Breeding for low cadmium accumulation cereals

The protective roles of Oryza glumaepatula and phytohormone in enhancing rice tolerance to cadmium stress by regulating gene expression, morphological, physiological, and antioxidant defense system

The heavy metal cadmium (Cd) is highly poisonous and has received significant attention from environmental scientists due to its harmful impacts on plants. Oryza glumaepatula is a wild rice that contains useful genes against biotic and abiotic stresses. Therefore, the current study used SG007, a single-segment substitution line (SSSL), generated by crossing O. glumaepatula with an elite rice cultivar (HJX74), to evaluate the resistance potential against Cd. Moreover, we assessed the efficacy of strigolactone GR24 (1 μM) against Cd toxicity (100 μM) by investigating physiological, biochemical, and molecular mechanisms in both cultivars (i.e., SG007 and HJX74). The findings of this study revealed that Cd toxicity declined the chlorophyll a, chlorophyll b, and carotenoids by 50%, 20%, and 44% in SG007, and 58%, 39%, and 59% in HJX74 by enhancing electrolyte leakage (EL), malondialdehyde (MDA), and hydrogen peroxide (H2O2) by 113%, 184%, and 119% in SG007 and 248%, 273% and 195% in HJX74, respectively. GR24 improved growth under Cd stress in both cultivars, and SG007 exhibited better plant growth parameters, antioxidant enzymatic activities, nitric oxide synthase (NOS), and nitric oxide (NO) levels than HJX74 under Cd toxicity. GR24 with SG007 regulated expressions of Cd transporters and reduced the cytological disruptions in cell organelles. The combined utilization of SG007 and GR24 reduced Cd accumulation and oxidative stress and improved plant growth parameters and enzymatic activities. In conclusion, our study highlights the potential of utilizing SG007 in conjunction with GR24 as a practical strategy to mitigate Cd pollution in rice. The results not only underscore the beneficial effects of strigolactone GR24 in alleviating Cd-induced stress but also emphasize the valuable genetic traits of O. glumaepatula in developing rice lines with enhanced tolerance to heavy metals, offering broader implications for sustainable agriculture and crop improvement in contaminated environments.

Multiomics and biotechnologies for understanding and influencing cadmium accumulation and stress response in plants

Cadmium (Cd) is one of the most toxic heavy metals faced by plants and, additionally, via the food chain, threatens human health. It is principally dispersed through agro-ecosystems via anthropogenic activities and geogenic sources. Given its high mobility and persistence, Cd, although not required, can be readily assimilated by plants thereby posing a threat to plant growth and productivity as well as animal and human health. Thus, breeding crop plants in which the edible parts contain low to zero Cd as safe food stuffs and harvesting shoots of high Cd-containing plants as a route for decontaminating soils are vital strategies to cope with this problem. Recently, multiomics approaches have been employed to considerably enhance our understanding of the mechanisms underlying (i) Cd toxicity, (ii) Cd accumulation, (iii) Cd detoxification and (iv) Cd acquisition tolerance in plants. This information can be deployed in the development of the biotechnological tools for developing plants with modulated Cd tolerance and detoxification to safeguard cellular and genetic integrity as well as to minimize food chain contamination. The aim of this review is to provide a current update about the mechanisms involved in Cd uptake by plants and the recent developments in the area of multiomics approach in terms of Cd stress responses, as well as in the development of Cd tolerant and low Cd accumulating crops.

Investigation of elemental composition in red teff grains using inductively coupled plasma optical emission spectroscopy (ICP OES), Sire District, Arsi zone, Ethiopia

BACKGROUND

Minerals are important not only for better plant growth and development but also for human and animal nutrition. It is known that east and west Gojam in the Amhara region and east and west Shoa areas in the Oromia region Ethiopia's most teff growing areas. However, there is no information on the mineral content and nutritional worth of Teff Sire district, Arsi zone, Ethiopia. Since ICP OES is a powerful technique to examine elemental compositions even in lower concentration, it is used in this work to investigate the elemental composition of red teff samples.

METHODS

The elemental compositions of red Teff grain samples were determined using ICP-OES from three sites: S1, S2, and S3 of Sire district, Arsi zone, Ethiopia. Wet digestion of the teff samples was carried out by weighing 0.5 g red teff sample and digested with 8 ml HNO3 and 2 ml H2O2 (30%) for 3:00 h at a temperature of 100℃ on hot plate. The investigations of method validation, limit of detection and limit of quantification were also carried out.

RESULTS

The average amount of elements in red teff sample obtained as 172-280 mg/kg Fe, 13-76 mg/kg Mn, 8.2-8.5 mg/kg Cu, 24-26 mg/kg Zn, and toxic trace elements 0.12-0.29 mg/kg Pb and 0.15-0.22 mg/kg Cd. The limit of detection found in ranges from 0.21 mg Kg-1 to 10.44 mg Kg-1 whereas quantification limit resulted in 0.7 mg Kg-1 to 34.8 mg Kg-1 for the metals under consideration. The method was validated by its linear range in the concentration range of 0.028-1.4 ppm or 0.056-2.8 ppm and excellent recovery result was achieved in the range of 90-120%.

CONCLUSION

This study aimed to investigate the mineral content in red teff cultivated in Ethiopia specifically Arsi zone by using ICP OES. From the obtained results, Iron was the first abundant essential element in red teff compared to Mn, Cu and Zn. The level of trace elements: Cd and Pb in the samples slightly above the acceptable limit, possibly due to agricultural practices like usage of fertilizers, pesticides, and other industrial products. Overall, this red teff elemental composition information contributes to the nutrition database and food safety in Ethiopia and beyond.

Melatonin enhances cadmium tolerance in rice via long non-coding RNA-mediated modulation of cell wall and photosynthesis

In plants, melatonin (MLT) is a versatile signaling molecule involved in promoting plant development and mitigating the damage caused by heavy metal exposure. Long non-coding RNAs (lncRNAs) are essential components in the plant's response to various abiotic stress, functioning within the gene regulatory network. Here, a hydroponic experiment was performed to explore the involvement of lncRNAs in MLT-mediated amelioration of cadmium (Cd) toxicity in rice plants. The results demonstrated that applying 250 mg L-1 MLT in a solution containing 10 μM Cd leads to an effective reduction of 30.0% in shoot Cd concentration. Remarkably, the treatment resulted in a 21.2% improvement in potassium and calcium uptake, a 164.5% enhancement in net photosynthetic rate, and a 33.2% decrease in malondialdehyde accumulation, resulting increases in plant height, root length, and biomass accumulation. Moreover, a transcriptome analysis revealed 2510 differentially expressed transcripts, including the Cd transporters (-3.82-fold downregulated) and the Cd tolerance-associated genes (1.24-fold upregulated). Notably, regulatory network prediction uncovered 6 differentially expressed lncRNAs that act as competitive endogenous RNA or in RNA complex interactions. These key lncRNAs regulate the expression of target genes that are involved in pectin and cellulose metabolism, scavenging of reactive oxygen species, salicylic acid-mediated defense response, and biosynthesis of brassinosteroids, which ultimately modify the cell wall for Cd adsorption, safeguard photosynthesis, and control hormone signaling to reduce Cd toxicity. Our results unveiled a crucial lncRNA-mediated mechanism underlying MLT's role in Cd detoxification in rice plants, providing potential applications in agricultural practices and environmental remediation.

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.

Screening of low-Cd accumulating early rice cultivars coupled with phytoremediation and agro-production: Bioavailability and bioaccessibility tests

Previous studies have focused on total cadmium (Cd) accumulation in rice or its transformation in soil, but only a few have examined the entire soil-rice-human system. This study investigated the Cd bioaccessibility and bioavailability for humans from grains of early rice cultivars grown in a Cd-polluted field and further combined with multi-traits to discover and evaluate the optimum safe production and phytoremediation potential cultivars. The results revealed that Cd concentration in polished rice was <0.20 mg kg^-1 in 79 % of early rice cultivars, implying that Cd levels in rice might be reduced by cultivar selection. Furthermore, the higher values of root to straw translocation factor indicates the maximal accumulation of Cd in straw and with highest soil to straw accumulation factor (>1.0) in 66.67 % of cultivars. However, bioaccessibility and bioavailability varied greatly among cultivars with corresponding values ranging from 5.68 to 7.67 % and 1.87 to 5.71 ng g^-1, respectively. Despite the fact that polynomial fitting revealed a statistically significant relationship between Cd content in polished rice and bioavailable Cd in humans (R² = 0.718, P = 0.025), poor goodness of fit for bioaccessibility, bioavailability, and toxicity varied even within low-Cd accumulating cultivars. As a result of multi trait analysis and bioavailability, Zhuliangyou4024 (ER-9), Lingliangyou211 (ER-3), and Yonxian15 (ER-28) were found to be the three best early rice cultivars with higher essential nutrients, less total and bioavailable Cd, and relative high phytoremediation potential and are suitable for healthy rice production and soil remediation.

The influence of barley genotype and growing conditions on zinc and cadmium accumulation

Zinc and cadmium accumulation of in plant tissues of barley of the line 999-93 and of its regenerant forms resulting from cell selection was assessed. The scheme of the experiment: 1) control; 2) acidic; 3) cadmium. We used the method of inversion voltammetry to assess the share of zinc and cadmium in plant samples. To assess the share of active and total forms of zinc and cadmium in soil samples collected from root rhizosphere we used the method of atomic absorption spectroscopy. Barley plants accumulated zinc from 0.2 to 79.7 mg∙kg-1 of dry phytomass. The genotype of plants has little influence on zinc accumulation, and plant organs participate in equal measure in zinc accumulation. As for the soil background with cadmium excess, there is a tendency to lessening zinc absorption in all the plant organs, and it is not connected with the plant’s genotype. Unlike zinc, cadmium gets accumulated mostly in the root system. In the control background the share of cadmium in roots was 0.2–0.4 mg∙kg-1; in the acidic one – 0.2–3.6 mg∙kg-1; in the cadmium one – 5.5–9.5 mg∙kg-1. In barley grain grown on soil with excess of cadmium we did not find any IPC excess of cadmium. On backgrounds of the same acidity, the more cadmium concentration grew, the less zinc concentration in grain was, mostly it concerns the original genotype, to a smaller degree it concerns the regenerant line on the selective medium with cadmium and aluminum. Coupling accumulation of zinc and cadmium took place mostly on acidic background, it was characteristic of barley with the original genotype and the regenerant selected in vitro as cadmium-resistant; on control background coupling accumulation is characteristic of aluminum-resistant regenerant. These regenerant genotypes had a tendency to eliminating cadmium and absorbing zinc.

The Rice Cation/H+ Exchanger Family Involved in Cd Tolerance and Transport

Cadmium (Cd), a heavy metal toxic to humans, easily accumulates in rice grains. Rice with unacceptable Cd content has become a serious food safety problem in many rice production regions due to contaminations by industrialization and inappropriate waste management. The development of rice varieties with low grain Cd content is seen as an economic and long-term solution of this problem. The cation/H⁺ exchanger (CAX) family has been shown to play important roles in Cd uptake, transport and accumulation in plants. Here, we report the characterization of the rice CAX family. The six rice CAX genes all have homologous genes in Arabidopsis thaliana. Phylogenetic analysis identified two subfamilies with three rice and three Arabidopsis thaliana genes in both of them. All rice CAX genes have trans-member structures. OsCAX1a and OsCAX1c were localized in the vacuolar while OsCAX4 were localized in the plasma membrane in rice cell. The consequences of qRT-PCR analysis showed that all the six genes strongly expressed in the leaves under the different Cd treatments. Their expression in roots increased in a Cd dose-dependent manner. GUS staining assay showed that all the six rice CAX genes strongly expressed in roots, whereas OsCAX1c and OsCAX4 also strongly expressed in rice leaves. The yeast (Saccharomyces cerevisiae) cells expressing OsCAX1a, OsCAX1c and OsCAX4 grew better than those expressing the vector control on SD-Gal medium containing CdCl₂. OsCAX1a and OsCAX1c enhanced while OsCAX4 reduced Cd accumulation in yeast. No auto-inhibition was found for all the rice CAX genes. Therefore, OsCAX1a, OsCAX1c and OsCAX4 are likely to involve in Cd uptake and translocation in rice, which need to be further validated.

Phytoexclusion of heavy metals using low heavy metal accumulating cultivars: A green technology

Heavy metal (HM) pollution of farmland is a serious problem worldwide and consumption of HM-contaminated food products poses significant public health risks. Phytoexclusion using low HM accumulating cultivars (LACs) is a promising and practical technology to mitigate the risk of HM contamination of agricultural products grown in polluted soils, and does not alter cultivation practices, is easy to apply, and is economical. This review provides an overview of the major scientific advances accomplished in the field of LACs worldwide. The LACs concept and identification criteria are presented, and the known LACs among currently cultivated grain crops and vegetables are re-evaluated. The low HM accumulation by LACs is affected by crop ecophysiological features and soil physicochemical characteristics. Taking low Cd accumulating cultivars as an example, it is known that they can efficiently exclude Cd from entering their edible parts in three ways: 1) decrease in root Cd uptake by reducing organic acids secretion in the rhizosphere and transport protein production; 2) restriction of Cd translocation from roots to shoots via enhanced Cd retention in the cell wall and Cd sequestration in vacuoles; and 3) reduction in Cd translocation from shoots to grains by limiting Cd redirection and remobilization mediated through nodes. We propose an LAC application strategy focused on LACs and optimized to work with other agronomic measures according to the classification of HM risk level for LACs, providing a cost-effective and practical solution for safe utilization of large areas of farmland polluted with low to moderate levels of HMs.

Breeding crops by design for future agriculture

Plant breeding is both the science and art of developing elite crop cultivars by creating and reassembling desirable inherited traits for human benefit. From the bulk selection of wild plants for cultivation during early civilization to Mendelian genetics and genomics-assisted breeding in modern society, breeding methodologies have evolved over the last thousand years. In the past few decades, the "Green Revolution" through breeding of semi-dwarf wheat and rice varieties, and the use of heterosis and transgenic crops have dramatically enhanced crop productivity and helped prevent widespread famine (Hickey et al., 2019). Integration of these technologies can significantly improve breeding efficiency in the development of super crop varieties (Li et al., 2018). For example, a hybrid cotton variety CCRI63 and six related hybrid varieties account for nearly 90% of cotton production in the Yangtze River Basin (Wan et al., 2017; Wang et al., 2018). These varieties have successfully combined high yield, good quality, and biotic stress tolerance through the integration of conventional breeding, hybrid and genetically modified organism (GMO) technologies (Lu et al., 2019; Ma et al., 2019; Song et al., 2019). Unfortunately, such technology integration is not practical for most staple food crops, including rice and wheat, because of social or technical restrictions. Furthermore, plant breeding is still labor-intensive and time-consuming, and conventional breeding remains the leading approach for the release of commercial crop varieties worldwide. This is especially true for breeding cultivars and hybrids with high yield, good quality, and resistance to biotic or abiotic stresses (Liu et al., 2015; Gu et al., 2016). New germplasm, knowledge, and breeding techniques are required to breed the next generation of crop varieties.

Strigolactone GR24 improves cadmium tolerance by regulating cadmium uptake, nitric oxide signaling and antioxidant metabolism in barley (Hordeum vulgare L.)

Cadmium (Cd) in the food chain poses a serious hazard to human health. Therefore, a greenhouse hydroponic experiment was conducted to examine the potential of exogenously strigolactone GR24 in lessening Cd toxicity and to investigate its physiological mechanisms in the two barley genotypes, W6nk2 (Cd-sensitive) and Zhenong8 (Cd-tolerant). Exogenous application of 1 μM GR24 (strigol analogue) reduced the suppression of growth caused by 10 μM Cd, lowered plant Cd contents, increased the contents of other nutrient elements, protected chlorophyll, sustained photosynthesis, and markedly reduced Cd-induced H2O2 and malondialdehyde accumulation in barley. Furthermore, exogenous GR24 markedly increased NO contents and nitric oxide synthase activity in the Cd-sensitive genotype, W6nk2, effectively alleviating the Cd-induced repression of the activities of superoxide dismutase and peroxidase, increasing reduced glutathione (GSH) and ascorbic acid (AsA) pools and activities of AsA-GSH cycle including ascorbate peroxidase, glutathione peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase. The findings of the present study indicate that GR24 could be a candidate for Cd detoxification by decreasing Cd contents, balancing nutrient elements, and protecting barley plants from toxic oxidation via indirectly eliminating reactive oxygen species (ROS), consequently contributing to reducing the potential risk of Cd pollution.