how Hup systems influence other physiological processes.     

Effects of Temperature, Nitrogen Fertilization, and Plant Age on Nitrogen Fixation by Setaria italica Inoculated with Azospirillum brasilense (strain cd)

The association between the nitrogen-fixing bacterium Azospirillum brasilense (strain cd) and the grass Setaria italica was studied under different environmental and soil conditions. Highest acetylene reduction rates in intact plants were observed at the booting stage of Setaria (2350 nmol ethylene produced hour⁻¹ plant⁻¹) at 27 C. Higher temperatures, up to 32 C, enhanced ethylene reduction. Significant increases in shoot dry weight, panicle weight, and length were obtained in inoculated plants fertilized with suboptimal NH₄NO₃ levels. The increase in nitrogen content of plants inoculated with A. brasilense was shown to be due to N₂ fixation. This was demonstrated by growing plants in washed quartz sand with no combined nitrogen. The bacteria also increased branching and development of roots. It was concluded that inoculation of Setaria with A. brasilense may lead both to increases in plant yield and saving of nitrogen fertilizer.     

Expression of <Emphasis Type="Italic">MAX2</Emphasis> under <Emphasis Type="Italic">SCARECROW</Emphasis> promoter enhances the strigolactone/MAX2 dependent response of <Emphasis Type="Italic">Arabidopsis</Emphasis> roots to low-phosphate conditions

Main conclusion

MAX2/strigolactone signaling in the endodermis and/or quiescent center of the root is partiallysufficient to exert changes in F-actin density and cellular trafficking in the root epidermis, and alter gene expression during plant response to low Pi conditions.Strigolactones (SLs) are a new group of plant hormones that regulate different developmental processes in the plant via MAX2, an F-box protein that interacts with their receptor. SLs and MAX2 are necessary for the marked increase in root-hair (RH) density in seedlings under conditions of phosphate (Pi) deprivation. This marked elevation was associated with an active reduction in actin-filament density and endosomal movement in root epidermal cells. Also, expression of MAX2 under the SCARECROW (SCR) promoter was sufficient to confer SL sensitivity in roots, suggesting that SL signaling pathways act through a root-specific, yet non-cell-autonomous regulatory mode of action. Here we show evidence for a non-cell autonomous signaling of SL/MAX2, originating from the root endodermis, and necessary for seedling response to conditions of Pi deprivation. SCR-derived expression of MAX2 in max2-1 mutant background promoted the root low Pi response, whereas supplementation of the synthetic SL GR24 to these SCR:MAX2 expressing lines further enhanced this response. Moreover, the SCR:MAX2 expression led to changes in actin density and endosome movement in epidermal cells and in TIR1 and PHO2 gene expression. These results demonstrate that MAX2 signaling in the endodermis and/or quiescent center is partially sufficient to exert changes in F-actin density and cellular trafficking in the epidermis, and alter gene expression under low Pi conditions.

     

Effect of Trichoderma harzianum on microelement concentrations and increased growth of cucumber plants

The potential of the biocontrol agent Trichoderma harzianum strain T-203 to induce a growth response in cucumber plants was studied in soil and under axenic hydroponic growth conditions. When soil was amended with T. harzianum propagules, a 30% increase in seedling emergence was observed up to 8 days after sowing. On day 28, these plants exhibited a 95 and 75% increase in root area and cumulative root length, respectively, and a significant increase in dry weight (80%), shoot length (45%) and leaf area (80%). Similarly, an increase of 90 and 30% in P and Fe concentration respectively, was observed in T. harzianum inoculated plants. To better characterize the effect of T. harzianum during the early stages of root colonization, experiments were carried out in a gnotobiotic hydroponic system. An increased growth response was apparent as early as 5 days post-inoculation with T. harzianum, resulting in an increase of 25 and 40% in the dry weight of roots and shoots, respectively. Similarly a significant increase in the concentration of Cu, P, Fe, Zn, Mn and Na was observed in inoculated roots. In the shoots of these plants, the concentration of Zn, P and Mn increased by 25, 30 and 70%, respectively. Using the axenic hydroponic system, we showed that the improvement of plant nutritional level may be directly related to a general beneficial growth effect of the root system following T. harzianum inoculation. This phenomenon was evident from 5 days post-inoculation throughout the rest of the growth period, resulting in biomass accumulation in both roots and shoots.     

EDTA and Pb—EDTA accumulation in Brassica juncea grown in Pb—amended soil

Previous studies have shown that EDTA is necessary to solubilize soil Pb and facilitate its transport from the soil to the above ground plant tissues. These studies have also suggested that Pb is accumulated in the plant tissue with transpiration as the driving force. We conducted further studies to evaluate the relationship between EDTA soil treatment, plant transpiration, and plant accumulation of Pb and EDTA. Indian mustard (Brassica juncea) plants were grown in soils containing Pb at three different concentrations (1.5, 3.0 and 4.8 mmol/kg) for 5 weeks before being treated with EDTA concentrations ranging from 0 to 10 mmol/kg. Plant shoots and xylem sap were collected and analyzed for Pb and EDTA content using ICP and HPLC, respectively. Water loss was measured for 7 days following EDTA application. Transpiration was not affected at <5 mmol/kg EDTA but, at 10 mmol/kg EDTA transpiration decreased by 80%, whereas accumulation of Pb and EDTA increased. In the Sassafras soil, Pb and EDTA accumulation in the plant shoots continued to increase as the applied EDTA concentration increased, except at the highest level (10 mmol/kg). In soil amended with 4.8 mmol/kg Pb and 10 mmol/kg EDTA, the concentrations of EDTA and Pb in shoots decreased and visible signs of phytotoxicity were observed. The results presented herein support recent studies in hydroponic systems showing that EDTA and Pb are taken up by the plant and suggest that Pb is translocated in the plant as the Pb-EDTA complex. The results also show that the maximum Pb accumulation by plants occurs by maximizing the concentration of the Pb-EDTA complex based on the EDTA extractable soil Pb. This revised version was published online in July 2006 with corrections to the Cover Date.     

Concomitant Induction of Systemic Resistance to Pseudomonas syringae pv. lachrymans in Cucumber by Trichoderma asperellum (T-203) and Accumulation of Phytoalexins

Most studies on the reduction of disease incidence in soil treated with Trichoderma asperellum have focused on microbial interactions rather than on plant responses. This study presents conclusive evidence for the induction of a systemic response against angular leaf spot of cucumber (Pseudomonas syringae pv. lachrymans) following application of T. asperellum to the root system. To ascertain that T. asperellum was the only microorganism present in the root milieu, plants were grown in an aseptic hydroponic growth system. Disease symptoms were reduced by as much as 80%, corresponding to a reduction of 2 orders of magnitude in bacterial cell densities in leaves of plants pretreated with T. asperellum. As revealed by electron microscopy, bacterial cell proliferation in these plants was halted. The protection afforded by the biocontrol agent was associated with the accumulation of mRNA of two defense genes: the phenylpropanoid pathway gene encoding phenylalanine ammonia lyase (PAL) and the lipoxygenase pathway gene encoding hydroxyperoxide lyase (HPL). This was further supported by the accumulation of secondary metabolites of a phenolic nature that showed an increase of up to sixfold in inhibition capacity of bacterial growth in vitro. The bulk of the antimicrobial activity was found in the acid-hydrolyzed extract containing the phenolics in their aglycone form. High-performance liquid chromatography analysis of phenolic compounds showed a marked change in their profile in the challenged, preelicited plants relative to that in challenged controls. The results suggest that similar to beneficial rhizobacteria, T. asperellum may activate separate metabolic pathways in cucumber that are involved in plant signaling and biosynthesis, eventually leading to the systemic accumulation of phytoalexins.     

Signal Transduction Pathways in Mycorrhizal Associations: Comparisons with the Rhizobium-Legume Symbiosis

A number of genera of soil fungi interact with plant roots to establish symbiotic associations whereby phosphate acquired by the fungus is exchanged for fixed carbon from the plant. Recent progress in investigating these associations, designated as mycorrhizae (sing., mycorrhiza), has led to the identification of specific steps in the establishment of the symbiosis in which the fungus and the plant interact in response to various molecular signals. Some of these signals are conserved with those of the Rhizobium-legume nitrogen-fixing symbiosis, suggesting that the two plant-microbe interactions share a common signal transduction pathway. Nevertheless, only legume hosts nodulate in response to Rhizobium, whereas the vast majority of flowering plants establish mycorrhizal associations. The key questions for the future are: what are the signal molecules produced by mycorrhizal fungi and how are they perceived by the plant? Copyright 1998 Academic Press.