Physiological and biochemical responses of common vetch to the imazamox accumulation.

Common vetch (Vicia sativa L.) is a forage and grain legume, widely distributed throughout the world. Alterations induced by the herbicide imazamox on plant growth, acetohydroxyacid synthase activity, total free amino acids, as well as concentrations of valine, leucine, isoleucine and imazamox in young and mature leaves were investigated at 2 and 7 days after the herbicide application. Plant growth decreased significantly after 7 days of imazamox treatment. The herbicide was detected in both young and mature leaves inducing an inhibition of acetohydroxyacid synthase activity in the former and consequently decreasing valine and leucine contents in this organ. At the same time, the treatment caused an increase of total free amino acids in young leaves, presumably as result of proteolysis stimulation in such conditions. Given that these effects were not observed in mature leaves, we suggest a different sensitivity of the acetohydroxyacid synthase activity to imazamox depending on leaf age. Common vetch seems not to degrade imazamox since the herbicide was accumulated in shoot with increasing treatment time. To our knowledge, no physiological and biochemical studies of common vetch responses to imazamox have been previously reported.

Substrate role in the accumulation of heavy metals in sporocarps of wild fungi.

The distribution of neodymium, lead, thorium and uranium was investigated in about 100 samples of 12 different species of common, edible and non-edible mushrooms collected in unpolluted areas in the province of Ciudad Real, Central Spain. The quantitative analysis of heavy metals was performed by X-ray fluorescence spectrometry (a simple, accurate and non-destructive method). The concentration of these elements was related to three factors: mushroom specie, life style/substrate and study area. The results reveal considerable amounts of the four metals in all species analyzed as well as significant differences on the capability to accumulate these elements. The maximum absorption of Nd and Pb was found in the ectomycorrhizal Cantharellus cibarius, reaching values of 7.10 and 4.86 microg g(-1), respectively. Thorium and uranium were mainly accumulated (3.63 and 4.13 microg g(-1), respectively) in Hypholoma fasciculare although it is an epiphyte species, isolated from the mineral particles of soil. The distribution patterns of these metals in sporocarps of different habitats and locations showed no significant differences, except for thorium, mainly accumulated in mushrooms living on wood regarding these living on soil organic matter. The species-specific is therefore the determining factor for accumulation of Nd, Pb, Th and U, more than substrate, in this study.

Validamycin A improves the response of Medicago truncatula plants to salt stress by inducing trehalose accumulation in the root nodules.

In this work, the role of trehalose as an osmoprotectant against salt stress conditions was examined in root nodules of Medicago truncatula. For this purpose, we used validamycin A, a potent trehalase inhibitor, in order to induce trehalose accumulation. Validamycin A induced an increase of trehalose concentration in root nodules of M. truncatula by inhibiting trehalase activity; no effect on trehalose concentration was observed in roots and leaves. Trehalose accumulation was accompanied by a decrease in sucrose and starch content, indicating interference with carbohydrate partitioning in the plants. Under salinity conditions, sucrose accumulation appears to be induced in M. truncatula to protect nodule functioning by the inhibition of sucrose catabolism by sucrose synthase and alkaline invertase activities. However, trehalose accumulation induced by val A in nodules improved the response to salinity by increasing plant dry weight (PDW), and no effects of validamycin A on nitrogenase activity and PDW were observed in nonsalinized plants.

Trehalose and trehalase in root nodules of Medicago truncatula and Phaseolus vulgaris in response to salt stress.

Trehalose (alpha-D-glucopyranosyl-1,1-alpha-D-glucopyranoside), a non-reducing disaccharide, has been found in a wide variety of organisms playing an important role as an abiotic stress protectant. Plants may come into contact with trehalose from exogenous sources, such as in plant-rhizobia symbiosis in which the rhizobia have the capacity to produce trehalose. The aim of this work is to analyse how trehalose and trehalase respond to salt stress in root nodules of legumes. For this purpose, tissue expression of Medicago truncatula trehalase gene (MTTRE1) and the expression of MTTRE1 under salt stress were analysed by real-time quantitative reverse transcription-PCR method. Trehalase activity was determined and trehalose was also measured by gas chromatography. In addition, trehalase protein occurrence in different organs and at different developmental stages in Phaseolus vulgaris plants has been studied. MTTRE1 expression is induced in nodules compared with leaves and roots, indicating a transcriptional regulation of trehalase in the presence of the microsymbiont. Under salt stress conditions, trehalase activity is downregulated at the transcriptional level, allowing trehalose accumulation. The results found in this study led us to conclude that trehalase activity is induced in root nodules of legumes by the microsymbiont and that under salt stress conditions; trehalase activity is downregulated at the transcriptional level in M. truncatula nodules. This allows trehalose accumulation and supports the possible role of this disaccharide as a stabilizer against salt stress conditions.

Growth and nitrogen fixation in Lotus japonicus and Medicago truncatula under NaCl stress: nodule carbon metabolism.

Lotus japonicus and Medicago truncatula model legumes, which form determined and indeterminate nodules, respectively, provide a convenient system to study plant-Rhizobium interaction and to establish differences between the two types of nodules under salt stress conditions. We examined the effects of 25 and 50mM NaCl doses on growth and nitrogen fixation parameters, as well as carbohydrate content and carbon metabolism of M. truncatula and L. japonicus nodules. The leghemoglobin (Lb) content and nitrogen fixation rate (NFR) were approximately 10.0 and 2.0 times higher, respectively, in nodules of L. japonicus when compared with M. truncatula. Plant growth parameters and nitrogenase activity decreased with NaCl treatments in both legumes. Sucrose was the predominant sugar quantified in nodules of both legumes, showing a decrease in concentration in response to salt stress. The content of trehalose was low (less than 2.5% of total soluble sugars (TSS)) to act as an osmolyte in nodules, despite its concentration being increased under saline conditions. Nodule enzyme activities of trehalose-6-phosphate synthase (TPS) and trehalase (TRE) decreased with salinity. L. japonicus nodule carbon metabolism proved to be less sensitive to salinity than in M. truncatula, as enzymatic activities responsible for the carbon supply to the bacteroids to fuel nitrogen fixation, such as sucrose synthase (SS), alkaline invertase (AI), malate dehydrogenase (MDH) and phosphoenolpyruvate carboxylase (PEPC), were less affected by salt than the corresponding activities in barrel medics. However, nitrogenase activity was only inhibited by salinity in L. japonicus nodules.

Nitrogenase and antioxidant enzyme activities in Phaseolus vulgaris nodules formed by Rhizobium tropici isogenic strains with varying tolerance to salt stress.

Common bean plants inoculated with salt-tolerant Rhizobium tropici wild-type strain CIAT899 formed a more active symbiosis than did its decreased salt-tolerance (DST) mutant derivatives (HB8, HB10, HB12 and HB13). The mutants formed partially effective (HB10, HB12) or almost ineffective (HB8, HB13) nodules (Fix(d)) under non-saline conditions. The DST mutant formed nodules that accumulated more proline than did the wild-type nodules, while soluble sugars were accumulated mainly in ineffective nodules. Under salt stress, plant growth, nitrogen fixation, and the activities of the antioxidant defense enzymes of nodules were affected in all symbioses tested. Overall, mutant nodules showed lower antioxidant enzyme activities than wild-type nodules. Levels of nodule catalase appeared to correlate with symbiotic nitrogen-fixing efficiency. Superoxide dismutase and dehydroascorbate reductase seem to function in the molecular mechanisms underlying the tolerance of nodules to salinity.

Influence of carbon and nitrogen sources on growth, nitrogenase activity, and carbon metabolism of Gluconacetobacter diazotrophicus.

The effects of different carbon and nitrogen sources on the growth, nitrogenase activity, and carbon metabolism of Gluconacetobacter diazotrophicus were investigated. The amino acids asparagine, aspartic acid, and glutamic acid affected microbial growth and nitrogenase activity. Several enzymatic activities involved in the tricarboxylic acid cycle were affected by the carbon source used. In addition, glucose and gluconate significantly increased the oxygen consumption (respiration rate) of whole cells of G. diazotrophicus grown under aerobic conditions. Enzymes responsible for direct oxidation of glucose and gluconate were especially active in cells grown with sucrose and gluconate. The presence of amino acids in the apoplastic and symplastic sap of sugarcane stems suggests that these compounds might be of importance in the regulation of growth and nitrogenase activity during the symbiotic association. The information obtained from the plant-bacterium association together with the results of other biochemical studies could contribute to the development of biotechnological applications of G. diazotrophicus.