Mild Fe-deficiency improves biomass production and quality of hydroponic-cultivated spinach plants (Spinacia oleracea L.)

A promising product: Abscisic acid-producing bacterial agents for restricting cadmium enrichment in field vegetable crops

Soil heavy metal contamination and its enrichment in the edible parts of crops have gained global concern. In this study, a compound bacterial agent possessing the ability to produce the plant hormone, abscisic acid (ABA), was applied to contaminated farmland in Hunan province. Its application reduced the concentration of Cd in radish, cabbage, mustard, and lettuce by 15-144%. Accordingly, the Cd contents in these vegetables were found to be below the maximum limits set by GB 2762-2017. Meanwhile, bacteria agents also led to a significant increase in crops yield by 45-82%. Furthermore, the nutritional indices, including soluble sugar and soluble protein increased by 18-66%, as well as the antioxidant indices, including total phenolic, ascorbate content, and DPPH capacity, enhanced by 12-76%, 10-49% and 50-140%, respectively. In conclusion, the use of ABA-producing bacteria is anticipated to be a novel approach for the safe use of soil with moderate and low pollution.

Nitrate-induced AHb1 expression aggravates Cd toxicity in plants

Cadmium (Cd) causes severe toxicity in plants. However, the molecular mechanisms underlying plant resistance to Cd in relation to nitrogen (N) supply remain unclear. The non-symbiotic hemoglobin gene Hb1 plays an important role in scavenging nitric oxide (NO) in plants. In this study, there was no differential effect of Cd on the biomass of wild-type (WT) and AHb1-overexpressing (H7) plants when NH4+-N was used as a nitrogen source. However, under NO3--N conditions, Cd exerted less biomass stress on AHb1-silenced (L3) plants and more stress on H7 plants than on WT plants. The Cd tolerance index followed the order: L3 > WT > H7. However, there was no difference in Cd concentrations in the roots or shoots of the WT, L3, and H7 plants, indicating that differences in AHb1 expression were unrelated to Cd uptake. Further investigation showed that Cd exposure enhanced H2O2 accumulation and aggravated oxidative damage in H7 plants. The application of an NO donor effectively reversed growth inhibition, H2O2 burst, and oxidative stress induced by Cd in H7 plants. Thus, we suggest that NO3--induced AHb1 expression suppresses Cd-induced NO production in plants, increasing the ROS burst and exacerbating Cd toxicity.

Effect of the Interaction between Elevated Carbon Dioxide and Iron Limitation on Proteomic Profiling of Soybean

Elevated atmospheric CO2 (eCO2) and iron (Fe) availability are important factors affecting plant growth that may impact the proteomic profile of crop plants. In this study, soybean plants treated under Fe-limited (0.5 mM) and Fe-sufficient (20 mM) conditions were grown at ambient (400 μmol mol−1) and eCO2 (800 μmol mol−1) in hydroponic solutions. Elevated CO2 increased biomass from 2.14 to 3.14 g plant−1 and from 1.18 to 2.91 g plant−1 under Fe-sufficient and Fe-limited conditions, respectively, but did not affect leaf photosynthesis. Sugar concentration increased from 10.92 to 26.17 μmol g FW−1 in roots of Fe-sufficient plants and from 8.75 to 19.89 μmol g FW−1 of Fe-limited plants after exposure to eCO2. In leaves, sugar concentration increased from 33.62 to 52.22 μmol g FW−1 and from 34.80 to 46.70 μmol g FW−1 in Fe-sufficient and Fe-limited conditions, respectively, under eCO2. However, Fe-limitation decreases photosynthesis and biomass. Pathway enrichment analysis showed that cell wall organization, glutathione metabolism, photosynthesis, stress-related proteins, and biosynthesis of secondary compounds changed in root tissues to cope with Fe-stress. Moreover, under eCO2, at sufficient or limited Fe supply, it was shown an increase in the abundance of proteins involved in glycolysis, starch and sucrose metabolism, biosynthesis of plant hormones gibberellins, and decreased levels of protein biosynthesis. Our results revealed that proteins and metabolic pathways related to Fe-limitation changed the effects of eCO2 and negatively impacted soybean production.

Alleviation of Cd phytotoxicity and enhancement of rape seedling growth by plant growth-promoting bacterium Enterobacter sp. Zm-123

The present study aims to investigate the impact of a metal-tolerant bacterium on metal detoxification and rape seedling growth promotion under Cd stress. The results showed that the isolated bacterium Enterobacter sp. Zm-123 has capability to resist Cd (200 mg/L), produce IAA (26.67 mg/L) and siderophores (82.34%), and solubilize phosphate (137.5 mg/L), etc. Zm-123 inoculation significantly enhanced the fresh weight of rape seedlings from 9.47 to 19.98% and the root length from 10.42 to 57.05% compared with non-inoculation group under different concentrations of Cd (0, 0.5, 1, 3, 5 mg/L) (p < 0.05). It also significantly increased the content of chlorophyll, soluble sugar, soluble protein, and proline (p < 0.05) in rape seedlings. Moreover, a significant elevation in catalase (CAT) and peroxidase (POD) activities and a significant reduction in malondialdehyde (MDA), electrolyte leakage (EL), and Cd content in rape seedlings were detected owing to Zm-123 inoculation (p < 0.05). The combined results imply that strain Zm-123 can alleviate the Cd phytotoxicity and promote the rape seedling growth by improving the physiological activity and antioxidant level, which can be potentially applied to protect plants from Cd toxicity.

Abscisic acid-generating bacteria can reduce Cd concentration in pakchoi grown in Cd-contaminated soil

Contamination of vegetable plants with cadmium (Cd) has become a serious issue in recent years. In the present study, pakchoi (Brassica chinensis L.) grown in Cd-contaminated soil inoculated with abscisic acid (ABA)-generating bacteria, Azospirillum brasilense and Bacillus subtilis, showed 28%-281% and 26%-255% greater biomass, and 40%-79% and 43%-77% lower Cd concentrations, respectively, than those of the control_{bacteria-free} plants. These treatments also alleviated the Cd-induced photosynthesis inhibition and oxidative damage (indicated by malondialdehyde [MDA], H₂O₂, and O₂^• -). Furthermore, the application of bacteria also remarkably improved the levels of antioxidant-related compounds (total phenolics, total flavonoids, ascorbate, and 2,2-diphenyl-1-picrylhydrazyl [DPPH] activity) and nutritional quality (soluble sugar and soluble protein) in the Cd-supplied plants. Based on these results, we conclude that the application of ABA-generating bacteria might be an alternative strategy for improving the biomass production and quality of vegetable plants grown in Cd-contaminated soil.

The mechanism of hydrogen sulfide mitigation of iron deficiency-induced chlorosis in strawberry (Fragaria × ananassa) plants

A study was carried out to assess the mitigation mechanism of exogenously applied sodium hydrosulfide (NaHS) as a donor of H₂S on strawberry seedlings under iron deficiency. The ameliorative effects of NaHS on oxidative damage, ion hemostasis and uptake, and availability of Fe were investigated by spraying solution of 0.2 mM NaHS or 0.2 mM NaHS plus 0.2 mM hypotaurine (HT), a scavenger of H₂S to plant leaves. Iron deficiency was created using 0.1 mM FeSO₄ instead of 0.1 mM EDTA-Fe in Hoagland's nutrient solution. After a 28-day treatment, strawberry plants exhibited leaf interveinal chlorosis under Fe deficiency, but these apparent symptoms of iron deficiency were overcome by foliar application of NaHS. Exogenously applied NaHS enhanced chlorophyll contents and available iron and Fe accumulation in young leaves, but application of H₂S scavenger hypotaurine with NaHS did not change those parameters under Fe deficiency. This clearly shows that NaHS improved iron availability in the strawberry plants. Furthermore, exogenously applied NaHS increased endogenous H₂S and iron levels in the roots and leaves. Moreover, NaHS enhanced the levels of zinc (Zn²⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) in both leaves and roots of the strawberry plants grown at Fe deficiency, except for Zn in roots which decreased significantly. This also suggests that NaHS maintains the levels of inorganic ions restricted by Fe deficiency. Fe deficiency increased electrolyte leakage (EL) and the levels of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) in plant leaves. Exogenous NaHS reduced the accumulation of H₂O₂, MDA, and EL and upregulated the activities of key antioxidant enzymes. Overall, NaHS improved Fe uptake and activation by improving endogenous H₂S, maintained balance of mineral nutrients and activities of the antioxidant enzymes, and reduced the generation of MDA and H₂O₂ as well as electrolyte leakage caused by Fe deficiency. So NaHS proved to be effective in ameliorating iron chlorosis caused by iron deficiency.

Effects of split applications of nitrogen fertilizers on the Cd level and nutritional quality of Chinese cabbage

Cadmium (Cd) contamination in soil is an increasingly serious problem. Management of plant nutrients has been proposed as a potentially promising strategy for minimizing Cd accumulation in crops grown in contaminated soil. This study investigated the effects of split applications of nitrogen (N) fertilizers on the Cd concentration in Chinese cabbage (Brassica chinensis L.) plants grown in Cd-contaminated soil. Compared with single applications, split applications of ammonium or urea resulted in significantly lower Cd concentrations, and higher biomass production and antioxidant-associated nutritional quality in the edible plant parts. However, when nitrate was used as the N fertilizer, there were no significant differences between the split and single applications for the same parameters. We conclude that a split application could be more beneficial than a single application method when ammonium or urea is used as the N fertilizer for vegetable cultivation in Cd-contaminated soil.

Transcriptional Characterization of a Widely-Used Grapevine Rootstock Genotype under Different Iron-Limited Conditions

Iron chlorosis is a serious deficiency that affects orchards and vineyards reducing quality and yield production. Chlorotic plants show abnormal photosynthesis and yellowing shoots. In grapevine iron uptake and homeostasis are most likely controlled by a mechanism known as "Strategy I," characteristic of non-graminaceous plants and based on a system of soil acidification, iron reduction and transporter-mediated uptake. Nowadays, grafting of varieties of economic interest on tolerant rootstocks is widely used practice against many biotic and abiotic stresses. Nevertheless, many interspecific rootstocks, and in particular those obtained by crossing exclusively non-vinifera genotypes, can show limited nutrient uptake and transport, in particular for what concerns iron. In the present study, 101.14, a commonly used rootstock characterized by susceptibility to iron chlorosis was subjected to both Fe-absence and Fe-limiting conditions. Grapevine plantlets were grown in control, Fe-deprived, and bicarbonate-supplemented hydroponic solutions. Whole transcriptome analyses, via mRNA-Seq, were performed on root apices of stressed and unstressed plants. Analysis of differentially expressed genes (DEGs) confirmed that Strategy I is the mechanism responsible for iron uptake in grapevine, since many orthologs genes to the Arabidopsis "ferrome" were differentially regulated in stressed plant. Molecular differences in the plant responses to Fe absence and presence of bicarbonate were also identified indicating the two treatments are able to induce response-mechanisms only partially overlapping. Finally, we measured the expression of a subset of genes differentially expressed in 101.14 (such as IRT1, FERRITIN1, bHLH38/39) or known to be fundamental in the "strategy I" mechanism (AHA2 and FRO2) also in a tolerant rootstock (M1) finding important differences which could be responsible for the different degrees of tolerance observed.

Iron nutrition, biomass production, and plant product quality

One of the grand challenges in modern agriculture is increasing biomass production, while improving plant product quality, in a sustainable way. Of the minerals, iron (Fe) plays a major role in this process because it is essential both for plant productivity and for the quality of their products. Fe homeostasis is an important determinant of photosynthetic efficiency in algae and higher plants, and we review here the impact of Fe limitation or excess on the structure and function of the photosynthetic apparatus. We also discuss the agronomic, plant breeding, and transgenic approaches that are used to remediate Fe deficiency of plants on calcareous soils, and suggest ways to increase the Fe content and bioavailability of the edible parts of crops to improve human diet.

Short-term alteration of nitrogen supply prior to harvest affects quality in hydroponic-cultivated spinach (Spinacia oleracea)

BACKGROUND

Quality-associated problems, such as excessive in planta accumulation of oxalate, often arise in soillessly cultivated spinach (Spinacia oleracea). Maintaining a higher level of ammonium (NH₄⁺) compared to nitrate (NO₃⁻) during the growth period can effectively decrease the oxalate content in hydroponically cultivated vegetables. However, long-term exposure to high concentrations of NH₄⁺ induces toxicity in plants, and thus decreases the biomass production. Short-term application of NH₄⁺ before harvesting in soilless cultivation may provide an alternative strategy to decrease oxalate accumulation in spinach, and minimise the yield reduction caused by NH₄⁺ toxicity.

RESULT

The plants were pre-cultured in 8 mmol L⁻¹ NO₃⁻ nutrient solution. Next, 6 days before harvest, the plants were transferred to a nutrient solution containing 4 mmol L⁻¹ NO₃⁻ and 4 mmol L⁻¹ NH₄⁺. This new mix clearly reduced oxalate accumulation, increased levels of several antioxidant compounds, and enhanced antioxidant capacity in the edible parts of spinach plants, but it did not affect biomass production. However, when the 8 mmol L⁻¹ NO₃⁻ was shifted to either nitrogen-free, 4 mmol L⁻¹ NH₄⁺ or 8 mmol L⁻¹ NH₄⁺ treatments, although some of the quality indexes were improved, yields were significantly reduced.

CONCLUSIONS

Short-term alteration of nitrogen supply prior to harvest significantly affects quality and biomass of spinach plants, and we strongly recommend to simultaneously use NO₃⁻ and NH₄⁺ in hydroponic cultivation, which improves vegetable quality without decreasing biomass production.