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

Physiological and molecular responses of strawberry plants to Cd stress

Cadmium (Cd), a toxic metal element, can be absorbed by plants via divalent metal ion transporters, thereby retarding plant growth and posing a threat to human health. Strawberries are popular and economically valuable berry species that are sensitive to soil pollutants, especially Cd. However, the mechanisms underlying Cd stress responses in strawberry plants remain largely unclear. Here, we investigated the physiological and molecular basis of Cd stress responses in strawberry plants using the diploid strawberry 'Yellow Wonder' as a material. The results indicated that Cd stress induced oxidative damage, repressed photosynthetic efficiency, and interfered with the accumulation and redistribution of trace elements. Furthermore, Cd stress reduced the concentrations of indoleacetic acid, trans-zeatin riboside and gibberellic acid while increasing the concentration of abscisic acid, thus altering the phytohormone signaling pathway in strawberry plants. Cd stress also inhibited the expression of genes involved in nitrogen uptake and assimilation while promoting the energy supply for plant survival under Cd toxicity. Moreover, the flavonoid biosynthesis pathway was induced, and the anthocyanin concentration increased, thereby improving the free radical scavenging capacity of strawberry plants under Cd toxicity. Additionally, we identified several transcription factors and functional genes as hub genes based on a weighted gene coexpression network analysis. These results collectively provide a theoretical foundation for strawberry breeding and ensuring agriculture and food safety.

Molecular-Assisted Breeding of Cadmium Pollution-Safe Cultivars

Cadmium (Cd) contamination in edible agricultural products, especially in crops intended for consumption, has raised worldwide concerns regarding food safety. Breeding of Cd pollution-safe cultivars (Cd-PSCs) is an effective solution to preventing the entry of Cd into the food chain from contaminated agricultural soil. Molecular-assisted breeding methods, based on molecular mechanisms for cultivar-dependent Cd accumulation and bioinformatic tools, have been developed to accelerate and facilitate the breeding of Cd-PSCs. This review summarizes the recent progress in the research of the low Cd accumulation traits of Cd-PSCs in different crops. Furthermore, the application of molecular-assisted breeding methods, including transgenic approaches, genome editing, marker-assisted selection, whole genome-wide association analysis, and transcriptome, has been highlighted to outline the breeding of Cd-PSCs by identifying critical genes and molecular biomarkers. This review provides a comprehensive overview of the development of Cd-PSCs and the potential future for breeding Cd-PSC using modern molecular technologies.

A Genome-Wide Identification and Comparative Analysis of the Heavy-Metal-Associated Gene Family in Cucurbitaceae Species and Their Role in Cucurbita pepo under Arsenic Stress

The heavy-metal-associated (HMA) proteins are a class of PB1-type ATPases related to the intracellular transport and detoxification of metals. However, due to a lack of information regarding the HMA gene family in the Cucurbitaceae family, a comprehensive genome-wide analysis of the HMA family was performed in ten Cucurbitaceae species: Citrullus amarus, Citrullus colocynthis, Citrullus lanatus, Citrullus mucosospermus, Cucumis melo, Cucumis sativus, Cucurbita maxima, Cucurbita moschata, Cucurbita pepo, and Legenaria siceraria. We identified 103 Cucurbit HMA proteins with various members, ranging from 8 (Legenaria siceraria) to 14 (Cucurbita pepo) across species. The phylogenetic and structural analysis confirmed that the Cucurbitaceae HMA protein family could be further classified into two major clades: Zn/Co/Cd/Pb and Cu/Ag. The GO-annotation-based subcellular localization analysis predicted that all HMA gene family members were localized on membranes. Moreover, the analysis of conserved motifs and gene structure (intron/exon) revealed the functional divergence between clades. The interspecies microsynteny analysis demonstrated that maximum orthologous genes were found between species of the Citrullus genera. Finally, nine candidate HMA genes were selected, and their expression analysis was carried out via qRT-PCR in root, leaf, flower, and fruit tissues of C. pepo under arsenic stress. The expression pattern of the CpeHMA genes showed a distinct pattern of expression in root and shoot tissues, with a remarkable expression of CpeHMA6 and CpeHMA3 genes from the Cu/Ag clade. Overall, this study provides insights into the functional analysis of the HMA gene family in Cucurbitaceae species and lays down the basic knowledge to explore the role and mechanism of the HMA gene family to cope with arsenic stress conditions.

Hydrogen peroxide mediates cadmium accumulation in the root of a high cadmium-accumulating rice (Oryza sativa L.) line

Hydrogen peroxide (H₂O₂) is a vital signaling molecule in response to cadmium (Cd) stress in plants. However, the role of H₂O₂ on Cd accumulation in root of different Cd-accumulating rice lines remains unclear. Exogenous H₂O₂ and 4-hydroxy-TEMPO (H₂O₂ scavenger) were applied to investigate the physiological and molecular mechanisms of H₂O₂ on Cd accumulation in the root of a high Cd-accumulating rice line Lu527-8 through hydroponic experiments. Interestingly, it was found Cd concentration in the root of Lu527-8 increased significantly when exposed to exogenous H₂O₂, while reduced significantly when exposed to 4-hydroxy-TEMPO under Cd stress, proving the role of H₂O₂ in regulating Cd accumulation in Lu527-8. Lu527-8 showed more Cd and H₂O₂ accumulation in the roots, along with more Cd accumulation in cell wall and soluble fraction, than the normal rice line Lu527-4. In particular, more pectin accumulation, especially low demethylated pectin, was observed in the root of Lu527-8 when exposed to exogenous H₂O₂ under Cd stress, resulting in more negative functional groups with greater capacity to binding Cd in the root cell wall of Lu527-8. It indicated that H₂O₂-induced cell wall modification and vacuolar compartmentalization contributes greatly to more Cd accumulation in the root of the high Cd-accumulating rice line.

Cadmium toxicity in medicinal plants: An overview of the tolerance strategies, biotechnological and omics approaches to alleviate metal stress

Medicinal plants, an important source of herbal medicine, are gaining more demand with the growing human needs in recent times. However, these medicinal plants have been recognized as one of the possible sources of heavy metal toxicity in humans as these medicinal plants are exposed to cadmium-rich soil and water because of extensive industrial and agricultural operations. Cadmium (Cd) is an extremely hazardous metal that has a deleterious impact on plant development and productivity. These plants uptake Cd by symplastic, apoplastic, or via specialized transporters such as HMA, MTPs, NRAMP, ZIP, and ZRT-IRT-like proteins. Cd exerts its effect by producing reactive oxygen species (ROS) and interfere with a range of metabolic and physiological pathways. Studies have shown that it has detrimental effects on various plant growth stages like germination, vegetative and reproductive stages by analyzing the anatomical, morphological and biochemical changes (changes in photosynthetic machinery and membrane permeability). Also, plants respond to Cd toxicity by using various enzymatic and non-enzymatic antioxidant systems. Furthermore, the ROS generated due to the heavy metal stress alters the genes that are actively involved in signal transduction. Thus, the biosynthetic pathway of the important secondary metabolite is altered thereby affecting the synthesis of secondary metabolites either by enhancing or suppressing the metabolite production. The present review discusses the abundance of Cd and its incorporation, accumulation and translocation by plants, phytotoxic implications, and morphological, physiological, biochemical and molecular responses of medicinal plants to Cd toxicity. It explains the Cd detoxification mechanisms exhibited by the medicinal plants and further discusses the omics and biotechnological strategies such as genetic engineering and gene editing CRISPR- Cas 9 approach to ameliorate the Cd stress.

Silicon-enhanced tolerance to cadmium toxicity in soybean by enhancing antioxidant defense capacity and changing cadmium distribution and transport

Cadmium (Cd) is a widely distributed heavy metal that is toxic to plants and humans. Although silicon (Si) has been reported to reduce Cd accumulation and toxicity in plants, evidence on the functions of Si and its mechanisms in the possible alleviation of soybean are limited. Therefore, a controlled experiment was conducted to investigate the impacts and mechanisms of Si on Cd retention in soybean. Here, we determined the growth index, Cd distribution, and antioxidant activity systems of Si, as well as expression levels of differentially expressed genes (DEGs) in Si under Cd stress, and conducted RNA-seq analysis. We not only found that Si can significantly promote soybean plant growth, increase plant antioxidant activities, and reduce the Cd translocation factor, but also revealed that a total of 636 DEGs were shared between CK and Cd, CK and Cd + Si, and Cd and Cd + Si. Moreover, several genes were significantly enriched in antioxidant systems and Cd distribution and transport systems. Therefore, the expression status of Si-mediated Cd stress response genes is likely involved in improving oxidative stress and changing Cd uptake and transport, as well as improving plant growth that contributes to Si alleviating Cd toxicity in plants. Moreover, numerous potential target genes were identified for the engineering of Cd-tolerant cultivars in soybean breeding programs.