The symbiosis phenotype and expression patterns of five nodule-specific genes of Astragalus sinicus under ammonium and salt stress conditions

Simultaneous determination of six active metabolites in Astragalus mongholicus (Fisch.) Bge. under salt stress by ultra-pressure liquid chromatography with tandem mass spectrometry

Astragalus membranaceus Bge. var. mongholicus (Bge.) Hsiao (A. mongholicus, family Leguminosae) is one of the most important traditional Chinese herbs because it contains lots of bioactive metabolites, which have beneficial and pharmacological effects on health. Simultaneously, it has been proved to be a salt-tolerant plant-one of the potential species to control the soil salinization. Therefore, a sensitive and specific ultra-pressure liquid chromatography coupled with tandem mass spectrometric (UPLC-MS/MS) method was developed and validated for the simultaneous determination of six main bioactive metabolites, astragaloside IV, cycloastragenol, calycosin-7-O-β-d-glucoside, calycosin, ononin and formononetin in different organs of A. mongholicus. The detection was accomplished by multiple-reaction monitoring (MRM) scanning via electrospray ionization source operating in the positive ionization mode. Calibration curves offered linear ranges of two orders of magnitude with R(2) > 0.99. The method was fully validated for the linearity, intra-day and inter day precisions, accuracy, recovery, matrix effect and stability. Then this method was successfully applied to detect the content of major bioactive metabolites in different plant organs of A. mongholicus under salt stress. Significant variations in the content of six bioactive metabolites were observed after been processed by different levels of salinity in different part of plant. The results support for further exploration of the salt-tolerant mechanisms in A. mongholicus and its possibility as the species that control the soil salinization. Meanwhile, we established a UPLC-MS/MS assay of the trace components in seedling of A. mongholicus in this study.

Toxicogenomic Responses of the Model Legume Medicago truncatula to Aged Biosolids Containing a Mixture of Nanomaterials (TiO₂, Ag, and ZnO) from a Pilot Wastewater Treatment Plant

Toxicogenomic responses in Medicago truncatula A17 were monitored following exposure to biosolids-amended soils. Treatments included biosolids produced using a pilot wastewater treatment plant with either no metal introduced into the influent (control); bulk/ionic TiO2, ZnO, and AgNO3 added to influent (bulk/dissolved treatment); or Ag, ZnO, and TiO2 engineered nanomaterials added to influent (ENM treatment) and then added to soil, which was aged in the field for 6 months. In our companion study, we found inhibition of nodulation in the ENM but not in the bulk/dissolved treatment. Gene expression profiling revealed highly distinct profiles with more than 10-fold down-regulation in 239 genes in M. truncatula roots from the ENM treatment, while gene expression patterns were similar between bulk/dissolved and control treatments. In response to ENM exposure, many of the identified biological pathways, gene ontologies, and individual genes are associated with nitrogen metabolism, nodulation, metal homeostasis, and stress responses. Expression levels of nine genes were independently confirmed with qRT-PCR. Exposure to ENMs induced unique shifts in expression profiles and biological pathways compared with bulk/dissolved treatment, despite the lack of difference in bioavailable metal fractions, metal oxidation state, and coordination environment between ENM and bulk/dissolved biosolids. As populations of Sinorhizobium meliloti Rm2011 were similar in bulk/dissolved and ENM treatments, our results suggest that inhibition of nodulation in the ENM treatment was primarily due to phytotoxicity, likely caused by enhanced bioavailability of Zn ions.

RNA-Seq and microarrays analyses reveal global differential transcriptomes of Mesorhizobium huakuii 7653R between bacteroids and free-living cells

Mesorhizobium huakuii 7653R occurs either in nitrogen-fixing symbiosis with its host plant, Astragalus sinicus, or free-living in the soil. The M. huakuii 7653R genome has recently been sequenced. To better understand the complex biochemical and developmental changes that occur in 7653R during bacteroid development, RNA-Seq and Microarrays were used to investigate the differential transcriptomes of 7653R bacteroids and free-living cells. The two approaches identified several thousand differentially expressed genes. The most prominent up-regulation occurred in the symbiosis plasmids, meanwhile gene expression is concentrated to a set of genes (clusters) in bacteroids to fulfill corresponding functional requirements. The results suggested that the main energy metabolism is active while fatty acid metabolism is inactive in bacteroid and that most of genes relevant to cell cycle are down-regulated accordingly. For a global analysis, we reconstructed a protein-protein interaction (PPI) network for 7653R and integrated gene expression data into the network using Cytoscape. A highly inter-connected subnetwork, with function enrichment for nitrogen fixation, was found, and a set of hubs and previously uncharacterized genes participating in nitrogen fixation were identified. The results described here provide a broader biological landscape and novel insights that elucidate rhizobial bacteroid differentiation, nitrogen fixation and related novel gene functions.

Systemin-dependent salinity tolerance in tomato: evidence of specific convergence of abiotic and biotic stress responses

Plants have evolved complex mechanisms to perceive environmental cues and develop appropriate and coordinated responses to abiotic and biotic stresses. Considerable progress has been made towards a better understanding of the molecular mechanisms of plant response to a single stress. However, the existence of cross-tolerance to different stressors has proved to have great relevance in the control and regulation of organismal adaptation. Evidence for the involvement of the signal peptide systemin and jasmonic acid in wound-induced salt stress adaptation in tomato has been provided. To further unravel the functional link between plant responses to salt stress and mechanical damage, transgenic tomato (Lycopersicon esculentum Mill.) plants constitutively expressing the prosystemin cDNA have been exposed to a moderate salt stress. Prosystemin over-expression caused a reduction in stomatal conductance. However, in response to salt stress, prosystemin transgenic plants maintained a higher stomatal conductance compared with the wild-type control. Leaf concentrations of abscissic acid (ABA) and proline were lower in stressed transgenic plants compared with their wild-type control, implying that either the former perceived a less stressful environment or they adapted more efficiently to it. Consistently, under salt stress, transgenic plants produced a higher biomass, indicating that a constitutive activation of wound responses is advantageous in saline environment. Comparative gene expression profiling of stress-induced genes suggested that the partial stomatal closure was not mediated by ABA and/or components of the ABA signal transduction pathway. Possible cross-talks between genes involved in wounding and osmotic stress adaptation pathways in tomato are discussed.

A nodule-specific plant cysteine proteinase, AsNODF32, is involved in nodule senescence and nitrogen fixation activity of the green manure legume Astragalus sinicus

Asnodf32, encoding a nodule-specific cysteine proteinase in Astragalus sinicus, is probably involved in nodule senescence. To obtain direct evidence of its role in nodule senescence, Agrobacterium rhizogenes-mediated RNA interference was applied to A. sinicus hairy roots. Real-time qRT-PCR was used to estimate the efficiency of suppression. The senescent phenotype of transgenic nodules was examined with paraffin-embedded slides, TUNEL (TdT-mediated dUTP nick-end labeling) assay, and transmission electron microscopy, and the bacteroid nitrogen fixation activity was also measured. It was found that silencing of Asnodf32 delayed root nodule and bacteroid senescence. The period of bacteroid active nitrogen fixation was significantly extended. Interestingly, nodules enlarged in length were also observed on Asnodf32-silenced hairy roots. The results reported here indicate that Asnodf32 plays an important role in the regulation of root nodule senescence.