Changes in Tillering, Nutritional Status, and Biomass Yield of Panicum Maximum Used for Cadmium Phytoextraction

Although several grasses have been evaluated for cadmium (Cd) phytoextraction, there are no studies assessing how Cd is accumulated and distributed in the tissues of Panicum maximum grown in mildly polluted soils. The evaluation of tillering, nutritional status and biomass yield of this grass, mainly along successive shoot regrowths, is not well studied so far. Thus, P. maximum Jacq. cv. Massai was grown for two periods in an Oxisol presenting bioavailable Cd concentrations varying from 0.04 (control) to 10.91 mg kg−1 soil. Biomass yield of leaves and stems´ growth have decreased under the highest Cd exposure, but it did not occur in the regrowth period, indicating that Cd-induced toxicity is stronger in the early stages of development of P. maximum. The tillering was not compromised even the basal node presenting Cd concentrations higher than 100 mg kg−1 DW. We identified a restriction on Cd transport upwards from basal node, which was the main local of Cd accumulation. Apparently, P, K, Mg, S and Cu are involved in processes that restrict Cd translocation and confer high tolerance to Cd in P. maximum. The Cd-induced nutritional disorders did not negatively correlate with factors used to calculate phytoextraction efficiency. However, the nutritional adjustments of P. maximum to cope with Cd stress restricted the upward Cd transport, which decreased the phytoextraction efficiency from the available Cd concentration of 5.93 mg kg−1 soil.

Nitrogen Application Practices to Reduce Cd Concentration in Rice (Oryza Sativa L.) Grains

Cd pollution in paddy soils creates challenges in rice grain production, thereby threatening food security. The effectiveness of different base-tillering-panicle urea application ratio and the combined basal application of urea and Chinese milk vetch (CMV, Astragalus sinicus L.) in minimizing Cd accumulation in rice grains was explored in a Cd-contaminated acidic soil via a field experiment. The results indicated that under similar N application rates, an appropriate amount of urea applied at the panicle stage or the combined basal application of urea and CMV decreased Cd absorption by rice roots and its accumulation in rice grains, as compared with that of conventional N application (control). Furthermore, under a 3:4:3 base-tillering-panicle urea application ratio or for basal application of CMV at high levels, Cd concentrations in brown rice were significantly lower (40.7% and 34.1%, respectively) than that of control. Cd transport coefficient from root to straw was significantly higher than that of control when an appropriate amount of urea was applied at the panicle stage or urea and CMV were applied basally, whereas the Cd transport coefficient from straw to brown rice was relatively lower. Moreover, soil pH, or the concentrations of CEC and CaCl2-Cd under different N fertilizer treatment was not significantly different. However, rice grain yield increased by 29.4% with basal application of a high amount of CMV compared with that of control. An appropriate amount of urea applied at the panicle stage or the combined basal application of urea and CMV decreased Cd absorption by rice roots and inhibited its transport from straw to brown rice, thus reducing Cd concentration in brown rice. Therefore, combined with the key phase of Cd accumulation in rice, a reasonable urea application ratio or a basal application of high amounts of CMV can effectively reduce Cd concentration in brown rice.

Iminodisuccinic Acid Relieved Cadmium Stress in Rapeseed Leaf by Affecting Cadmium Distribution and Cadmium Chelation With Pectin

Rapeseed (Brassica napus. L) is a nutritious vegetable, while cadmium (Cd) pollution threatens the growth, productivity, and food security of rapeseed. By studying the effects of iminodisuccinic acid (IDS), an easily biodegradable and environmental friendly chelating agent, on Cd distribution at the organ and cellular level, we found IDS promoted dry matter accumulation of rapeseed and increased the contents of photosynthetic pigment in leaves. Inhibited root-shoot Cd transport resulted in higher activity of antioxidant enzymes and decreased hydrogen peroxide (H2O2) and malondialdehyde (MDA) accumulation in leaves, which indicated that IDS contributed to alleviating Cd-caused oxidative damage in leaf cells. Additionally, IDS increased Cd distribution in cell wall (CW) and relieved Cd toxicity in organelle of leaves, while IDS did not change the contents of different CW components. The improved Cd fixation in leaf CW was mainly attributed to enhanced demethylation of covalently bound pectin (CSP) and Cd chelation with CSP.

RNA-Seq Identification of Cd Responsive Transporters Provides Insights into the Association of Oxidation Resistance and Cd Accumulation in Cucumis sativus L.

Greenhouse vegetable production (GVP) has grown rapidly and has become a major force for cucumber production in China. In highly intensive GVP systems, excessive fertilization results in soil acidification, increasing Cd accumulation and oxidative stress damage in vegetables as well as increasing health risk of vegetable consumers. Therefore, enhancing antioxidant capacity and activating the expression level of Cd transporter genes seem to be feasible solutions to promote plant resistance to Cd stress and to reduce accumulated Cd concentration. Here, we used transcriptomics to identify five cucumber transporter genes (CsNRAMP1, CsNRAMP4, CsHMA1, CsZIP1, and CsZIP8) in response to cadmium stress, which were involved in Cd transport activity in yeast. Ionomics, gene expression, and REDOX reaction level association analyses have shown that the transcript of CsNRAMP4 was positively correlated with Cd accumulation and antioxidant capacity of cucumber roots. The expression level of CsHMA1 was negatively correlated with Cd-induced antioxidant capacity. The overexpression of CsHMA1 significantly relieved Cd stress-induced antioxidant activities. In addition, shoots with high CsHMA2 expression remarkably presented Cd bioaccumulation. Grafting experiments confirmed that CsHMA1 contributed to the high antioxidant capacity of cucumber, while CsHMA2 was responsible for the transport of Cd from the roots to the shoots. Our study elucidated a novel regulatory mechanism for Cd transport and oxidative damage removal in horticultural melons and provided a perspective to regulate Cd transport artificially by modulating Cd accumulation and resistance in plants.

Comparing the Effects of Calcium and Magnesium Ions on Accumulation and Translocation of Cadmium in Rice

Rice is one of China's most important food crops, and it is considered the primary source of human exposure to cadmium (Cd) pollution. A hydroponic experiment was performed to investigate the effect of calcium (Ca) and magnesium (Mg) on the absorption, distribution, and translocation of Cd in rice. Under the concentration gradient of Ca, Mg, and Cd, the concentrations of Cd in rice tissues were determined. The results revealed that the existence of Ca and Mg in the environment could benefit rice growth and limit the accumulation and translocation of Cd in plants. Cd concentrations in rice plants were as orders: roots > stems > leaves ≈ panicles ≈ husks > grains. While Cd content in rice grains decreased significantly under high Ca and Mg concentrations, this pattern was not observed under low and medium concentrations. Ca2+ and Mg2+ ions significantly influenced the translocation of Cd in the environment-rice system. Under the Ca (Mg)-deficient and Ca (Mg)-rich conditions, the husk and panicle played an essential role in hindering Cd transport to the rice grain, respectively. At the same concentration, the effect of Ca on the decrease of Cd bioconcentration was greater than that of Mg. An apparent antagonism was observed between Cd and Ca (Mg) in different parts of the rice plant. Altogether, the results of this study indicate that it was possible to plant and grow rice in Cd-polluted soil and that the accumulation and translocation of Cd in rice plants could be reduced by optimizing soil nutrient elements.

Selection for Low-cadmium Cultivars and Cadmium Subcellular Distribution Comparison between Two Selected Cultivars of Eggplant (Solanum Melongena L.)

Excessive accumulation of cadmium (Cd) in vegetables poses a serious threat to human health; therefore, it is urgent to screen and cultivate vegetable cultivars with low Cd accumulation in the edible parts. Eggplant has a high tendency for Cd accumulation, but research on its low Cd accumulation cultivars is still rare. In this study, to screen low-Cd cultivars, 30 eggplant cultivars were screened using soils containing 0.22 mg/kg, 2.9 mg/kg (low-Cd), and 4.7 mg/kg of Cd (high-Cd). MYCQ and ZGQ were confirmed as low-Cd cultivars, BXGZ and WCCQ were confirmed as high-Cd cultivars, and a 2.52–3.88 fold difference in Cd concentration was observed in their fruits. The subcellular distribution revealed that the root cell wall and vacuole Cd concentrations of a typical low-Cd cultivar (MYCQ) were significantly higher than those of a typical high-Cd cultivar (BXGZ); however, the Cd concentrations in the cell wall and vacuole in fruits, leaves, and stems were significantly lower in MYCQ than in BXGZ. These results indicated that the low-Cd cultivars of eggplant could lessen Cd toxicity through the elevated Cd retention and sequestration levels of root cell walls and vacuoles, thus reducing Cd transport from roots to aboveground tissues, leading to low Cd accumulation. The findings of this study can provide a physiological and biochemical foundation for the screening and breeding of low-Cd cultivars of fruit vegetables and demonstrates that the application of low-Cd cultivars is necessary for food safety in humans.

Genome-wide Identification and Expression Analysis of NRAMP Transporter Genes in Cucumis sativus and Citrullus lanatus

Natural resistance-associated macrophage proteins (NRAMPs) are able to transport various metal ions across cell membranes, which play an important role in plant normal growth and development. Here, a survey of cucumber (Cucumis sativus) and watermelon (Citrullus lanatus) genomes found a total of five CsNRAMPs and four ClNRAMPs, respectively. Based on the phylogenetic relationships, CsNRAMPs and ClNRAMPs were clustered into three groups (I, II and III). Five orthologous pairs were identified between cucumber and watermelon genome, and they were clustered on the same branch of the phylogenetic tree. The number of introns in CsNRAMPs and ClNRAMPs ranged from 3 to 13 and the genes from group Ι were more fragmented than those in group II. Subsequently, analysis of promoter sequences found that five putative transcription factors could act on NRAMPs. Moreover, CsNRAMPs and ClNRAMPs were differentially regulated by deficiencies of Fe, Mn, Cu or Zn, along with toxicities of Fe, Mn, Cu, Zn or Cd. Functional analysis by heterologous expression in yeast indicated that CsNRAMP4 and ClNRAMP3 participate in Cd transport. Overall, the comprehensive analysis of CsNRAMPs and ClNRAMPs reported herein may pave the way for further investigations examining the regulation and functions of this gene family in cucumber and watermelon.