Molecular and biochemical mechanisms underlying boron-induced alleviation of cadmium toxicity in rice seedlings

Predicting Cd accumulation in rice and identifying nonlinear effects of soil nutrient elements based on machine learning methods

The spatial mismatch of Cd content in soil and rice causes difficulties in environmental management for paddy soil. To investigate the influence of soil environment on the accumulation of Cd in rice grain, we conducted a paired field sampling in the middle of the Xiangjiang River basin, examining the relationships between soil properties, soil nutrient elements, Cd content, plant uptake factor (PUFCd), and translocation factors in different rice organs (root, shoot, and grain). The total soil Cd (CdT) and available Cd (CdA) contents and PUFCd showed large spatial variability with ranges of 0.31-6.19 mg/kg, 0.03-3.07 mg/kg, and 0.02-3.51, respectively. Soil pH, CdT, CdA, and the contents of soil nutrient elements (Mg, Mn, Ca, P, Si, and B) were linearly correlated with grain Cd content (Cdg) and PUFCd. The decision tree analysis identified nonlinear effects of Si, Zn and Fe on rice Cd accumulation, which suggested that low Si and high Zn led to high Cdg, and low Si and Fe caused high PUFCd. Using the soil nutrient elements as predictor variables, random forest models successfully predicted the Cdg and PUFCd and performed better than multiple linear regressions. It suggested the impacts of soil nutrient elements on rice Cd accumulation should receive more attention.

Boron decreases cadmium accumulation in water spinach by enhancing cadmium retention in the root cell walls

Cadmium (Cd) contamination and boron (B) deficiency are two major challenges associated with the farmland soils in Southern China. Therefore, this study was conducted to examine the impacts of B supply on Cd accumulation in water spinach (Ipomoea aquatica) using a cultivar (T308) with high Cd accumulation. The study further investigated the physiological mechanism behind the changes in Cd accumulation due to B supply. The findings revealed that B supply substantially reduced the Cd concentration in the leaves of water spinach by 41.20% and 37.16% under the Cd stress of 10 μM and 25 μM, respectively. Subcellular distribution of Cd showed that the Cd content as well as its proportion in root cell wall (RCW) increased significantly after B supply. Fourier transform infrared spectroscopy showed significant enrichment of negatively charged groups (such as -OH, -COOH, and -NH2) in the RCW after B supply. Overall, B supply also enhanced covalently bound pectin (CSP) content as well as the Cd content linked with CSP under Cd stress. These observations revealed that B regulated the Cd chelation in RCW, thereby reducing the amassment of Cd in water spinach.

Effect of boron supply on the uptake and translocation of cadmium in Capsicum annuum

Large areas of soil in southern China are contaminated with cadmium (Cd) and are deficient in boron (B). Previously, we suggested that B supplementation could reduce Cd accumulation in hot peppers (Capsicum annuum L.); however, the physiological mechanisms underlying this reduction remain unclear. In this study, the uptake and translocation of Cd in hot pepper plants were investigated using hydroponic experiments with different B and Cd treatments. A pot experiment was performed to verify whether B decreased the Cd concentration in hot peppers by minimizing the Cd translocation rate. The results of the dose- and time-dependent experiments showed that B supplementation reduced root Cd uptake and root-to-shoot Cd translocation. Additionally, B supplementation increased the root length, diameter, volume, surface area, and number of root forks and tips, as well as improving the relative absorbance of carboxyl groups under Cd exposure, leading to enhanced Cd fixation in the cell walls of the roots. As a result, the fruit Cd concentration decreased because B inhibited Cd translocation from the roots. Overall, the results demonstrate that B supplementation can reduce Cd accumulation in hot peppers by promoting normal root growth and development and by limiting the uptake and translocation of Cd.

Transcriptomic analysis reveals key genes and pathways corresponding to Cd and Pb in the hyperaccumulator Arabis paniculata

Soil and water are increasingly at risk of contamination from the toxic heavy metals lead (Pb) and cadmium (Cd). Arabis paniculata (Brassicaceae) is a hyperaccumulator of heavy metals (HMs) found widely distributed in areas impacts by mining activities. However, the mechanism by which A. paniculata tolerates HMs is still uncharacterized. For this experiment, we employed RNA sequencing (RNA-seq) in order to find Cd (0.25 mM)- and Pb (2.50 mM)-coresponsive genes A. paniculata. In total, 4490 and 1804 differentially expressed genes (DEGs) were identified in root tissue, and 955 and 2209 DEGs were identified in shoot tissue, after Cd and Pb exposure, respectively. Interestingly in root tissue, gene expression corresponded similarly to both Cd and Pd exposure, of which 27.48% were co-upregulated and 41.00% were co-downregulated. Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) analyses showed that the co-regulated genes were predominantly involved in transcription factors (TFs), cell wall biosynthesis, metal transport, plant hormone signal transduction, and antioxidant enzyme activity. Many critical Pb/Cd-induced DEGs involved in phytohormone biosynthesis and signal transduction, HM transport, and transcription factors were also identified. Especially the gene ABCC9 was co-downregulated in root tissues but co-upregulated in shoot tissues. The co-downregulation of ABCC9 in the roots prevented Cd and Pb from entering the vacuole rather than the cytoplasm for transporting HMs to shoots. While in shoots, the ABCC9 co-upregulated results in vacuolar Cd and Pb accumulation, which may explain why A. paniculata is a hyperaccumulator. These results will help to reveal the molecular and physiological processes underlying tolerance to HM exposure in the hyperaccumulator A. paniculata, and aid in future efforts to utilize this plant in phytoremediation.

Root cell wall remodeling: A way for exopolysaccharides to mitigate cadmium toxicity in rice seedling

Exopolysaccharides (EPS) are macromolecules with environment beneficial properties. Currently, numerous studies focus on the absorption of heavy metals by EPS, but less attention has been paid to the effects of EPS on the plants. This study explored the effects of EPS from Lactobacillus plantarum LPC-1 on the structure and function of cell walls in rice seedling roots under cadmium (Cd) stress. The results showed that EPS could regulate the remodeling process of the cell walls of rice roots. EPS affects the synthesis efficiency and the content of the substances that made up the cell wall, and thus plays an essential role in limiting the uptake and transport of Cd in rice root. Furthermore, EPS could induce plant resistance to heavy metals by regulating the lignin biosynthesis pathway in rice roots. Finally, the cell wall remodeling induced by EPS likely contributes to plant stress responses by activating the reactive oxygen species (ROS) signaling.

Interaction between Boron and Other Elements in Plants

Boron (B) is an essential mineral nutrient for growth of plants, and B deficiency is now a worldwide problem that limits production of B deficiency-sensitive crops, such as rape and cotton. Agronomic practice has told that balanced B and other mineral nutrient fertilizer applications is helpful to promote crop yield. In recent years, much research has reported that applying B can also reduce the accumulation of toxic elements such as cadmium and aluminum in plants and alleviate their toxicity symptoms. Therefore, the relation between B and other elements has become an interesting issue for plant nutritionists. Here we summarize the research progress of the interaction between B and macronutrients such as nitrogen, phosphorus, calcium, potassium, magnesium, and sulfur, essential micronutrients such as iron, manganese, zinc, copper, and molybdenum, and beneficial elements such as sodium, selenium, and silicon. Moreover, the interaction between B and toxic elements such as cadmium and aluminum, which pose a serious threat to agriculture, is also discussed in this paper. Finally, the possible physiological mechanisms of the interaction between B and other elements in plants is reviewed. We propose that the cell wall is an important intermediary between interaction of B and other elements, and competitive inhibition of elements and related signal transduction pathways also play a role. Currently, research on the physiological role of B in plants mainly focuses on its involvement in the structure and function of cell walls, and our understanding of the details for interactions between B and other elements also tend to relate to the cell wall. However, we know little about the metabolic process of B inside cells, including its interactions with other elements. More research is needed to address the aforementioned research questions in future.

Boron supplying alters cadmium retention in root cell walls and glutathione content in Capsicum annuum

Large areas of farmland in southern China are facing environmental problems such as cadmium (Cd) contamination and boron (B) deficiency. The aim of this study was to investigate the biochemical and molecular mechanisms underlying the reduction in Cd accumulation in hot pepper (Capsicum annuum) by B application. A hydroponic experiment was conducted to compare the subcellular distribution of Cd, transcriptome profile, degree of pectin methylation, and glutathione (GSH) synthesis in the roots of hot pepper under different B and Cd conditions. Boron supply promoted root cell wall biosynthesis and pectin demethylation by upregulating related genes and increasing cell wall Cd concentration by 28%. In addition, with the application of B, the proportion of Cd in root cell walls increased from 27% to 37%. Boron supplementation upregulated sulfur metabolism-related genes but decreased cysteine and GSH contents in the roots. As a result, shoot Cd concentration decreased by 27% due to the decrease in GSH, a critical long-distance transport carrier of Cd. Consequently, B supply could reduce the uptake, translocation, and accumulation of Cd in hot pepper by retaining Cd in the root cell walls and decreasing GSH content.