Xuecheng Sun

Xylem transport and gene expression play decisive roles in cadmium accumulation in shoots of two oilseed rape cultivars (Brassica napus).

Cadmium (Cd) is a toxic metal which harms human health through food chains. The mechanisms underlying Cd accumulation in oilseed rape are still poorly understood. Here, we investigated the physiological and genetic processes involved in Cd uptake and transport of two oilseed rape cultivars (Brassica napus). L351 accumulates more Cd in shoots but less in roots than L338. A scanning ion-selective electrode technique (SIET) and uptake kinetics of Cd showed that roots were not responsible for the different Cd accumulation in shoots since L351 showed a lower Cd uptake ability. However, concentration-dependent and time-dependent dynamics of Cd transport by xylem showed L351 exhibited a superordinate capacity of Cd translocation to shoots. Additionally, the Cd concentrations of shoots and xylem sap showed a great correlation in both cultivars. Furthermore, gene expression levels related to Cd uptake by roots (IRT1) and Cd transport by xylem (HMA2 and HMA4) were consistent with the tendencies of Cd absorption and transport at the physiological level respectively. In other words, L351 had stronger gene expression for Cd transport but lower for Cd uptake. Overall, results revealed that the process of Cd translocation to shoots is a determinative factor for Cd accumulation in shoots, both at physiological and genetic levels.

Antioxidant enzyme systems and the ascorbate–glutathione cycle as contributing factors to cadmium accumulation and tolerance in two oilseed rape cultivars (Brassica napus L.) under moderate cadmium stress

Oilseed rape (Brassica napus L.) with high tolerance to cadmium (Cd) may be used in the phytoremediation of Cd-contaminated fields. However, the mechanisms responsible for Cd accumulation and tolerance in oilseed rape are still poorly understood. Here, we investigated the physiological and molecular processes involved in Cd tolerance of two oilseed rape cultivars with different Cd accumulation abilities. The total Cd accumulation in cultivar L351 was higher than cultivar L338, particularly with increasing concentrations of Cd exposure. L338 was a more pronounced Cd-sensitive cultivar than L351, while higher activities of antioxidant enzymes (CAT, APX, GR, DHAR) as well as higher contents of GSH and AsA were all observed in L351 under Cd treatments, especially at high levels. No differences were found in SOD activities between the two cultivars under the same Cd treatments, suggesting that SOD was not the key factor in relation to the differences of Cd tolerance and accumulation between them. Gene expression levels of BnFe-SOD, BnCAT, BnAPX, BcGR and BoDHAR in roots of L351 were relatively higher than that in L338 under Cd exposure as well as BnCAT and BcGR in leaves. It is concluded that antioxidant enzymes and the ascorbate-glutathione cycle play important roles in oilseed rape Cd accumulation and tolerance.

Genotypic differences in nitrate uptake, translocation and assimilation of two Chinese cabbage cultivars [Brassica campestris L. ssp. Chinensis (L.)].

A hydroponic trial was conducted to investigate genotypic differences in nitrate uptake, translocation and assimilation between low nitrate accumulator L18 and high accumulator H96 of Chinese cabbage [Brassica campestris L. ssp. Chinensis (L.)]. The results suggested that H96 could uptake more nitrate than L18 in the root but lower transport inside leaf cells and assimilation in the leaf. It was showed that root morphology parameters - length, surface area and volume of H96 were 18.0%, 31.6% and 46.5% higher than L18. Nitrate transporters NRT1.1 and NRT2.1 transcription levels were 41.6% and 269.6% higher than those of L18 respectively. NRT1.1 and NRT2.1 expression amount in leaf blade of two cultivars were opposite to in the root, L18 NRT1.1 and NRT2.1 expressions were 279.2% and 80.0% higher than H96. In addition, nitrate assimilation capacity of L18 was significantly higher than H96 in leaf. It was showed that Nitrate Reductase (NR; EC 1.7.1.1) activity and NIA expression level of L18 leaf were 234 0.4% and 105.4% higher than those of H96, Glutamine Synthetase (GS; EC 6.3.1.2) activity, Gln1 and Gln2 expression levels in the leaf blade of L18 were 43.9%, 331.5% and 124.8% higher than those of H96 respectively. Nitrate assimilation products-Glu, total amino acid, soluble protein content in the leaf of L18 were all significantly higher than those of H96. The results above suggested that nitrate accumulation differences were due to differential capacities to uptake, mechanisms for nitrate transport in leaves and assimilate nitrate. Comparing contribution of three aspects in nitrate accumulation, translocation and assimilation were contributed more in low nitrate concentration in the leaf blade.

Differences in responses of soil microbial properties and trifoliate orange seedling to biochar derived from three feedstocks

PurposeThe aim of the study was to examine the remediation effect of biochar derived from three feedstocks on soil acidification.Materials and methodsThe effects of biochar derived from peanut hull, rice straw and rape straw on soil acidity, chemical and microbial properties, nutrients absorption, and growth of orange seedlings planted in an acidic soil were studied in a greenhouse experiment.Results and discussionSoil pH was increased 0.70, 0.92, and 0.63 by peanut hull, rice straw, and rape straw biochars. However, only peanut hull biochar significantly increased plant growth and the biomass of trifoliate orange seedlings. Soil microbial biomass C and basal respiration were increased by peanut hull and rice straw biochar, and the geometric mean of enzyme activities (GMea) were increased by the three biochars, peanut hull biochar result in the highest increase. Rice straw and rape straw biochars had more abundant mineral nutrient, led to greater influence on soil and plant nutrient contents than peanut hull biochar. However, peanut hull biochar resulted in higher plant nutrients accumulation due to the improvement of plant biomass.ConclusionsAlthough the three biochars effectively neutralized soil acidity, only peanut hull biochar raised plant growth and the biomass of orange seedlings significantly, and the increase of soil microbial properties and enzyme activity would be the key factors for the improvement of plant growth.

INTERACTION OF MOLYBDENUM AND PHOSPHORUS SUPPLY ON UPTAKE AND TRANSLOCATION OF PHOSPHORUS AND MOLYBDENUM BY BRASSICA NAPUS

A hydroponic trial was conducted to assess interaction of molybdenum (Mo) and phosphorus (P) on uptake and translocation of P and Mo by Brassica napus. Molybdenum was applied at four rates (0, 0.01, 0.1 and 1 mg L−1) and P at three rates (1, 30, and 90 mg L−1) in nutrient solution. The results indicated that P increased shoot growth and 0.01 mg L−1 Mo improved the growth of shoots and roots. Molybdenum increased shoot P uptake and root P concentration and uptake when higher P was provided, and had a stimulating effect on P translocation from shoots to roots. P increased shoot Mo concentration and uptake, decreased those in roots, and enhanced Mo transport from roots to shoots. These results implied that both Mo and P had beneficial effects on Mo and P absorption and translocation and co-application of them were necessary to promote growth and utilization of Mo and P for Brassica napus.

Impact of molybdenum on Chinese cabbage response to selenium in solution culture

Molybdenum (Mo) and selenium (Se) are both essential micronutrients for animals and humans. Increasing Mo and Se contents in food crops offers an effective approach to reduce Mo and Se deficiency problems. A hydroponic trial was conducted to investigate the interactions of Mo and Se on uptake, transfer factors (TF shoot ) as well as distribution coefficients (DC) of Mo and Se on Chinese cabbage (Brassica campestris L. ssp. Pekinensis). In Experiment 1 three concentrations of Mo (0.01, 0.1 and 1 mg L−1) and four concentrations of Se (0, 0.01, 0.1 and 1 mg L−1) were arranged with a randomized block design. In Experiment 2, there were three treatments, 0.1 mg L−1 Mo, 0.1 mg L−1 Se and a combination of 0.1 mg L−1 Mo + 0.1 mg L−1 Se. Experiment 1 showed that Se decreased Mo concentrations in shoots and roots. The impact of Mo on Chinese cabbage response to uptake of Se varied, depending on whether the root Se concentration was saturated or not; Experiment 2 showed that there is a strong antagonism between Mo and Se on nutrition uptake when Mo and Se deficiencies persist for long periods; Mo and Se were easily translocated from solution to plants and from roots to shoots. The results will also be of help in cultivating Mo-enriched and Se-enriched crops.

Effects of tungsten on uptake, transport and subcellular distribution of molybdenum in oilseed rape at two different molybdenum levels

Due to the similarities of molybdenum (Mo) with tungsten (W) in the physical structure and chemical properties, studies involving the two elements have mainly examined their competitive relationships. The objectives of this study were to assess the effects of equimolar W on Mo accumulation, transport and subcellular distribution in oilseed rape at two Mo levels with four treatments: Mo1 (1μmol/L Mo, Low Mo), Mo1+W1 (1μmol/L Mo+1μmol/LW, Low Mo with Low W), Mo200 (200μmol/L Mo, High Mo) and Mo200+W200 (200μmol/L Mo+200μmol/L Mo, High Mo with high W). The fresh weight and root growth were inhibited by equimolar W at both low and high Mo levels. The Mo concentration and accumulation in root was increased by equimolar W at the low Mo level, but that in the root and shoot was decreased at the high Mo level. Additionally, equimolar W increased the Mo concentrations of xylem and phloem sap at low Mo level, but decreased that of xylem and increased that of phloem sap at the high Mo level. Furthermore, equimolar W decreased the expression of BnMOT1 in roots and leaves at the low Mo level, and only decreased its expression in leaves at the high Mo level. The expression of BnMOT2 was also decreased in root for equimolar W compared with the low Mo level, but increased compared with high Mo level. Moreover, equimolar W increased the proportion of Mo in cell wall fraction in root and that of soluble fraction in leaves when compared with the low Mo level. The results suggest that cell wall and soluble fractions might be responsible for the adaptation of oilseed rape to W stress.

Nitric Oxide Mediates Molybdenum-Induced Antioxidant Defense in Wheat under Drought Stress

Molybdenum (Mo) has been reported to alleviate drought stress by enhancing antioxidant defense in plants, but the underlying mechanism remains unclear. Here, we hypothesized that Mo mediates nitric oxide (NO)-induced antioxidant defense through Mo-enzymes, particularly by nitrate reductase (NR) in wheat under drought stress. The 30-day-old wheat seedlings cultivated in -Mo (0 μM Mo) and +Mo (1 μM Mo) Hoagland solutions were detached and then pretreated with Mo-enzyme inhibitors, NO scavengers, NO donors or their combinations according to demands of complementary experiment under 10% polyethylene glycol 6000 (PEG)-stimulated drought stress (PSD). Mo supplementation increased the activities and transcripts of antioxidant enzymes, decreased H2O2 and MDA contents, and elevated NO production, implying that Mo-induced antioxidant defense may be related to NO signal. Complementary experiment showed that NO production was induced by Mo, while suppressed by Mo-enzyme inhibitors and NO scavengers, but restored by NO donors, suggesting that Mo-induced increase of NO production may be due to the regulation by Mo-enzymes. Further experiment indicated that the increased activities and transcripts of antioxidant enzymes induced by Mo were suppressed by Mo-enzyme inhibitors and NO scavengers, and NO donors could eliminate their suppressing effects. Moreover, Mo application increased NR activity and inhibitors of Mo-enzymes inhibited NR activity in wheat leaves under PSD, suggesting that NR might involve in the regulation of Mo-induced NO production. These results clearly indicate that NO mediates Mo-induced antioxidant defense at least partially through the regulation of NR.

Comparison of cadmium absorption, translocation, subcellular distribution and chemical forms between two radish cultivars (Raphanus sativus L.)

Cadmium (Cd) absorption and accumulation vary greatly not only among plant species but also among cultivars within the same species. In order to better understand the mechanisms of Cd absorption, transportation and distribution, we examined the differences of Cd absorption, translocation, subcellular distribution and chemical forms between L19, a Cd-tolerant genotype, and H4, a Cd-sensitive genotype, using kinetic analysis and soil culture experiment. Kinetic assays showed that the different Cd concentrations between the two cultivars might be ascribed to root absorption and translocation from root to shoot. The investigations of subcellular distribution and chemical forms verified that Cd concentrations of all subcellular fractions in H4 were all higher than in L19. Meanwhile, most of the Cd was associated with cell walls in the root of H4, but the Cd in the root of L19 and leaf of the two cultivars was mainly stored in soluble fraction, which could be one possible mechanism of tolerance to Cd toxicity. In addition, Cd fractions extracted by 1M NaCl and 2% HAC were predominant in root and leaf of both cultivars and the concentrations and proportions extracted by water and 80% ethanol in root and 1M NaCl in leaf were all higher in H4 than in L19. These results indicate that the Cd in H4 is more active than L19, which could be responsible for the sensitivity of H4 to Cd damage.

Molybdenum Affects Photosynthesis and Ionic Homeostasis of Chinese Cabbage under Salinity Stress

A pot trial was conducted to clarify the effects of molybdenum (Mo) on photosynthesis and ionic homeostasis of Chinese cabbage under salinity stress. Mo was applied at three levels (0, 0.15, 0.3 mg kg−1). Ten days after sowing, 500 ml of 0.8% of NaCl solution was added to half of the plants for each treatment every 10th day for three consecutive times. The results revealed that fresh weight was significantly increased by application of Mo under salt stress; contents of chlorophyll a, chlorophyll b, carotene, and total chlorophyll were all raised by application of Mo; photosynthesis rate was enhanced by nonstomatal factors by application of Mo; and the ratios of potassium/sodium ions (K+/Na+), calcium/sodium ions (Ca2+/Na+), and magnesium/sodium ions (Mg2+/Na+) were all increased by application of Mo under salt stress. The study suggests that the application of Mo enhances salinity stress tolerance in Chinese cabbage by increasing the photosynthesis rate and the ionic homeostasis adjustment.